TITLE:   GCN CIRCULAR
NUMBER:  25631
SUBJECT: Chandra observations of GW170817 ~740-743 days since merger
DATE:    19/09/03 15:17:27 GMT
FROM:    Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>

A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U Bath),  D.
Coppejans, G. Terreran, W. Fong, K. D. Alexander, A. Baldeschi, K.
Paterson (Northwestern
U.), E. Berger, P. K. Blanchard, T. Eftekhari, G. Hosseinzadeh, S. Gomez,
V. A. Villar, P. K. G. Williams (Harvard U), M. Nicholl (U Edinburgh), R.
Chornock (Ohio U), P. S. Cowperthwaite (Carnegie Observatories), D.
Giannios (Purdue U.), A. MacFadyen (NYU), A. Kathirgamaraju (UC Berkeley)
report:

We report on Chandra observations of GW170817 acquired ~2 yrs after the
binary neutron star merger. The Chandra X-ray Observatory started observing
GW170817 August 27, 2019 at 14:16:40 UT (t ~ 740 d after merger) for an
exposure time of 40.0 ks (PI Troja; program 20500691, observation ID 21372).
Another two epochs of Chandra observations were acquired on August 29, 2019
at 13:46:01 UT (ID 22736) and on August 30, 2019 at 11:46:44 UT (ID 22737)
for an exposure time of 33.6 ks and 25.3 ks, respectively.

A preliminary reduction shows that GW170817 is detected in the merged
exposure with a ~3 sigma significance and a net count-rate of (1.03 +\-
0.33)e-4 c/s (0.5-8 keV). Assuming negligible intrinsic absorption and a
Galactic neutral hydrogen column density N_h = 7.8E+20cm-2 (Kalberla et
al., 2005), we infer a best-fit photon index Gamma=1.0+/-1.3 and an
unabsorbed flux of  ~3e-15 erg/s/cm2 (0.3-10 keV). The photon index is
loosely constrained by the observations and we therefore adopt the best-fit
photon index from the entire Chandra data set acquired in the first two
years of observations of GW170817 (Gamma=1.6) for the spectral calibration.
We find an unabsorbed flux of (2.9 +\- 1.0)e-15 erg/s/cm2. These results
are consistent within the uncertainties with the extrapolation of the
off-axis structured jet model by Wu & MacFadyen (ApJ, 869, 55) at the
current epoch.

We thank the entire Chandra team for making these observations possible.