NICER observations of the tidal disruption event AT2019teq >3 years after optical discovery reveal X-ray spectral hardening and brightening indicative of an actively forming corona

ATel #15724; Yukta Ajay (IISER Tirupati), Dheeraj Pasham (MIT), Muryel Guolo (JHU)
on 26 Oct 2022; 19:23 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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AT2019teq/ZTF19accmaxo is a tidal disruption event (TDE) at a redshift of 0.0878 identified by the Zwicky Transient Facility (ZTF) on 20 October 2019 02:47:36 UT. Swift-XRT observations between December 2019 to May 2020 (55 - 200 days post discovery) revealed a soft X-ray spectrum that can be modeled with a powerlaw with an index of 3.52 (+0.53,-0.45; ATel#15657). Subsequent XMM-Newton observations in September 2022 (roughly 1054 days after discovery) showed a similar X-ray spectrum with a best-fit powerlaw index of 2.99+-0.01 (ATel #15657).

NICER carried out two observations of the source on 18 October 2022 (MJD 59872.79 to 59872.99) and 21 October 2022 (MJD 59873.04 to 59873.57), approximately 1100 days after its optical discovery. The 4.5 ks of NICER data shows a 0.3 - 1.5 keV mean background-subtracted count rate of 0.69+/-0.02 cps, for which we performed a combined spectral fitting using a TBabs*zashift(clumin*powerlaw)+gaussian model in XSPEC. The TBabs nH value was fixed at 0.045e+22, and the centroid energy for the gaussian component was constrained to be between 0.5 - 0.6 keV with a fixed width of 0.0 keV to model the Oxygen line in the Earth’s atmosphere (background feature). The best fit powerlaw index was 1.82+/-0.12, which indicates that the spectrum has significantly hardened within a timespan of less than two months. The observed 0.3 - 10 keV NICER luminosity and flux were (3.95 +0.35 -0.23)e+43 ergs/s and (2.11 +0.16 -0.15)e-12 ergs/cm^2/s, respectively. In comparison to the previously observed values from Swift (6.9+/-0.4 e+42 ergs/cm^2/s, 3.6+/-0.2 e-13 ergs/s) and XMM (1.92+/-0.02 e+43 ergs/s, 1.0+/-0.1 e-12 ergs/cm^2/s), the latest 0.3-10 keV luminosity suggests an overall increasing trend.

The recent NICER data suggests that the X-ray corona may be actively forming now and presents a rare opportunity to capture the transition from an X-ray soft to an X-ray hard state. We encourage multiwavelength observations, particularly in the radio. Further Swift (XRT+UVOT) and NICER observations are planned.

References: Nordin J., Brinnel V., Giomi M., Santen J.~V., Gal-Yam A., Yaron O., Schulze S., 2019, TNSTR, 2019-2172 Yao Y., Guolo M., 2022, ATel, 15657