[Paper Review] Rapid accretion state transitions following the tidal disruption event AT2018fyk

T. Wevers, Dheeraj R. Pasham+8 more|UvA-DARE (University of Amsterdam)|Jan 12, 2021

Astrophysical Phenomena and Observations119 references56 citations

TL;DR

The paper presents two years of multi-wavelength monitoring of the tidal disruption event AT2018fyk, identifying three accretion states and two transitions, analogous to stellar-mass black hole outbursts, in a supermassive black hole system.

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$_{ m bol} \sim $few $ imes 10^{-2}$ L$_{ m Edd}$, 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 $α_{ox}$) 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$_{ m Edd}$. 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.

Motivation & Objective

• Investigate how the accretion rate in a TDE evolves from quiescent to near-Eddington and back and how this maps to SMBH accretion physics.
• Characterize the UV–X-ray spectral evolution and timing properties across the event to identify state transitions.
• Compare the observed SMBH accretion state behavior with the known states of stellar-mass black holes.
• Assess whether TDEs can probe accretion state transitions on timescales accessible for SMBHs.
• Evaluate how the bolometric Eddington fraction relates to spectral states and variability in AT2018fyk.

Proposed method

• Multi-wavelength monitoring of AT2018fyk over ~2 years using X-ray, UV, optical, and radio observations (XMM-Newton, Swift, NICER, Chandra, Magellan, ATCA).
• Spectral energy distribution modelling of host galaxy to subtract host flux and obtain intrinsic source SED.
• X-ray spectral modelling with two-component (thermal plus power-law) fits to track disk and corona contributions.
• Measurement of UV–X-ray spectral slope alpha_ox and its evolution with bolometric Eddington fraction f_Edd,bol.
• Timing analysis of X-ray light curves to study low- and high-frequency variability and transitions between states.

Experimental results

Research questions

• RQ1 Do TDEs exhibit accretion-state transitions in SMBHs similar to those in stellar-mass black holes?
• RQ2 How does the UV–X-ray spectral slope alpha_ox evolve with bolometric Eddington fraction during a TDE?
• RQ3 What are the spectral and timing signatures of the proposed accretion states in AT2018fyk (soft, hard, quiescent) and their transitions?
• RQ4 Can the observed behavior in AT2018fyk constrain scale invariance and disc-corona physics across seven orders of black hole mass?

Key findings

• AT2018fyk shows three accretion states and two transitions over ~2 years, analogous to stellar-mass black hole outbursts.
• The X-ray spectral state transitions from soft (thermal-dominated) to hard (power-law-dominated) around L_bol ≈ a few × 10^-2 L_Edd, with corona strengthening ~100–200 days after UV/optical peak.
• The UV-to-X-ray spectral slope alpha_ox softens as the outburst progresses, and the X-ray timing shows a shift from long-timescale variability to a high-frequency component after transition to the hard state.
• A second accretion state transition occurs ~500 days after peak, from hard to quiescent, indicated by a collapse of bolometric emission below ~10^-3.4 L_Edd.
• The observed accretion-state behavior in AT2018fyk closely resembles accretion states seen in stellar-mass BHs, supporting scale-invariance of accretion processes in SMBHs.
• The study demonstrates TDEs as a tool to study accretion physics across seven orders of magnitude in black hole mass.