The First Tidal Disruption Flare in ZTF: From Photometric Selection to Multi-wavelength Characterization

Abstract

We present Zwicky Transient Facility (ZTF) observations of the tidal disruption flare AT2018zr/PS18kh reported by Holoien et al. and detected during ZTF commissioning. The ZTF light curve of the tidal disruption event (TDE) samples the rise-to-peak exceptionally well, with 50 days of g-and r-band detections before the time of maximum light. We also present our multi-wavelength 2 van Velzen et al.follow-up observations, including the detection of a thermal (kT ≈ 100 eV) X-ray source that is two orders of magnitude fainter than the contemporaneous optical/UV blackbody luminosity, and a stringent upper limit to the radio emission. We use observations of 128 known active galactic nuclei (AGN) to assess the quality of the ZTF astrometry, finding a median host-flare distance of 0. 2 for genuine nuclear flares. Using ZTF observations of variability from known AGN and supernovae we show how these sources can be separated from TDEs. A combination of light-curve shape, color, and location in the host galaxy can be used to select a clean TDE sample from multi-band optical surveys such as ZTF or LSST.

To TDE or not to TDE: the luminous transient ASASSN-18jd with TDE-like and AGN-like qualities

“…We compare the optical and UV spectra of ASASSN-18jd to well-studied TDEs, other extragalactic transients, and objects with similar spectra in Figures 15 and 16. In Figure 15, we compare two early-time optical spectra of ASASSN-18jd to known TDEs (Holoien et al 2014(Holoien et al , 2016aHung et al 2017;Blagorodnova et al 2019;Hung et al 2019), as well as the composite quasar spectrum from Vanden Berk et al (2001), an early-time spectrum of the TDE candidate PS16dtm (Blanchard et al 2017), an early-time spectrum of the rapid turn-on event ZTF18aajupnt/AT 2018dyk (Frederick et al 2019), and the spectrum of a WR star with strong He ii and N iii-v features (Yaron & Gal-Yam 2012). In Figure 16, we compare a UV spectrum of ASASSN-18jd to TDEs with published UV spectra (Cenko et al 2016;Brown et al 2018;Blagorodnova et al 2019;Hung et al 2019), the composite quasar spectrum from Vanden Berk et al (2001), and an "N-rich" quasar spectrum (Batra & Baldwin 2014).…”

“…Some recently discovered TDEs have shown deviations from monotonic fading: ASASSN-18ul showed a light curve plateau ∼40 d after peak brightness (Wevers et al 2019b) and ASASSN-19bt showed a short flare before reaching peak brightness (Holoien et al 2019b). In a more extreme example, PS18kh showed a UV re-brightening ∼50 d after peak (Holoien et al 2019a) and then, after a seasonal gap due Sun constraints, appeared to return to near-peak luminosity (van Velzen et al 2019a). However, these deviations from monotonic fading were still "smooth", with little short-timescale variation, which is qualitatively different from the bumpiness in the light curve of ASASSN-18jd.…”

“…Its X-ray flux increased around ∼200 d before decaying ∼350 d after discovery, which is believed to have been caused by ei-ther inefficient circularization that resulted in delayed accretion, or material surround the TDE becoming optically thin a few months after discovery (Gezari et al 2017b;Holoien et al 2018). The optically-detected TDEs PS18kh (Holoien et al 2019a;van Velzen et al 2019a) and ASASSN-19bt (Holoien et al 2019b) have also shown X-ray emission, but only the TDE candidate ASASSN-18ul (Wevers et al 2019b) exhibits a X-ray light curve similar to that of ASASSN-18jd. ASASSN-18ul shows a relatively flat X-ray light curve with small timescale variations where the X-ray luminosity varies by more than an order of magnitude (∼10 42 −10 43.2 erg s −1 ).…”

“…The light curves of TDEs usually peak and decay monotonically, but several TDEs show deviations from a monotonic decay. Examples are PS18kh/AT 2018zr (Holoien et al 2019a;van Velzen et al 2019a), which rebrightened multiple times after its initial decay, ASASSN-18ul/AT 2018fyk (Wevers et al 2019b), which showed an extended plateau in its light curve, and ASASSN-19bt (Holoien et al 2019b), which showed a short flare ∼30 d before reaching peak brightness. Additionally, there is some evidence that TDEs may fade more slowly as the SMBH mass increases Wevers et al 2017;van Velzen et al 2019b).…”

“…This publication makes use of data obtained from the Weizmann Interactive Supernova Data Repository (WIS-eREP, Yaron & Gal-Yam 2012). This paper has been typeset from a T E X/L A T E X file prepared by the author.…”