Upper limits from five years of VERITAS blazar observations

Upper limits from five years of blazar observations with the VERITAS Cherenkov telescopes

Abstract

Between the beginning of its full-scale scientific operations in 2007 and 2012, the VERITAS Cherenkov telescope array observed more than 130 blazars; of these, 26 were detected as very-high-energy (VHE; E100 GeV) sources. In this work, we present the analysis results of a sample of 114 undetected objects. The observations constitute a total live-time of 570 hours. The sample includes several unidentified Fermi-Large Area Telescope (LAT) sources (located at high Galactic latitude) as well as all the sources from the second Fermi-LAT catalog which are contained within the field of view of the VERITAS observations. We have also performed optical spectroscopy measurements in order to estimate the redshift of some of these blazars that do not have a spectroscopic distance estimate. We present new optical spectra from the Kast instrument on the Shane telescope at the Lick observatory for 18 blazars included in this work, which allowed for the successful measurement or constraint on the redshift of four of them. For each of the blazars included in our sample we provide the flux upper limit in the VERITAS energy band. We also study the properties of the significance distributions and we present the result of a stacked analysis of the data-set, which shows a 4 excess.

Subject headings:
BL Lacertae objects: general – galaxies: active – gamma rays: galaxies – radiation mechanisms: non-thermal

1. Introduction

The current generation of imaging atmospheric Cherenkov telescopes (IACTs), sensitive to very-high-energy (VHE; E100 GeV) -ray photons, has significantly increased our knowledge of blazars. They represent by far the class of objects which dominates the VHE extragalactic sky. Among the 66 extragalactic VHE sources currently detected,86 about 90 of them are blazars.

In the framework of the unified model of active galactic nuclei (AGN), blazars are radio-loud AGN, characterized by a pair of relativistic jets of plasma emitted along the polar axis of the super-massive black-hole powering the system, aligned along the line of sight of the observer (see Urry & Padovani, 1995). The observational properties of blazars include a spectral energy distribution (SED) characterized by a non-thermal continuum from radio to -rays, extreme temporal variability and strong polarization. These properties can be explained by considering that the emission from the jet, enhanced by relativistic effects, dominates the SED (Angel & Stockman, 1980). Spectroscopic measurements in optical and UV reveal that two distinct sub-classes of blazars exist: BL Lac objects, characterized by a featureless optical/UV spectrum, and flat-spectrum radio-quasars (FSRQs), which show instead broad emission lines. The traditional division between the two classes of objects is an equivalent width of the emission lines equal to 5 (see Stickel et al., 1991). The two sub-classes are also characterized by different luminosity and redshift distributions. The FSRQs are on average brighter and located at higher redshifts (see e.g. Padovani, 1992; Massaro et al., 2009). In the unified AGN model, this dichotomy is associated with a similar dichotomy seen in radio-galaxies. FSRQs are considered the blazar version of FR II radio-galaxies (Fanaroff & Riley, 1974), while BL Lac objects correspond to FR I (Urry & Padovani, 1995).

Two broad non-thermal components characterize blazar SEDs. The first, peaking between millimeter and X-rays, is attributed to synchrotron emission by a population of electrons/positrons in the blazar jet. The second, peaking in -rays, is associated, in leptonic models, with inverse Compton scattering between the same leptons and their own synchrotron emission (synchtron-self-Compton model, SSC, Konigl, 1981), or an external photon field, such as the emission from the dusty torus, the accretion disc or the broad-line-region (see Sikora et al., 1994). Alternatively, in hadronic scenarios, the second SED component is attributed to synchrotron emission by protons, or by secondary particles produced in p- interactions (Mücke & Protheroe, 2001). The position of the first peak is used to further classify BL Lac objects into low and high-frequency-peaked BL Lac objects (LBL/HBL), depending on whether the peak frequency is located in infrared/optical or UV/X-rays, respectively. The transition between LBL and HBL is smooth, and a population of intermediate-frequency-peaked BL Lac objects exists as well (IBL, with << Hz, see Laurent-Muehleisen et al., 1998). While BL Lac objects present a variety of synchrotron peak frequencies, FSRQs are almost all characterized by a low-frequency peak. This classification can be seen as a more recent version of the older classification (see e.g. Padovani & Giommi, 1995b) of blazars into radio-selected objects (RBLs, which are more likely FSRQs and LBLs) and X-ray-selected objects (XBLs, which are more likely HBLs)87.

The measurement of blazar spectral properties at VHE is important not only to characterize the blazar emission itself, but also to indirectly study the extragalactic background-light (EBL) in the infrared and visible bands due to the absorption that it causes on VHE photons via pair-production, (see Salamon & Stecker, 1998). VHE observations can also be used to put limits on the strength of the intergalactic magnetic field according to the non-detection of the emission from the cascade triggered by the interaction of the pairs with the cosmic-microwave-background (see e.g. Taylor et al., 2011).

Even though the detection and the measurement of the VHE spectrum of a blazar is of paramount importance for the comprehension of the physics of relativistic jets in AGN and for cosmological studies, a non-detection in the VHE regime can also be extremely useful. It can constrain the source emission model, especially when the flux upper-limit is significantly lower than the extrapolation of the Fermi-LAT (Atwood et al., 2009) measurement in high-energy -rays (HE; 100 MeVE100 GeV) up to VHE, implying the presence of a spectral cut-off. It can also constrain the variability properties of the source at VHE if the blazar has been previously detected or is detected at a later time during a higher flux state. Additionally, for the upcoming Cherenkov Telescope Array (CTA, Actis et al., 2011), it will be useful to have the information from all past observations performed by the current generation of IACTs, in order to make predictions for expected outcomes (see e.g. Sol et al., 2013).

All three major IACTs (H.E.S.S., MAGIC and VERITAS) have published upper-limits on blazar VHE emission, including more than 70 sources in their lists (Aharonian et al., 2005, 2008b; Aleksić et al., 2011a; Aliu et al., 2012; Abramowski et al., 2014). Past IACTs, such as CAT, HEGRA and Whipple, have also presented the results from undetected blazars (Piron, 2000; Aharonian et al., 2004; Falcone et al., 2004; Horan et al., 2004), though their limits have in general been superseded by the current instruments. At higher energies, upper limits on blazars have also been estimated using the air-shower technique with Milagro (Williams, 2005).

The study of blazars is complicated by the uncertainty in their redshifts. In fact, the almost featureless spectra of BL Lac objects imply that a redshift estimate can be obtained only via absorption features from the host galaxy or clouds in the intergalactic medium, or via molecular emission lines (see Fumagalli et al., 2012), or via a less precise photometric estimate (comparing the luminosity of the host galaxy to samples of giant elliptical galaxies). If the blazar belongs to a group of galaxies, its redshift can also be estimated by studying the non-active companions (see Muriel et al., 2015). For VHE studies, the knowledge of the redshift of the blazar is very important, because the absorption by the EBL increases with the distance of the object. Currently, the farthest detected VHE blazar is the gravitationally-lensed quasar S3 0218+357 (Mirzoyan, 2014b) at , closely followed by the FSRQ PKS 1441+25 (Ahnen et al., 2015; Abeysekara et al., 2015) at , while the farthest, persistent (i.e. detected not only during episodic flaring activity) VHE blazar is PKS 1424+240 (, see Acciari et al., 2010; Furniss et al., 2013; Archambault et al., 2014). To improve the constraints on the distance of some VHE candidates, new redshift estimates obtained with the Kast spectrograph at Lick Observatory (see Section 2 and Appendix A) are presented together with the VHE upper limits from VERITAS.

In this paper we present the results of the analysis of most of the non-detected blazars observed by VERITAS from 2007 (the beginning of full-scale scientific operations) to August 2012 (before the upgrade of the VERITAS array, see Kieda, 2013). VERITAS upper limits on six VHE candidates were presented by Aliu et al. (2012). The sample also includes several unidentified Fermi-LAT objects (located at high Galactic latitude, and most probably associated with unidentified AGN, see Ackermann et al., 2012a; Mirabal et al., 2012; Doert & Errando, 2014).

The paper is organized as follows: in Section 2 we provide the details of the properties of the sample and the source selection; the details of the VERITAS data analysis and results are provided in Section 3; a stacked analysis of the data is presented in Section 4, studying the full data-set as well as sub-data-sets defined by redshifts and blazar classes; the conclusions are in Section 5.

2. The sample and new redshift estimates

Blazars targeted by VERITAS as VHE source candidates were selected according to a variety of criteria. Early source selections were based on blazar X-ray or radio catalogs, while more recent candidates have also been selected on the basis of their Fermi-LAT spectral characteristics (Abdo et al., 2009a, 2010a; Nolan et al., 2012) or on their association with clusters of HE -rays (see Archambault et al., 2013). The target list includes:

  • all the nearby () HBL/IBL recommended as potential VHE emitters by Stecker et al. (1996); Perlman (2000) and Costamante & Ghisellini (2002)

  • the X-ray brightest nearby () HBL in the Sedentary (Giommi et al., 2005) and ROXA (Turriziani et al., 2007) surveys

  • four distant () BL Lac objects recommended by Costamante & Ghisellini (2002) and Costamante (2007)

  • all nearby () blazars detected by EGRET (Mukherjee, 2001)

  • several FSRQs recommended as potential VHE emitters by Perlman (2000) and Padovani et al. (2002)

  • two high-frequency-peaked FSRQs (B2 0321+33 and Mrk 1218, see Perlman, 2000; Falcone et al., 2004), which have also been classified as Seyfert-1 galaxies (see Osterbrock & Dahari, 1983; Abdo et al., 2009b)

  • the brightest Fermi-LAT sources after extrapolation into the VERITAS energy band (Abdo et al., 2009a, 2010a; Nolan et al., 2012)

  • sources associated with clusters of HE -rays, but not included in Fermi-LAT catalogs, similar to VER J0521+211 (Archambault et al., 2013).

In addition, several targets have been observed by VERITAS only as targets of opportunity (ToO), following flare alerts by multi-wavelength partners (see Errando, 2011).

