Optical spectroscopic observations of -ray blazar candidates IV
Results of the 2014 follow-up campaign
The extragalactic -ray sky is dominated by the emission arising from blazars, one of the most peculiar classes of radio-loud active galaxies. Since the launch of Fermi several methods were developed to search for blazars as potential counterparts of unidentified -ray sources (UGSs). To confirm the nature of the selected candidates, optical spectroscopic observations are necessary. In 2013 we started a spectroscopic campaign to investigate -ray blazar candidates selected according to different procedures. The main goals of our campaign are: 1) to confirm the nature of these candidates, and 2) whenever possible determine their redshifts. Optical spectroscopic observations will also permit us to verify the robustness of the proposed associations and check for the presence of possible source class contaminants to our counterpart selection. This paper reports the results of observations carried out in 2014 in the Northern hemisphere with Kitt Peak National Observatory (KPNO) and in the Southern hemisphere with the Southern Astrophysical Research (SOAR) telescopes. We also report three sources observed with the Magellan and Palomar telescopes. Our selection of blazar-like sources that could be potential counterparts of UGSs is based on their peculiar IR colors and on their combination with radio observations both at high and low frequencies (i.e., above and below 1 GHz) in publicly available large radio surveys. We present the optical spectra of 27 objects. We confirm the blazar-like nature of 9 sources that appear to be potential low-energy counterparts of UGSs. Then we present new spectroscopic observations of 10 active galaxies of uncertain type associated with Fermi sources, classifying all of them as blazars. In addition, we present the spectra for five known -ray blazars with uncertain redshift estimates and three BL Lac candidates that were observed during our campaign. We also report the case for WISE J173052.85-035247.2, candidate counterpart of the source 2FGL J1730.6-0353, which has no radio counterpart in the major radio surveys. We confirm that our selection of -ray blazars candidates can successfully indentify low-energy counterparts to Fermi unassociated sources and allow us to discover new blazars.
Blazars are the largest population of active galactic nuclei (AGNs) detected in the range (abdo10 ; nolan12 ). Their non-thermal emission extends from radio to TeV energies and it is coupled with very rapid variability, high and variable polarization, superluminal motion, high luminosities (e.g., urry95 ) and peculiar infrared (IR) colors (paper1 ). Since 1978, well before the establishment of the unification scenario for the AGNs, their peculiar properties were described in terms of emission arising from particles accelerated in a relativistic jet closely pointed along our line of sight (blandford78 ).
Adopting the nomenclature of the blazar subclasses described in the Roma-BZCAT: Multi-frequency Catalogue of Blazars111 http://www.asdc.asi.it/bzcat/ (e.g., massaro09 ; massaro11 ), we distinguish between BL Lac objects (i.e., BZBs) and the flat spectrum radio quasars indicated as BZQs. The former present optical spectra with emission and/or absorption lines of rest frame equivalent width Å (e.g., stickel91 ; stoke91 ; laurent99 ) while the latter show typical quasar-like optical spectra with strong and broad emission lines. In the Roma-BZCAT there are also several BZBs indicated as BL Lac candidates; these sources were indicated as BL Lacs in literature and thus reported in the catalog but, lacking their optical spectra, a correct classification is still uncertain (see also sdss ).
We developed several methods to search for -ray blazar candidates that could be counterparts of the unidentified -ray sources (UGSs, 2FGL nolan12 ) on the basis of the peculiar IR colors of known -ray blazars (e.g. ugs1 ; ugs2 ; ugs3 ), discovered thanks to all-sky survey of the Wide-Field Infrared Survey Explorer (WISE; wright10 ), or using radio observations in combination with IR colors (paper5 ) as well as at low frequencies in the MHz regime (paper3 ; ugs6 ). In addition, multifrequency analysis based on X-ray follow-up observations (e.g., mirabal09 ; paggi13 ; takeuchi13 ; stroh13 ; acero13 ) and radio campaigns (e.g., petrov13 ) were performed to search for potential UGS counterparts.
Since the positional uncertainty of Fermi is a few tenths of a degree, all the proposed methods and the multifrequency follow-up observations are primarily useful to decrease the number of potential counterparts for the UGSs. There could be possible contamination by different source classes in these selection procedures of -ray blazar candidates (e.g., stern13 ) and such degeneracy can be only removed with optical spectroscopic observations (e.g., masetti13 ; shaw13a ; shaw13b ; paggi14 ; sdss ). Furthermore, a detailed knowledge of the number of UGSs is of paramount importance for instance to provide constraints on dark matter models (abdo14 ). Many UGSs could be blazars, but how many of them are actually blazars is still unknown due in part to the incompleteness of catalogs used for the associations (ackermann11a ).
