ASASSN-14ae: A Tidal Disruption Event at 200 Mpc
ASASSN-14ae is a candidate tidal disruption event (TDE) found at the center of SDSS J110840.11+340552.2 ( Mpc) by the All-Sky Automated Survey for Supernovae (ASAS-SN). We present ground-based and Swift follow-up photometric and spectroscopic observations of the source, finding that the transient had a peak luminosity of erg s and a total integrated energy of ergs radiated over the months of observations presented. The blackbody temperature of the transient remains roughly constant at K while the luminosity declines by nearly 1.5 orders of magnitude during this time, a drop that is most consistent with an exponential, with days. The source has broad Balmer lines in emission at all epochs as well as a broad He II feature emerging in later epochs. We compare the color and spectral evolution to both supernovae and normal AGN to show that ASASSN-14ae does not resemble either type of object and conclude that a TDE is the most likely explanation for our observations. At , ASASSN-14ae is the lowest-redshift TDE candidate discovered at optical/UV wavelengths to date, and we estimate that ASAS-SN may discover of these events every year in the future.
keywords:accretion, accretion disks — black hole physics — galaxies: nuclei
When a star’s orbit brings it within the tidal disruption radius of a supermassive black hole (SMBH), the tidal shear forces become more powerful than the star’s self-gravity and the star breaks apart. Roughly half of the mass of the star is ejected while the rest of the stellar material remains bound to the black hole and is accreted. These tidal disruption events (TDEs) result in a short-lived ( yr) accretion flare (Lacy et al., 1982; Phinney, 1989; Rees, 1988; Evans & Kochanek, 1989). For , the initial fallback rate is super-Eddington and the eventual rate at which material returns to pericenter becomes a power law (Evans & Kochanek, 1989; Phinney, 1989). While it is commonly assumed that the resulting luminosity is proportional to this rate of return to pericenter, this is only a crude approximation to the complex physics associated with evolution of the accretion stream (Kochanek, 1994), and the exact return rates depend on, for example, the structure of the star (e.g., Lodato & Rossi, 2011).
In the most luminous phases, TDE emission is likely dominated by a photosphere formed in the debris rather than any direct emission from a hot accretion disk (Evans & Kochanek, 1989; Loeb & Ulmer, 1997; Ulmer, 1999; Strubbe & Quataert, 2009). Only in the late phases, as the debris becomes optically thin, will there be any direct emission from the disk. The exact balance likely depends on the viewing angle, as illustrated by the simulations of Guillochon, Manukian & Ramirez-Ruiz (2014). Observationally, TDEs would be expected to show spectral characteristics and light curve evolution that would distinguish them from both supernovae (SNe) and normal active galactic nuclei (AGN), and the detection and study of TDEs remains a useful avenue for studying the properties of SMBHs despite their low predicted frequency ( per galaxy; van Velzen & Farrar, 2014), as the light emitted during the TDE flare may be sensitive to the black hole spin and mass (e.g., Ulmer, 1999; Graham et al., 2001).
