X-ray flaring of the nuclear transient AT2019avd/eRASStJ082337+042303/ZTF19aaiqmgl: Swift and NICER observations
ATel #14036; Dheeraj Pasham (MIT), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Michael Loewenstein, Elizabeth Ferrara (UMCP, NASA/GSFC), and Diego Altamirano (Southampton), Michael T. Wolff (NRL)
on 21 Sep 2020; 22:56 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)
Subjects: X-ray, AGN, Transient, Tidal Disruption Event
The extragalactic nuclear transient AT2019avd (z=0.0296) was discovered on 9 February 2019 by the Zwicky Transient Facility (ZTF). After reaching an optical peak around 20 February 2019 it declined steadily for about 11 months. Thereafter, it started rising again in the optical and reached a second peak in April 2020 (see the link to ZTF light curve below). eROSITA discovered a soft X-ray flare spatially coincident with AT2019avd (ATel #13712) on 28 April 2020, close to the second optical peak of AT2019avd. Its 0.2-2 keV flux of 1.5e-12 ergs/c/cm^2 on 28 April 2020 meant an increase of almost two orders of magnitude compared to XMM-Newton upper limit of 1.7e-14 ergs/s/cm^2 from 2015 (ATel#13712).
More recent Swift/XRT observations from 16 September 2020 revealed that the X-ray flux has increased by another factor of 10 between May and September 2020 (~ 4 months). Following this X-ray brightening we observed AT2019avd with NICER for a total of 6.8 ks between 19-20 September 2020. Unlike the eROSITA spectrum from April 2020, the average NICER energy spectrum cannot be fully described by a single blackbody. Instead, a model with two thermal components (bbody in Xspec) having temperatures of 0.094+-0.002 and 0.22+-0.01 keV provides a better fit. In addition to the 0.094 keV component reported by eROSITA (ATel#13712), a new ``warmer'' component seems to have appeared. Even with two thermal components in the spectral model, systematic residuals can be seen between 1-2 keV that resemble an absorption line or edge. Detailed spectral analysis is currently being carried out. The mean 0.2-2 keV unabsorbed X-ray flux using tbabs*zashift*edge(bbody+bbody) in XSPEC is 1.65e-11 ergs/s/cm^2. Combined with the archival XMM-Newton observation from 2015, this confirms that the X-ray flux has increased by a factor of at least 1000 over the last 5 years. The X-ray flux in our NICER observations is also variable on time scales as short as ~ 2 hours.
The properties are unusual for a typical AGN, and the optical spectra (Trakhtenbrot et al. 2020; AstroNote 2020-105; Gezari et al. 2020, ATel #13718) are also atypical of a tidal disruption event. It may be an extreme example of the Bowen line nuclear transients (Trakhtenbrot et al. 2019), but observations especially in the optical and the radio bands are necessary to uncover the true nature of this transient. Further NICER and XMM-Newton observations are planned in the coming weeks.
NICER carries out prompt follow-up observations of transients and tracks alerts from LIGO and other X-ray-bright extragalactic transients. 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.
ZTF optical light curve: https://alerce.online/object/ZTF19aaiqmgl
