TITLE: GCN CIRCULAR NUMBER: 22692 SUBJECT: Chandra observations of GW170817 260 days since merger: first statistically significant evidence for an X-ray decay DATE: 18/05/08 01:44:24 GMT FROM: Kate Alexander at Harvard A. Hajela (Northwestern/CIERA), K. D. Alexander, T. Eftekhari (Harvard/CfA), R. Margutti, W. Fong (Northwestern/CIERA), E. Berger (Harvard/CfA) report "The Chandra X-ray Observatory started another round of follow-up observations of GW170817. The first observation (ID 21080) was taken on May 03, 2018 at 10:41:26 UT (t ~ 259 d after merger) for a total exposure time of 50.8 ks (PI Wilkes; program 19408644). We find that an X-ray source is detected at the location of GW170817 with a count-rate of (7.7 +/- 1.3)e-4 cts/s (0.5 - 8 keV) and 13.8-sigma significance. We modeled the spectrum with an absorbed simple power-law and find a best-fitting photon index, Gamma = 1.4 +/- 0.3 and no evidence for intrinsic absorption. The galactic neutral hydrogen column density in the direction of the transient is, N_H = 7.8E+20cm-2 (Kalberla et al., 2005). For these parameters, the unabsorbed flux is (1.3 +/- 0.3)e-14 ergs cm-2 s-1 (0.3 - 10 keV). A second observation (ID 21090) was acquired on May 05, 2018 at 01:25:30 UT (~ 261 days after merger) for a total exposure time of 46.0 ks. GW170817 is detected with 14.8-sigma significance and a count-rate of (8.3 +/- 1.4)e-4 cts/s (0.5 - 8 keV). The best-fitting photon-index is Gamma = 1.3 +/- 0.3. The corresponding unabsorbed flux is (1.5 +0.5/-0.3) e-14 ergs cm-2 s-1 (0.3 - 10 keV). The best fitting photon index from a joint spectral fit is Gamma = 1.43 +/- 0.15, consistent with the photon index measured at ~160 days. We conclude that there is no evidence for the passage of the synchrotron cooling frequency through the Chandra band. The total 0.5-8 keV count-rate inferred from the two observations is (8.0+/-0.9)e-4 cts/s. The X-ray source shows significant fading compared to the previous epoch at ~160 days (inferred rate of (1.50 +\-0.12)e-3 cts/s, Margutti et al., 2018). Applying a simple binomial test, we find that the detected fading is significant at a level > 4 sigma. We can thus reject the hypothesis of a random fluctuation of the X-ray count-rate, and conclude that these Chandra observations provide the first statistically significant evidence for a decaying X-ray emission from GW170817. We thank the entire Chandra team for making these observations possible."