AT 2019avd: Luminosity of non-absorbed X-rays in the 0.3-2 keV band. Black dots, light blue stars, and red stars represent NICER, XRT/Swift, and LETG/Chandra data, respectively. Credit: Wang et al, 2023
An international team of astronomers used an array of space telescopes to observe a particular nuclear transient known as AT 2019avd. Results of the observation campaign, presented in an article published on December 21 on the pre-print server arXivprovide important information about the properties and behavior of this transient.
Nuclear astrophysics is essential to understanding supernova explosions, and in particular the synthesis of chemical elements that evolved after the Big Bang. Therefore, the detection and investigation of transient nuclear events could be essential to advance our knowledge in this area.
With a redshift of 0.028, AT 2019avd is a peculiar nuclear transient discovered by the Zwicky Transient Facility (ZTF) in 2009. The transient has been detected in various wavelengths, from radio to soft X-rays, and has recently presented two continuous flaring episodes. with different profiles, spanning two years.
Previous studies of AT 2019avd have suggested, based on its ultra-soft X-ray spectrum and optical spectral lines, that it could be a tidal disturbance event (TDE). Typically, TDEs occur when a star passes close enough to a supermassive black hole and is separated by tidal forces from the black hole, causing the disruption process. However, two optical flares observed from this transient were found to be atypical for TDEs.
To determine the true nature of AT 2019avd, a team of astronomers led by Yanan Wang of the Chinese Academy of Sciences, used NASA’s Swift and Chandra spacecraft, as well as NASA’s Interior Composition Explorer. neutron star (NICER) on board the International Space Station (ISS) to carry out a monitoring campaign for this transient spanning more than 1,000 days.
Observations of AT 2019avd showed that it exhibits strong X-ray variability on short (lasting hundreds to thousands of seconds) and long (years) timescales. Additionally, the monitoring campaign revealed some unique properties of this transient.
First, a rapid drop in the brightness of AT 2019avd occurred about 225 days after the peak X-ray emission, which exceeded two orders of magnitude. This drop in brightness was accompanied by spectral hardening of the X-rays and was followed by the possible ejection of an optically thick radio stream.
Observations revealed a softer-to-brighter relationship throughout the flare as the spectrum hardens as brightness decreases. It was also found that when the luminosity decreases by more than one magnitude, the black body temperature remains constant and the photon index decreases with luminosity.
According to the study, the fractional root mean square (rms) amplitude of the detected X-ray variability is high with an average of 43% and its evolution is linked to the spectral state. Astronomers assume that the variability can be attributed to some intermediate clumped flows.
The authors of the article noted that the results obtained do not allow them to draw definitive conclusions about the TDE nature of AT 2019avd. They plan to monitor this transient further to see if it will eventually evolve into the standard hard state and how long it will take to chart a full evolution of the accretion process.
More information:
Yanan Wang et al, Rapid dimming followed by a state transition: a study of the highly variable nuclear transient AT 2019avd over more than 1,000 days, arXiv (2023). DOI: 10.48550/arxiv.2312.13543
Journal information:
arXiv
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