TITLE:   GCN CIRCULAR
NUMBER:  30558
SUBJECT: GRB 210726A: Chandra detection of the X-ray afterglow 
DATE:    21/07/30 23:26:02 GMT
FROM:    Alicia Rouco Escorial at CIERA  <alicia.rouco.escorial@northwestern.edu>

A. Rouco Escorial, W. Fong, K. Paterson (Northwestern University), R. Margutti (UC Berkeley) and E. Berger (Harvard University) report:



“We initiated observations with the Chandra X-ray Observatory of the short-duration GRB 210726A (Bernardini et al., GCN 30523; Tohuvavohu, GCN 30535; Palmer, GCN 30536) starting on 2021 July 29 16:04:38 UT, with a median observation time of ~3 days post-trigger. We obtained one ACIS-S observation under the Proposal 22400461 (ObsID 23445; PI: Fong), with an effective exposure time of ~24.6 ks.

We obtain a ~7sigma detection of GRB 210726A with a total net source counts of 76+/-10 (0.5-7 keV). The Chandra position of the short GRB is fully consistent with the enhanced Swift-XRT position (90% confidence; XRT GRB catalogue, Evans et al., 2009). The Chandra position is:

RA(J2000)= 12h53m09.7s
Dec(J2000)=19d11m24.6s

with a total positional uncertainty of 0.81 arcsec (largely dominated by Chandra's absolute astrometric uncertainty).

Based on our MCMC modeling, the XRT and Chandra afterglow flux, starting at ~120.89 s post-burst, can be modeled with a broken power-law decline characterized by a second segment with a decay index (F~t^alpha) of alpha2=-0.62 (-0.05,+0.04) starting at ~418 s post-burst. In particular, the Chandra detection roughly follows the latest decline rate based on XRT data alone..

Further Chandra observations are planned. We thank the Chandra Director, Pat Slane, and staff for the rapid planning and scheduling of these observations.”