At present candidate TDEs can be divided into two observational classes based on the wavelength at which they were discovered. The first consists of those found in UV and X-ray surveys, such as candidates in NGC5905 (Komossa & Bade, 1999) and IC3599 (Grupe et al., 1995; Brandt, Pounds & Fink, 1995), a candidate in the galaxy cluster A1795 (Donato et al., 2014), Swift J164449.3+573451 (Burrows et al., 2011; Bloom et al., 2011; Levan et al., 2011; Zauderer et al., 2011), Swift J0258.4+0516 (Cenko et al., 2012b), and GALEX candidates D1-9, D3-13, and D23H-1 (Gezari et al.,, 2008; Gezari et al., 2009). These typically do not have strong optical emission. The second consists of those found in optical surveys, including PTF10iya (Cenko et al., 2012a); SDSS TDE 1 and TDE 2 (van Velzen et al., 2011); PS1-10jh (Gezari et al., 2012); PS1-11af (Chornock et al., 2014); and PTF09ge, PTF09axc, PTF09djl, PTF10iam, PTF10nuj, and PTF11glr (Arcavi et al., 2014). While the X-ray candidates currently outnumber the optical and UV candidates, variable AGN activity can also mimic the behavior expected for TDEs in the X-ray, making it difficult to disentangle true TDEs from AGN (van Velzen et al., 2011). Modern wide-field optical transient surveys, such as the All-Sky Automated Survey for Supernovae (ASAS-SN
Here we describe the discovery and follow-up observations of ASASSN-14ae, a potential TDE. The transient was discovered by ASAS-SN, a long-term project to monitor the whole sky on a rapid cadence to find nearby supernovae and other bright transients (see Shappee et al. 2013 for details), such as AGN activity (e.g., Shappee et al., 2013), extreme stellar flares (e.g., Schmidt et al., 2014), outbursts in young stellar objects (e.g., Holoien et al., 2014), and cataclysmic variable stars (e.g., Stanek et al., 2013; Kato et al., 2013). Our transient source detection pipeline was triggered on 2014 January 25, detecting a new source with mag (Prieto et al., 2014). The object was also detected on 2014 January 26 at roughly the same magnitude, but is not detected ( mag) in data obtained on 2014 January 1 and earlier. A search at the object’s position in the Sloan Digital Sky Survey Data Release 9 (SDSS DR9; Ahn et al., 2012) catalog revealed the source of the outburst to be the inclined spiral galaxy SDSS J110840.11+340552.2 at redshift , corresponding to a luminosity distance of Mpc ( km s Mpc, , ), and that the ASAS-SN source position was consistent with the center of the host galaxy. Follow-up images obtained on 2014 January 27 with the Las Cumbres Observatory Global Telescope Network (LCOGT) 1-m telescope at McDonald Observatory (Brown et al., 2013), the 2-m Liverpool Telescope (LT) (Steele et al., 2004), and the Swift UltraViolet and Optical Telescope (UVOT; Roming et al., 2005) confirmed the detection of the transient. After astrometrically aligning an LT image of the source in outburst with the archival SDSS image of the host galaxy, we measure an offset of pixels ( arcseconds) between the position of the brightest pixel in the host galaxy in the LT image and the position of the brightest pixel in the SDSS image. This indicates that the source of the new flux is consistent with the center of the galaxy, providing evidence for a TDE interpretation. Figure 1 shows the ASAS-SN -band reference and subtracted images of the source as well as SDSS pre-discovery and LT -band images.
The archival SDSS spectrum of the host is that of an early-type spiral with little evidence of emission lines from an AGN, although it does show [O III] 5007 in emission indicating that there is some recent star-formation. A transient classification spectrum obtained on 2014 January 29 with the Dual-Imaging Spectrograph (DIS) mounted on the Apache Point Observatory (APO) 3.5-m telescope showed a blue continuum as well as a broad ( km/s) H line. The blue continuum and H emission suggested that this transient was likely a young Type II SN, but the proximity to the galactic nucleus and its absolute magnitude at discovery ( mag from the ASAS-SN host-subtracted image) made a tidal capture event a potential alternative. We decided to start a follow-up campaign in order to fully characterize this interesting transient.
In §2 we describe pre-outburst archival observations, including both photometry and spectroscopy of the host galaxy, as well as new data taken of the transient during our follow-up campaign. In §3 we analyze these data and describe the properties of the transient. Finally, in §4 we compare these properties to those of supernovae, AGN, and other proposed TDEs to examine the nature of the object.
2 Observations and Survey Data
In this section we summarize the available archival data of the transient host galaxy as well as our new photometric and spectroscopic observations of ASASSN-14ae.
2.1 Archival Photometry and Spectroscopy
We retrieved archival reduced images in of SDSS J110840.11+340552.2 from SDSS DR9. We then measured the fluxes in a 50 aperture radius (the same aperture used to measure the source in follow-up data, chosen to match the Swift PSF and to minimize the effects of seeing variations on the photometry) to use for galaxy SED modeling and for subtracting the host galaxy fluxes from the transient fluxes. We also retrieved near-IR images from the Two-Micron All Sky Survey (2MASS; Skrutskie et al., 2006) and measured aperture magnitudes of the host galaxy in the same fashion. The measured SDSS and 2MASS magnitudes of the host galaxy are presented in Table 1.
