Long-term Evolution of the Short-term X-ray Variability of the Jetted TDE Swift J1644+57

TL;DR

This study investigates the evolution of short-term X-ray variability in the TDE Swift J1644+57, revealing different physical mechanisms in various states and estimating the black hole mass, highlighting the importance of follow-up observations in time-domain astronomy.

Contribution

It provides a detailed analysis of the variability properties of Swift J1644+57 across different states, introducing new insights into the physical mechanisms and variability patterns of TDEs.

Findings

Flux distribution is lognormal in the normal state but deviates in the dipping state.

Different variability patterns observed in the dipping state with steeper PSDs and larger rms.

Detected a significant soft X-ray lag of ~50 s with high coherence.

Abstract

The short-term X-ray variability of tidal disruption events (TDEs) and its similarities with active galactic nuclei (AGN) are poorly understood. In this work, we show the diversity of TDE's short-term X-ray variability, and take Swift J1644+57 as an example to study the evolution of various properties related to the short-term X-ray variability, such as the X-ray flux distribution, power spectral density (PSD), rms variability, time lag and coherence spectra. We find that the flux distribution of Swift J1644+57 has a lognormal form in the normal state, but deviates from it significantly in the dipping state, thereby implying different physical mechanisms in the two states. We also find that during the first two XMM-Newton observations in the dipping state, Swift J1644+57 exhibited different variability patterns, which are characterized by steeper PSDs and larger rms than the normal state. A significant soft X-ray lag is detected in these two observations, which is ~50 s between 0.3-1 keV and 2-10 keV with a high coherence. Using the 2-10 keV rms of 0.10-0.50, the black hole mass of Swift J1644+57 is estimated to be $(0.6-7.9)\times10^{6}M_{\odot}$, but the variation of rms as the TDE evolves introduces a large uncertainty. Finally, we discuss the value of conducting similar studies on other TDEs, especially in the coming era of time-domain astronomy when a lot more TDEs will be discovered in X-rays promptly. This also heralds a significant increase in the demand for deep follow-up observations of X-ray selected TDEs with X-ray telescopes of large effective areas and long orbital periods.