Rapid accretion state transitions following the tidal disruption event AT2018fyk

Thomas Wevers; Dheeraj R. Pasham; Sjoert van Velzen; James C. A. Miller-Jones; Phil Uttley; Keith Gendreau; Ronald Remillard; Zaven Arzoumanian; Michael Loewenstein; Ani Chiti

doi:10.3847/1538-4357/abf5e2

paper

Rapid accretion state transitions following the tidal disruption event AT2018fyk

arXiv:2101.04692 · doi:10.3847/1538-4357/abf5e2

Abstract

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 $_{bol} \sim$ few $\times 1 0^{- 2}$ L $_{Edd}$ , 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 $α_{o x}$ ) 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 $_{Edd}$ . 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.

arXiv commentAccepted version following referee comments. 2 new figures compared to previous arxiv version (Figs 9 and 10). Data will be available from the journal webpages, or upon request to the authors

Rapid accretion state transitions following the tidal disruption event AT2018fyk

Abstract

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