Thus, motivated by these arguments, we started an optical spectroscopic campaign in 2013 aiming to confirm the real nature of the proposed low-energy counterparts of UGSs selected according to our methods.
In this paper we report the results of observations carried out since January 2014 in both hemispheres. We mainly used the Kitt Peak National Observatory (KPNO) for our targets in the Northern hemisphere in addition to two more observations performed at Palomar. For targets mainly visible in the Southern sky, we present spectra obtained with the Southern Astrophysical Research (SOAR) telescope and one more observation carried out at Magellan. Preliminary results for our exploratory program in the Northern hemisphere obtained with the Telescopio Nazionale Galileo (TNG), the Multiple Mirror Telescope (MMT) and the Observatorio Astronómico Nacional (OAN) in San Pedro Mártir (México) were already presented in Paggi et al. (2014). In addition, the results of our observations carried out in the 2013 campaign with SOAR and KPNO are described by Landoni et al. (2014) and Massaro et al. (2014b), respectively.
The paper is organised as follows: Section 2 contains the different methods that we employed for the sample selection, in Section 3 we present our observational setup and we discuss the data reduction procedures. Then in Section 4 we describe the results of our analysis for different types of sources in our sample. Finally, Section 5 is devoted to summary and conclusions. We use cgs units unless otherwise stated.
2 Selecting the sample
The surveys and the catalogs used to search for the counterparts of our targets are listed in the following. These symbols are
also reported in Table 1.
Below 1GHz we used
the VLA Low-Frequency Sky Survey Discrete Source Catalog (VLSS; cohen07 , - V)
and the recent revision VLLSr222http://heasarc.gsfc.nasa.gov/W3Browse/all/vlssr.html
(lane14 ), the Westerbork Northern Sky Survey (WENSS; rengelink97 , - W),
the Sydney University Molonglo Sky Survey (SUMSS; mauch03 , - S),
the Parkes-MIT-NRAO Surveys (PMN; wright94 , - Pm),
the Parkes Southern Radio Source catalog (PKS; wright90 , - Pk),
and the Low-frequency Radio Catalog of Flat-spectrum Sources (LORCAT; lorcat , - L).
At higher radio frequencies we also verified the counterparts in the NRAO VLA Sky Survey 333http://heasarc.gsfc.nasa.gov/W3Browse/radio-catalog/nvss.html (NVSS; condon98 , - N),
the Australia Telescope 20 GHz Survey (AT20G; murphy10 , - A),
the Combined Radio All-Sky Targeted Eight-GHz Survey (CRATES; healey07 , - c).
In the infrared, we queried the WISE all-sky survey in the
AllWISE Source catalog444http://wise2.ipac.caltech.edu/docs/release/allwise/ (wright10 , - w)
and the Two Micron All Sky Survey (2MASS; skrutskie06 , - M)
since each WISE source is automatically matched to the closest 2MASS potential counterpart (see cutri12 , for details).
Then, we also searched for optical counterparts, with or without possible spectra available,
in the Sloan Digital Sky Survey Data Release 9 (SDSS DR9; e.g. ahn12 , - s),
in the Six-degree-Field Galaxy Redshift Survey (6dFGS; jones04 ; jones09 , - 6).
At high-energies, in the X-rays, we searched the ROSAT all-sky survey
in both the ROSAT Bright Source Catalog (RBSC; voges99 , - X)
and the ROSAT Faint Source Catalog (RFSC; voges00 , - X),
as well as XMM-Newton Slew Survey (XMMSL; saxton08 ; warwick12 , - x),
the Deep Swift X-Ray Telescope Point Source Catalog (1XSPS; evans14 , - x),
the Chandra Source Catalog (CSC; evans10 , - x) and the Swift X-ray survey for all Fermi UGSs (stroh13 ).
Note that we use the same symbol for the X-ray catalogs of XMM-Newton, Chandra and Swift.
Our sample lists a total of 27 sources divided as follows: 9 are UGSs for which the analysis based on the IR colors of blazar candidates indicated them as blazar-like sources that were observed during our campaign; 10 are indeed classified as active galaxies of uncertain type (AGUs) according to The Second Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope (2LAC; ackermann11a ) for which no optical spectra were available when the catalog was released and have been observed as part of our campaign. Some sources have been also selected based on low-frequency radio information (ugs3 ). The remaining 8 sources are BL Lac candidates and BL Lacs, either detected or not by Fermi listed in the Roma-BZCAT for which no optical spectroscopic information were found in literature (massaro11 ) or with uncertain/unknown redshift estimate.
