Radio observations of the TDE AT2019qiz

ATel #13310; Andrew O'Brien (University of Wisconsin-Milwaukee), David Kaplan (University of Wisconsin-Milwaukee), Tara Murphy (University of Sydney), Wenfei Yu (Shanghai Astronomical Observatory), Wenda Zhang (Astronomical Institute, Czech Academy of Sciences)
on 25 Nov 2019; 16:59 UT
Credential Certification: David L. Kaplan (kaplan@gravity.phys.uwm.edu)

Tweet

AT2019qiz is a tidal disruption event discovered optically using the ALeRCE broker with ZTF data on 2019-09-19, then classified as a rising TDE using Keck I/LRIS spectra on 2019-09-25 (ATel #13131). Follow up observations with Swift/UVOT have traced the UV/optical brightening of AT2019qiz (ATel #13146) and subsequent decay (ATel #13193). These observations, plus another with INTEGRAL (ATel #13170) and regular Swift/XRT monitoring through 2019-11-18, have not yet yielded any X-ray detections.

We have conducted radio observations with the Australia Telescope Compact Array (ATCA) during unallocated Director's time (project code CX442).

The first observation was on 2019-10-21 at 5.5 and 9.0 GHz. We detected a 5-sigma point source of 0.21 +/- 0.08 mJy at 9.0 GHz offset by (-7.2 +/- 6.2, -1.6 +/- 4.5) arcsec (dRA, dDEC) from the recorded AT2019qiz optical position. We note that there is a bright optical source offset from the nucleus of the host galaxy present in many archival images and our radio detection sits approximately between the host galaxy nucleus and this unknown optical source. Since these radio observations were conducted in a very compact array configuration (H168) which produced a large synthesized beam (42" x 22" at 9.0 GHz), we are unable to confidently associate the entirety of this radio flux to AT2019qiz. No significant detection was made at 5.5 GHz giving a flux density upper limit of 0.45 mJy.

The second observation was on 2019-11-17 at 17.0 and 19.0 GHz. We detect a point source coincident with the AT2019qiz position to <0.1" with high significance in both bands: 1.35 +/- 0.04 and 1.24 +/- 0.04 mJy (42 and 21-sigma), respectively. This observation was made in a more extended array configuration (750D) which gave a synthesized beam size of 3.56" x 0.34" at 19.0 GHz. Due to the superior resolution compared to the earlier radio observation, we are confident that this emission is associated with the nucleus of the host galaxy 2MASX J04463790-1013349, although we require further epochs of radio data to disentangle potential quiescent AGN emission from AT2019qiz.

Assuming minimal evolution between our two observations, the strongly increasing spectral energy distribution from 9.0 GHz to 17 GHz (alpha=2.9) is inconsistent with typical AGN but consistent with the optically thick portion of a TDE (e.g., Alexander et al. 2016, ApJ, 819, L25), suggesting that we are seeing the TDE itself. Furthermore, a quiescent optical spectrum of the host (Jones et al. 2009, MNRAS, 399, 683) does not display any obvious signs of AGN activity. Additional spectral and/or temporal monitoring are encouraged to resolve this issue.

We thank the staff at the Australia Telescope National Facility for approving our requests to use Director's time. AO and DK were supported by NSF grant AST-1816492.