AT2019azh: an unusually long-lived, radio-bright thermal tidal disruption event

TL;DR

This paper presents extensive radio observations of the long-lived TDE AT2019azh, revealing unusual radio brightening and fluctuations, and provides insights into the nature of the outflow, contributing valuable data to TDE studies.

Contribution

It offers the first detailed radio monitoring of AT2019azh over two years, showing slow brightening and fluctuations, and suggests a non-relativistic, possibly spherical or mildly collimated outflow.

Findings

AT2019azh exhibited slow radio brightening over two years.

Detected fluctuations in the synchrotron energy index at 450 days.

First radio detection of a non-relativistic TDE relative to optical discovery.

Abstract

Tidal disruption events (TDEs) occur when a star is destroyed by a supermassive black hole at the center of a galaxy, temporarily increasing the accretion rate onto the black hole and producing a bright flare across the electromagnetic spectrum. Radio observations of TDEs trace outflows and jets that may be produced. Radio detections of the outflows from TDEs are uncommon, with only about one third of TDEs discovered to date having published radio detections. Here we present over two years of comprehensive, multi-radio frequency monitoring observations of the tidal disruption event AT2019azh taken with the Very Large Array (VLA) and MeerKAT radio telescopes from approximately 10 days pre-optical peak to 810 days post-optical peak. AT2019azh shows unusual radio emission for a thermal TDE, as it brightened very slowly over two years, and showed fluctuations in the synchrotron energy index of the optically thin synchrotron emission from 450 days post-disruption. Based on the radio properties, we deduce that the outflow in this event is likely non-relativistic and could be explained by a spherical outflow arising from self-stream intersections, or a mildly collimated outflow from accretion onto the supermassive black hole. This data-set provides a significant contribution to the observational database of outflows from TDEs, including the earliest radio detection of a non-relativistic TDE to date, relative to the optical discovery.