We discuss our spectroscopic analysis in the following subsections while in Table 1 we summarize our results and report the multifrequency notes for each source with the only exceptions of those listed in the Roma-BZCAT. The finding charts for all the sources are shown in Figures 1-3 using the USNO-B1 Catalog (monet03 ) and the Digitized Sky Survey555http://archive.eso.org/dss/dss. We highlight that some of the sources observed during our campaign have also been observed at different observatories and groups. However we re-observed these targets for two main reasons: 1) when our observations were scheduled and performed these spectra were not yet published; 2) due to the well-known BL Lac variability in the optical energy range, there is always the chance to observe the source during a quiescent or low state and detect some emission and/or absorption features that could allow us to constrain their redshifts.
3 Observations and Data Reduction
Optical spectra of most of the sources accessible from the Northern hemisphere were acquired in remote observing mode with the KPNO Mayall 4-m telescope using the R-C spectrograph, while the sources in the Southern hemisphere were observed remotely with the SOAR 4-m telescope using the High Throughput Goodman spectrograph. We also present the optical spectrum of the source WISE J024440.30-581954.5, observed on UT 2014 February 3 in visitor mode at Las Campanas Observatory using the Magellan 6.5-m telescope in combination with the IMACS instrument. Finally, we present optical spectra of WISE J025333.64+321720.8 and WISE J085654.85+714623.8 obtained in visitor mode on UT 2014 February 22 with the Double Spectrograph (DBSP) at the Hale 200-inch Telescope at Palomar Observatory. Our scientific goal, i.e. the classification of the selected targets, is best achieved with broad spectral coverage. We thus adopted a slit width of 1.2 (1) and the low resolution gratings yielding a dispersion of 3 (2) pixel for KPNO (SOAR). Our observations took place the nights UT 2014 June 4 and 5 at KPNO and on UT 2014 April 24 at SOAR during grey time. The average seeing for both runs was about 1 and conditions were clear. Additional observations were made on 2014 February 3 with the 6.5m Baade Magellan telescope using the Inamori Magellan Areal Camera and Spectrograph (IMACS; Bigelow98 ). The f/2 camera was used in combination with the 300 l mm grism (blaze angle 17.5 degrees) and a 0.7 slit to yield spectra with dispersion of 1.34 Å pixel and FWHM resolution of 4 Å. Conditions were photometric and the seeing was generally . The whole set of spectroscopic data acquired at these telescopes was optimally extracted (horne1986 ) and reduced following standard procedures with IRAF (tody86 ). For each acquisition we performed bias subtraction, flat field correction and cosmic rays rejection. Since for each target we secured two individual frames, we averaged them according to their signal to noise ratios (SNR). For questionable spectral features, we have exploited the availability of two individual exposures in order to better reject spurious signals. The wavelength calibration was achieved using the spectra of He-Ne-Ar or Hg-Ar lamps that assure coverage of the entire range. To take into account drift and flexures of the instruments during the night, we took an arc frame before each target to guarantee a good wavelength solution for the scientific spectra. The achieved accuracy is about 0.3 (0.4) rms for KPNO (SOAR). Although our program did not require accurate photometric precision, we observed a spectrophotometric standard star to perform relative flux calibration on each spectrum. Finally, we corrected the whole sample for the Galactic absorption assuming values computed using the Schlegel et al. (1998) and Cardelli et al. (1989) relations. To better detect faint spectral features and measure redshifts, we normalised each spectrum to its continuum. For the Palomar observations, we observed the candidates through a 1.5 slit with the 5500 dichroic to split the light across the blue and red arms of DBSP. Both sides have had resolving powers , and the data were reduced as above.