The sources included in our sample are listed in Table 1 (for sources without VHE detection, 82 targets) and Table 2 (for known VHE emitters, detected by other instruments, or by VERITAS after 2012 and during flaring activity, 11 targets). Sources are listed in order of increasing Right Ascension (R.A.). For every target, we indicate the name, the coordinates (R.A. and Dec, in J2000), the catalog redshift , the blazar class, the VERITAS dead-time corrected exposure (see Section 3), the average zenith angle of VERITAS observations, the dates of VERITAS observations (in MJD) and on which basis the source was selected as a VHE candidate. Names and coordinates are taken from the SIMBAD database.88 The references for the redshift estimates (and their uncertainties) and the blazar class are provided in the table notes. For every source, the archival SED from the ASDC SED Builder tool89 has been visually inspected. It is used to classify all BL Lac objects marked as HBLs. The total number of hours of VERITAS data analyzed is about 570, which represents about 60% of a single VERITAS yearly observing season, i.e. about one tenth of all good quality VERITAS data taken from 2007 to 2012.

The field-of-view (FoV) of the VERITAS telescope array is about and for every observation there is a chance that, in addition to the targeted blazar, other -ray sources are contained within the FoV. For every target included in our sample we checked if other known -ray sources (included in the 2-year Fermi-LAT catalog, 2FGL, see Nolan et al., 2012) were present in the FoV. Twenty-one 2FGL sources were indirectly observed by VERITAS through proximity to the blazar of interest, and are listed in Table 3. We indicate as well the counterpart name (from the 2FGL catalog), the coordinates (R.A. and Dec) of the counterpart, the redshift and the blazar sub-class (if known). The majority of these additional 2FGL sources are AGN without classification.

2.1. New redshift measurements

In order to measure the distance of some -ray blazars, we observed 18 of the VERITAS targets using the dual-arm Kast spectrograph at the Cassegrain focus of the Shane telescope at Lick Observatory. For all the observations presented here, the instrument was configured with the 600/4310 grism on the blue arm, and the 600/7500 grating on the red arm, the D55 dichroic, and a slit. The dichroic crossover creates an instrumental gap located at 5500 and affects approximately 200  of the spectrum. In the spectroscopic figures 2 - 7 (Appendix A), this gap is shown on each spectrum. While the absolute fluxes are shown in the plots, the flux calibration is the least certain aspect of the spectra. In several spectra there is a gap in flux across the dichroic; this is purely the result of calibrations and is not intrinsic to the AGN. The wavelength coverage is from 3450 to 8000 , but tellurics and fringing mask features above 6850 . We do not show this contaminated portion in the spectra. The targets, observation dates and exposures are summarized in Table 7. The corresponding standard star and the signal-to-noise ratio () are also included in this table. The data were reduced following standard techniques with the Low-Redux pipeline90. Each spectrum was inspected visually for absorption or emission features. Features are noted in the spectral plots, Figures 2 - 7, and in Table 7. For 14 of the sources observed, there are no spectral features that allow redshift measurements or redshift limits. By definition, BL Lac objects have no or weak spectral lines, so this high rate of non-detections is somewhat expected. Below are the sources for which features were found that allow for redshift determinations (see Figure 2):

  • RGB J0250+172

    Observations of the source were obtained on August 15, 2010 (UT) and resulted in the detection of galactic features at . We detected Ca II (H, equivalent width (EW) ; K, ), G band () and Mg I () absorption.

    In the literature, Bauer et al. (2000) quotes a redshift of z=1.10 for RGB J0250+172; however, there is no information on spectroscopic lines provided within the reference. Nilsson et al. (2003) presents optical images of BL Lac objects, including RGB J0250+172. They find that the object is clearly resolved and state that is too high because it results in a host galaxy that is exceedingly bright (). Based on fits to the observed light profile, a redshift estimate of is provided, which is similar to the value measured within this work.

  • 1ES 1118+424

    Observations of the source were obtained on February 14, 2013 (UT) and resulted in the detection of galactic features at . We detected Ca II (H, ; K, ), G band (), Ca I () and Mg I () absorption lines.

    In the literature, the redshift for 1ES 1118+424 is quoted as from a private communication (see Falomo & Kotilainen, 1999). However, Falomo & Kotilainen (1999) derive a lower limit of based on images taken using the Nordic Optical Telescope, where the source is unresolved. They simulate an elliptical host galaxy with and an effective radius of 10 kpc to determine the lowest redshift at which it would not be resolved. These galactic parameters are what they find from other BL Lac objects in their study, and they note that assuming a less luminous and smaller host galaxy would result in a lower redshift estimate.

  • RBS 1366 (=1E 1415+25.9)

    Observations of the source obtained on May 30, 2014 (UT) resulted in the detection of galactic features at . We detected Ca II (H, ; K, ), G band (), Ca I () and Mg I () absorption.

    Halpern et al. (1986) also measure a redshift of based on Ca II, G band, Fe I, Mg I and Na absorption. This source displays the significant variability associated with BL Lac objects. The spectrum taken in 2013 has a lower overall flux than the spectrum taken in 2014 (see Table 7), indicating that the source might have been in different flux states.

  • 1ES 2321+419

    Observations of the source obtained on October 28, 2014 (UT) resulted in the detection of absorption features at . We detected Ca II (H, ; K, ) and Mg II (2796 , ; 2803 , ) absorption. Because the Ca II absorption is narrow, and there are no other galactic features, only a lower limit can be set on the redshift of the source. Additionally, there is potentially Mg II absorption at a higher redshift, .

    In the literature, Falomo & Kotilainen (1999) derive a lower limit for this source of using the same technique as for 1ES 1118+424. While our value is not inconsistent, it is considerably lower than that placed based on assumptions about the host galaxy.

3. VERITAS observations and data analysis

The VERITAS (Very Energetic Radiation Imaging Telescope Array System) telescope array is composed of four IACTs of 12-m diameter each, located at the Fred Lawrence Whipple Observatory, on the slopes of Mount Hopkins, in southern Arizona (31 40 N, 110 57 W). Each telescope has a segmented mirror which focuses light onto a camera composed of 499 photomultipliers located at the focal plane. The instrument FoV is 3.5. For further details on the VERITAS instrument see Holder et al. (2006); Holder (2011).

The telescopes measure the faint Cherenkov light induced by the electromagnetic showers triggered by the interaction of the -ray photons with the Earth atmosphere. Similar cascades triggered by cosmic rays are also detected by VERITAS, and can be rejected by applying specific cuts on the shape of the Cherenkov image (Hillas, 1985).

{ThreePartTable}{TableNotes}
  • a

    Unconstrained redshifts are indicated with a hyphen (). If the redshift value is uncertain it is followed by a colon ().
    Redshift references: 91 Fischer et al. (1998);92 Rau et al. (2012); 93Laurent-Muehleisen et al. (1998); 94Perlman et al. (1996); 95Lawrence et al. (1986); 96Shaw et al. (2013); 97Meisner & Romani (2010);98Marcha et al. (1996);99Böhringer et al. (2000);100Sbarufatti et al. (2005);101Cohen et al. (1987);102this work;103Piranomonte et al. (2007);104Bauer et al. (2000);105Lavaux & Hudson (2011);106Carswell et al. (1974);107Falomo (1991);108Osterbrock & Dahari (1983);109Stickel et al. (1989);110Plotkin et al. (2010);111Nilsson et al. (2003);112Stocke et al. (1991); 113Cao et al. (1999); 114Sbarufatti et al. (2009);115Burbidge et al. (1977);116White et al. (2000);117Padovani & Giommi (1995a);118Sandrinelli et al. (2013);119Jannuzi et al. (1993);120Henstock et al. (1997);121Shaw et al. (2009); 122Eracleous & Halpern (2004);123Stickel et al. (1988); 124Sbarufatti et al. (2006); 125Smith et al. (1976);126Falomo & Kotilainen (1999) b Blazars of unknown type are indicated with a hyphen ().
    Blazar type references: 127Laurent-Muehleisen et al. (1999); 128Giommi et al. (2012); 129Chandra et al. (2012); 130Ackermann et al. (2012b); 131Nieppola et al. (2006); 132Abdo et al. (2009b); 133Massaro et al. (2012); 134Rani et al. (2011); 135Osterbrock & Dahari (1983); 136Impey & Neugebauer (1988); 137Cornwell et al. (1986); 138Ajello et al. (2014); 139Massaro et al. (2003); 140Padovani et al. (2002); 141Komossa et al. (2006); 142Li et al. (2010); 143Drinkwater et al. (1997); 144Abdo et al. (2010b); 145Lister et al. (2011);146Smith et al. (1976) c Source selection references: 1FGL, Abdo et al. (2010a); 2FGL, Nolan et al. (2012); B97, Brinkmann et al. (1997); CG02, Costamante & Ghisellini (2002); C07, Costamante (2007); F04, Falcone et al. (2004); M01, Mukherjee (2001); N06, Nieppola et al. (2006); ROXA, Turriziani et al. (2007); S96, Stecker et al. (1996); SHBL, Giommi et al. (2005); P00, Perlman (2000); P02, Padovani et al. (2002); ToO, Target of Opportunity

