The first tidal disruption flare in ZTF: from photometric selection to multi-wavelength characterization

TL;DR

This paper reports the discovery and detailed multi-wavelength follow-up of the first tidal disruption flare detected by ZTF, demonstrating how to distinguish TDEs from other nuclear transients using light-curve and host galaxy data.

Contribution

It provides the first ZTF TDE detection with comprehensive photometric and multi-wavelength characterization, and develops criteria to differentiate TDEs from other nuclear transients.

Findings

ZTF captured the rise-to-peak of the TDE with 50 days of pre-peak data.

A faint thermal X-ray source was detected, significantly dimmer than optical/UV emission.

Methodology to distinguish TDEs from AGN and supernovae based on light-curve and host galaxy properties.

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 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.