|J112325.37252857.0||2FGL J1123.32527||11:23:25.37||25:28:57.0||2014-04-23||300||N,w,M,6,U,g – (=0.145784-QSO-Jones+09)||QSO||0.148|
|J125949.80374858.1||2FGL J1259.83749||12:59:49.80||37:48:58.2||2014-04-23||600||S,N,w,U||BL Lac||?|
|J132840.61472749.2||2FGL J1328.54728||13:28:40.43||47:27:48.7||2014-04-23||600||S,rf,w,M,U,g,u,x - SED in Takeuchi+13||BL Lac||?|
|J134042.01041006.8||2FGL J1340.50412||13:40:42.01||04:10:07.2||2014-06-05||1200||N,F,w,U,u,x - SED in Takeuchi+13||BL Lac||?|
|J134706.88295842.4||2FGL J1347.02956||13:47:06.88||29:58:42.5||2014-04-23||900||S,N,rf,w,M,U||BL Lac||?|
|J173052.85035247.2||2FGL J1730.60353||17:30:52.86||03:52:47.2||2014-06-05||900||w,M||BL Lac||0.776|
|J183535.34+134848.8||2FGL J1835.4+1349||18:35:35.35||+13:48:48.8||2014-06-05||600||V,T,N,87,rf,w,M,U||BL Lac||?|
|J074642.31475455.0||2FGL J0746.54758||07:46:42.31||47:54:55.2||2014-04-23||600||Pm,S,A,c,rf,w,M||BL Lac||?|
|J084502.48545808.4||2FGL J0844.85459||08:45:02.47||54:58:08.6||2014-04-23||600||Pm,A,rf,U,X||BL Lac||?|
|J114141.80140754.6||2FGL J1141.71404||11:41:41.84||14:07:53.5||2014-04-23||900||L,Pm,N,c,w,M,U||BL Lac||?|
|J123824.39195913.8||2FGL J1238.11953||12:38:24.40||19:59:13.5||2014-04-23||900||Pm,N,A,c,rf,w,M,g,X||BL Lac||?|
|J140609.60250809.2||2FGL J1406.22510||14:06:09.60||25:08:09.3||2014-04-23||600||L,N,w,M,U,u,x - SED in Takeuchi+13||BL Lac||?|
|J162638.15763855.5||2FGL J1626.07636||16:26:38.18||76:38:55.5||2014-04-23||300||Pm,S,c,rf,w,M,g||BL Lac||0.1050|
|J184931.74+274800.8||2FGL J1849.5+2744||18:49:31.69||+27:48:00.9||2014-06-04||1800||W,N,87,c,rf,w,M||BL Lac||?|
|J195945.66472519.3||2FGL J1959.94727||19:59:45.48||47:25:19.3||2014-04-23||600||S,w,M,U,g,u,X,x - SED in Takeuchi+13||BL Lac||0.519|
|J024440.30581954.5||BZBJ02445819||02:44:40.31||58:19:54.6||2014-02-03||600||BL Lac candidate at z=0.265||BL Lac||0.265|
|J121752.08+300700.6||BZBJ1217+3007||12:17:52.09||+30:07:00.7||2014-06-04||300||BL Lac at z=0.13?||BL Lac||?|
|J135949.71374600.7||BZBJ13593746||13:59:49.72||37:46:00.8||2014-04-23||300||BL Lac||BL Lac||?|
|J155333.56311830.9||BZBJ15533118||15:53:33.56||31:18:31.0||2014-04-23||300||BL Lac (z=0.132-BLLac-Masetti+13)||BL Lac||?|
|J164924.98+523515.0||BZBJ1649+5235||16:49:25.00||+52:35:15.0||2014-06-05||900||BL Lac candidate at z=?||BL Lac||?|
|J170209.63+264314.7||BZBJ1702+2643||17:02:09.64||+26:43:14.8||2014-06-05||1200||BL Lac candidate at z=?||BL Lac||?|
|J180945.39+291019.8||BZBJ1809+2910||18:09:45.39||+29:10:20.0||2014-06-05||900||BL Lac candidate at z=?||BL Lac||?|
|J203923.51+521950.1||BZBJ2039+5219||20:39:23.50||+52:19:49.9||2014-06-04||600||BL Lac candidate at z=0.053||BL Lac||?|
4.1 Unidentified Gamma-ray Sources
Here we provide details for the low-energy counterparts of the 9 UGSs observed in our sample.
All these WISE-selected counterparts where found in the analysis performed by Massaro et al. (2013a)
to have IR WISE colors similar to known -ray blazars.
All of them were predicted to be BZB-like sources having the IR colors more consistent with those of the Fermi BZBs
rather than the BZQs (see ugs1 , for more details).
The spectra of the whole UGSs listed in Table 1 are shown in Figure 4-12. The blazar counterpart for one of these sources, WISE J173052.85-035247.2 (candidate counterpart
of 2FGL J1730.6-0353) appears intriguing.
On the basis of our optical spectra, we classify the source as a BL Lac object and we put a
lower limit on its redshift of 0.776 based on the detection of an intervening doublet system of Mg II ( - with - ; see also Figure 9).
However, this source
is not associated with any NVSS sources, as expected for BL Lac objects.