These are 50 radius aperture magnitudes from SDSS and 2MASS.
There are no archival Spitzer, Herschel, Hubble Space Telescope (HST), Chandra, or X-ray Multi-Mirror Mission (XMM-Newton) observations of the source. The host galaxy is not detected in the ROSAT All-Sky Survey with an upper flux limit of erg s cm in the keV band (Voges et al., 1999), providing further evidence that the galaxy is inactive. We also retrieved archival mid-IR photometry from the Wide-field Infrared Survey Explorer (WISE; Wright et al., 2010). From the WISE and measurements we calculate that the host galaxy has mag, and this blue mid-IR color is further evidence against AGN activity (e.g., Assef et al., 2013).
We used the code for Fitting and Assessment of Synthetic Templates (FAST v1.0; Kriek et al., 2009) to fit stellar population synthesis (SPS) models to the 50 SDSS and 2MASS magnitudes of the host galaxy. The fit was made assuming a CCM extinction law (Cardelli, Clayton & Mathis, 1988) with , an exponentially declining star-formation history, a Salpeter IMF, and the Bruzual & Charlot (2003) models. We obtained a good SPS fit (reduced ), with the following parameters: mag, , age Gyr, and SFR yr. We note that these properties do not appear to be consistent with SDSS J110840.11+340552.2 being an EA galaxy, as many of the hosts of the TDE candidates in Arcavi et al. (2014) were. The FAST estimate of mag incorporates both Galactic and host extinction and this value is consistent with the Galactic extinction ( mag based on Schlafly & Finkbeiner 2011). In fits to the transient SED, we find no evidence for additional extinction, even though the Swift UV data, particularly the band which lies on top of the 2200 Å extinction curve feature, is a powerful probe for additional dust (though this depends on the strength of the UV bump in the dust law). In the analyses of the event’s SED which follow we only correct for this Galactic extinction.
We also obtained the spectrum of SDSS J110840.11+340552.2 from SDSS-DR9. The archival spectrum is dominated by absorption lines (e.g., Balmer lines, Ca I G-band, Mg I, Na I, Ca H&K, and the 4000 Å break) that are characteristic of intermediate-age and old stellar populations. This is consistent with the results of the FAST fit to the SED of the host. The spectrum does not show strong emission lines, except for the detection of an unresolved [O III] 5007 line with km s and integrated luminosity erg s. This is likely a sign of a low level of recent star formation, indicating the galaxy could host core-collapse supernova events, but without detecting other emission lines (e.g., H, H, [N II]) we cannot constrain the rate of star formation. We note that the [O III]/H and [N II]/H ratios may indicate that the host contains a very weak Type 2 AGN, consistent with the analysis done in Arcavi et al. (2014). However, other factors (e.g., the WISE photometry) argue against significant activity.
We use the km s of the [O III] line from the latest spectroscopic epoch, presented in §2.3, to estimate a velocity dispersion for the galaxy and place an upper limit on the mass of the black hole of using the M- relation from Gültekin et al. (2009). SDSS reports a velocity dispersion of km s, which is well below the velocity resolution of the SDSS spectrograph of km s, so we will conservatively regard the SDSS estimate as a limit of km s. Using the same M- relation from Gültekin et al. (2009) and the SDSS resolution of 100 km s gives an upper limit of , consistent with the limit we derive from the width of the [O III] line. Finally, from the FAST fit, we have , which is consistent with a bulge mass of (Mendel et al., 2014). Using the - relation from McConnell & Ma (2013) gives , which is again consistent with the limits derived from the host and transient spectra.
2.2 New Photometric Observations
After detection of the transient, we were granted a series of Swift X-ray Telescope (XRT; Burrows et al., 2005) and UVOT target-of-opportunity (ToO) observations. The Swift UVOT observations of ASASSN-14ae were obtained in 6 filters: (5468 Å), (4392 Å), (3465 Å), (2600 Å), (2246 Å), and (1928 Å) (Poole et al., 2008). We used the UVOT software task uvotsource to extract the source counts from a 50 radius region and a sky region with a radius of 400. The UVOT count rates were converted into magnitudes and fluxes based on the most recent UVOT calibration (Poole et al., 2008; Breeveld et al., 2010). The UVOT Vega magnitudes are shown along with other photometric data in Figure 2.