    RBS 0042 00 18 27.8 +29 47 32 0.100:147 HBL 7.1 15 4731/32/33/40 SHBL 4741/42/46/73 5089/90/91 5131/43 RBS 0082 00 35 14.7 +15 15 04 1.28:148 HBL 6.3 19 5100/01/02 SHBL 5119/20/29/30 1ES 0037+405 00 40 13.8 +40 50 05 - HBL 36.0 14 4767/68/71/72 ToO 4773/89/91 4800/02/22/29/46 5156/57/58/59 5457/70/71/72 5475/76/77/78/79 5495/96/97/98 5500/01/23/26 5546/54/56 1RXS J0045.3+2127 00 45 19.2 +21 27 43 - HBL 1.2 22 5512 N06, 1FGL RGB J0110+418 01 10 04.9 +41 49 51 0.096149 IBL150 4.0 13 4832/33 P00 5866/67/68/96 1ES 0120+340 01 23 08.7 +34 20 51 0.272151 HBL 5.9 16 4383/84/93/94 SHBL, CG02 4414/37/5171 5470/73/99 5512/26 QSO 0133+476 01 36 58.6 +47 51 29 0.859152 FSRQ153 0.8 40 4508 ToO B2 0200+30 02 03 45.6 +30 41 30 0.761154 - 1.8 21 5569/70/88 ToO CGRaBS J0211+1051 02 11 13.1 +10 51 35 0.20:155 LBL156 4.0 31 5588/89/90/91 ToO 5595/99 RGB J0214+517 02 14 17.9 +51 44 52 0.049157 IBL158 5.1 22 4773/89/90/91/94 P00, CG02 4800/5156/81 5201/04/5489 RBS 0298 02 16 30.9 +23 15 13 0.289159 HBL 3.1 14 4731/32/33/73 SHBL RBS 0319 02 27 16.6 +02 02 00 0.457160 HBL 0.3 31 4412 SHBL AO 0235+16 02 38 38.9 +16 36 59 0.94161 LBL162 4.3 21 4737/38/39/42/45 1FGL, ToO RGB J0250+172 02 50 38.0 +17 12 08 0.243163 IBL164 5.1 22 5144/59/5472 1FGL 5528/42/71/98 2FGL J0312.8+2013 03 12 23.0 +20 07 50 - - 9.7 14 5209 2FGL 5830/33/34/40 5855/56/57/58 5860/61/62 RGB J0314+247 03 14 02.7 +24 44 33 0.056165 LBL166 3.1 28 4441 P00 4761/62/63/64/73 RGB J0314+063 03 14 23.9 +06 19 57 - HBL 0.3 26 5868 SHBL RGB J0321+236 03 22 00.0 +23 36 11 - IBL167 9.2 12 5501/02/03/04 1FGL 5507/08/10/11 5512/13/14 B2 0321+33 03 24 41.2 +34 10 46 0.061168 9.1 12 4409/12/16/37 P00, F04 FSRQ/ 4438/39/40/47   NLS1169 4448/49/50/64 1FGL J0333.7+2919 03 33 49.2 +29 16 32 - IBL170 0.6 3 5571 1FGL 1RXS J044127.8+150455 04 41 27.4 +15 04 56 0.109171 HBL 10.1 21 4747/48/49/89/90 SHBL 4831/32/33 4847/49/51/79 4880/82/91 2FGL J0423.3+5612 04 23 27.0 +56 12 24 - - 1.6 28 5855/5926 2FGL 1FGL J0423.8+4148 04 23 56.1 +41 50 03 - - 1.0 15 5599 1FGL 1ES 0446+449 04 50 07.3 +45 03 12 0.203:172 IBL173 7.1 22 4734/42/61/62 S96 4763/64/65 5209/13/34/35 5838 RGB J0505+612 05 05 58.7 +61 13 36 - - 9.2 32 5502/03/29/30 1FGL 5535/36/40/41 5543/44/57/58/72 1FGL J0515.9+1528 05 15 47.3 +15 27 17 - - 3.9 18 5480/81/82 1FGL 5558/59 2FGL J0540.4+5822 05 40 26.0 +58 22 54 - - 1.3 30 5856/5926/27 2FGL RGB J0643+422 06 43 26.8 +42 14 19 0.080174 HBL 1.2 23 4439/4790/4892 B97 RGB J0656+426 06 56 10.7 +42 37 02 0.061175 IBL176 9.4 16 4746/66/67/77 P00 4778/79/90/4800 1ES 0735+178 07 38 07.4 +17 42 19 0.424177 IBL178 5.2 18 5531/32/58/59 5574/87/5602 BZB J0809+3455 08 09 38.9 +34 55 37 0.082179 HBL 1.6 13 5928/29/30 1FGL PKS 0829+046 08 31 48.9 +04 29 39 0.174180 IBL181 2.4 30 4822/4921/5181 M01 Mrk 1218 08 38 10.9 +24 53 43 0.028182 FSRQ/ 5.9 13 4423/25/39/40 F04 Sy1183 4448/49/50/52 OJ 287 08 54 48.9 +20 06 31 0.306184 LBL185 10.2 19 4438/39/40/48 CG02, M01 4449/50/52/65/66 5233/35/37/66 5302 B2 0912+29 09 15 52.4 +29 33 24 0.36:186 HBL 11.7 10 5571/72/74/75 1FGL 5576/87/88/89/90 5630/44/72/73 1ES 0927+500 09 30 37.6 +49 50 26 0.188187 HBL 11.7 23 4466/4770/71 SHBL, ROXA 4800/01/02/03/06 4807/20/21/22 5247/75 RBS 0831 10 08 11.4 +47 05 22 0.343188 HBL 1.6 18 5531/59/89 SHBL, ROXA RGB J1012+424 10 12 44.3 +42 29 57 0.36:189 IBL190 1.7 15 5589/5931/86 ROXA 1ES 1028+511 10 31 18.5 +50 53 36 0.36:191 HBL 24.1 23 4412/13/15/4530 4828/29/30/31 4832/59/83 4905/06/11/21 4922/23/27/28 SHBL, CG02 5292/93/95/98 ROXA 5301/03 5919/23/27/31/45 5946/58/70/79/82 6000/02/09/27 6035/38 RGB J1037+571 10 37 44.3 +57 11 56 >0.62:192 IBL193 3.7 26 5241/42/43/46 1FGL RGB J1053+494 10 53 44.1 +49 29 56 0.140194 HBL 7.8 24 4879/81/82/88/91 1FGL 4921/22 5157/58/59 RBS 0921 10 56 06.6 +02 52 14 0.236195 HBL 2.7 30 4821/22/51 SHBL RBS 0929 11 00 21.1 +40 19 28 - HBL 4.4 16 5333/5587/88/89 SHBL, ROXA 1ES 1106+244 11 09 16.2 +24 11 20 0.482196 HBL 1.0 17 5981/82 C07 RX J1117.1+2014 11 17 06.3 +20 14 07 0.139197 HBL 9.1 16 4940/41/42/5538 5540/41/42/43/44 SHBL, CG02 5543/63/64/65/71 1FGL, ToO 1ES 1118+424 11 20 48.1 +42 12 12 0.230198 HBL 6.4 17 5212/32/74/75 SHBL, S96 5276/89/90/91 S4 1150+497 11 53 24.5 +49 31 09 0.334199 FSRQ200 3.8 24 5701/02/03/04/05 ROXA, ToO 5706/07/08/09/10 RGB J1231+287 12 31 43.6 +28 47 50 1.03201 HBL 5.1 17 5239/66/91/98 1FGL 5300/03 1ES 1239+069 12 41 48.3 +06 36 01 0.150202 HBL 1.9 26 4979/80 S96 PG 1246+586 12 48 18.8 +58 20 29 >0.73:203 IBL204 9.6 29 5595/ 5601/03/05 1FGL 5621/24/25/29 5630/31 1ES 1255+244 12 57 31.9 +24 12 40 0.141205 HBL 26.0 16 4531/34/68/80 SHBL, S96 4581/82/83/84 4585/86/87/91 4907/11/23/50 4970/79/80 5207/08/36 5591/5617 5947/49/53/59 5972/78/89 6016/42/44/45 6072/73/74 BZB J1309+4305 13 09 25.5 +43 05 06 0.691206 HBL 9.4 15 5594/96/98 1FGL 5600/02/04/06 5620/22/23/25 1FGL J1323.1+2942 13 23 02.4 +29 41 35 - FSRQ207 8.4 14 4832/34/92/93 1FGL, ToO 4909/14/79 5596/97/99 5602/05/48/49 RX J1326.2+2933 13 26 15.0 +29 33 31 0.431208 HBL 8.4 14 Same as above ROXA, C07 RGB J1341+399 13 41 05.2 +39 59 46 0.169209 HBL 2.7 26 4938/78 ROXA, N06 5972/6045 RGB J1351+112 13 51 20.8 +11 14 53 >0.619210 HBL 6.2 24 5210/21/39/40 1FGL 5241/43/46 5293/97/98 RX J1353.4+5601 13 53 28.1 +53 00 57 0.370211 HBL 2.5 26 5589/90/6002 ROXA, N06 RBS 1350 14 06 59.2 +16 42 06 >0.623212 HBL 4.5 22 5269/97/98 1FGL 5300/01 RBS 1366 14 17 56.7 +25 43 56 0.237213 HBL 10.0 17 4591/92/93/94 4596/99 SHBL, CG02 4611/12/13/15/16 ROXA 6046 1ES 1421+582 14 22 38.9 +58 01 56 0.683214 HBL 3.4 28 5324/25/26/28/30 SHBL 5333/35/50/51 RGB J1439+395 14 39 17.5 +39 32 43 0.344215 HBL 1.5 12 5620/6002 SHBL, ROXA 1RXS J144053.2+061013 14 40 52.9 +06 10 16 0.396216 IBL217 2.5 27 5731/32/34/35/36 1FGL RBS 1452 15 01 01.8 +22 38 06 0.235218 IBL219 4.1 21 5297/98/99 1FGL RGB J1532+302 15 32 02.3 +30 16 29 0.065220 HBL 6.5 17 4939/40/67/68 P00 4970/75/76/77 RGB J1533+189 15 33 11.3 +18 54 29 0.305221 HBL 2.9 20 5648/77 SHBL, ROXA 5705/06/20 N06 1ES 1533+535 15 35 00.9 +53 20 37 0.89:222 HBL 1.0 23 4256/6002 SHBL RGB J1610+671B 16 10 04.1 +67 10 26 0.067223 HBL 6.6 36 4908/38/5268 P00 5292/93/94/95 1ES 1627+402 16 29 01.3 +40 08 00 0.272224 13.1 16 4229/35/36 P02, F04 4537/38/39/40/57 HBL/ 4559/60/61/62/63 NLS1225 4564/65/69/83 4954 GB6 J1700+6830 17 00 09.3 +68 30 07 0.301226 FSRQ227 0.8 37 4914/16 1FGL, ToO PKS 1717+177 17 19 13.0 +17 45 06 >0.58228 LBL229 5.1 18 4909/11/17/18 1FGL 4920/21/22 PKS 1725+045 17 28 25.0 +04 27 05 0.2966230 FSRQ231 0.3 27 4412 M01 PKS 1749+096 17 51 32.8 +09 39 01 0.32232 LBL233 0.3 22 6072 ToO RGB J1838+480 18 39 49.2 +48 02 34 0.30:234 IBL235 0.5 26 6090 2FGL RGB J1903+556 19 03 11.6 +55 40 39 >0.58:236 IBL237 1.0 27 5099 1FGL 1FGL J1926.8+6153 19 26 41.9 +61 54 41 - - 1.3 31 5706/07 1FGL PKS 2233-148 22 36 34.1 -14 33 22 >0.49:238 LBL239 0.3 49 6100 ToO 3C 454.3 22 53 57.7 +16 08 54 0.859240 FSRQ241 1.0 24 5504/31 ToO RGB J2322+346 23 22 44.0 +34 36 14 0.098242 IBL243 2.9 10 4731/36/39 P00 4745/46/47 1ES 2321+419 23 23 52.1 +42 10 59 >0.45244 HBL 4.2 21 4773/76 S96 4802/03/30/31 5091 B3 2322+396 23 25 17.9 +39 57 37 >1.05245 LBL246 1.0 16 5118/30 1FGL 1FGL J2329.2+3755 23 29 14.2 +37 54 15 - - 3.3 11 5470/71/72/75/76 1FGL 5477/78/80 1RXS J234332.5+343957 23 43 33.8 +34 40 04 0.366247 HBL 1.5 17 5912 SHBL List of sources observed by VERITAS \insertTableNotes

