NICER X-ray observations of the young tidal disruption flare candidate AT2019dsg

ATel #12825; Dheeraj Pasham (MIT), Ronald Remillard (MIT), Michael Loewenstein (NASA/GSFC), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Jon M. Miller (U. Michigan), Erin Kara (UMD/MIT), James F. Steiner (SAO) on behalf of NICER team
on 31 May 2019; 15:26 UT
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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Following the detection of X-rays with Swift/XRT (ATel #12777), NICER made several exposures of the tidal disruption flare (TDF) candidate AT2019dsg between 2019 May 21 (MJD 58624) and May 28 (MJD 58630), yielding a total exposure near 11 ks. We investigated the average energy spectrum derived from good time intervals with particularly low background (roughly 9.5 ks of exposure) with a variety of spectral models that included thermal and Comptonization components typically used for accreting black holes.

The X-ray source is extremely soft, with significant detection limited to the range 0.3-1.0 keV. Fits with a simple accretion disk model ("ezdisk") modified by interstellar absorption, i.e., tbabs*zashift(ezdisk) in XSPEC, produce very good results (i.e., no Comptonization term is needed), with reduced chi squared values close to unity. The spectral fits yield an absorption column density of 8.8(+-0.7)e20 cm**-2. This value is slightly higher than the Galactic column of 6.5e20 cm**-2 along the line of sight of AT2019dsg. The inner disk temperature was 0.072+-0.002 keV, and is similar to the best-fit value of 0.06+-0.01 keV from an earlier, lower signal-to-noise, spectrum obtained with Swift/XRT (ATel #12777). The implied unabsorbed X-ray (0.3-1.0 keV) flux is 5.3(+0.5,-0.4)E-12 erg/s/cm**2 which translates to a luminosity of 3(+-0.3)e44 erg/s in 0.1-10 keV band in source frame. Using models with zero torque at the inner boundary for the disk, i.e., the ezdiskbb (Zimmerman et al. 2005) and the diskpn models (Gierlinski et al. 1999) with inner disk radius fixed at 6 gravitational radii, resulted in similar absorbing column and disk temperatures. The best-fit normalization value for diskpn was 1.28(+0.4, -0.3).

With a pure blackbody model (tbabs*zashift(bbodyrad) in XSPEC) we obtain fits of equally good quality. The best-fit absorbing column, temperature of the blackbody, and the size of the X-ray photosphere are 7.5(+-0.7)e20 cm**-2, 0.066(+0.001,-0.002) keV, and 6.8(+0.9,-0.7)e11 cm, respectively. The best-fit unabsorbed flux in the 0.3-1.0 keV band and the 0.1-10 keV luminosity are 4.4(+0.4,-0.4)e-12 erg/s/cm**2 and 1(+0.16,-0.12)e44, respectively. The best-fit photospheric radius corresponds to the innermost stable circular orbit of a 8e5 solar mass non-spinning black hole.

The X-ray detections and extremely soft spectra with a luminosity of ~1e44 erg/sec are consistent with the TDF classification that is favored for this source.

We have requested additional Swift (XRT+UVOT) monitoring and more NICER observations are planned. We encourage follow-up observations of this rare X-ray (and also radio; ATel #12798) bright TDF candidate.

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.