The XRT was operating in Photon Counting mode (Hill et al., 2004) during our observations. The data from all epochs were reduced and combined with the software tasks xrtpipeline and xselect to obtain an image in the 0.310 keV range with a total exposure time of s. We used a region with a radius of 20 pixels (471) centered on the source position to extract source counts and a source-free region with a radius of 100 pixels (2357) for background counts. We do not detect X-ray emission from ASASSN-14ae to a 3-sigma upper limit of counts s. To convert this to a flux, we assume a power law spectrum with and Galactic H I column density (Kalberla et al., 2005), yielding an upper limit of erg cm s. At the host distance of Mpc, this corresponds to an upper limit of erg s ( ) on the average X-ray luminosity. The constraints for the individual Swift epochs are on average times weaker, and we only consider the combined X-ray limit.
In addition to the Swift observations, we obtained images with the LCOGT 1-m at the MacDonald Observatory and images with the LT 2-m telescope. We measured aperture photometry
Figure 2 shows the UV and optical light curves of ASASSN-14ae from MJD 56682.5 (the epoch of first detection) through our latest epoch of observations on MJD 56828 (146 days after first detection) without extinction correction or host flux subtraction. Also shown are the SDSS magnitudes and synthesized Swift UVOT magnitudes of the host galaxy extrapolated from the host SED fit. With the host flux included, the light curve shows ASASSN-14ae brightened much more strongly in the blue and UV filters than in the red bands, with the largest increase in the Swift band (2246 Å), where it brightened by . The brightness also appears to be declining at a faster rate with respect to the host in bluer filters. We further analyze this light curve and compare it to SNe and TDEs in the literature in §3.1.
After correcting our photometric measurements for Galactic extinction, we construct SEDs for 5 follow-up epochs. These are shown with the extinction-corrected SDSS archival data and host SED fit from FAST in Figure 3.
2.3 New Spectroscopic Observations
We obtained seven low- and medium-resolution optical spectra of ASASSN-14ae spanning more than four months between 2014 January 29 and 2014 June 6. The spectra were obtained with DIS mounted on the Apache Point Observatory 3.5-m telescope (range Å, ) and with the Multi-Object Double Spectrographs (MODS; Pogge et al. 2010) on the 8.4-m Large Binocular Telescope (LBT) on Mount Graham (range Å, ). The spectra from DIS were reduced using standard techniques in IRAF and the spectra from MODS were reduced using a custom pipeline written in IDL
The main characteristics of the spectra of ASASSN-14ae are the blue continuum, consistent with the photometric measurements, and the detection of broad Balmer lines in emission, which are not present in the host galaxy spectrum. The H line has km s at all epochs and does not show a P-Cygni absorption trough. The blue continuum present in the first follow-up spectrum from 2014 January 29 becomes progressively weaker over time, with the spectrum from 2014 April 29 showing only slight emission above the host at wavelengths shorter than Å. While the latest spectrum from 2014 June 6 appears to show more blue continuum emission than the previous epoch, the flux calibration and host subtraction for this spectrum are uncertain, and the corresponding Swift photometry indicates the UV emission of the source should be fading. The broad H emission feature becomes stronger relative to the continuum (higher equivalent width) after the initial spectrum and continues to show strong emission in all later epochs. Other broad emission features can be seen as well, including a He II 4686 line which has become stronger in equivalent width relative to the Balmer lines in the latest spectrum. We further analyze the features of these spectra in comparison to SNe, AGN, and TDEs in §3.3.