    {ThreePartTable}{TableNotes}
  • a

    If the redshift value is uncertain it is followed by a colon (). Redshift references: 248 private communication from Perlman, see Falomo & Kotilainen (1999); 249 Laurent-Muehleisen et al. (1998); 250 Cao et al. (1999); 251 Bade et al. (1994); 252 Burbidge & Kinman (1966); 253 Marziani et al. (1996); 254 Thompson et al. (1990); 255 Landoni et al. (2014); 256 Aleksić et al. (2014a); 257 Meisner & Romani (2010) b Selection references: see Table 1 c VHE detection references (see as well for the blazar subclass classification): 258 Aleksić et al. (2015a); 259 Aharonian et al. (2008a); 260 Mirzoyan (2014c); 261 Mirzoyan (2014a); 262 Aleksić et al. (2011b); 263 Holder (2014a); 264 Albert et al. (2008); 265 Aleksić et al. (2014c); 266 Abramowski et al. (2013a); 267 Aleksić et al. (2014b); 268 Cortina (2013); 269 Aleksić et al. (2014a); 270 Holder (2014b); 271 Aleksić et al. (2012)

    1ES 0033+595 00 35 52.6 +59 50 05 0.086: HBL 22.6 31 4411/18/19/20 CG02, P00 1 4421/38/40/48 4464/66/76 4734/36/37/74 4775/76/77 4803/04/06 5866/67 RGB J0152+017 01 52 33.5 +01 46 40 0.080 HBL 8.4 35 4421/22/37/38 CG02 2 4439/40/47/48 4449/50/64/65/78 5537/5832/33/68 RGB J0847+115 08 47 13.0 +11 33 50 0.198 HBL 12.1 25 4499/4505/07/08 SHBL 3 4522/23/24/25/26 5303 5502/03/31/59 RX J1136.5+6737 11 36 30.1 +67 37 04 0.134 HBL 7.7 37 4860/61/91/92 4 4918/21 CG02, SHBL 5292/94/99 ROXA 5303 PKS 1222+216 12 24 54.4 +21 22 47 0.432 FSRQ 25.8 16 4939/5182 ToO 5a, 5b 5318/19/20/21 5322/24/25/26 5327/28/30/33 5622/24/25/26 5631/33/34 3C 279 12 56 11.1 -05 47 22 0.536 FSRQ 8.3 40 5623 ToO 6a, 6b 5707/08/09/10 5711/15/17 5923/24/25/26/27 6016 PKS 1510-089 15 12 52.2 -09 06 22 0.361 FSRQ 14.9 42 4909/11/48 ToO 7a, 7b 5976/77/78/79 5980/81/82/83 5984/87 RGB J1725+118 17 25 04.3 +11 52 16 >0.35: HBL 10.0 23 4593/94 CG02 8 4615/16/17/18 4619/20/21/22 0FGL J2001.0+4352 20 01 13.5 +43 53 03 0.18: HBL 4.9 25 5143/44/46/51/52 1FGL 9 5326/52/57 RGB J2243+203 22 43 54.7 +20 21 04 >0.39: IBL 4.1 17 5094/98/99 1FGL 10 5101/16/28/29 B3 2247+381 22 50 06.6 +38 25 58 0.118 HBL 6.0 13 5092/93/95/96/97 1FGL 11 5832/89 List of known VHE sources observed but not detected by VERITAS in 2007-2012 \insertTableNotes
    {ThreePartTable}{TableNotes} a Coordinates are provided for the counterpart. If the Fermi-LAT source is not associated with any lower-energy blazar, coordinates from the 2FGL catalog are given instead. b Unconstrained redshifts are indicated with a hyphen ().
    Redshift references: 272 Shaw et al. (2012); 273 Shaw et al. (2013); 274 Kraus & Gearhart (1975); 275 Halpern et al. (1986); 276 Plotkin et al. (2010); 277 Healey et al. (2008); 278 Hewitt & Burbidge (1993); 279 White et al. (1988); 280 Glikman et al. (2007); 281 Afanas’Ev et al. (2005); 282 Sowards-Emmerd et al. (2005). c Blazars of unknown type are indicated with a hyphen ().
    Blazar type references: 283 Shaw et al. (2012); 284 Kraus & Gearhart (1975); 285 Ajello et al. (2014); 286 Ghisellini et al. (2011); 287 Laurent-Muehleisen et al. (1999); 288 Hewitt & Burbidge (1993); 289 Plotkin et al. (2010); 290 Glikman et al. (2007); 291 Afanas’Ev et al. (2005); 292 Maselli et al. (2010). d See Tables 1 and 2 for information on the exposure and the zenith angle of the observations.
    2FGL J0047.9+2232 BWE 0045+2218 00 48 02.5 +22 34 53 1.161 FSRQ RGB J0045+214 2FGL J0148.6+0127 PMN J0148+0129 01 48 33.8 +01 29 01 0.940 - RGB J0152+017 2FGL J0158.4+0107 - 01 58 25.4 +01 07 31 - - RGB J0152+017 2FGL J0205.4+3211 1Jy 0202+319 02 05 04.9 +32 12 30 1.466 FSRQ B2 0200+30 2FGL J0212.1+5318 - 02 12 09.4 +53 18 19 - - RGB J0214+517 2FGL J0213.1+2245 1RXS J021252.2+224510 02 12 52.8 +22 44 52 0.459 HBL RBS 0298 2FGL J0326.1+2226 TXS 0322+222 03 25 36.8 +22 24 00 2.06 FSRQ RGB J0321+236 2FGL J0440.4+1433 TXS 0437+145 04 40 21.1 +14 37 57 - - 1RXS J044127.8+150455 2FGL J0856.3+2058 TXS 0853+211 08 56 39.7 +20 57 43 >0.388 - OJ 287 2FGL J0929.5+5009 QSO J0929+5013 09 29 15.4 +50 13 36 0.370 IBL 1ES 0927+500 2FGL J1058.4+0133 4C 01.28 10 58 29.6 +01 33 58 0.888 FSRQ RBS 0921 2FGL J1059.0+0222 PMN J1059+0225 10 59 06.0 +02 25 12 - - RBS 0921 2FGL J1141.0+6803 1RXS J114118.3+680433 11 41 18.0 +68 04 33 - - RX J1136.5+6737 2FGL J1239.5+0728 PKS 1236+077 12 38 24.6 +07 30 17 0.400 FSRQ 1ES 1239+069 2FGL J1245.1+5708 GB6 J1245+5710 12 45 10.0 +57 09 54 >0.521 LBL PG 1246+586 2FGL J1303.1+2435 VIPS J13030+2433 13 03 03.2 +24 33 56 0.993 LBL 1ES 1255+244 2FGL J1359.4+5541 VIPS J13590+5544 13 59 05.7 +55 44 29 1.014 FSRQ RX J1353.4+5601 2FGL J1722.7+1013 TXS 1720+102 17 22 44.6 +10 13 36 0.732 FSRQ RGB J1725+118 2FGL J1727.9+1220 PKS 1725+123 17 28 07.1 +12 15 39 0.583 FSRQ RGB J1725+118 2FGL J1927.5+6117 S4 1926+611 19 27 30.4 +61 17 33 - LBL 1FGL J1926.8+6153 2FGL J1959.9+4212 1RXS J195956.1+421339 19 59 56.1 +42 13 39 - - 0FGL J2001.0+4352 List of 2FGL sources in the VERITAS field of view of sources listed in Tables 1 and 2 \insertTableNotes
    The results presented in this paper have been obtained using a set of -hadron separation cuts specifically optimized for the detection of soft spectrum sources (differential spectrum parametrized by a power-law function with ). The spectral index assumed is in line with the typical value of observed for VHE blazars (see Şentürk et al., 2013).
    All the observations presented in this paper were made using the ‘wobble’ observing strategy (Fomin et al., 1994). Here, the telescopes point 0.5 away from the target, alternatively in each of the four cardinal directions, to enable background estimation from the same field of view. This procedure ensures a similar acceptance for both the source (ON) and the background (OFF) regions. Regions overlapping bright stars are excluded from background estimates. The ratio of the ON over the OFF region size defines the background normalization parameter . The dead time of the telescope array is explicitly calculated and is approximately 10 for the observations described here. The exposure values provided in Table 1 are all corrected for dead time, and represent the effective live-time of VERITAS observations.
    The VERITAS observations here have an average length of twenty minutes (referred to as a run), before switching targets or wobble directions. For quality assurance all the runs with a length lower than ten minutes were excluded, often being associated with technical problems, resulting in the early termination of the run. Additionally, all observations characterized by non-optimal weather conditions or malfunctioning hardware were excluded from the run selection. On certain occasions one of the VERITAS telescopes can be non-operational due to technical problems; all the runs analyzed in this paper have at least three telescopes in operation. Runs with all four telescopes in operation represent the bulk () of the data.
    In the standard configuration, VERITAS observations are not performed under bright-moonlight conditions (Moon illumination of full Moon). Since 2012, the VERITAS collaboration has started a new observing program in order to extend the duty cycle of the observatory and perform observations also under bright moonlight (Archambault et al., 2015). None of the data presented in this paper were taken under bright moonlight conditions. Observations performed under moderate moonlight (Moon illumination ) are included, and analyzed in the same manner as dark-time observations, with appropriate instrument response functions to account for the increased night-sky background.
    The significance at the source location is computed using Equation 17 in Li & Ma (1983). The upper limit on the VHE flux is estimated according to Rolke et al. (2005) at the confidence level. It is first calculated assuming three different values of the spectral index (, and ) in order to estimate the decorrelation energy (the energy at which the upper limit estimate depends the least on the spectral index). The upper limit is then recomputed at the reference energy assuming a spectral index . The threshold of the analysis (which depends mainly on the zenith angle of the observations) is also calculated. For every source we verified that, not only the overall significance is lower than standard deviations (), but that no flares have been detected, i.e. that none of the sources was detected at more than during any single run.
    For sources which are detected by Fermi-LAT (76% of the sample), the flux is extrapolated into the VERITAS energy band, taking into account the absorption from the EBL using the model by Franceschini et al. (2008), which is in agreement with the most recent observational constraints (Abramowski et al., 2013b). The extrapolated flux is then compared to the VERITAS upper limit. If the VERITAS measurement is lower than the extrapolation, it means that an additional cut-off should be present in the -ray component between the Fermi-LAT and the VERITAS energy bands.
    The results of the analysis are reported in Tables 4 and 5 (for known VHE sources) and 6 (for 2FGL sources in the VERITAS field of view). For every target, we provide the significance, the number of ON and OFF counts, the value of the parameter (ratio of the ON over OFF region size), the threshold of the analysis , the decorrelation energy , the differential flux upper limit at , the integral flux upper limit (above , provided in Crab units, following Hillas et al., 1998)293, the ratio between the VERITAS differential upper limit and the extrapolation of the Fermi-LAT detection (, evaluated at ). Note that the values of the decorrelation and threshold energies are provided with three decimal values to ease any extrapolation to other energy bands, but they are known only to the second decimal value.
    All the results presented in this paper have been cross-checked using a separate analysis, which provided consistent results for the single upper limit values, significance distributions (Section 3.2) and stacked analysis (Section 4).

