The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk-Corona System
Kulkarni, S. R.; Sunyaev, R.; Cenko, S. Bradley; Walton, Dominic J.; Miller, Jon M.; Rusholme, Ben; Stern, Daniel; Sollerman, Jesper; García, Javier A.; van Velzen, Sjoert; Gezari, Suvi; Hammerstein, Erica; Graham, Matthew J.; Masci, Frank J.; Sharma, Yashvi; Gilfanov, Marat; Medvedev, Pavel; Alexander, Kate D.; Nicholl, Matt; Yao, Yuhan; Smith, Roger; Pasham, Dheeraj R.; Guolo, Muryel; Mahabal, Ashish A.; Bloom, Joshua S.; Miller-Jones, James C. A.; Lu, Wenbin; Gendreau, Keith C.; Harrison, Fiona; Ravi, Vikram; Kool, Erik C.; Purdum, Josiah
United States, Russia, Germany, United Kingdom, Netherlands, Australia, Sweden
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.