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

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Peer-reviewed

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Abstract

jats:titleAbstract</jats:title> jats:pWe 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≈10jats:sup7</jats:sup> jats:italicM</jats:italic> jats:sub⊙</jats:sub> black hole (jats:italicM</jats:italic> jats:subBH</jats:sub> 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 jats:italicδ</jats:italic> jats:italict</jats:italic>≈272 days during which the X-ray flux drops by a factor of 10. In the joint NICER+NuSTAR observation (jats:italicδ</jats:italic> jats:italict</jats:italic> = 264 days, harder state), we observe a prominent nonthermal component up to 30 keV and an extremely broad emission line in the iron jats:italicK</jats:italic> band. The bolometric luminosity of AT2021ehb reaches a maximum of jats:inline-formula jats:tex-math

</jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> mml:msubsup mml:mrow mml:mn6.0</mml:mn> </mml:mrow> mml:mrow mml:mo−</mml:mo> mml:mn3.8</mml:mn> </mml:mrow> mml:mrow mml:mo+</mml:mo> mml:mn10.4</mml:mn> </mml:mrow> </mml:msubsup> mml:mo%</mml:mo> mml:msub mml:mrow mml:miL</mml:mi> </mml:mrow> mml:mrow mml:miEdd</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac898aieqn1.gif" xlink:type="simple" /> </jats:inline-formula> 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.</jats:p>

Description

Keywords

5101 Astronomical Sciences, 51 Physical Sciences

Journal Title

Astrophysical Journal

Conference Name

Journal ISSN

0004-637X
1538-4357

Volume Title

Publisher

American Astronomical Society
Sponsorship
National Aeronautics and Space Administration (NASA) (80NSSC22K0574)