  • 3.1. Notes on individual sources

    Among the blazars targeted by VERITAS between 2007 and 2012, eleven of them were later identified as VHE emitters. They are listed in Table 2. The VERITAS upper limits are useful in these cases to constrain the properties of the VHE emission during low-flux states, as well as the variability properties of the source. The discussion of these blazar observations, in order of R.A., follows:

    • 1ES 0033+595 (HBL, )
      VHE emission from 1ES 0033+595 was discovered by MAGIC (Aleksić et al., 2015a). The flux, measured from 24 hours of observations taken from August to October 2009, is Crab above 290 GeV294. The observed spectral index during the MAGIC observations is . The VERITAS upper limit ( Crab above 290 GeV) is fully consistent with the MAGIC measurement. The VHE variability of the source has been demonstrated by more recent VERITAS observations, which detected a bright VHE flare (with integral flux higher than 10% Crab) from the source during September 2013, with simultaneous X-ray and ultraviolet coverage by Swift (Benbow, 2015). A paper presenting the results of this multi-wavelength campaign is currently in preparation.

    • RGB J0152+017 (HBL, )
      This source has been known as a VHE emitter since 2008 (Aharonian et al., 2008a), when it was detected by H.E.S.S. at a flux of Crab above 240 GeV, during hours of observation. VERITAS observed the blazar in three different seasons: 2007-2008 (covering the H.E.S.S. period), 2010-2011 and 2011-2012. The VERITAS upper limit ( Crab above 240 GeV) is fully consistent with the H.E.S.S. detection.

    • RGB J0847+115 (HBL, )
      VHE emission from this blazar was announced by MAGIC in 2014, at a flux corresponding to Crab above 200 GeV (Mirzoyan, 2014c). No spectral information is currently available, but the MAGIC collaboration reported a preliminary classification of the source as an extreme-HBL, with synchrotron peak-frequency in hard-X-rays, and inverse-Compton peak-frequency at TeV energies. Evidence of VHE and optical variability was also claimed by MAGIC. The VERITAS upper limit ( of the Crab Nebula flux above 180 GeV) is marginally consistent with the preliminary flux estimate by MAGIC, and could be related to variability of the VHE emission.

    • RX J1136.5+6737 (HBL, )
      The MAGIC collaboration recently reported the detection of this source at a flux of Crab above 200 GeV, in hours of observations between January and April 2014 (Mirzoyan, 2014a). No spectral information is available at the present time. The VERITAS upper limit ( Crab above 290 GeV) is fully consistent with the preliminary flux estimate by MAGIC.

    • PKS 1222+216 (FSRQ, )
      VHE emission from this FSRQ was detected by MAGIC in 2010 at a flux of the order of the Crab Nebula flux (Aleksić et al., 2011b). The detection of this VHE flare is of paramount importance for blazar physics: the rapid variability, and the fact that VHE photons can escape the bright photon field present in FSRQs was used to put constraints on the location of the -ray emitting region in blazars. The VERITAS non-detection constrains the low-flux state at a level of Crab above 180 GeV.
      During May 2014, PKS 1222+216 underwent another -ray flare, and VERITAS detected VHE emission at a flux of Crab (Holder, 2014a). A paper describing the VERITAS detection in 2014 is currently in preparation.

    • 3C 279 (FSRQ, )
      This quasar is the first of its class detected as a VHE emitter (Albert et al., 2008). VERITAS observations during 2011 were triggered by flaring activity observed at lower wavelengths (optical, X-rays and HE -rays). The same flare triggered observations with the MAGIC telescopes (Aleksić et al., 2014c), which resulted as well in no VHE detection (flux upper limit equal to Crab above 260 GeV). The VERITAS flux upper limit ( Crab above 260 GeV) is similar to the one measured with the MAGIC telescopes.

    • PKS 1510-089 (FSRQ, )
      Two VHE flares from this quasar have been reported so far: the first during March-April 2009, seen by H.E.S.S. (around 0.6% of the Crab Nebula flux above 260 GeV, see Abramowski et al., 2013a), the second during February 2012, seen by MAGIC (around 1% of the Crab Nebula flux above 260 GeV, see Aleksić et al., 2014b). The VERITAS observations presented in this paper are quasi-simultaneous with both flares (see Table 2 for details). For the 2009 flare, VERITAS observations were taken a few days before the VHE flare seen by H.E.S.S.. For the 2012 flare, VERITAS observations were taken every night from February 19 to February 27, covering the Fermi-LAT flare. The VERITAS upper limit is 2.9% of the Crab Nebula flux above 260 GeV.

    • RGB J1725+118 (HBL, )
      The discovery of VHE emission from this blazar was recently reported by MAGIC (Cortina, 2013) at a flux of Crab above 140 GeV during observations in May 2013 triggered by an elevated optical state. No spectral information is available at the present time. The VERITAS upper limit ( Crab above 200 GeV) is consistent with the MAGIC measurement.

    • 0FGL J2001.0+4352 (HBL, )
      Early results from Fermi-LAT indicated that this source was a good candidate for IACTs, in particular due to its hard GeV spectrum (Abdo et al., 2009a). The MAGIC collaboration detected the source at a flux of Crab during a single night ( hours on July 16, 2010, see Aleksić et al., 2014a). The VERITAS upper limit clearly indicates that this blazar is variable at VHE, and that its baseline flux is below Crab above 200 GeV.

    • RGB J2243+203 (HBL, )
      VHE emission from this source was detected by VERITAS during December 2014, following a trigger from a high Fermi-LAT flux (Holder, 2014b; Abeysekara, 2015). Preliminary analysis indicates that the flux from the flaring blazar was at Crab above 180 GeV. The upper limits computed from 2009 observations indicate that the VHE emission from this source is variable, being significantly lower (2.1% Crab above 170 GeV) than the 2014 detection.

    • B3 2247+381 (HBL, )
      This source was detected by MAGIC in 14 hours of observations from September to October 2010, at a flux of Crab above 170 GeV (Aleksić et al., 2012). The MAGIC observations were triggered by a high optical state, and there is evidence of variability in the simultaneous X-ray light curve. VERITAS observations do not cover the MAGIC detection, nor the high optical flux state measured by the Tuorla observatory. The non-detection by VERITAS (flux upper limit equal to Crab above 170 GeV) constrains the low-state flux of this blazar to be lower than the MAGIC detection, suggesting that it may have been related to a VHE high-flux state.

    In addition to these known VHE emitters, we discuss a few other targets with noteworthy histories:

    • 1ES 0037+405 (HBL, unknown)
      VERITAS observations of this target were taken as a self-triggered ToO. During observations of the Andromeda galaxy (M31, see Bird, 2015), a 4 hotspot coincident with this blazar was observed in the reconstructed sky-map. However, further observations did not confirm the hotspot, and the cumulative significance is 1.5 in 36 hours.

    • OJ 287 (LBL, )
      This blazar is one of the most studied objects of its kind due to a clear periodicity in its optical lightcurve, with a period of about twelve years. The VERITAS observations presented in this work cover the last active phase in Fall 2007 (from December 4, 2007 to January 1, 2008), with additional observations during 2010. The VHE upper limits are comparable to the ones measured with the MAGIC telescopes and presented by Seta et al. (2009).