We note that, even if deeper radio observations detect emission from the source, blazars are
traditionally defined as radio-loud sources on the basis of current radio data. All confirmed blazar in
Roma-BZCAT are indeed detected at 1.4 GHz with fluxes above a few mJy, and radio-quiet blazars are extremely
rare objects (londish04 ; paggi13 ). In particular, only 14 BZBs out of the 1220 present in the Roma-BZCAT have a radio flux density at 1.4GHz lower than 2.5 mJy
that is the average flux limit of the NVSS survey666http://heasarc.gsfc.nasa.gov/W3Browse/radio-catalog/nvss.html.
There are two sources, WISE J174507.82+015442.4 and WISE J174526.95+020532.6, potential counterparts to 2FGL J1745.6+0203 found on the basis of the IR color selection method. According to our multifrequency investigation we note that only WISE J174507.82+015442.4 has a radio counterpart, thus we conclude that WISE J174526.95+020532.6 is a normal radio-quiet quasar contaminating our selection. However we cannot firmly establish the real blazar nature of WISE J174507.82+015442.4 since the lack of multi-wavelength radio observations did not allow us to verify the flatness of its radio spectrum. For both objects we determined their redshifts based on broad emission lines (H and H are both present, see Figures 10-11). Their redshifts are reported in Table 1.
4.2 Gamma-ray Active Galaxies of Uncertain type
In this subsection we discuss the AGUs in our observed sample. The multifrequency notes relative to each source are reported in Table 1, as previously done for the UGSs. The spectra of the whole AGU sample are shown in Figure 13-22. Our spectroscopic observations confirm that 8 out of 10 sources are BL Lac objects while the remaining two, having quasar-like optical spectra and flat radio spectra, are indeed BZQs. The optical spectra of these two BZQs, WISE J025333.64+321720.8 and WISE J085654.85+714623.8, contain broad emission lines identified as Mg II and H (see Figures 13 and 16 for more details), and permit us to measure their redshifts of 0.859 and 0.542, respectively. In particular, the identifications of [OII], H and [OIII] doublet in the optical spectrum of WISE J025333.64+321720.8 are uncertain due to low SNR. The optical spectrum of WISE J084502.48-545808.4, candidate counterpart of 2FGL J0844.8-5459, show an absorption feature ( = 6364 with = 2.2 ) that we are not able to clearly identify (see Figure 15). We classify the WISE J162638.15-763855.5 counterpart to 2FGL J1626.0-7636 as a BL Lac since its emission lines have EW 5 . The detection of emission ([O I] with , the [S II] doublet - with - ) and absorption lines (G band, Mg I with and Na I with ), enable us to measure a redshift of 0.1050 (see Figure 20). The optical spectra of the AGUs associated with the -ray source 2FGL J1849.5+2744 are also published in Shaw et al. (2013a). The source is listed as a BZB in the 2LAC catalog, although the optical spectrum was not yet available. The optical spectrum collected by us for this object (WISE J184931.74+274800.8) is nearly featureless (see Figure 21), so we are not able to confirm the lower limit estimate (i.e., 1.466) of this source proposed by Shaw et al. (2013a) on the basis of Mg II absorption doublet. Thanks to our optical spectrum of the WISE J195945.66-472519.3, candidate counterpart to 2FGL J1959.9-4727, we can put a lower limit on its redshift of 0.519 based on the detection of an intervening doublet system of Mg II ( - with - ; see also Figure 22).
4.3 BL Lac objects
Details for the BL Lacs in our sample are listed below. Table 1 reports their Roma-BZCAT name and the name of the WISE counterpart with the coordinates. Multifrequency notes are not presented in this table since they are already extensively discussed in the Roma-BZCAT. All the BZBs that are also detected by Fermi belong to the sample named: locus (i.e. the three-dimensional region in the WISE color space occupied by the -ray-emitting blazars, well separated from the regions occupied by other classes, ugs1 ), with the only exception being BZB J2039+5219. They were used in D’Abrusco et al. (2013) to build the method to search for blazar-like sources within the positional uncertainty region of the Fermi UGSs. Thus all their WISE counterparts have the IR colors consistent with the Fermi blazar population. There are five objects listed in Table 1 as BL Lac candidates for which no optical spectra are present in literature that allowed a firm classification. Our spectroscopic observations confirmed that all the BL Lac candidates have featureless optical spectra as shown in Figures 23-30. In addition, the remaining three objects, namely BZB J1217+3007, BZB J1359-3746 and BZB J1553-3118, that were indeed classified BZBs with an uncertain redshift estimate when the Roma-BZCAT was released. For one BZB, BZB J0244-5819, we have been able to estimate a lower limit on its redshift, z0.265, on the basis of absorption features in its optical spectrum. Our observation of WISE J024440.30-581954.5 show the doublet Ca H+K ( - with - ), G band, Mg I ( ) and Na I ( ) absorption features that could be due to the host galaxy and/or to intervening systems (see Figure 23). We note that BZB J1359-3746, BZB J1649+5235 and BZB J1809+2910 also have published optical spectra for their candidate counterparts released in Shaw et al. (2013b). In addition, Shaw et al. (2013b) reported the spectrum of BZB J1359-3746, with the detection of the Calcium break that allowed them to determine a redshift of 0.334. We cannot confirm this result since our optical spectrum of WISE J135949.71-374600.7 shows a featureless continuum (see Figure 25). We also present the spectrum of the low-energy counterpart of BZB J1553-3118, which was also published in Masetti et al. (2013) with a redshift of 0.132. In our lower signal to noise observation, its IR counterpart WISE J135949.71-374600.7 appears featureless, as shown in Figure 26. The optical spectrum of WISE J203923.51+521950.1, candidate counterpart of BZB J2039+5219, show an absorption feature at with . We do not have a clear identification for this feature (see also Figure 30).
