Swift and NICER continue to detect the tidal disruption event AT2021ehb in X-rays 500+ days after optical discovery: Evidence for a second X-ray spectral hardening phase

ATel #15524; Dheeraj Pasham (MIT), Yuhan Yao (Caltech), Thomas Wevers (ESO), Keith Gendreau (NASA/GSFC), and Elizabeth Ferrara (NASA/GSFC).
on 23 Jul 2022; 02:12 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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AT2021ehb/ZTF21aanxhjv is a tidal disruption event (TDE) identified by the Zwicky Transient Facility (ZTF) on 1 March 2022 (Gezari et al 2021, TNS AstroNote 103). Similar to many previous TDEs the X-rays started to rise post optical peak. Yao et al. (2022) have recently performed a detailed multiwavelength study using data acquired between March 2021 and May 2022. In short, in X-rays, the source rose by three orders of magnitude over a timespan of 300 days after which it suddenly dropped by an order of magnitude within 3 days. Over this 300 day time the source first went from soft to hard and the sudden flux drop was accompanied by an X-ray softening.

New NICER X-ray observations were taken on 20 July 2022, roughly 180 days after the X-ray drop (and 507 days post optical discovery). Additional Swift XRT exposures have been taken throughout July 2022. Compared with the observations in January-April 2022, AT2021ehb has become harder and the observed 0.3-10 keV flux has increased by a factor of ~3 to (1.9 -0.1 +0.2) E-12 erg/cm^2/s. We modeled this new NICER 0.3-4.0 keV spectrum with tbabs*zashift*simpl(diskbb) model in XSPEC and find the best-fit power-law index, the Compton scattering fraction, and the disk temperature to be 2.5+-0.3, 0.35(+0.18,-0.10), and 0.14+-0.02 keV, respectively. For comparison these values on 25 January 2022 (shortly after the X-ray softening accompanied by flux drop) were 2.92+-0.15, 0.16+-0.03, and 0.125+-0.008, respectively. These most recent parameters indicate that the corona is getting gradually energized (see Fig. 18 of Yao et al. 2022 for comparison). This behavior of soft-->hard-->soft-->hard X-ray spectral change has never been seen in a TDE before. We encourage follow-up especially in the X-ray and radio band to search for strong relativistic iron lines and jet emission.

Swift and NICER are planning to continue to monitor AT2021ehb with additional ToO and DDT observations. The Swift schedule of AT2021ehb can be found here: https://www.swift.psu.edu/operations/obsSchedule.php?t=14217. A weekly updated NICER schedule can be found here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. To coordinate multiwavelength observations please contact drreddy@mit.edu and yyao@caltech.edu.

NICER carries out prompt follow-up observations of X-ray-bright extragalactic transients and tracks alerts from LIGO/VIRGO and other facilities. NICER is a 0.2-12 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, Transient Name Server AstroNote, 103, 1
Yao, Y., Lu, W., Guolo, M., et al. 2022, https://arxiv.org/abs/2206.12713
All the errorbars quoted in this telegram represent 90% uncertainties.