Kavli Affiliate: Ronald Remillard
| First 5 Authors: Yanan Wang, Dheeraj R. Pasham, Diego Altamirano, Andres Gurpide, Noel Castro Segura
| Summary:
The tidal disruption of a star around a supermassive black hole (SMBH) offers
a unique opportunity to study accretion onto a SMBH on a human-timescale. We
present results from our 1000+ days NICER, Swift and Chandra monitoring
campaign of AT 2019avd, a nuclear transient with TDE-like properties. Our
primary finding is that approximately 225 days following the peak of X-ray
emission, there is a rapid drop in luminosity exceeding two orders of
magnitude. This X-ray drop-off is accompanied by X-ray spectral hardening,
followed by a 740-day plateau phase. During this phase, the spectral index
decreases from 6.2+-1.1 to 2.3+-0.4, while the disk temperature remains
constant. Additionally, we detect pronounced X-ray variability, with an average
fractional root mean squared amplitude of 47%, manifesting over timescales of a
few dozen minutes. We propose that this phenomenon may be attributed to
intervening clumpy outflows. The overall properties of AT 2019avd suggest that
the accretion disk evolves from a super-Eddington to a sub-Eddington luminosity
state, possibly associated with a compact jet. This evolution follows a pattern
in the hardness-intensity diagram similar to that observed in stellar-mass
black holes, supporting the mass invariance of accretion-ejection processes
around black holes.
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