GCN Circular 31187

A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander
(Northwestern U.), T. Laskar (Radboud U.), R. Chornock (UC Berkeley), E.
Berger (Harvard), V. Ashley Villar (PSU)  report:

We have acquired X-ray observations of GW170817 with the Chandra X-ray
Observatory (CXO) at t ~ 1573 days (~ 4.3 years) after the binary neutron
star merger. We report here the preliminary results from the first epoch
(ObsID 23869; PI Margutti; program 22510329), for a total exposure time of
28.5 ks (out of a total of 100 ks requested; we will report on the results
from the complete data set in the next few days).

After aligning the images to a common astrometric solution as described in
Hajela et al., 2021, we find evidence for X-ray emission at the location of
GW170817 with a statistical significance of  3.8 sigma (Gaussian
equivalent) corresponding to a net count-rate of  (1.31 +/- 0.72)e-04 c/s
(0.5 - 8 keV). Assuming a simple absorbed power-law spectral model with a
photon index Gamma ~ 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), we derive an unabsorbed
flux of ~ 1.7e-15 erg/cm2/s (0.3-10 keV), corresponding to a luminosity of
~3.4e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018).

Based on the extrapolation of the jet afterglow models calculated with
JetFit (gammaB=12) and the universal post jet break models with start time
tstart=196 days as calculated in Hajela et al., 2021, and the observed
background, the expected number of photons within a 1��� region at the
location of GW170817  is ~0.55 (0.5-8 keV, exposure time of 28.5 ks). The
observed number of counts after Xspec filtering corresponds to a ~10%
chance fluctuation probability for universal post jet break models (~12%
for JetFit). For this choice of tstart and gammaB,  the total probability
of observing at least as many counts as those collected for GW170817 at
t>900 days based on jet afterglow models as a result of a chance
fluctuation is ~6e-5 (~ 4.0 sigma Gaussian equivalent, for the universal
post jet break model) and ~3e-5 (~ 4.2 sigma Gaussian equivalent for
JetFit). The remaining ~75 ks of planned CXO observations will further
constrain the statistical evidence of an excess of X-ray emission in
GW170817 with respect to jet afterglow models as reported in Hajela et al.,
2021.

We thank the entire Chandra team for scheduling and executing these
observations.