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

Yuhan Yao, Wenbin Lu, Muryel Guolo, Dheeraj R. Pasham, Suvi Gezari, Marat Gilfanov, Keith C. Gendreau, Fiona Harrison, S. Bradley Cenko, S. R. Kulkarni, Jon M. Miller, Dominic J. Walton, Javier A. García, Sjoert van Velzen, Kate D. Alexander, James C. A. Miller-Jones, Matt Nicholl, Erica Hammerstein, Pavel Medvedev, Daniel Stern, Vikram Ravi, R. Sunyaev, Joshua S. Bloom, Matthew J. Graham, Erik C. Kool, Ashish A. Mahabal, Frank J. Masci, Josiah Purdum, Ben Rusholme, Yashvi Sharma, Roger Smith, Jesper Sollerman

August 2022

Abstract

We present X-ray, UV, optical, and radio observations of the nearby ( $a p p r o x 78$ 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},Modot $b l a c k h o l e ($ M{rm BH} $i n f e r r e d f r o m h o s t g a l a x y s c a l i n g r e l a t i o n s) . H i g h - c a d e n c e S w i f t a n d N I C E R m o n i t o r i n g r e v e a l s a d e l a y e d X - r a y b r i g h t e n i n g . T h e s p e c t r u m f i r s t u n d e r g o e s a g r a d u a l$ {rm soft }rightarrow{rm hard} $t r a n s i t i o n a n d t h e n s u d d e n l y t u r n s s o f t a g a i n w i t h i n 3 d a y s a t$ delta tapprox 272 $d a y s d u r i n g w h i c h t h e X - r a y f l u x d r o p s b y a f a c t o r o f t e n . I n t h e j o i n t N I C E R + N u S T A R o b s e r v a t i o n ($ delta t =264 $d a y s, h a r d e r s t a t e), w e o b s e r v e a p r o m i n e n t n o n - t h e r m a l c o m p o n e n t u p t o 30 k e V a n d a n e x t r e m e l y b r o a d e m i s s i o n l i n e i n t h e i r o n K b a n d . T h e b o l o m e t r i c l u m i n o s i t y o f A T 2021 e h b r e a c h e s a m a x i m u m o f$ 6.0^{+10.4}{-3.8}% L{rm Edd} $w h e n t h e X - r a y s p e c t r u m i s t h e h a r d e s t . D u r i n g t h e d r a m a t i c X - r a y e v o l u t i o n, n o r a d i o e m i s s i o n i s d e t e c t e d, t h e U V / o p t i c a l l u m i n o s i t y s t a y s r e l a t i v e l y c o n s t a n t, a n d t h e o p t i c a l s p e c t r a a r e f e a t u r e l e s s . W e p r o p o s e t h e f o l l o w i n g i n t e r p r e t a t i o n s : (i) t h e$ {rm soft }rightarrow{rm hard} $t r a n s i t i o n m a y b e c a u s e d b y t h e g r a d u a l f o r m a t i o n o f a m a g n e t i c a l l y d o m i n a t e d c o r o n a; (i i) h a r d X - r a y p h o t o n s e s c a p e f r o m t h e s y s t e m a l o n g s o l i d a n g l e s w i t h l o w s c a t t e r i n g o p t i c a l d e p t h ($ 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.

Type

Journal article

http://arxiv.org/abs/2206.12713v2