    • 1FGL J1323.1+2942 (FSRQ, unknown) and
      RX J1326.2+2933 (HBL, )
      Although the angular distance between the two sources is only 43, they are not the same blazar. VERITAS can resolve the two objects, and they have been targeted by VERITAS independently (see last column of Table 1). Since they are well within the VERITAS FoV, the exposures on these two objects have been merged into a single dataset.

    • B2 0912+29 (HBL, )
      This blazar shows the highest significance in our dataset ( in 11.7 hours). This excess was confirmed at the same significance level by the cross-check analysis chain. Further observations were taken during the 2013 and 2014 observing seasons, but the initial excess did not increase. While VHE blazars are known to be variable, and one could interpret the lack of a detection in 2013-2014 as due to variability, we also note that the probability of a excess reduces to only when 103 trials (the sources from Tables 1 and 3) are taken into account, and it is thus not enough to make any claim.

    3.2. Significance distributions

    In Fig. 1 we present the distribution of the significances for all the sources presented in our work. Given that the blazar population at VHE is not homogeneous (see the Introduction), and depends on both the blazar sub-class (which is correlated with the energy of the high-energy SED peak) and the blazar redshift (which implies a different level of EBL absorption), significance distributions are produced as a function of these two parameters. For the redshift division (left plot of Fig. 1), redshifts lower or higher than 0.6 were considered, along with unknown redshift. Concerning the division of blazar sub-classes (right plot of Fig. 1), the sources were categorized as HBLs, IBLs/LBLs/FSRQs, and blazars of unknown type. The Gaussian distribution expected from a sample with average and is overlaid on the significance distribution. A fit of the histogram with a Gaussian function instead yields and .

    Source name ON OFF E E UL UL UL/
    [TeV] [TeV] [10 cmsTeV] [% C.U.]
    RBS 0042 0.02 1239 6455 0.192 0.182 0.345 08.7 2.2 0.2 (z=0.1)
    RBS 0082 0.16 1680 9062 0.185 0.166 0.264 16.9 1.8 45.0
    1ES 0037+405 1.50 7515 64132 0.115 0.166 0.322 29.3 6.3
    1RXS J0045.3+2127 2.04 224 1116 0.172 0.166 0.297 54.6 8.9 3.0/14.8 (z=0.1/0.5)
    RGB J0110+418 -0.02 801 4810 0.167 0.182 0.299 22.4 3.4
    1ES 0120+340 1.47 1174 5215 0.214 0.166 0.283 24.6 3.4 1.4
    QSO 0133+476 1.24 114 496 0.202 0.417 0.728 11.8 17.4 1.9e4
    B2 0200+30 1.38 495 2775 0.167 0.151 0.273 51.9 6.9 59.6
    CGRaBS J0211+1051 1.01 977 5659 0.167 0.200 0.318 20.8 3.6 3.4
    RGB J0214+517 0.33 1113 5877 0.187 0.182 0.336 15.9 3.7
    RBS 0298 1.76 606 3245 0.173 0.240 0.435 21.5 9.2
    RBS 0319 -0.52 61 393 0.167 0.219 0.351 38.4 8.6 44.9
    AO 0235+16 0.63 704 4116 0.167 0.182 0.311 18.7 3.2 9.6
    RGB J0250+172 -0.06 1274 5637 0.226 0.166 0.316 13.5 2.7 1.4
    2FGL J0312.8+2013 -0.36 2124 9046 0.238 0.166 0.257 10.0 1.0 0.5/0.6 (z=0.1/0.5)
    RGB J0314+247 1.07 691 3967 0.167 0.240 0.460 16.4 8.5
    RGB J0314+063 1.18 76 326 0.200 0.182 0.294 76.7 11.0
    RGB J0321+236 1.24 3065 17948 0.167 0.138 0.233 31.2 2.6 2.0/2.9 (z=0.1/0.5)
    B2 0321+33 -0.03 2190 8223 0.267 0.166 0.272 09.7 1.2 15.9
    1FGL J0333.7+2919 0.37 158 606 0.223 0.138 0.226 101.0 7.6 2.7/7.4 (z=0.1/0.5)
    1RXS J044127.8+150455 1.83 2351 10636 0.212 0.182 0.338 17.5 4.1
    2FGL J0423.3+5612 0.67 283 1522 0.178 0.240 0.457 16.4 8.3 1.9/32.6 (z=0.1/0.5)
    1FGL J0423.8+4148 -0.22 240 1462 0.167 0.166 0.274 46.4 5.7 0.6/2.2 (z=0.1/0.5)
    1ES 0446+449 -1.49 1482 10866 0.142 0.219 0.363 04.7 1.2
    RGB J0505+612 -1.50 2167 9896 0.227 0.219 0.377 04.1 1.2 0.6/5.7 (z=0.1/0.5)
     1FGL J0515.9+1528 -0.54 1149 6734 0.173 0.151 0.283 16.7 2.5 0.9/3.8 (z=0.1/0.5)
     2FGL J0540.4+5822 0.43 226 1315 0.167 0.240 0.459 16.5 8.5 2.8/49.4 (z=0.1/0.5)
     RGB J0643+422 0.46 240 1282 0.181 0.200 0.369 32.7 9.5
    RGB J0656+426 1.16 1960 9607 0.198 0.200 0.322 20.2 3.6
    1ES 0735+178 -1.20 1259 6865 0.190 0.166 0.260 09.0 0.9 0.5
    BZB J0809+3455 -0.24 252 1537 0.167 0.151 0.251 39.8 4.0
    PKS 0829+046 -0.77 465 2465 0.196 0.240 0.379 10.1 2.7 0.6
    Mrk 1218 2.44 1589 8994 0.165 0.166 0.280 43.1 5.7
    OJ 287 0.97 2197 12966 0.166 0.182 0.296 17.4 2.6 3.1
    B2 0912+29 3.49 3466 19492 0.167 0.138 0.228 45.9 3.6 1.6
    1ES 0927+500 -0.18 2378 11404 0.209 0.182 0.346 11.0 2.8
    RBS 0831 -0.31 394 2403 0.167 0.166 0.297 29.5 4.8
    RGB J1012+424 0.18 270 1324 0.167 0.219 0.316 43.5 6.7 22.2
    1ES 1028+511 1.16 4610 27154 0.167 0.182 0.305 12.4 2.0 1.4
    RGB J1037+571 -1.53 790 3798 0.221 0.200 0.331 05.7 1.1 2.5 (z=0.6)
    RGB J1053+494 -0.76 1397 8567 0.167 0.200 0.386 06.4 2.2 0.5
    RBS 0921 0.84 633 3534 0.173 0.240 0.354 20.1 4.2
    RBS 0929 -0.62 923 4678 0.202 0.166 0.319 11.3 2.4 0.5/2.7 (z=0.1/0.5)
    1ES 1106+244 -1.65 200 1151 0.197 0.151 0.257 14.3 1.5 3.3
    RX J1117.1+2014 0.16 2545 12950 0.190 0.151 0.281 12.5 1.8 0.18
    1ES 1118+424 0.39 1685 9703 0.172 0.151 0.267 22.4 2.8 0.39
    S4 1150+497 -0.53 749 4589 0.167 0.182 0.315 12.9 2.3 70.0
    RGB J1231+287 0.74 1258 6664 0.185 0.138 0.243 36.6 3.5 12.2
    1ES 1239+069 -0.86 224 1429 0.167 0.240 0.369 08.7 2.1
    PG 1246+586 0.23 2123 12670 0.167 0.200 0.363 09.5 2.4 14.5 (z=0.73)
    1ES 1255+244 2.24 5127 29732 0.167 0.166 0.315 12.4 2.5
    BZB J1309+4305 0.54 2359 14020 0.167 0.151 0.298 17.8 3.2 7.9
    1FGL J1323.1+2942 0.24 1781 14622 0.121 0.151 0.243 18.0 1.6 1.2/3.7 (z=0.1/0.5)
    RX J1326.2+2933 1.36 1771 14150 0.127 0.166 0.256 17.8 1.7
    RGB J1341+399 0.00 381 2286 0.167 0.200 0.405 12.7 5.1
    RGB J1351+112 1.44 1715 8994 0.184 0.151 0.248 30.0 2.8 2.9 (z=0.62)
    RX J1353.4+5601 0.65 569 3163 0.175 0.200 0.339 24.8 5.3
    RBS 1350 0.55 1387 8190 0.167 0.151 0.248 27.9 2.6
    RBS 1366 1.89 1789 9843 0.173 0.200 0.327 17.1 3.3 7.9
    1ES 1421+582 0.17 674 4016 0.167 0.219 0.378 13.2 3.8
    RGB J1439+395 0.80 404 2321 0.167 0.151 0.246 62.4 5.7 5.0
    1RXS J144053.2+061013 -0.09 424 2556 0.167 0.200 0.343 13.8 3.1 16.7
    RBS 1452 0.03 1232 7474 0.165 0.151 0.254 25.9 2.7 0.5
    RGB J1532+302 -0.56 812 3648 0.227 0.166 0.348 10.7 3.0
    RGB J1533+189 -1.44 653 4460 0.167 0.151 0.293 08.5 1.5
    1ES 1533+535 0.54 191 973 0.188 0.182 0.324 41.7 8.9
    RGB J1610+671B -1.75 1318 6510 0.214 0.263 0.516 01.6 1.1
    1ES 1627+402 -0.33 2191 13244 0.167 0.182 0.360 07.3 2.1
    GB6 J1700+6830 -1.98 81 610 0.167 0.316 0.528 03.5 2.2 161
    PKS 1717+177 0.42 934 4343 0.212 0.182 0.290 19.3 2.6 3.1(z=0.58)
    PKS 1725+045 -0.58 41 271 0.167 0.200 0.329 37.5 7.3 127
    PKS 1749+096 -1.00 64 438 0.167 0.166 0.257 43.6 4.3 1.6
    RGB J1838+480 0.27 87 329 0.167 0.200 0.346 52.3 12.1 4.2
    RGB J1903+556 0.19 164 968 0.167 0.240 0.398 22.4 7.0 22.2 (z=0.58)
    1FGL J1926.8+6153 0.62 231 1326 0.167 0.240 0.408 21.6 7.4 1.1/12.5 (z=0.1/0.5)
    PKS 2233-148 0.06 32 190 0.167 0.501 0.829 07.8 15.0 1.4e3 (z=0.49)
    3C 454.3 -1.21 220 981 0.25 0.138 0.250 23.0 2.5 0.6
    RGB J2322+346 -0.42 518 2926 0.181 0.182 0.296 19.2 2.8 1.4
    1ES 2321+419 2.05 992 3686 0.250 0.219 0.414 23.7 9.4 15.9 (z=0.5)
    B3 2322+396 -0.63 131 705 0.197 0.166 0.256 49.2 4.8 80 (z=1.05)
    1FGL J2329.2+3755 -0.13 847 5106 0.167 0.166 0.254 27.3 2.6 1.0/3.4 (z=0.1/0.5)
    1RXS J234332.5+343957 0.80 341 1952 0.167 0.151 0.250 53.5 5.2 3.2
    Table 4Analysis results and flux upper limits for the non-detected AGN observed by VERITAS
    Source name ON OFF E E UL UL UL/
    [TeV] [TeV] [10 cmsTeV] [% C.U.]
    1ES 0033+595 3.23 4560 20321 0.214 0.288 0.490 10.0 5.4 0.8 (z=0.086)
    RGB J0152+017 1.83 1720 8693 0.188 0.240 0.376 14.1 3.6 1.2
    RGB J0847+115 -0.03 3391 19889 0.171 0.182 0.338 8.63 2.0 0.3
    RX J1136.5+6737 1.10 1324 7271 0.177 0.288 0.519 7.86 5.2 1.3
    PKS 1222+216 3.40 7482 39770 0.180 0.182 0.257 24.1 2.2 0.2
    3C 279 0.10 1479 8849 0.167 0.263 0.430 5.5 2.1 2.6
    PKS 1510-089 2.00 2691 13698 0.167 0.263 0.496 4.7 2.9 1.1
    RGB J1725+118 2.29 1979 10833 0.079 0.200 0.316 18.7 3.2 0.6/2.6 (z=0.1/0.5)
    0FGL J2001.0+4352 0.76 1102 6449 0.167 0.200 0.384 15.5 5.2 0.3 (z=0.2)
    RGB J2243+203 -0.12 1111 6458 0.173 0.166 0.263 19.3 2.1 0.3 (z=0.39)
    B3 2247+381 -0.93 1447 8042 0.185 0.166 0.315 8.8 1.8 0.5
    Table 5Results and upper limits for the known VHE sources
    Source name ON OFF E E UL UL UL/
    [TeV] [TeV] [10 cmsTeV] [% C.U.]
    2FGL J0047.9+2232 0.32 123 2058 0.0770 0.200 0.314 70.5 11.6 1.6e4
    2FGL J0148.6+0127 1.15 1239 17272 0.063 0.240 0.412 25.1 8.9 6.7e3
    2FGL J0158.4+0107 -1.62 582 13194 0.0625 0.240 0.435 16.6 7.1 150/2.2e3 (z=0.1/0.5)
    2FGL J0205.4+3211 1.76 232 3743 0.0524 0.166 0.290 102 15.4 5.4e5
    2FGL J0212.1+5318 -1.22 389 8089 0.0512 0.200 0.377 37.8 11.9 1.0/9.1 (z=0.1/0.5)
    2FGL J0213.1+2245 0.57 501 7460 0.0655 0.182 0.295 63.3 9.2 29.3
    2FGL J0326.1+2226 0.22 1342 25891 0.0515 0.151 0.249 92.9 9.0 2.5e4
    2FGL J0440.4+1433 0.20 1028 19458 0.0525 0.182 0.340 50.3 12.1 60/416 (z=0.1/0.5)
    2FGL J0856.3+2058 0.19 1686 21961 0.0764 0.182 0.297 37.1 5.6 53 (z=0.5)
    2FGL J0929.5+5009 3.28 1712 17578 0.0884 0.200 0.349 40.2 9.6 28
    2FGL J1058.4+0133 1.19 319 5882 0.0506 0.240 0.373 51.7 12.9 985
    2FGL J1059.0+0222 -0.37 533 7097 0.0764 0.240 0.359 25.8 5.7 30/240 (z=0.1/0.5)
    2FGL J1141.0+6803 0.17 1195 12124 0.0981 0.288 0.525 7.5 5.1 0.7/18.9 (z=0.1/0.5)
    2FGL J1239.5+0728 1.41 198 2766 0.0644 0.240 0.368 55.1 13.1 67
    2FGL J1245.1+5708 2.34 1418 24171 0.0545 0.219 0.369 40.7 10.8 276 (z=0.52)
    2FGL J1303.1+2435 0.33 3021 57951 0.0518 0.166 0.322 36.2 7.8 117
    2FGL J1359.4+5541 1.74 507 5493 0.0850 0.219 0.347 50.5 10.8 1.4e5
    2FGL J1722.7+1013 1.73 693 14010 0.0462 0.200 0.332 72.2 14.5 433
    2FGL J1727.9+1220 -0.56 1816 23431 0.0789 0.200 0.315 15.3 2.5 41
    2FGL J1927.5+6117 -1.94 127 1964 0.077 0.240 0.412 12.1 4.3 5.1/62 (z=0.1/0.5)
    2FGL J1959.9+4212 1.82 458 9118 0.037 0.219 0.347 64.4 13.8 24/176 (z=0.1/0.5)
    Table 6Results and upper limits for the 2FGL sources in the VERITAS field of view

