GCN Circular 31231
Subject
Chandra observation of GW170817 at ~1575 days since merger (all epochs)
Date
2021-12-14T23:20:36Z (3 years ago)
From
Aprajita Hajela at Northwestern U <AprajitaHajela2015@u.northwestern.edu>
A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander
(Northwestern U.), J. S. Bright (UC Berkeley), T. Laskar (Radboud U.), E.
Berger (Harvard U.) report:
Chandra X-ray Observatory (CXO) observed GW170817 at t ~ 1575 days (~ 4.3
years) after the binary neutron star merger in four distinct exposures
(ObsID 23869, 26223, 24336, 24337; PI Margutti; program 22510329). We
reported the preliminary results from the first epoch (ObsID 23869) in GCN
31187 (Hajela et al.). Here, we report results from all four observations
acquired for a total exposure time of ~ 98ks.
We merged all four observations after reprocessing and correcting for the
absolute astrometry (as described in Hajela et al. 2021), and found 5 total
photons in 0.5-8 keV energy range in an 1��� region at the position of
GW170817. This corresponds to a net 0.5-8 keV count-rate of ~4.3e-5 c/s,
which translates to a detection of X-ray emission at the location of
GW170817 with a statistical significance of ~3.2 sigma (Gaussian
equivalent). We joint-fitted all the four observations with an absorbed
power law model with photon index fixed to ~ 1.6, no intrinsic absorption
(NH,int = 0 cm-2 as found in e.g. Hajela et al. 2019), and Galactic neutral
hydrogen column density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005). The
derived unabsorbed flux is ~1.3e-15 erg/cm2/s (0.3-10 keV).
From a preliminary analysis we find that this measurement is in slight
excess to, but statistically consistent with, the expectations from the
off-axis jets models of Hajela et al., 2021. However, we also note that,
when compared to the Chandra observations of GW170817 obtained in March
2020 (t=939.31 days since merger) and Dec 2020+Jan2021 (t=1234.11 days
since merger), and using a Binomial test for variability in the count phase
space, we find no statistically significant evidence of fading of the X-ray
source associated with GW170817 in the last ~650 days.
We thank the entire Chandra team for scheduling and executing these
observations.