5 Summary and conclusions
We present here the results of our 2014 spectroscopic campaign carried out in the Northern hemisphere with the KPNO and Palomar telescopes, and in the Southern hemisphere with the SOAR and Magellan telescopes. The main goal of our campaign is to confirm the nature of sources selected for having IR colors or low radio frequency spectra (i.e., below 1GHz) similar to known Fermi-detected blazars and lying within the positional uncertainty regions of the UGSs through optical spectroscopic observations. Given the positional uncertainty of the UGSs, there could be a possible contamination by different source classes in these selection procedures of -ray blazar candidates (e.g., stern13 ) and spectroscopic observations are the only way to remove such degeneracy (e.g., masetti13 ; shaw13a ; shaw13b ; paggi14 ; sdss ) and discover new gamma-ray blazars. The confirmation of the blazar nature of these selected objects will improve/refine future associations for the Fermi catalog and will also yield to understand the efficiency and completeness of the association method based on the WISE colors once our campaign is completed. Our spectroscopic observations could potentially allow us to obtain redshift estimates for the UGS candidate counterparts. During our campaign we also observed several AGUs, as defined according to the Fermi catalogs (see e.g., ackermann11a ; nolan12 ), to verify if they are indeed blazars. In addition we observed several sources that already belong to the Roma-BZCAT but were classified as BL Lac candidates due to the lack of optical spectra available in literature, or were BL Lac objects with uncertain redshift estimates, when the Roma-BZCAT v4.1 was released.
We observed a total of 27 targets. The results of this spectroscopic campaign are reported as follows:
In the sample of potential counterparts for the UGS, selected based on their IR colors (paper1 ; ugs1 ; ugs2 ) and on the basis of their flat radio spectra below 1 GHz (ugs3 ; ugs6 ; lorcat ), we confirm the blazar nature of 8 of 9 UGSs. Among them, six are clearly BL Lacs presenting featureless optical spectra. The remaining two are QSOs. One potential counterparts for 2FGL J1745.6+0203 found on the basis of the IR color selection method, namely WISE J174526.95+020532.6, is a QSO that probably contaminates our selection method.
We classify WISE J173052.85-035247.2 as a BL Lac although this source is not associated with any NVSS counterpart, as expected for BL Lac objects. The detection of an intervening doublet system of Mg II enabled us to set a lower limit on its redshift of 0.776.
All the sources that belong to our AGU subsample have a blazar nature. Two of them are QSOs, while for one of them, namely WISE J162638.15-763855.5, we have been able to detect some features in the optical spectrum, both in emission and in absorption, leading to a redshift measurement of 0.1050. The AGU associated with the WISE J184931.74+274800.8 has been classified as a BZB in the 2LAC paper but at that time there was no optical spectra. Our observations confirm its BL Lac nature. We have been also able to set a lower limit for the AGU associated with WISE J195945.66-472519.3 of 0.519 thanks to the detection in its optical spectrum of a Mg II intervening system.
Within the Roma-BZCAT sources we found five BL Lac candidates that thanks to the collected optical spectra are all confirmed BL Lacs. For one of them, BZB J0244-5819, we have been able to set a lower limit on their redshifts on the basis of absorption systems (Ca H+K, G band, Mg I and Na I) that could be due to the host galaxy and/or to intervening systems. For the remaining four BZBs listed in the Roma-BZCAT with uncertain redshift estimates, we were not able to obtain any redshift values.
