by In a fascinating discovery, an international team of astronomers has closely examined a peculiar nuclear transient called AT 2019avd using a set of space telescopes. The team’s observational campaign, which lasted over 1,000 days, provided valuable insights into the properties and behavior of this transient. The results of their study presented in a paper published on the pre-print server arXiv on December 21.
Understanding supernova explosions and the synthesis of chemical elements that evolved after the Big Bang is crucial in the field of nuclear astrophysics. Consequently, detecting and investigating nuclear transient events like AT 2019avd is essential for advancing our knowledge in this area.
AT 2019avd, discovered by the Zwicky Transient Facility in 2009, has a redshift of 0.028. It has been detected across various wavelengths, from radio to soft X-rays, and has recently shown two continuous flaring episodes with different profiles, spanning over two years.
Previous studies of AT 2019avd have suggested that it may be a tidal disruption event (TDE) based on its ultra-soft X-ray spectrum and optical spectral lines. TDEs occur when a star passes close enough to a supermassive black hole and is torn apart by the black hole’s tidal forces. However, the two optical flares observed from AT 2019avd were found to be atypical for TDEs.
To determine the true nature of AT 2019avd, astronomer Yanan Wang of the Chinese Academy of Sciences led a team that utilized NASA’s Swift and Chandra spacecraft, as well as the Neutron star Interior Composition Explorer (NICER) onboard the International Space Station (ISS), to conduct a monitoring campaign spanning more than 1,000 days.
The observations of AT 2019avd revealed that it displayed high X-ray variability on both short and long timescales. The monitoring campaign also unveiled some unique characteristics of this transient.
One significant observation was a rapid drop in the luminosity of AT 2019avd, exceeding two orders of magnitude. And approximately 225 days after the peak of X-ray emission. This drop in luminosity was accompanied by X-ray spectral hardening and possibly the ejection of an optically-thick radio outflow.
Additionally, the observations indicated a “softer-when-brighter” relation throughout the flare, with the spectrum becoming harder as the luminosity decreases. It observed that when the luminosity decreased by over one magnitude. And the blackbody temperature remained constant while the photon index decreased.
The study found that the fractional root-mean-square (RMS) amplitude of the detected X-ray variability was high. Also averaging 43%, and its evolution linked to the spectral state. The astronomers propose that the variability may be attributes to intervening clumpy outflows.
Despite their findings, the authors of the paper emphasize that they are unable to draw definitive conclusions about the TDE nature of AT 2019avd. They plan to continue monitoring this transient to determine. If it will eventually evolve into the standard hard state and to track the full evolution of the accretion process.
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1. Source: Coherent Market Insights, Public sources, Desk research
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