T. Laskar (Radboud University), R. Margutti (UC Berkeley), K. D. Alexander
(Northwestern), E. Berger (Harvard), and R. Chornock (UC Berkeley) report:
"We observed GRB 210905A (Sonbas et al., GCN 30765) with Chandra/ACIS,
starting on 2021 November 24 at 13:08:25 UT (80.7 days after the GRB
trigger), for a total exposure of 20 ks. We detect a point source at the
position of the Swift/XRT afterglow with a 0.5-8 keV (observed) count rate
of ~ 3.1e-4 counts/s. Using spectral parameters from fitting the Swift/XRT
PC-mode spectrum at a redshift of z=6.318 (Tanvir et al., GCN 30771), we
find a preliminary unabsorbed flux of ~ 5.4e-15 erg/(s cm^2) in the 0.3-10
keV band (observer frame).
This measurement confirms a potential break in the Swift/XRT observations
at ~ 30 days after the trigger. The post-break decline rate is consistent
with -2. Interpreting this break as a jet break, assuming EKiso ~
Egamma,iso, and using the isotropic-equivalent gamma-ray energy from
Konus-Wind (Frederiks et al., GCN 30780), the inferred opening angle is ~ 9
deg * (E/1.1e54 erg) / (n/ (1particle/cm^3))] ^(-3/8) for a uniform density
environment, leading to a beaming-corrected energy of ~ 1.3e52 erg. Further
analysis is underway.
We thank Harvey Tananbaum and Pat Slane for approving our DDT request and
the entire CXC staff for arranging and executing the observations."