    text

    Figure 1.— Left: stacked significance distribution of the sources included in our sample, classified according to their redshift. Sources with unknown z are in blue, sources with are in red and sources with are in grey. The Gaussian function represents the expectation from a randomly distributed sample, with mean equal to zero, and variance equal to 1. Right: same as left panel, but for sources classified according to the AGN type. Unidentified sources are in blue, IBL/LBL/FSRQ in red and HBL in grey.

    4. Stacked analysis

    Motivated by the skew in the significance distribution and in order to study if there is any evidence of emission from a population of blazars below the VERITAS sensitivity level, a stacked analysis of the data-set is performed. For every source the -ray excess (ON - OFF) and its uncertainty (the excess divided by the significance) are calculated. We then compute the sum of the excesses, and its uncertainty (the square root of the sum of the squared uncertainties), whose ratio provides the significance of the stacked excess. Sources known as VHE emitters are excluded from the stacked analysis, which only includes sources listed in Tables 1 and 3.

    The stacked analysis indicates that there is evidence of VHE emission at a level of , corresponding to an excess of 1990 -rays. The same study is then performed for sub-samples of the overall data-set. The majority of the excess () comes from nearby () HBLs. On the other hand, the stacked analysis including only non-HBL sources located at an unknown distance or results in a stacked significance of . However, because nearby HBLs are considered the most likely VHE candidates, there are more of them and they often have deeper exposures. So this study has more sensitivity to the nearby HBLs. Indeed, the VERITAS exposure on HBLs located at is about 196 hours. By assuming that the stacked excess comes from a constant signal from all sources in 570 hours, one would expect a excess in 196 hours. The excess from the HBLs is thus compatible with this expectation, and it is not possible to claim that the stacked excess is dominated by a particular blazar population.

    The MAGIC collaboration has also reported evidence for VHE emission from a stacked sample of IBL/HBL sources (Aleksić et al., 2011a), detecting a signal at a significance level of from an exposure of 394 hours. The following sources included in the present work are also part of the MAGIC sample: 1ES 0120+340, 1RXS J044127.8+150455, 1ES 0927+500, 1ES 1028+511, RX J1117.1+2014, RX 1136.5+6737, and RBS 1366. The four sources with the highest significance in the MAGIC publication (1ES 0033+595, 1ES 1011+496, B2 1215+30, and 1ES 1741+196) were, notably, later confirmed as VHE emitters, either during flaring activity, or by increasing the integration time.

    5. Conclusions

    The results from the analysis of the observations of non-detected blazars targeted by VERITAS from 2007 to 2012 have been presented. In addition, -ray sources from the 2FGL catalog which were within the field of view of these VERITAS observations were included in this study. For all the 114 sources included in this data-set we provided the VERITAS upper limit at VHE. Given that the redshift estimate of blazars is particularly important for VHE extragalactic astronomy, due to the -ray absorption on the EBL, we also presented the results from optical spectroscopy of 18 of these targets, determining the redshift for three of them, and providing a lower limit for the redshift of one of the sources.

    We have presented the results from a stacked analysis of the data-set, showing that there is some evidence of signal with a significance level of .

    In the near future, the sensitivity of VHE astronomy will be significantly increased thanks to the Cherenkov Telescope Array (CTA), which will be capable of detecting sources with VHE fluxes of the order of 0.001 Crab units, about a factor of ten better than current IACTs (Actis et al., 2011). Among the scientific goals of CTA, an important endeavor will be to increase the number of known VHE blazars in order to perform population studies. Among the VERITAS targets presented in this work, the sources with the highest significance could be considered as primary candidates for observations with CTA, which may be able to detect many of them on the basis of the extrapolation of their Fermi-LAT spectra to higher energies. The non-detection of a number of later detected VHE blazars emphasizes the variable nature of these sources, highlighting the importance of monitoring observations in order to increase the likelihood of catching the sources at detectable VHE states.

    This research is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, and by NSERC in Canada. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. The VERITAS Collaboration is grateful to Trevor Weekes for his seminal contributions and leadership in the field of VHE gamma-ray astrophysics, which made this study possible. MF acknowledges support by the Science and Technology Facilities Council [grant number ST/L00075X/1].

    Appendix A New redshift estimates

    In this appendix, we present optical spectra of 18 blazars taken at Lick Observatory in an attempt to spectroscopically measure their redshift. The sources selected for spectroscopy were selected independently from the sources selected in the main text, so the overlap is not complete. The spectra we show were taken between August, 2010 and October, 2014. During the observations at Lick we often observed the same source more than once. Duplicate observations are noted in Table 7, but we only show one spectrum per source in the figures. Four of these spectra (shown in Figure 2) have host galaxy features which allow an accurate redshift determination, and are discussed in the main text.