3.1 Light Curve Analysis
After correcting both the host and transient fluxes for Galactic extinction, we produced host-subtracted light curves for all 9 photometric filters. From these data we calculate peak absolute magnitudes and decline rates for all Swift filters, which are reported in Table 2. Comparison with luminous supernovae SN 2008es (Miller et al., 2009) and SN 2009kf (Botticella et al., 2010), both of which had absolute -band magnitudes roughly equal to or greater than that of ASASSN-14ae, shows that the UV decline rates of these highly luminous supernovae are much faster than what we observe for ASASSN-14ae, indicating that a supernova explanation for the event is disfavored.
|Filter||Absolute Magnitude||Magnitude Uncertainty||Decline Rate (mag/100 days)||Decline Rate Uncertainty|
ASASSN-14ae’s UV-UV and UV-optical color evolution are also atypical of hydrogen-rich supernovae with broad lines. Figure 6 shows the full host-subtracted UV-UV and UV-U color evolution for ASASSN-14ae in all Swift filters and Figure 7 compares the and colors to those of SN 2008es, a super-luminous SN IIL (Miller et al., 2009; Gezari et al., 2009), and SN 2012aw, a normal SN IIP (Bayless et al., 2013), which were also heavily observed with Swift. For SN 2008es we applied cross-filter K-corrections to obtain the rest-frame colors assuming a blackbody with K (Miller et al., 2009; Gezari et al., 2009), but we did not apply these corrections to SN 2012aw or ASASSN-14ae as they are much lower redshift. ASASSN-14ae shows almost no change in color and became only slightly redder in during the days shown in the Figure. In contrast, SN 2008es became redder in both colors over time, while SN 2012aw became significantly redder in both colors over the first days after discovery and then remained roughly constant in later epochs. ASASSN-14ae looks like neither of these, and all its and colors show little change over the time shown in Figure 6, implying the most likely SN types that could produce the observed spectra of ASASSN-14ae are unlikely to be the sources of the transient.
3.2 SED Analysis
Using the host-subtracted fluxes of ASASSN-14ae we fit the transient SEDs with blackbody curves using Markov Chain Monte Carlo (MCMC) methods. The evolution of the source’s SED along with the best-fit blackbody curves are shown in Figure 8. At early epochs, the blackbody fit is not able to replicate the apparent excess in the (2246 Å) filter. This excess is not created by the extinction correction and corresponds to no obvious emission line. Using the best-fit blackbody curves we estimate the temperature and luminosity evolution of ASASSN-14ae. The derived estimates, along with 90% confidence errors and the values of the best-fit blackbody curve, are given in Table 3 and shown in Figure 9. When there were Swift observations the temperature was estimated as a free parameter. When the UV data were not available, the temperature was constrained by a prior shown by the solid line in Figure 9 that roughly tracks the epochs with UV data. There are some degeneracies between the temperature and luminosity because much of the luminosity is farther in the UV than our data cover for these temperatures, resulting in relatively large uncertainties in some cases. In general, the temperature of the source falls from K to K during the first days of the outburst, then rises again to K over the next days, and then remains roughly constant for the rest of the period shown. Conversely, the luminosity fades steadily over the 150-day period shown. Integrating over the luminosity curve using the epochs with directly estimated temperatures gives a value of ergs for the total energy radiated by ASASSN-14ae during this time. This only requires accretion of of mass, where is the radiative efficiency.
|MJD||Best-Fit||Luminosity ( )||Temperature ( K)||Radius ( cm)|
The results are given only for epochs with Swift data, where the temperature can be estimated without a prior.
After the first days, the luminosity evolution is well fit as an exponential with days as shown in Figure 9. This differs from most TDE models where the luminosity evolution is described as a power law with (e.g., Strubbe & Quataert, 2009; Lodato & Rossi, 2011). However, this temperature and luminosity behavior would be highly unusual for a supernova, which typically exhibit a temperature that drops considerably within days of the explosion along with either a relatively constant luminosity (Type IIP; e.g., Botticella et al., 2010) or a declining luminosity (Type IIn, IIL, Ic; e.g., Miller et al., 2009; Inserra et al., 2013; Graham et al., 2014).
While it is unlikely we are seeing direct emission from a thin disk, we can model the data using the surface brightness profile of a thin disk (Shakura & Sunyaev, 1973). We make the disk infinite, where the location of the inner edge is unimportant given our wavelength coverage. Adding an outer edge could be used to make the profile rise more steeply towards shorter wavelengths. Figure 10 shows the implied luminosity of the disk in Eddington units, where the estimate of depends on the black hole mass , the disk inclination factor and the radiative efficiency in the sense that raising the black hole mass, making the disk inclined to the line of site or lowering the radiative efficiency will reduce the observed luminosity relative to the Eddington limit. In general, the SED of a thin disk fits the data significantly worse than a blackbody. Like the bolometric luminosity, the estimated thin disk luminosity drops exponentially with time rather than as a power law, following a plateau at for the first days. Raising the black hole mass scale to would allow at peak. The differences compared to the Eddington limit shown in Figure 9 arise from the enormous increase in the unobserved hard UV emission which we discuss in §3.3 using the broad emission lines.
