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

September 2022 • 2022ApJ...937....8Y

Authors • Yao, Yuhan • Lu, Wenbin • Guolo, Muryel • Pasham, Dheeraj R. • Gezari, Suvi • Gilfanov, Marat • Gendreau, Keith C. • Harrison, Fiona • Cenko, S. Bradley • Kulkarni, S. R. • Miller, Jon M. • Walton, Dominic J. • García, Javier A. • van Velzen, Sjoert • 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 • 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 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 ${6.0}_{-3.8}^{+10.4} \% {L}_{\mathrm{Edd}}$ 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.


IPAC Authors

Frank Masci

Senior Scientist

Ben Rusholme

Chief Engineer