Optical Discovery of Probable Stellar Tidal Disruption Flares

Abstract

Using archival SDSS multi-epoch imaging data (Stripe 82), we have searched for flares due to the tidal disruption of stars by super-massive black holes in non-active galaxies. Two candidate tidal disruption events (TDEs) are identified, using a pipeline with high rejection efficiency for supernovae (SNe) and Active Galactic Nucleus (AGN) flares, and minimal selection bias. The flares have optical black-body temperatures 2 × 10 4 K and their cooling rates are very low; their observed peak luminosities are M g = −18.3 and −20.4 (νL ν = 5 × 10 42 , 4 × 10 43 erg s −1 , in the rest-frame), qualitatively consistent with expectations for tidal disruption flares. The properties of the TDE candidates are examined using i) SDSS imaging to compare them to other flares observed in the search, ii) UV emission measured by GALEX and iii) spectra of the hosts and of one of the flares. Our pipeline excludes optically identifiable AGN hosts, and our variability monitoring over 9 years provides strong evidence that these are not flares in hidden AGNs: 1) the luminosity increases in the candidate TDE flares are estimated to be greater by factors 4 and 15 than in any of the variable AGNs monitored, and 2) the TDE candidates' hosts are much quieter in the seasons not including the primary flare than are hosts of AGN flares (less activity on average in each of the non-primary-flare seasons than in 95% of AGNs, with a cumulative probability estimated to be 10 −5 for a flaring AGN to have all other seasons as quiet as for the TDE candidates). The spectra and color evolution of the flares are unlike any SN observed to date, and their strong late-time UV emission is particularly distinctive. These features, along with the high resolution with which they are placed at the nucleus, argue against these being first cases of a previously-unobserved class of SNe or more extreme examples of known SN types. Taken together, the observed properties are difficult to reconcile with a SN or AGN-flare explanation, although an entirely new process specific to the inner few-hundred parsecs of non-active galaxies cannot be excluded. Based on our observed rate, we infer that hundreds or thousands of TDEs will be present in current and next-generation optical synoptic surveys. Using the approach outlined here, a TDE candidate sample with O (1) purity can be selected using geometric resolution and host and flare color alone, demonstrating that a campaign to create a large sample of tidal disruption events, with immediate and detailed multi-wavelength follow-up, is feasible. A by-product of this work is quantification of the power-spectrum of extreme flares in AGNs.

The X-Ray Through Optical Fluxes and Line Strengths of Tidal Disruption Events

“…Our model continua slopes are somewhat shallower, L λ ∝λ −3 , than the Rayleigh-Jeans tail, L λ ∝λ −4 , seen in PS1-10jh (Gezari et al 2012). Nevertheless, our slope is consistent with HST UV data for ASASSN-14li (Cenko et al 2016), as well as the spectral indices measured in TDE1/TDE2 (van Velzen et al 2011), in the absence of strong dust extinction.…”

“…There have also been TDE candidates found in the ultraviolet (UV; Gezari et al 2006Gezari et al , 2009, and in the optical in SDSS (van Velzen et al 2011;van Velzen & Farrar 2014), Pan-STARRS1 (Gezari et al 2012;Chornock et al 2014), ASASSN (Holoien et al 2014(Holoien et al , 2016a(Holoien et al , 2016b, PTF (Cenko et al 2012a;Arcavi et al 2014), and ROTSE (Vinkó et al 2015). These events typically rise to a peak (observer frame) R-band luminosity of ∼2×10 43 erg s −1 on a timescale of ∼months (Arcavi et al 2014), with a late time fall consistent with t −5/3 .…”

XMMSL1 J074008.2-853927: a tidal disruption event with thermal and non-thermal components

“…This radiation signature has been observed in the soft X-ray Bade et al 1996;Komossa & Greiner 1999b;Esquej et al 2007;Saxton et al 2012;Maksym et al 2010;Lin et al 2015) and UV (Gezari et al 2006(Gezari et al , 2008(Gezari et al , 2009 bands. Optical flares have also been detected (van Velzen et al 2011;Cenko et al 2012;Gezari et al 2012;Arcavi et al 2014) from candidate TDE, with an apparent temperature of a few ×10 4 K, cooler than that expected from an accretion disc (e.g. Bonning et al 2007).…”

A tidal disruption-like X-ray flare from the quiescent galaxy SDSS J120136.02+300305.5

“…The accretion of tidally disrupted stellar debris may significantly contribute to the low-luminosity end of the AGN luminosity function (Milosavljevic et al 2006). To date, several candidate tidal disruption events (TDE) have been identified, based on a luminous flare seen at soft X-ray (e.g., Komossa & Bade 1999;Greiner et al 2000;Esquej et al 2007;Cappelluti et al 2009), UV (e.g., Gezari et al 2006;Gezari et al 2008) or optical (e.g., Cenko et al 2012;van Velzen et al 2011) wavelengths. The first X-ray tidal disruption flares were identified in the course of the ROSAT all-sky survey (Komossa & Bade 1999;Komossa & Greiner 1999;Grupe et al 1999;Greiner et al 2000).…”