Kavli Affiliate: Ronald Remillard
| First 5 Authors: Thomas Wevers, Dheeraj R. Pasham, Sjoert van Velzen, James C. A. Miller-Jones, Phil Uttley
| Summary:
Following a tidal disruption event (TDE), the accretion rate can evolve from
quiescent to near-Eddington levels and back over months – years timescales.
This provides a unique opportunity to study the formation and evolution of the
accretion flow around supermassive black holes (SMBHs). We present two years of
multi-wavelength monitoring observations of the TDE AT2018fyk at X-ray, UV,
optical and radio wavelengths. We identify three distinct accretion states and
two state transitions between them. These appear remarkably similar to the
behaviour of stellar-mass black holes in outburst. The X-ray spectral
properties show a transition from a soft (thermal-dominated) to a hard
(power-law dominated) spectral state around L$_{rm bol} sim $few $ times
10^{-2}$ L$_{rm Edd}$, and the strengthening of the corona over time
$sim$100–200 days after the UV/optical peak. Contemporaneously, the spectral
energy distribution (in particular, the UV-to-X-ray spectral slope
$alpha_{ox}$) shows a pronounced softening as the outburst progresses. The
X-ray timing properties also show a marked change, initially dominated by
variability at long ($>$day) timescales while a high frequency ($sim$10$^{-3}$
Hz) component emerges after the transition into the hard state. At late times
($sim$500 days after peak), a second accretion state transition occurs, from
the hard into the quiescent state, as identified by the sudden collapse of the
bolometric (X-ray+UV) emission to levels below 10$^{-3.4}$ L$_{rm Edd}$. Our
findings illustrate that TDEs can be used to study the scale (in)variance of
accretion processes in individual SMBHs. Consequently, they provide a new
avenue to study accretion states over seven orders of magnitude in black hole
mass, removing limitations inherent to commonly used ensemble studies.
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