3.3 Spectral Analysis
The spectra of ASASSN-14ae show broad Balmer lines in emission with a blue continuum, including an important contribution of the host galaxy (especially in the red) at late times (see Figure 4). We subtracted the SDSS host galaxy spectrum from ASASSN-14ae spectra in order to compare to other objects and for analyzing the spectral line profiles. Figure 11 shows a comparison of ASASSN-14ae host-subtracted spectra at two different epochs after discovery with the spectra of SNe II (SN 2010jl, ASASSN-13co, and SN 2008es) and a broad-line AGN (SDSS J1540-0205). SN 2010jl is a luminous SN IIn ( mag, Stoll et al., 2011), SN 2008es is a super-luminous SN IIL ( mag, Miller et al., 2009), and ASASSN-13co is a luminous SN IIP ( mag, Holoien et al., 2013). At the earliest epoch, the spectrum of ASASSN-14ae is similar to ASASSN-13co and SN 2008es, dominated by a blue continuum. At the later epoch, the spectrum of ASASSN-14ae stays blue, but the spectra of the SN II ASASSN-13co and SN 2008es become significantly redder, consistent with the comparison in color evolution illustrated in Figure 7. The spectral lines also show differences. In ASASSN-14ae the H is broad ( km s) at all times and does not show the P-Cygni profile that is characteristic of SNe II with broad lines. The spectrum of ASASSN-14ae is quite different from the Type IIn SN 2010jl (Stoll et al., 2011; Zhang et al., 2012) at early and late times, both in line profiles (SNe IIn have narrower lines with km s) and continuum shape. The spectrum of the low-ionization broad-line AGN SDSS J1540-0205 (Strateva et al., 2003) does not resemble the spectrum of ASASSN-14ae in the earlier epoch but shows interesting similarities with the spectrum of ASASSN-14ae at days after discovery. In particular, it has a complex H line profile, which is thought to be produced by emission from the accretion disk (Strateva et al., 2003).
In Figure 12 we show the evolution of the H line profile of ASASSN-14ae as a function of time. In the first epoch, 4 days after discovery, the line can be well-fit with a Gaussian profile centered at km s and with km s and integrated luminosity erg s. However, the line peak evolves to the red and the shape becomes significantly asymmetric in later epochs between days after discovery. The peak of the profile in these epochs is at km s and the blue/red wing of the line reaches km s, showing a strong red asymmetry. At 70 days after discovery, the profile again becomes more symmetric and can be relatively well-fit using a gaussian with km s , km s and integrated luminosity erg s. In the spectrum from 2014 April 29, not shown in Figure 12, the H line has km s and its integrated luminosity has only decreased by a factor of three since the first epoch, to erg s. In the April 29 spectrum we also detect broad He II 4686 with km s and erg s. In the latest spectrum from 2014 June 6, this He II line has become stronger relative to the Balmer lines, but has the same FWHM.
In summary, the spectra of ASASSN-14ae seem to be inconsistent with the spectra of SN II and the H emission line profile shows strong evolution during the event. Compared to the spectra of TDEs in the literature, the most similar to ASASSN-14ae is SDSS TDE2 (van Velzen et al., 2011), which showed a broad H line with km s. The spectra of PS1-10jh (Gezari et al., 2012) showed a He II 4686 line in emission with km s and the spectra of PS1-11af (Chornock et al., 2014) did not show any emission lines. The recent paper by Arcavi et al. (2014) presents spectra of multiple TDE candidates and shows that their spectra fall on a continuum, with some events being more He-rich and others being more H-rich. The spectra of ASASSN-14ae resembles the other TDE candidates, and with both strong He and H emission, it appears to fall in the middle of the proposed continuum of spectral properties.
