The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk-Corona System

Creators: Yao, Yuhan; Lu, Wenbin; Guolo, Muryel; Pasham, Dheeraj R.; Gezari, Suvi; Gilfanov, Marat; Gendreau, Keith C.; Harrison, Fiona A.; Cenko, S. Bradley; Kulkarni, S. R.; Miller, Jon M.; Walton, Dominic J.; García, Javier A.; Velzen, Sjoert van; Alexander, Kate D.; Miller-Jones, James C. A.; Nicholl, Matt; Hammerstein, Erica; Medvedev, Pavel; Stern, Daniel; Ravi, Vikram; Sunyaev, R.; Bloom, Joshua S.; Graham, Matthew J.; Kool, Erik C.; Mahabal, Ashish A.; Masci, Frank J.; Purdum, Josiah N.; Rusholme, Ben; Sharma, Yashvi; Smith, Roger; Sollerman, Jesper

Abstract

We present X-ray, UV, optical, and radio observations of the nearby (≈78 Mpc) tidal disruption event AT2021ehb/ZTF21aanxhjv during its first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxy hosting a ≈ 107 M_⊙ black hole (M_(BH) inferred from host galaxy scaling relations). High-cadence Swift and Neutron Star Interior Composition Explorer (NICER) monitoring reveals a delayed X-ray brightening. The spectrum first undergoes a gradual soft → hard transition and then suddenly turns soft again within 3 days at δt ≈ 272 days during which the X-ray flux drops by a factor of 10. In the joint NICER+NuSTAR observation (δt = 264 days, harder state), we observe a prominent nonthermal component up to 30 keV and an extremely broad emission line in the iron K band. The bolometric luminosity of AT2021ehb reaches a maximum of 6.0^(+10.4)_(3.8)%L_(Edd) when the X-ray spectrum is the hardest. During the dramatic X-ray evolution, no radio emission is detected, the UV/optical luminosity stays relatively constant, and the optical spectra are featureless. We propose the following interpretations: (i) the soft → hard transition may be caused by the gradual formation of a magnetically dominated corona; (ii) hard X-ray photons escape from the system along solid angles with low scattering optical depth (∼ a few) whereas the UV/optical emission is likely generated by reprocessing materials with much larger column density—the system is highly aspherical; and (iii) the abrupt X-ray flux drop may be triggered by the thermal–viscous instability in the inner accretion flow, leading to a much thinner disk.

Additional Information

We are grateful to the NuSTAR, NICER, Swift, XMM-Newton, and VLA teams for making this observing campaign possible. We thank the anonymous referee for constructive comments and suggestions. We thank Julian Krolik for providing comments on an early version of this manuscript. We thank Erin Kara, Renee Ludlam, Guglielmo (Gullo) Mastroserio, and Riley Connors for helpful discussions on the NuSTAR and NICER spectral fitting. We thank Murray Brightman and Hannah Earnshaw for discussions on disk reflection and super-Eddington accretion. Y.Y. acknowledges support from NASA under award No. 80NSSC22K0574. W.L. is supported by the Lyman Spitzer, Jr. Fellowship at Princeton University. This work was supported by the Australian government through the Australian Research Council's Discovery Projects funding scheme (DP200102471). M.N. is supported by the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation program (grant agreement No. 948381) and by a Fellowship from the Alan Turing Institute. E.C.K. acknowledges support from the G.R.E.A.T. research environment funded by Vetenskapsrådet, the Swedish Research Council, under project No. 2016-06012, and support from The Wenner-Gren Foundations. This work has made use of data from the NuSTAR mission, a project led by Caltech, managed by NASA/JPL, and funded by NASA. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS), jointly developed by the ASI Science Data Center (ASDC, Italy) and Caltech (USA). This work is based on observations obtained with the Samuel Oschin Telescope 48 inch and the 60 inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the National Science Foundation under grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, IN2P3, University of Warwick, Ruhr University Bochum, and Northwestern University. Operations are conducted by COO, IPAC, and UW. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant No. 12540303 (PI: Graham). SED Machine is based upon work supported by the National Science Foundation under grant No. 1106171. This work used observations with the eROSITA telescope on board the SRG observatory. The SRG observatory was built by Roskosmos with the participation of the Deutsches Zentrum ${\rm{f}}\ddot{{\rm{u}}}{\rm{r}}$ Luft- und Raumfahrt (DLR). The SRG/eROSITA X-ray telescope was built by a consortium of German Institutes led by MPE, and supported by DLR. The SRG spacecraft was designed, built, launched, and is operated by the Lavochkin Association and its subcontractors. The eROSITA data used in this work were processed using the eSASS software system developed by the German eROSITA consortium and proprietary data reduction and analysis software developed by the Russian eROSITA Consortium. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Additional details

Views

Downloads

	All versions	This version
Views	0	0
Downloads	0	0
Data volume	0 Bytes	0 Bytes

More info on how stats are collected....

Resource type: Journal Article
Publisher: American Astronomical Society
Published in: Astrophysical Journal, 937(1), 8, ISSN: 0004-637X.