J/ApJ/937/8 UV and optical LCs of the TDE AT2021ehb (Yao+, 2022)
The tidal disruption event AT2021ehb: evidence of relativistic disk reflection,
and rapid evolution of the disk-corona system.
Yao Y., Lu W., Guolo M., Pasham D.R., Gezari S., Gilfanov M.,
Gendreau K.C., Harrison F., Cenko S.B., Kulkarni S.R., Miller J.M.,
Walton D.J., Garcia J.A., van Velzen S., Alexander K.D.,
Miller-Jones J.C.A., Nicholl M., Hammerstein E., Medvedev P., Stern D.,
Ravi V., Sunyaev R., Bloom J.S., Graham M.J., Kool E.C., Mahabal A.A.,
Masci F.J., Purdum J., Rusholme B., Sharma Y., Smith R., Sollerman J.
<Astrophys. J., 937, 8 (2022)>
=2022ApJ...937....8Y 2022ApJ...937....8Y
ADC_Keywords: X-ray sources; Photometry, ultraviolet; Optical
Keywords: Tidal disruption ; X-ray transient sources ;
Supermassive black holes ; Time domain astronomy ;
High energy astrophysics ; Accretion
Abstract:
We present X-ray, UV, optical, and radio observations of the nearby
(∼78Mpc) tidal disruption event AT2021ehb/ZTF21aanxhjv during its
first 430 days of evolution. AT2021ehb occurs in the nucleus of a
galaxy hosting a ∼107M☉ black hole (MBH 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
δt∼272d during which the X-ray flux drops by a factor of 10. In
the joint NICER+NuSTAR observation (δt=264d, harder state), we
observe a prominent nonthermal component up to 30keV 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%LEdd 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.
Description:
In this paper, we present an in-depth study of the X-ray, UV, optical,
and radio emission of AT2021ehb, using observations obtained from
2021 March 1 to 2022 May 31.
We obtained ZTF mosaic camera forced photometry on the Palomar Oschin
Schmidt 48 inch (P48) telescope in the g and the r bands using the
median position of all ZTF alerts up to MJD59550
(RAJ200=03:07:47.82,DEJ2000=40:18:40.85). See Section 2.1.1
We obtained additional ugri photometry using the Spectral Energy
Distribution Machine (SEDM) on the robotic Palomar 60 inch telescope
(P60), and the optical imager on the Liverpool Telescope (LT).
See Section 2.2.
We obtained low-resolution optical spectroscopic observations using
the Low Resolution Imaging Spectrograph (LRIS) on the Keck I
telescope, the Double Spectrograph (DBSP) on the 200 inch Hale
telescope, the integral field unit (R∼100) spectrograph of SEDM, and
the De Veny Spectrograph on the Lowell Discovery Telescope (LDT). We
also obtained a medium-resolution spectrum using the Echellette
Spectrograph and Imager (ESI) on the Keck II telescope.
See Section 2.3.
AT2021ehb was observed by the X-Ray Telescope (XRT) and the
Ultra-Violet/Optical Telescope (UVOT) on board Swift under our GO
program 1619088 (as ZTF21aanxhjv; target ID 14217; PI: Gezari) and a
series of time-of-opportunity (ToO) requests (PI: Yao).
The first four UVOT epochs (obsID 14217001-14217005) were conducted
with UBV+All UV filters. Subsequent observations were conducted with
U+All UV filters.
See Section 2.4.
AT2021ehb was observed by NICER under Director's Discretionary Time
(DDT) programs on 2021 March 26, 2021 Jul 2-7, and from 2021 Nov 13 to
2022 Mar 29 (PIs: Yao, Gendreau, Pasham).
See Section 2.5.
We obtained two epochs of follow-up observations with XMM-Newton/EPIC
under our Announcement of Opportunity program (PI: Gezari), on
2021 Aug 4 (obsID 0882590101) and 2022 Jan 25 (obsID 0882590901).
See Section 2.6.
The location of AT2021ehb was scanned by eROSITA as part of the
planned eight all-sky surveys. See Section 2.7.
We obtained NuSTAR observations under a preapproved ToO program
(PI: Yao; obsID 80701509002) and a DDT program (PI: Yao;
obsID 90801501002). The first epoch was conducted from 2021 Nov 18.8
to 19.9 with an exposure time of 43.2ks. The second epoch was
conducted from 2022 Jan 10.4 to 12.1 with an exposure time of 77.5ks.
See Section 2.8
We began a monitoring program of AT2021ehb using the Very Large Array
(VLA) under program 20B-377 (PI Alexander). AT2021ehb was not detected
in any of our observations. See Section 2.9.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
----------------------------------------------------------
03 07 47.80 +40 18 40.5 AT2021ehb = ZTF21aanxhjv
----------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table8.dat 38 362 UV and optical photometry of AT2021ehb
--------------------------------------------------------------------------------
See also:
B/xmm : XMM-Newton Observation Log (XMM-Newton Science Operation Center, 2012-)
III/218 : ELODIE archive (Prugniel+, 2001)
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
II/328 : AllWISE Data Release (Cutri+ 2013)
V/154 : Sloan Digital Sky Surveys (SDSS), Release 16 (DR16) (Ahumada+, 2020)
J/MNRAS/397/1177 : Swift-XRT observations of GRBs (Evans+, 2009)
J/ApJ/788/48 : X-ray through NIR photometry of NGC 2617 (Shappee+, 2014)
J/A+A/593/A68 : PTF12os & iPTF13bvn spectra & light curves (Fremling+, 2016)
J/A+A/594/A116 : HI4PI spectra and column density maps (HI4PI team+, 2016)
J/ApJS/222/15 : WATCHDOG: all-sky database of Gal. BHXBs (Tetarenko+, 2016)
J/ApJ/844/46 : Phot. of the transient event iPTF16fnl (Blagorodnova+, 2017)
J/A+A/631/A147 : Transient processing & analysis using AMPEL (Nordin+, 2019)
J/ApJ/872/198 : UV-Opt LC of flare AT2018zr (van Velzen+, 2019)
J/ApJ/886/152 : ZTF early observations of Type Ia SNe. I. LCs (Yao+, 2019)
J/ApJ/900/46 : UV, optical and IR LC of supernova SN2019dge (Yao+, 2020)
J/ApJ/909/102 : SDSSJ143359.16+400636.0 nucleus LRIS sp. (Brightman+, 2021)
J/ApJS/266/9 : Young Supernova Experiment (YSE) DR1 (Aleo+, 2023)
Byte-by-byte Description of file: table8.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 F10.4 d MJD [59250.16/59679.14] Modified Julian Date (UT)
12- 15 A4 --- Inst Instrument identifier (1)
17- 20 A4 --- Filt Filter identifier (uvw1,uvw2,uvm2,UBV,ugri)
22- 29 F8.4 uJy Flux [-23.21/720.7] Flux in Filt
31- 38 F8.4 uJy e_Flux [1.85/181.1] Uncertainty in Flux
--------------------------------------------------------------------------------
Note (1): Instrument as follows:
UVOT = The Ultra-Violet/Optical Telescope on board Swift (165 occurrences)
ZTF = ZTF mosaic camera forced photometry on the Palomar Oschin Schmidt
48 inch (P48) telescope (145 occurrences)
SEDM = The Spectral Energy Distribution Machine on the robotic Palomar 60 inch
telescope (P60; 31 occurrences)
LT = the optical imager on the Liverpool Telescope (21 occurrences)
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 08-Aug-2024