If we assume that the H and He II emission are driven by photoionization and recombination, we can gain some insight into the hard UV continuum and the physical conditions of the line emitting region. In particular, if and are the case B recombination and line emission rate coefficients, and and are the energies of the ionization edge and the line, then we can estimate the luminosity at the ionization edge as , which for and erg s implies ionizing luminosities of and erg s, respectively, as shown in Figure 13. If we compare these estimates, we see that the SED probably requires some additional hard UV emission beyond that expected from the blackbody fits, but definitely has a sharp cutoff at wavelengths only somewhat shorter than the blackbody predictions. This assumes an emission line gas covering fraction near unity, and these ionizing luminosities can be shifted upwards as . However, the covering fractions for H and He are unlikely to differ enormously, so the SED likely must fall towards shorter wavelengths independent of the exact value of . This is also consistent with the X-ray flux limit from §2.2.
For comparison, Figure 13 shows the SED of a thin disk, raising the black hole mass to so that an disk with efficiency is consistent with the observed UV emission near MJD (as compared with Figure 10). The inner regions of the disk are much hotter than the blackbody, so the SED continues to rise into the hard UV, producing far more ionizing flux than is required. This is true even when we add an inner edge at where is the gravitational radius of the black hole. Thus, while there must be some excess hard emission compared to a blackbody, it is likely less than for a thin disk. Note that the effects of the inner disk edge only affect the very hard UV and X-ray emission, which is why we could ignore the inner edge in the SED models from §3.2. The different shape of the thin disk model where we have the optical and near-UV SED also shows why the blackbody models are a better fit to the directly observed SEDs. Extrapolating the SED following the thin disk model would increase the total energy budget and accreted mass by roughly an order of magnitude.
We can also estimate the gas mass associated with the line emission if we assume that the line widths are related to orbital velocities. The emission radius for a velocity of km/s is of order
and the H luminosity is of order
where and are the volumetric rate and line energy and we assume a spherical geometry. This implies a characteristic number density of order
which implies that the line emission should almost instantaneously track the hard UV emission because the recombination times are short. The mass associated with the emission line region is then
which is an order of magnitude larger than the amount of accreted mass needed to power the transient.
The transient ASASSN-14ae, discovered by ASAS-SN on 2014 January 25, had a peak absolute -band magnitude of and position consistent to within arcseconds of the center of SDSS J110840.11+340552.2. However, it does not appear to be consistent with either a supernova or a normal AGN outburst. Its colors remain blue over 140 days since detection, rather than showing the rapid reddening seen in super-luminous SNe with similar absolute magnitudes, and ASASSN-14ae’s temperature has remained relatively constant at K for the duration of the outburst while declining steadily in luminosity at a rate best fit by an exponential decay curve, behavior which is inconsistent with nearly all SNe. Finally, spectra of ASASSN-14ae show a strong blue continuum and broad emission features, including Balmer lines and a He II line, and its spectral evolution does not appear to match either those of SNe or AGN. While highly unusual AGN activity or a strange SN cannot be ruled out completely, the observational characteristics of ASASSN-14ae disfavor both of these scenarios.
Archival photometry, spectroscopy, and SED fitting indicate that SDSS J110840.11+340552.2 appears to be an early-type galaxy with a generally intermediate-to-old-aged stellar population but some signs of recent star formation. While the recent star formation indicates that the galaxy could host a core-collapse SN, the supernova explanation is disfavored by photometric and spectroscopic observations, as previously mentioned. SDSS J110840.11+340552.2 shows spectral emission features indicating only a weak AGN host at best, and its mid-IR colors from WISE are inconsistent with significant AGN activity, further disfavoring normal AGN activity as an explanation for ASASSN-14ae.
Conversely, many of the observed properties of ASASSN-14ae are consistent with other TDE candidates discussed in the literature. The blue colors, slow decline rate, and color evolution have been seen in many TDE candidates, and these transients are predicted to show a largely constant temperature and steadily declining luminosity curve. ASASSN-14ae’s spectra also do not appear to be highly unusual for TDEs, and in fact are a very close match to spectra of the SDSS candidate TDE 2 (van Velzen et al., 2011) and the PTF candidate PTF09djl (Arcavi et al., 2014). With both strong He and H emission lines, ASASSN-14ae appears to fall in the middle of the He-rich-to-H-rich continuum proposed by Arcavi et al. (2014).
