Astronomers Investigate a Peculiar Nuclear Transient

A team of international astronomers has observed a peculiar nuclear transient known as AT 2019avd using a set of space telescopes. This transient, discovered in 2009 by the Zwicky Transient Facility, has been detected in various wavelengths, from radio to soft X-rays, and has exhibited two continuous flaring episodes with different profiles, spanning over two years.

Previous studies suggested that AT 2019avd might be a tidal disruption event (TDE), which occurs when a star passes close to a supermassive black hole and is disrupted by the black hole’s tidal forces. However, two optical flares observed from this transient were atypical for TDEs.

To determine the true nature of AT 2019avd, the astronomers employed NASA’s Swift and Chandra spacecraft, as well as the Neutron star Interior Composition Explorer (NICER) onboard the International Space Station (ISS) for a monitoring campaign spanning more than 1,000 days.

The observations revealed that AT 2019avd exhibits high X-ray variability on both short and long timescales. A rapid drop in luminosity occurred approximately 225 days following the peak of X-ray emission, which exceeded two orders of magnitude. This drop in luminosity was accompanied by X-ray spectral hardening and was followed by the possible ejection of an optically-thick radio outflow.

The study found a softer-when-brighter relation throughout the flare, with the spectrum hardening as the luminosity decreases. It was also observed that when the luminosity decreases by over one magnitude, the blackbody temperature remains constant, and the photon index decreases with the luminosity.

The fractional root-mean-square (rms) amplitude of the detected X-ray variability was high, with an average of 43%, and its evolution was related to the spectral state. The astronomers assume that the variability may be attributed to some intervening clumpy outflows.

The obtained results do not allow the researchers to draw final conclusions on the TDE nature of AT 2019avd. They plan to continue monitoring this transient to see whether it will eventually evolve to the standard hard state and how long it will take to observe the full evolution of the accretion process.