- (Abdo et al. 2010) Abdo, A. A., Ackermann, M., Ajello, M. et al. 2010 ApJS, 188, 405
- (Abdo et al. 2014) Abdo A. A, et al., 2014, in preparation
- (Ackermann et al. 2011a) Ackermann, M., Ajello, M., Allafort, A. et al. 2011a ApJ, 743, 171
- (Ackermann et al. 2011b) Ackermann, M. et al. 2011b ApJ, 741, 30
- (Ackermann et al. 2012) Ackermann, M., Ajello, M., Allafort, A. et al. 2012 ApJ, 753, 83
- (Acero et al. 2013) Acero, F., Donato, D., Ojha, R. et al. 2013 ApJ, 779, 133
- (Ahn et al. 2012) Ahn, C. P., Alexandroff, R., Allende Prieto, C. et al. 2012, ApJS, 203, 21
- (Bigelow et al. 1998) Bigelow, B. C., Dressler, A. M., Shectman, S. A., & Epps, H. W. 1998, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 3355, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, ed. S. D’Odorico, 225
- (Bonnarel et al. 2000) Bonnarel, F., Fernique, P., Bienaymé, O., Egret, D., Genova, F., Louys, M., Ochsenbein, F., Wenger, M., Bartlett, J. G., 2000, A&AS, 143, 33
- (Blandford & Rees 1978) Blandford, R. D., Rees, M. J., 1978, PROC. ”Pittsburgh Conference on BL Lac objects”, 328
- (Cardelli et al. 1989) Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245
- (Cohen et al. 2007) Cohen, A. S., Lane, W. M., Cotton, W. D. et al. 2007 AJ, 134, 1245
- (Condon et al. 1998) Condon, J. J., Cotton, W. D., Greisen, E. W. et al. 1998, AJ, 115, 1693
- (Cutri et al. 2012) Cutri et al. 2012 wise.rept, 1C
- (D’Abrusco et al. 2012) D’Abrusco, R., Massaro, F., Ajello, M., Grindlay, J. E., Smith, Howard A. & Tosti, G. 2012 ApJ, 748, 68
- (D’Abrusco et al. 2013) D’Abrusco, R., Massaro, F., Paggi, A. et al. 2013 ApJS, 206, 12
- (Doert & Errando 2014) Doert, M. & Errando, M. 2014 ApJ, 782, 41
- (Evans et al. 2010) Evans, I. N., Primini, F. A., Glotfelty, K. J. et al. 2010 ApJS, 189, 37
- (Evans et al. 2014) Evans, P. A., Osborne, J. P., Beardmore, A. P. et al. 2014 ApJS, 210, 8
- (Ghirlanda et al. 2010) Ghirlanda, G. et al. 2010 MNRAS, 407, 791
- (Hassan et al. 2013) Hassan, T., Mirabal, N., Contreras, J. L., Oya, I. 2013 MNRAS, 428, 220
- (Healey et al. 2007) Healey, S. E., Romani, R. W., Taylor, G. B. et al. 2007 ApJS, 171, 61
- (Horne 1986) Horne, K. 1986, PASP, 98, 609
- (Jones et al. 2004) Jones, H. D. et al. 2004 MNRAS, 355, 747
- (Jones et al. 2009) Jones, H. D. et al. 2009 MNRAS, 399, 683
- (Kovalev 2009) Kovalev, Y. Y. 2009 ApJ, 707L, 56
- (Lane et al. 2014) Lane, W. M., Cotton, W. D., van Velzen, S. et al. 2014 MNRAS, 440, 327
- (Laurent-Muehleisen et al. 1999) Laurent-Muehleisen, S.A. et al., 1999, ApJ, 525, 127
- (Londish et al. 2004) Londish D., Heidt J., Boyle B. J., Croom S. M., Kedziora-Chudczer L., 2004, MNRAS, 352, 903
- (Mahony et al. 2010) Mahony, E. K. et al. 2010 ApJ, 718, 587
- (Masetti et al. 2013) Masetti, N., Sbarufatti, B., Parisi, P. 2013 A&A, 559A, 58
- (Massaro et al. 2009) Massaro, E., Giommi, P., Leto, C. et al. 2009 A&A, 495, 691
- (Massaro et al. 2011a) Massaro, F., D’Abrusco, R., Ajello, M., Grindlay, J. E. & Smith, H. A. 2011a ApJ, 740L, 48
- (Massaro et al. 2011b) Massaro, E., Giommi, P., Leto, C. et al. 2011b “Multifrequency Catalogue of Blazars (3rd Edition)”, ARACNE Editrice, Rome, Italy
