Article

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

Yuhan YaoDivision of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USAWenbin LuDepartment of Astronomy, University of California, Berkeley, CA 94720-3411, USAMuryel GuoloDepartment of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USADheeraj R. PashamKavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USASuvi GezariSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USAM. GilfanovSpace Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997, RussiaKeith C. GendreauAstrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAFiona A. HarrisonDivision of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USAS. B. CenkoAstrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAS. R. KulkarniDivision of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USAJ. M. MïllerDepartment of Astronomy, The University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48103, USAD. J. WaltonCentre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UKJavier A. GarcíaDr. Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Sternwartstr. 7, D-96049 Bamberg, GermanySjoert van VelzenLeiden Observatory, Leiden University, Postbus 9513, 2300 RA, Leiden, The NetherlandsK. D. AlexanderCenter for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USAJames C. A. Miller-JonesInternational Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, AustraliaM. NichollBirmingham Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UKErica HammersteinDepartment of Astronomy, University of Maryland, College Park, MD 20742, USAП. С. МедведевSpace Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997, RussiaDaniel SternJet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USAVikram RaviDivision of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USAR. SunyaevMax-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85741 Garching, GermanyJ. S. BloomLawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 50B-4206, Berkeley, CA 94720, USAM. J. GrahamDivision of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USAErik C. KoolThe Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691, Stockholm, SwedenAshish A MahabalCenter for Data Driven Discovery, California Institute of Technology, Pasadena, CA 91125, USAFrank J. MasciIPAC, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USAJosiah PurdumCaltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USABen RusholmeIPAC, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USAY. SharmaDivision of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USARoger M. SmithCaltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USAJ. SollermanThe Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691, Stockholm, Sweden

2022en

ABI

Abstract

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≈10 7 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 δ t ≈272 days during which the X-ray flux drops by a factor of 10. In the joint NICER+NuSTAR observation ( δ t = 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>6.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>10.4</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>%</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>Edd</mml:mi> </mml:mrow> </mml:msub> </mml:math> 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.

Identifiers

DOI: 10.3847/1538-4357/ac898a

Citations and references

Cited by 20 references

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