NICER X-ray observations of the tidal disruption event AT2020ksf/ZTF20abgbdpr/Gaia20cjk/SRGeJ213527.3-181634
ATel #14248; Dheeraj Pasham (MIT) and Keith Gendreau (NASA/GSFC)
on 7 Dec 2020; 00:26 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)
Subjects: X-ray, Transient, Tidal Disruption Event
AT2020ksf was discovered by Gaia on 21 April 2020 and subsequently by the Zwicky Transient Facility (ZTF) on 18 June 2020. It is spatially coincident with the nucleus of a galaxy at a redshift of 0.0923 (ATel#14246). It has been classified as a Tidal Disruption Event (TDE) based on the broad H-alpha and HeII 4686A complex in its optical spectrum taken on 20 November 2020 (ATel#14246). The transient has been steadily declining in the optical since its discovery in April.
Following Swift/XRT detection of X-ray emission at a flux of 1.2e-12 erg/s/cm^2 on 26 November 2020 NICER observed AT2020ksf for a total of 4.6 ks over a period of 2 days between 4-6 December 2020.
We modeled the average 0.3-1 keV spectrum with a blackbody model (tbabs*zashift(cflux*bbody)). The resulting absorbing column and blackbody temperature are well-constrained to (2.45+-0.05)e20 cm^-2 and 0.094+-0.002 keV, respectively. The source flux is higher during the NICER epoch with absorbed [unabsorbed] X-ray flux values of 2.44e-12 [(4.5+-0.3)e-12] erg/s/cm^2. The X-ray source was not present in eROSITA all sky images taken in 8-9 May 2020 with a 0.3-2 keV upper limit of ~ 1.7e-13 erg/s/cm^2. However, eROSITA detected emission on 9 November 2020 with a flux of 1.7e-12 erg/s/cm^2 (ATel#14246). Thus, the latest NICER data implies an overall increase by a factor of roughly 26 over ~8 months (assuming eROSITA upper limit from May 2020 is the unabsorbed flux).
We encourage follow-up especially in the radio wavebands. NICER is planning further monitoring observations and a weekly updated schedule can be found here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/nicer_sts_current.html. To coordinate multiwavelength observations please contact drreddy@mit.edu
NICER carries out prompt follow-up observations of X-ray-bright extragalactic transients and tracks alerts from LIGO/VIRGO and other facilities. 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/ZTF20abgbdpr