- (Massaro et al. 2012a) Massaro, F., D’Abrusco, R., Tosti, G., Ajello, M., Paggi, A., Gasparrini, 2012a ApJ, 752, 61
- (Massaro et al. 2012b) Massaro, F., D’Abrusco, R., Tosti, G., Ajello, M., Gasparrini, D., Grindlay, J. E. & Smith, Howard A. 2012b ApJ, 750, 138
- (Massaro et al. 2012c) Massaro, F., D’Abrusco, R., Paggi, A., Tosti, G., Gasparrini, D. 2012c ApJ, 750L, 35
- (Massaro et al. 2013a) Massaro, F., D’Abrusco, R., Paggi, A. et al. 2013a ApJS, 206, 13
- (Massaro et al. 2013b) Massaro, F., D’Abrusco, R., Giroletti, M. et al. 2013b ApJS, 207, 4
- (Massaro et al. 2014a) Massaro, F., Masetti, N., D’Abrusco, R. et al. 2014a AJ in press
- (Massaro et al. 2014b) Massaro, F., Giroletti, M., D’Abrusco, R. et al. 2014b ApJS, 213, 3
- (Mauch et al. 2003) Mauch, T., Murphy, T., Buttery, H. J. et al. 2003 MNRAS, 342, 1117
- (Mirabal & Halpern 2009) Mirabal, N. 2009 ApJ, 701, 129
- (Mirabal et al. 2012) Mirabal, N.; Frías-Martinez, V.; Hassan, T.; Frías-Martinez, E. 2012 MNRAS, 424L, 64
- (Monet et al. 2003) Monet, D. G. et al. 2003 AJ, 125, 984
- (Murphy et al. 2010) Murphy, T. et al. 2010 MNRAS, 402, 2403
- (Nolan et al. 2012) Nolan, P. L., Abdo, A. A., Ackermann, M. et al. 2012 ApJS, 199, 31
- (Nori et al. 2014) Nori, M., Giroletti, M., Massaro, F., D’Abrusco, R., Paggi, A. & Tosti, G. et al. 2014 ApJS 212, 3
- (Paggi et al. 2013) Paggi, A., Massaro, F., D’Abrusco, R. et al. 2013 ApJS, 209, 9
- (Paggi et al. 2014) Paggi, A., Milisavljevic, D., Masetti, N. et al. 2014 AJ, 147, 112
- (Petrov et al. 2013) Petrov, L., Mahony, E. K., Edwards, P. G. et al. 2013 MNRAS, 432, 1294
- (Rengelink et al. 1997) Rengelink, R., Tang, Y., de Bruyn, A. G. et al. 1997, A&A Suppl. 124, 259
- (Saxton et al. 2008) Saxton, R. D. 2008 A&A, 480, 611
- (Schlegel et al. 1998) Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525
- (Shaw et al. 2013a) Shaw, M. S., Romani, R. W.; Cotter, G. et al. 2013 ApJ, 764, 135
- (Shaw et al. 2013b) Shaw, M. S., Filippenko, A. V., Romani, R. W. et al. 2013 AJ, 146, 127
- (Stern & Assef 2013) Stern, D. & Assef, R. J. 2013 ApJ, 764L, 30
- (Skrutskie et al. 2006) Skrutskie, M. F. et al. 2006, AJ, 131, 1163
- (Stickel et al. 1991) Stickel, M. et al. 1991 ApJ, 374, 431
- (Stoke et al. 1991) Stocke et al. 1991, ApJS, 76, 813
- (Stroh et al. 2013) Stroh, M. C. & Falcone, A. D. 2013, ApJS, 207, 28
- (Takeuchi et al. 2013) Takeuchi, Y., Kataoka, J., Maeda, K. et al. 2013 ApJS, 208, 25
- (Taylor 2005) Taylor, M. B. 2005, ASP Conf. Ser., 347, 29
- (Tody 1986) Tody, D.1986 SPIE, 627, 733
- (Urry & Padovani 1995) Urry, C. M., & Padovani, P. 1995, PASP, 107, 803
- (Voges et al. 1999) Voges, W. et al. 1999 A&A, 349, 389
- (Voges et al. 2000) Voges, W. et al. 2000 IAUC, 7432R, 1.
- (Warwick et al. 2012) Warwick, R. S.; Saxton, R. D.; Read, A. M. 2012 A&A, 548A, 99
- (Wright et al. 1990) Wright, A. & Otrupcek, R. 1990 PKS, C, 0
- (Wright et al. 1994) Wright, A. E., Griffith, M. R., Burke, B. F., Ekers, R. D. 1994 ApJS, 91, 111
- (Wright et al. 2010) Wright, E. L., et al. 2010 AJ, 140, 1868