Swift and NICER monitoring of AT2021ehb
ATel #15217; Yuhan Yao (Caltech), Dheeraj R. Pasham (MIT), Keith C. Gendreau, Zaven Arzoumanian, Elizabeth C. Ferrara, S. Bradley Cenko (GSFC)
on 9 Feb 2022; 16:46 UT
Credential Certification: Yuhan Yao (yyao@astro.caltech.edu)
Subjects: X-ray, Black Hole, Transient, Tidal Disruption Event
We report an update on the Swift and NICER observations of AT2021ehb/ZTF21aanxhjv --- a nearby tidal disruption event (TDE) at z=0.018 discovered by ZTF (Gezari et al. 2021).
X-rays from AT2021ehb were first detected by Swift/XRT on July 1 2021 (Yao et al. 2021). Ever since then, Swift had been observing this object with a cadence of 5--7 days, until the observatory entered into a safe mode on January 18 2022 (Cenko 2022). The 0.3--10 keV XRT spectra can be well described by a combination of multi-color disk and power-law. Throughout the XRT monitoring campaign, the hydrogen column density was consistent with the Galactic value of 9.97 E+20 cm^-2. From July 2021 to January 2022, the observed 0.3--10 keV luminosity increased from 3.1+/-0.8 E+41 erg/s to 3.5+/-0.5 E+43 erg/s. A gradual X-ray spectral hardening was clearly captured --- the fractional contribution of the power-law component with respect to the total observed 0.3--10 keV flux (hereafter defined as f_PL) increased from 35+/-6 % to 90+/-5 %, and the power-law photon index Gamma decreased from 3+/-0.3 to 2+/-0.3.
High-cadence X-ray observations with NICER were initiated on November 13 2021. Short-timescale (minutes to hours) variability with an amplitude of 1.2--2 was observed. In all GTI-binned NICER data, the 0.3--4 keV energy spectra can be described with the same continuum model as adopted in XRT. From November 13 to November 30 2021, the inferred observed 0.3--10 keV luminosity stayed at 8+/-2 E+42 erg/s. The luminosity rose to 1.3+/-0.5 E+43 erg/s around November 31 2021, and then further rose to 3.5+/-0.5 E+43 erg/s around December 25 2021. The X-ray brightening can be attributed to an increase of the power-law normalization factor (with f_PL increased from 78+/-5 % to 90+/-4 %) and an increase of the inner disk temperature (from 200+/-10 eV to 240+/-6 eV).
On January 19 2022, NICER caught a drastic decrease in X-ray flux, which was accompanied by a spectral softening. From January 19 to February 8 2022, the inferred observed 0.3--10 keV luminosity stayed around 2.5 E+42 erg/s, with T_in=146+/-4 eV, f_PL=68+/-5 %, and Gamma=2.6+/-0.1.
The X-ray spectral evolution of AT2021ehb before January 19 2022 was reminiscent of the soft-to-hard state transition seen in AT2018fyk (Wevers et al. 2021), while the recent detection of the late-time transition to a much softer state is novel for a TDE.
We note that AT2021ehb is the third nearest optically-selected TDE (after iPTF16fnl and AT2019qiz , both of which are faint in the X-ray; see Table 1 of Gezari 2021). As such it provides a unique opportunity to study a TDE with high signal-to-noise observations. For example, a previous Chandra/LETG observation showed evidence of a relativistic outflow (Miller et al. 2022a), and the recent XMM-Newton/EPIC observation showed interesting features in the Fe K band (Miller et al. 2022b). Future high-cadence multi-wavelength observations of this object are encouraged to capture the entire evolution, and to reveal the inner workings of this TDE.
NICER is a 0.25--12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
Gezari, S., Hammerstein, E., Yao, Y., et al. 2021, TNS AstroNote, 103, 1
Yao, Y., Brightman, M., Gezari, S., et al. 2021, TNS AstroNote, 183, 1
Cenko, B. 2022, GRB Coordinates Network, 31500, 1
Wevers, T., Pasham, D. R., van Velzen, S., et al. 2021, ApJ, 912, 151
Gezari, S. 2021, ARA&A, 59
Miller, J. M., Reynolds, M. T., Yun, S. B., et al. 2022a, ATel #15179
Miller, J. M., Reynolds, M. T., Zoghbi, A., et al. 2022b, ATel #15206
