X-ray flares from the stellar tidal disruption by a candidate supermassive black hole binary

Xinwen Shu,Fu-Guo Xie,Rongfeng Shen,Zhen Yan,Shuo Li,Liming Dou,Tinggui Wang,Ning Jiang,Fukun Liu,Wenjie Zhang,Luming Sun

doi:10.1038/s41467-020-19675-z

Abstract

Optical transient surveys have led to the discovery of dozens of stellar tidal disruption events (TDEs) by massive black hole in the centers of galaxies. Despite extensive searches, X-ray follow-up observations have produced no or only weak X-ray detections in most of them. Here we report the discovery of delayed X-ray brightening around 140 days after the optical outburst in the TDE OGLE16aaa, followed by several flux dips during the decay phase. These properties are unusual for standard TDEs and could be explained by the presence of supermassive black hole binary or patchy obscuration. In either scenario, the X-rays can be produced promptly after the disruption but are blocked in the early phase, possibly by a radiation-dominated ejecta which leads to the bulk of optical and ultraviolet emission. Our findings imply that the reprocessing is important in the TDE early evolution, and X-ray observations are promising in revealing supermassive black hole binaries.

Highlights

Optical transient surveys have led to the discovery of dozens of stellar tidal disruption events (TDEs) by massive black hole in the centers of galaxies
We show delayed X-ray brightening by a factor of >60, ~140 days after the optical flare, in the TDE OGLE16aaa, followed by several dips of X-ray emission during the afterwards decay phase. These properties are unusual among standard TDEs and are instead consistent with either the tidal disruption by supermassive black hole (SMBH) binary (SMBHB) or changes in absorption along the line of sight
Following the initial optical peak, OGLE16aaa was monitored at three UV bands (UVW2, λeff = 1928 Å; UVM2, λeff = 2246 Å; UVW1, λeff = 2600 Å) by the Ultraviolet and Optical Telescope (UVOT)[22], as well as the X-ray band by the X-ray Telescope (XRT)[23] on board the Swift observatory[24] (Supplementary Table 1)

Summary

Introduction

Optical transient surveys have led to the discovery of dozens of stellar tidal disruption events (TDEs) by massive black hole in the centers of galaxies. We show delayed X-ray brightening by a factor of >60, ~140 days after the optical flare, in the TDE OGLE16aaa, followed by several dips of X-ray emission during the afterwards decay phase These properties are unusual among standard TDEs and are instead consistent with either the tidal disruption by SMBH binary (SMBHB) or changes in absorption along the line of sight. In this context, the X-ray non-detections at the very early times could be due to obscuration, in which the reprocessed accretion radiation may power the bulk of observed UV and optical emission

Methods

Results

Conclusion