X-ray dimming and hardening, a newly launched X-ray outflow, and optical dimming of the nuclear transient AT2019avd/eRASStJ082337+042303/ZTF19aaiqmgl: NICER observations
ATel #14664; Dheeraj R. Pasham (MIT), Yanan Wang (Southampton), Ron Remillard (MIT), Keith Gendreau, Zaven Arzoumanian (NASA/GSFC), Diego Altamirano (Southampton), Michael Loewenstein (UMCP, NASA/GSFC)
on 27 May 2021; 16:00 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)
Subjects: X-ray, AGN, Transient, Tidal Disruption Event
AT2019avd (also known as eRASStJ082337+042303 or ZTF19aaiqmgl) is an ongoing peculiar optical and X-ray transient originating from the nucleus of a galaxy at a redshift of 0.0296 (ATel #13712; Malyali et al. 2021). It has a double peaked optical light curve extending over a temporal baseline of 815 days (see ZTF light curve link below) and has been X-ray active for at least the past year (since MJD 58967.7 or 28 April 2020; Malyali et al. 2021).
NICER has been performing high-cadence monitoring observations of AT2019avd since 19 September 2020, a few days after it was detected with Swift/XRT with a flux of roughly 8e-12 erg/s/cm^2 (Malyali et al. 2021). NICER has been obtaining exposures daily and, occasionally, multiple times per day.
Over the last 8 months NICER finds AT2019avd to be highly variable on timescales of a day, with the most extreme flare recording a factor of 12 change in flux over less than 4 hours. AT2019avd's unabsorbed 0.3-2.0 keV flux varied between 7e-13 erg/s/cm^2 and 2e-11 erg/s/cm^2 between 19 September 2020 and 22 May 2021. We fit NICER's 0.3-2.0 keV X-ray spectra with two models: 1) a blackbody plus a power law modified by absorption (tbabs*zashift(bbody+pow) in XSPEC) and 2) two blackbodies (tbabs * zashift (bbody+bbody) in XSPEC). Both models give similar reduced chi-squared values between 0.5 and 2.3, although the inferred power-law index with the former model is steep, with a value between 1.5 and 4.5. Under both models the occasional presence of strong emission and absorption lines in the energy range of 0.8-1.2 keV is evident. This could represent the newly launched outflow accompanying the X-ray spectral change. At present, the line energy of this outflow is steadily increasing while its strength is decreasing.
Over the last two months the source X-ray flux has been dropping in conjunction with its optical flux. The X-ray flux has remained highly variable during the decline phase, unlike the optical flux which is observed to show a smooth decline. The most recently measured unabsorbed 0.3-2.0 keV X-ray flux is roughly 2e-12 erg/s/cm^2. Optical spectra and radio observations are encouraged to uncover the true nature of this transient. Further NICER observations are planned in the coming weeks and the observing schedule may be found here:
https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html
NICER carries out prompt follow-up observations of transients based on multi-messenger alerts. NICER is a 0.25-12 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
