The gamma-ray source Fermi J1623-1752 and X-ray transient Swift J162315.0-175233 are likely GRB 210928A
ATel #14948; M. Axelsson (Stockholm Univ.), E. Bissaldi (Politecnico & INFN Bari), C. C. Cheung (NRL), G. La Mura (LIP), on behalf of the Fermi Large Area Telescope Collaboration
on 1 Oct 2021; 14:22 UT
Credential Certification: Magnus Axelsson (magaxe@kth.se)
Subjects: X-ray, Gamma Ray, Gamma-Ray Burst, Transient
Further analysis of the Fermi-LAT source Fermi J1623-1752 (Rani et al., ATel #14939) has revealed that it is likely a gamma-ray burst, which occurred on 2021 September 28 at 02:00:31 UT. This GRB triggered the Gamma-ray Burst Monitor (GBM) instrument onboard Fermi, but no alert was sent out. When the transient was discovered it was therefore labeled as a new gamma-ray transient, rather than a GRB.
The best LAT on-ground location is found to be RA, Dec = 245.88, -17.91 (degrees, J2000),
with an error radius of 0.15 deg (90% containment, statistical error only). This location is consistent with both the recurrent nova U Sco (currently in quiescence; Sokolovsky et al., ATel #14941) as well as the Swift X-ray transient reported by Page et al. (ATel #14945). If Fermi J1623-1752 is indeed GRB 210928A, the Swift transient is likely the afterglow.
The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.
