XMM-Newton detection of soft time lags in the TDE candidate AT 2018fyk

Abstract

In this letter we report a tentative detection of soft time lags (i.e. variability of softer photons lags behind the variability of harder photons) in one $XMM-Newton$ observation of the tidal disruption event (TDE) candidate AT 2018fyk while the source was in the hard spectral state. The lags are detected at $6.51\times 10^{-5}\mathrm{Hz}$. The amplitude of the lags with respect to 0.5$-$1 keV monotonically decreases with the photon energy, from $\sim 1200\mathrm{s}$ at 0.3$-$0.5 keV to $\sim -4200\mathrm{s}$ at 3$-$5 keV (in our convention a positive lag means lagging behind the reference band). We find that the amplitude is proportional to the logarithm of the energy separation between the examined band and the reference band. The energy-dependent covariance spectrum indicates that the correlated variability is more likely to be associated with the non-thermal radiation. The soft lags are difficult to reconcile with the reverberation scenario that are used to explain the soft lags in active galactic nuclei. On the other hand, the observed soft lags are consistent with the picture that the soft X-rays are down-scattered hard X-rays by the outflow as predicted by ``unification'' models of TDEs.

5 pages, 3 figures; accepted for publication in MNRAS Letters

References in corpus (13)

• Tidal Disruption Events
• Rapid accretion state transitions following the tidal disruption event AT2018fyk
• Spectral-timing evidence for a Very High State in the Narrow Line Seyfert 1 Ark 564
• X-ray Variability of the Seyfert 1 Markarian 335: Power Spectrum and Time Lags
• Evolution of the reverberation lag in GX 339-4 at the end of an outburst
• Fainter harder brighter softer: a correlation between ${\alpha }_{\mathrm{ox}}$ , X-ray spectral state and Eddington ratio in tidal disruption events
• The evolution of the X-ray phase lags during the outbursts of the black hole candidate GX 339-4
• Discovery of a soft X-ray lag in the Ultraluminous X-ray Source NGC 1313 X-1
• Energy Dependent time lags in the Seyfert 1 galaxy NGC 4593
• The X-ray continuum time-lags and intrinsic coherence in AGN
• The rare X-ray flaring activity of the Ultraluminous X-ray source NGC 4559 X7
• The long term X-ray time-lags of NGC 4051
• Evidence for Fe K${}_{\alpha }$ line and soft X-ray lag in NGC 7456 ULX-1

Cited by in corpus (2)

• Live to die another day: the rebrightening of AT2018fyk as a repeating partial tidal disruption event
• AT2018fyk: Candidate Tidal Disruption Event by a (Super)massive Black Hole Binary