TITLE:   GCN CIRCULAR
NUMBER:  29375
SUBJECT: Continued Chandra observations of GW170817 at ~3.3 years since merger
DATE:    21/01/30 18:21:47 GMT
FROM:    Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>

A. Hajela, R. Margutti, J. Bright, K. D. Alexander, W. Fong (Northwestern
U.), E. Berger (Harvard), R. Chornock, D. L. Coppejans, P. Blanchard
(Northwestern U.), V. A. Villar (Columbia U.), T. Eftekhari (Harvard U), A.
Kathirgamaraju (UC Berkeley), D. Giannios (Purdue U.), T. Laskar (Bath), J.
Zrake (Clemson), A. MacFadyen (NYU), K. Paterson (Northwestern U.) report:

We acquired 3 distinct exposures (ObsID 23870, 24923, and 24924) of
GW170817 with the Chandra X-ray Observatory (CXO) between January 18 and
January 27, 2021, under our program 22510329 (PI Margutti), for a total
exposure time of ~98 ks. We initiated these observations after we inferred
a potential excess of X-ray emission compared to our off-axis jet model in
our previous round of observations (ObsID 22677, 24887, 24888, and 24889;
results reported in Hajela et al., GCN 29055; also see Troja et al. GCN
27411). The 3 new observations combined with our previous round span
1209-1258 days post-merger and have a mean epoch of 1234 days.

We merge all 7 observations and find significant X-ray emission at the
location of GW170817 at the level of ~5 sigma (Gaussian equivalent). From a
joint spectral fit with a simple power-law spectral model using a photon
index Gamma ~ 1.6, no intrinsic absorption NHint = 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 estimate an unabsorbed flux of
2.4(+0.53 -0.82) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties),
corresponding to a luminosity of 4.8 (+1.1 -1.6) e+38 erg/s at the distance
of 40.7 Mpc (Cantiello et al., 2018). Based on the expectations from the
best-fitting off-axis jet model from Hajela+2019,  the chance probability
to obtain at ~1234 days after the merger an X-ray count-rate at least as
large as the measured count-rate is P=0.0014 (corresponding to 3.2 sigma,
Gaussian equivalent). Radio observations to date have not shown the same
excess (Alexander et al. GCN 29053), but more detailed analysis is in
progress.

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