Seventeen Tidal Disruption Events from the First Half of ZTF Survey Observations: Entering a New Era of Population Studies

Abstract

While tidal disruption events (TDEs) have long been heralded as laboratories for the study of quiescent black holes, the small number of known TDEs and uncertainties in their emission mechanism have hindered progress toward this promise. Here we present 17 new TDEs that have been detected recently by the Zwicky Transient Facility along with Swift UV and X-ray follow-up observations. Our homogeneous analysis of the optical/UV light curves, including 22 previously known TDEs from the literature, reveals a clean separation of light-curve properties with spectroscopic class. The TDEs with Bowen fluorescence features in their optical spectra have smaller blackbody radii, lower optical luminosities, and higher disruption rates compared to the rest of the sample. The small subset of TDEs that show only helium emission lines in their spectra have the longest rise times, the highest luminosities, and the lowest rates. A high detection rate of Bowen lines in TDEs with small photometric radii could be explained by the high density that is required for this fluorescence mechanism. The stellar debris can provide a source for this dense material. Diffusion of photons through this debris may explain why the rise and fade timescale of the TDEs in our sample are not correlated. We also report, for the first time, the detection of soft X-ray flares from a TDE on ∼day timescales. Based on the fact that the X-ray flares peak at a luminosity similar to the optical/UV blackbody luminosity, we attribute them to brief glimpses through a reprocessing layer that otherwise obscures the inner accretion flow.

Tidal Disruption Events and High-energy Neutrinos

“…More broadly, like most TDEs, the UV/optical emission of AT2019dsg was well-described by thermal emission from a blackbody of temperature ∼ 10 4.6 K and radius ∼ 10 14.5 cm, reaching a peak luminosity of ∼ 10 44.5 erg s −1 . This inferred temperature was somewhat hotter than a typical TDE [11]. Relative to the broader ZTF TDE population, AT2019dsg was both relatively bright and also relatively close (z=0.051, corresponding to a luminosity distance of 230 Mpc), meaning that AT2019dsg had second-brightest bolometric energy flux of all ZTF…”

“…TDEs [11]. Accounting for all neutrino follow-up campaigns listed in Figure 5, the probability of finding such a bright TDE by chance with our neutrino follow-up program is just 0.2%.…”

“…Relative to transients detected by the unbiased ZTF Bright Transient Survey [69,70], both AT2019dsg and AT2019fdr have atypically bright time-integrated optical fluxes. Indeed, they are outliers even relative to the populations of TDEs and flares in Narrow-Line Seyfert 1 galaxies, which are themselves more intrinsically luminous than supernovae [11,68]. These observations provide strong indications of an emerging trend, suggesting that some portion of the neutrino flux is indeed emitted by bright TDEs.…”

Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg

“…A 2 arcsec slit was placed on the target at the parallactic angle. One-dimensional spectra were extracted, reduced, and calibrated following standard procedures using the FLOYDS pipeline 2 (Valenti et al 2014). The FLOYDS spectra are not used in the analysis, due to a combination of low SNR, wavelength range covered, and proximity in time with higher quality spectra.…”

“…Arp 299B-AT1 Mattila et al 2018), and those where the relativistic/sub-relativistic nature of the outflow is less clear (e.g. van Velzen et al 2016;Alexander et al 2016). Stein et al (2020) have modelled the radio emission of AT 2019dsg as a synchrotron selfabsorbed sub-relativistic outflow that has a continuous injection of fresh relativistic electrons from a central engine.…”