A Late-Time Radio Flare following a Possible Transition in Accretion State in the Tidal Disruption Event AT 2019azh

TL;DR

This study presents radio follow-up observations of TDE AT 2019azh, revealing a late-time radio flare coinciding with X-ray variability, suggesting a possible accretion state transition similar to black hole X-ray binaries.

Contribution

It reports the first detection of a late-time radio flare in AT 2019azh and proposes a connection to accretion state changes, expanding understanding of TDE evolution.

Findings

Late-time radio flare observed approximately 110 days after X-ray peak.

Radio flare peaks at twice the pre-flare emission levels.

Comparison with other TDEs suggests similar accretion state transitions.

Abstract

We report here radio follow-up observations of the optical Tidal Disruption Event (TDE) AT 2019azh. Previously reported X-ray observations of this TDE showed variability at early times and a dramatic increase in luminosity, by a factor of $\sim 10$, about 8 months after optical discovery. The X-ray emission is mainly dominated by intermediate hard--soft X-rays and is exceptionally soft around the X-ray peak, which is $L_X \sim 10^{43} \rm \, erg \, s^{-1}$. The high cadence $15.5$ GHz observations reported here show an early rise in radio emission followed by an approximately constant light curve, and a late-time flare. This flare starts roughly at the time of the observed X-ray peak luminosity and reaches its peak about $110$ days after the peak in the X-ray, and a year after optical discovery. The radio flare peaks at $\nu L_{\nu} \sim 10^{38} \rm \, erg \, s^{-1}$, a factor of two higher than the emission preceding the flare. In light of the late-time radio and X-ray flares, and the X-ray spectral evolution, we speculate a possible transition in the accretion state of this TDE, similar to the observed behavior in black hole X-ray binaries. We compare the radio properties of AT 2019azh to other known TDEs, and focus on the similarities to the late time radio flare of the TDE ASASSN-15oi.