Astronomers inspect a peculiar nuclear transient

Astronomers inspect a peculiar nuclear transient
AT 2019avd: Unabsorbed X-ray luminosity in the 0.3–2 keV band. The black dots, the light blue stars and the red stars represent the NICER, XRT/Swift and LETG/Chandra data, respectively. Credit: Wang et al, 2023

Astronomers from around the world have utilized an array of space telescopes to observe an unusual nuclear transient called AT 2019avd. The findings of their observational campaign, presented in a recent paper published on the pre-print server arXiv, have provided valuable insights into the behavior and properties of this transient cosmic phenomenon.

Nuclear astrophysics plays a crucial role in our understanding of supernova explosions and the synthesis of chemical elements following the Big Bang. Therefore, the detection and investigation of nuclear transient events are essential for advancing our knowledge in this field.

Located at a redshift of 0.028, AT 2019avd is a peculiar nuclear transient that was first discovered by the Zwicky Transient Facility (ZTF) in 2009. This transient has been detected across various wavelengths, ranging from radio waves to soft X-rays, and has recently exhibited two distinct flaring episodes with different profiles over a span of two years.

Prior 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 typically occur when a star comes within close proximity to a supermassive black hole and is torn apart by the immense tidal forces, resulting in a disruptive process. However, the two optical flares observed from this transient have proven to be atypical for TDEs.

To uncover the true nature of AT 2019avd, a team of astronomers led by Yanan Wang from the Chinese Academy of Sciences has conducted a monitoring campaign of this transient spanning over 1,000 days. This campaign utilized NASA’s Swift and Chandra spacecraft, as well as the Neutron star Interior Composition Explorer (NICER) aboard the International Space Station (ISS).

The observations of AT 2019avd have revealed high X-ray variability on both short (lasting from hundreds to thousands of seconds) and long (years) timescales. Additionally, the monitoring campaign has unveiled several unique properties of this transient.

One notable observation is a rapid decline in luminosity of AT 2019avd, occurring approximately 225 days after the peak of X-ray emission. This decline exceeded two orders of magnitude and was accompanied by X-ray spectral hardening. It was followed by the potential ejection of an optically-thick radio outflow.

The observations also showed a “softer-when-brighter” relationship throughout the flare, with the X-ray spectrum becoming harder as the luminosity decreases. Furthermore, when the luminosity decreases by over one magnitude, the blackbody temperature remains constant while the photon index decreases.

The study suggests that 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. The astronomers propose that this variability may be attributed to intervening clumpy outflows.

The authors of the paper acknowledge that the obtained results do not allow for definitive conclusions regarding the TDE nature of AT 2019avd. They plan to continue monitoring this transient to determine whether it will eventually transition to the standard hard state and to observe the complete 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 1000+ days, arXiv (2023). DOI: 10.48550/arxiv.2312.13543

Journal information:

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Astronomers inspect a peculiar nuclear transient (2023, December 27)
retrieved 27 December 2023

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