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

Kavli Affiliate: Dheeraj R. Pasham

| First 5 Authors: Yuhan Yao, Wenbin Lu, Muryel Guolo, Dheeraj R. Pasham, Suvi Gezari

| Summary:

We present X-ray, UV, optical, and radio observations of the nearby
($approx78$ Mpc) tidal disruption event (TDE) AT2021ehb/ZTF21aanxhjv during
its first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxy
hosting a $approx 10^{7},M_odot$ black hole ($M_{rm BH}$ inferred from host
galaxy scaling relations). High-cadence Swift and NICER monitoring reveals a
delayed X-ray brightening. The spectrum first undergoes a gradual ${rm soft
}rightarrow{rm hard}$ transition and then suddenly turns soft again within 3
days at $delta tapprox 272$ days during which the X-ray flux drops by a
factor of ten. In the joint NICER+NuSTAR observation ($delta t =264$ days,
harder state), we observe a prominent non-thermal 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 $sim 0.05L_{rm Edd}$ 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
${rm soft }rightarrow{rm hard}$ 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 ($sim,$a
few) whereas the UV/optical emission is likely generated by reprocessing
materials with much larger column density — the system is highly aspherical;
(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.

| Search Query: ArXiv Query: search_query=au:”Dheeraj R. Pasham”&id_list=&start=0&max_results=10

Read More