AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system

TL;DR

This paper investigates the unusual variability of AT 2021hdr, proposing it is caused by a gas cloud being tidally disrupted by a binary supermassive black hole, distinct from typical tidal disruption events or AGN flares.

Contribution

It introduces the hypothesis that AT 2021hdr's behavior results from a gas cloud disrupted by a binary SMBH, with estimates of the binary's separation and merger timeline.

Findings

Oscillations every 60-90 days observed in multiple wavelengths.

VLBA shows no radio emission at milliarcsecond scale.

The galaxy is part of a merger unrelated to the oscillations.

Abstract

With a growing number of facilities able to monitor the entire sky and produce light curves with a cadence of days, in recent years there has been an increased rate of detection of sources whose variability deviates from standard behavior, revealing a variety of exotic nuclear transients. The aim of the present study is to disentangle the nature of the transient AT 2021hdr, whose optical light curve used to be consistent with a classic Seyfert 1 nucleus, which was also confirmed by its optical spectrum and high-energy properties. From late 2021, AT 2021hdr started to present sudden brightening episodes in the form of oscillating peaks in the Zwicky Transient Facility (ZTF) alert stream, and the same shape is observed in X-rays and UV from Swift data. The oscillations occur every about 60-90 days with amplitudes of around 0.2 mag in the g and r bands. Very Long Baseline Array (VLBA) observations show no radio emission at milliarcseconds scale. It is argued that these findings are inconsistent with a standard tidal disruption event (TDE), a binary supermassive black hole (BSMBH), or a changing-look active galactic nucleus (AGN); neither does this object resemble previous observed AGN flares, and disk or jet instabilities are an unlikely scenario. Here, we propose that the behavior of AT 2021hdr might be due to the tidal disruption of a gas cloud by a BSMBH. In this scenario, we estimate that the putative binary has a separation of about 0.83 mpc and would merge in about 70000 years. This galaxy is located at 9 kpc from a companion galaxy, and in this work we report this merger for the first time. The oscillations are not related to the companion galaxy.