    Target Name Obs. Date Obs. Date Exposure Signal to noise Standard Star z Fig. 
    [UT] [MJD - 50000] [s]
    RBS 0082 August 13, 2010 5421 3600 50, 77 BD+28 4211 3
    1ES 0033+595 August 22, 2012 6161 3600 20, 88 BD+28 4211
    1ES 0033+595 December 4, 2013 6630 3600 2.8, 34 G191B2B 3
    1RXS J0045.3+2127 August 22, 2012 6161 1800 56, 106 BD+28 4211
    1RXS J0045.3+2127 October 28, 2014 6958 1800 57, 121 Feige 110 3
    RGB J0250+172 August 15, 2010 5423 5400 21, 44 BD+28 4211 0.243 2
    1ES 0446+449 February 14, 2013 6337 3600 n/a, 65 HZ2 4
    RGB J0505+612 February 14, 2013 6337 0900 0.4, 4.3 HZ2 4
    2FGL J0540.4+5822 October 28, 2014 6958 3600 14, 46 G19B2B 4
    B2 0912+29 April 7, 2013 6389 3600 57, 179 Feige 34
    B2 0912+29 January 4, 2013 6299 3600 81, 144 Feige 34 5
    B2 0912+29 December 4, 2013 6630 3600 48, 102 G191B2B
    RBS 0929 April 7, 2013 6389 3600 13, 33 Feige 34 5
    RGB J1037+571 February 14, 2013 6337 3600 54, 128 Feige 34 5
    1ES 1118+424 February 14, 2013 6337 3600 1.5, 22 Feige 34 0.230 2
    PG 1246+586 April 7, 2013 6389 3600 80, 229 GD 153
    PG 1246+586 May 29, 2014 6806 3600 86, 175 HZ44
    PG 1246+586 May 30, 2014 6807 3600 80, 203 HZ44 6
    RBS 1366 April 7, 2013 6389 0900 7, 34 BD+33 2642
    RBS 1366 May 30, 2014 6807 3600 26, 72 BD+33 2642 0.237 2
    1RXS J144053.2+061013 January 4, 2013 6299 3800 13, 53 BD+33 2642 6
    RGB J1725+118 June 12, 2013 6455 3600 48, 129 BD+33 2642
    RGB J1725+118 May 29, 2014 6806 3600 81, 189 BD+33 2642
    RGB J1725+118 May 30, 2014 6807 3600 111, 205 BD+33 2642 6
    RGB J1903+556 June 13, 2013 6456 1800 18, 46 BD+28 4211 7
    BZB J2243+2021 August 13, 2010 5421 3600 92, 167 BD+28 4211 7
    1ES 2321+419 August 22, 2012 6161 3000 20, 33 BD+28 4211
    1ES 2321+419 October 28, 2014 6958 3600 52, 121 Feige 110 2

    Note. – See Table 1 in main text for coordinates.
    This is the average signal to noise per pixel. The first number is for the blue side CCD, between 3500 and 5400 A. The second number is for the red side CCD, between 5700 and 6800 Angstroms
    Several targets have spectra from multiple nights. Only one spectrum per target is shown; this column indicates the corresponding figure, if applicable.

    Table 7Blazars observed at Lick Observatory using the Shane 3m Kast Spectrograph

    Figure 2.— Spectra shown from top to bottom: RGB J0250+172 (August 15, 2010), 1ES 1118+424 (February 14, 2013), RBS 1366 (May 30, 2014), 1ES 2321+419 (October 28, 2014). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N. Solid gray lines indicate features at non-zero redshift.

    Figure 3.— Spectra shown from top to bottom: RBS 0082 (August 13, 2010), 1ES 0033+595 (December 4, 2013), 1RXS J0045.3+2127 (October 28, 2014). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

    Figure 4.— Spectra shown from top to bottom: 1ES 0446+449 (February 14, 2013), RGB J0505+612 (February 14, 2013), 2FGL J0540.4+5822 (October 28, 2014). The spectrum of 1ES 0446+449 (February 14, 2013) only includes the red arm data (5500-8000 Angstroms) because there were complications in reducing the blue arm data. Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

    Figure 5.— Spectra shown from top to bottom: B2 0912+29 (January 4, 2013), RBS 0929 (April 7, 2013), RGB J1037+571 (February 14, 2013). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

    Figure 6.— Spectra shown from top to bottom: PG 1246+586 (May 30, 2014), 1RXS J144053.2+061013 (January 4, 2013), RGB J1725+118 (May 30, 2014). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

    Figure 7.— Spectra shown from top to bottom: RGB J1903+556 (June 13, 2013), BZB J2243+2021 (August 13, 2010). Dashed lines indicate telluric and Galactic features. Red lines indicate the error array for each observation; some are not visible due to high S/N.

    Footnotes

    1. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
    2. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
    3. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
      wystan.benbow@cfa.harvard.edu
    4. affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
    5. affiliation: Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
      caajohns@ucsc.edu
    6. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
    7. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
    8. affiliation: Department of Physics, Washington University, St. Louis, MO 63130, USA
    9. affiliation: Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
    10. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
      wystan.benbow@cfa.harvard.edu
    11. affiliation: Now at Sorbonne Universités, UPMC Université Paris 06, Université Paris Diderot, Sorbonne Paris Cité, CNRS, Laboratoire de Physique Nucléaire et de Hautes Energies (LPNHE), 4 place Jussieu, F-75252, Paris Cedex 5, France
      matteo.cerruti@lpnhe.in2p3.fr
    12. affiliation: Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
    13. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    14. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
    15. affiliation: School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
    16. affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
    17. affiliation: Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
    18. affiliation: Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
    19. affiliation: Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
    20. affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
    21. affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
    22. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    23. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
      wystan.benbow@cfa.harvard.edu
    24. affiliation: School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
    25. affiliation: Department of Physics, California State University - East Bay, Hayward, CA 94542, USA
    26. affiliation: School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
    27. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
    28. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
    29. affiliation: Astronomy Department, Adler Planetarium and Astronomy Museum, Chicago, IL 60605, USA
    30. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    31. affiliation: Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
    32. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
    33. affiliation: Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
    34. affiliation: Physics Department, Columbia University, New York, NY 10027, USA
    35. affiliation: Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
      caajohns@ucsc.edu
    36. affiliation: Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, IA 52242, USA
    37. affiliation: Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
    38. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    39. affiliation: Department of Physics and Astronomy, DePauw University, Greencastle, IN 46135-0037, USA
    40. affiliation: Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
    41. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    42. affiliation: Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
    43. affiliation: Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19716, USA
    44. affiliation: School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
    45. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    46. affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
    47. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
    48. affiliation: School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430
    49. affiliation: School of Physics, National University of Ireland Galway, University Road, Galway, Ireland
    50. affiliation: Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
    51. affiliation: School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430
    52. affiliation: Physics Department, Columbia University, New York, NY 10027, USA
    53. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    54. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
    55. affiliation: School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430
    56. affiliation: Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
    57. affiliation: N.A.S.A./Goddard Space-Flight Center, Code 661, Greenbelt, MD 20771, USA
    58. affiliation: Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67 - Sucursal 28, (C1428ZAA) Ciudad Autónoma de Buenos Aires, Argentina
    59. affiliation: Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
    60. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    61. affiliation: Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
    62. affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
    63. affiliation: School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
    64. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
    65. affiliation: Department of Physical Sciences, Cork Institute of Technology, Bishopstown, Cork, Ireland
    66. affiliation: School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, 837 State Street NW, Atlanta, GA 30332-0430
    67. affiliation: Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado, AZ 85645, USA
      wystan.benbow@cfa.harvard.edu
    68. affiliation: Instituto de Astronomia y Fisica del Espacio, Casilla de Correo 67 - Sucursal 28, (C1428ZAA) Ciudad Autónoma de Buenos Aires, Argentina
    69. affiliation: Department of Physics and Astronomy, Barnard College, Columbia University, NY 10027, USA
    70. affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
    71. affiliation: School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
    72. affiliation: University of Maryland, College Park / NASA GSFC, College Park, MD 20742, USA
    73. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
    74. affiliation: Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam-Golm, Germany
    75. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    76. affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907, USA
    77. affiliation: Physics Department, McGill University, Montreal, QC H3A 2T8, Canada
    78. affiliation: DESY, Platanenallee 6, 15738 Zeuthen, Germany
    79. affiliation: Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
    80. affiliation: Physics Department, Columbia University, New York, NY 10027, USA
    81. affiliation: Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA
    82. affiliation: Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
      caajohns@ucsc.edu
    83. affiliation: Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
    84. affiliation: Institute for Computational Cosmology and Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
    85. affiliation: Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
    86. For a recent review, see e.g. Şentürk et al. (2013); for an updated list of known VHE sources see http://tevcat.uchicago.edu
    87. The classification of BL Lac objects as LBL/IBL/HBL is sometimes replaced by LSP/ISP/HSP (low/intermediate/high-synchrotron-peaked blazars, see Abdo et al., 2010b), making explicit reference to the synchrotron origin of the first component of the SED. For BL Lac objects, the two triplets of acronyms can be considered as synonyms.
    88. http://simbad.u-strasbg.fr/simbad/
    89. http://tools.asdc.asi.it/SED/
    90. http://www.ucolick.org/x̃avier/LowRedux/
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    293. The best-fit of the VHE emission from the Crab Nebula as measured with the Whipple 10-m telescope and presented in Hillas et al. (1998) is a power-law function with index and normalization cmsTeV at 1 TeV. The integral upper limits are computed from the differential ones and provided here as a reference. They can easily be recomputed for different values of , or for other definitions of the Crab unit. For example, using as a reference the MAGIC spectrum of the Crab nebula (Aleksić et al., 2015b), the Crab unit above GeV is 74 of the Whipple Crab unit above the same threshold.
    294. In order to ease the comparison between the results from different instruments, the integral fluxes provided in this section have been recomputed above the VERITAS threshold, when the spectral information is available, and are expressed in Crab units as defined in Hillas et al. (1998).

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