Chandra observations of GW170817 740-743 days since merger

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.