Thus, we conclude that ASASSN-14ae is probably a TDE. ASASSN-14ae appears to be the lowest redshift candidate TDE discovered at optical or UV wavelengths, and continues to emit well above host galaxy levels in the UV over 140 days since discovery. In the most recent spectra, the optical continuum is again dominated by the host, but with a prominent, broad H line and other, weaker Balmer and He II lines.
The amount of mass associated with the event is small, roughly of accreted material is sufficient to power the transient, and is associated with the line emission region. This suggests that this event is likely powered by tidally stripping the envelope of a giant rather than by the complete disruption of a main-sequence star, as described in MacLeod, Guillochon, & Ramirez-Ruiz (2012), similar to the case seen with black hole candidate ESO 243-49 HLX-1 (Webb et al., 2014). The duration of a TDE can be truncated by putting the star on an orbit bound to the black hole (Hayasaki et al., 2013), but this necessarily implies that a larger fraction of the stellar mass is on bound orbits and so should enhance the total energy release. On the other hand, disruptions of giants on parabolic orbits have the same asymptotic power law for the rate of return of material to pericenter, but they have a far higher peak. At its simplest, a constant density spherical star has a return rate proportional to where is the period at the surface, while a shell has a rate simply proportional to for in both cases. As a result, disruptions of giant envelopes should show both faster rises and declines.
The fact that ASAS-SN has discovered a likely TDE in the first year of real-time operations, despite only having two cameras operational during most of this period, indicates that we may find these events on a yearly basis now that we are operational in both the Northern and Southern hemispheres. Using the estimates for the local density of black holes from Shankar (2013) and an observable volume of Mpc expected for the fully operational (both hemispheres) ASAS-SN gives a total of black holes with masses in the mass range that will be observable by ASAS-SN. Given a TDE rate of per galaxy (van Velzen & Farrar, 2014), and assuming a 50% detection efficiency, we calculate an expected rate of roughly TDEs per year to be found in ASAS-SN data. Using the volumetric TDE rate from the same manuscript of gives a more optimistic rate of TDEs per year to be found in ASAS-SN data. These nearby events will be easily observable with a variety of telescopes and instruments, allowing us to study these TDEs across a broad wavelength range and at a level that cannot be done for TDEs discovered at higher redshifts. This will allow us to develop a catalog of well-observed TDE candidates that can be used for population studies, and if candidates are caught early enough, to examine the early behavior of these events. Although ASAS-SN is focused primarily on the discovery and observation of supernovae, it will also be an invaluable resource for studying TDEs and other bright transients in the future.
The authors thank A. Piro, F. Shankar, N. McConnell, M. Strauss, J. Greene, and J. Guillochon for useful comments and suggestions. We thank M. L. Edwards and the staff of the LBT Observatory for their support and assistance in obtaining the MODS spectra. We thank PI Neil Gehrels and the Swift ToO team for promptly approving and executing our observations. We thank LCOGT and its staff for their continued support of ASAS-SN.
Development of ASAS-SN has been supported by NSF grant AST-0908816 and the Center for Cosmology and AstroParticle Physics at the Ohio State University. JFB is supported by NSF grant PHY-1101216.
This research has made use of the XRT Data Analysis Software (XRTDAS) developed under the responsibility of the ASI Science Data Center (ASDC), Italy. At Penn State the NASA Swift program is support through contract NAS5-00136.
The LBT is an international collaboration among institutions in the United States, Italy and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; the Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia.
This publication used data obtained with the MODS spectrographs built with funding from NSF grant AST-9987045 and the NSF Telescope System Instrumentation Program (TSIP), with additional funds from the Ohio Board of Regents and the Ohio State University Office of Research.
The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.
This research was made possible through the use of the AAVSO Photometric All-Sky Survey (APASS), funded by the Robert Martin Ayers Sciences Fund.
Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/.
SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.
This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.
This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Appendix A Follow-up Photometry
All follow-up photometry is presented in Table 4 below. Photometry is presented in the natural system for each filter: magnitudes are in the AB system, while Swift filter magnitudes are in the Vega system.
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