NICER observations of late-time X-ray brightening of the tidal disruption event candidate AT2019azh

ATel #13221; Dheeraj Pasham (MIT), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Erin Kara (MIT)
on 23 Oct 2019; 20:43 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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The transient AT2019azh was discovered on 2019 February 22 by the ASASSN sky survey. Spatial coincidence with the nucleus of a galaxy at redshift of 0.022 combined with the blue optical spectrum (ATel#12529) suggested the source could be a tidal disruption event (TDE) candidate (ATel#12568, #12870).

Swift/XRT tracked the evolution of this transient in X-rays between March and May 2019. During this time the X-rays (0.3-8 keV) were more or less steady with low and high count rate values of 2e-3 and 10e-3 counts/sec, respectively. After a gap of roughly 100 days, Swift observed AT2019azh again on October 11 during which the count rate increased by a factor of more than 20 (mean count rate of 0.11 cps). For comparison, the optical light curve peaked around 2019 March 19 and has been steadily declining ever since (see, for example, http://lasair.roe.ac.uk/object/ZTF17aaazdba/).

Following the Swift-detected X-ray brightening, NICER made several short exposures of AT2019azh spread over a 16 hour time frame on 2019 October 19. This yielded a total useful exposure of 3 ks with a mean count rate of 6 counts/sec in the 0.25-1 keV band. The source is variable on a few hours timescale with a fractional amplitude of 20%. The spectrum is soft, with essentially all source counts below 1 keV. We fit the 0.3-1 keV energy spectrum with a disk blackbody modified by absorption (phabs*diskbb in XSPEC). This gave a best fit chi-squared of 45 with 47 degrees of freedom. The model parameters, i.e., the disk temperature and the absorbing column, are 0.052+-0.002 keV and (0.056+-0.006)e22 cm^-2, respectively. The disk temperature is similar to previous X-ray bright TDEs. The unabsorbed flux is 1.1e-11 ergs/s/cm^2 in the 0.3-1 keV band, which corresponds to a luminosity of 1.4e43 erg/s. This late-time X-ray brightening is similar to the TDE ASASSN-15oi and may be signalling the formation of an accretion disk around the black hole. NICER is planning to continue monitoring this transient and the observing schedule can be found here: http://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html

NICER can carry out prompt follow-up observations of transients and is planning to systematically follow up alerts from LIGO and other X-ray bright extra-galactic transients in the future.

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.