{
  "bibcode": "2018GCN.22692....1H",
  "body": "A. Hajela (Northwestern/CIERA), K. D. Alexander, T. Eftekhari\n (Harvard/CfA), R. Margutti, W. Fong (Northwestern/CIERA), E. Berger\n(Harvard/CfA) report\n\n\"The Chandra X-ray Observatory started another round of follow-up\nobservations of GW170817. The first observation (ID 21080) was taken on May\n03, 2018 at 10:41:26 UT (t ~ 259 d after merger) for a total exposure time\nof 50.8 ks (PI Wilkes; program 19408644).   We find that an X-ray source is\ndetected at the location of GW170817 with a count-rate of (7.7 +/- 1.3)e-4\ncts/s (0.5 - 8 keV) and 13.8-sigma significance. We modeled the spectrum\nwith an absorbed simple power-law and find a best-fitting photon index,\n Gamma = 1.4 +/- 0.3 and no evidence for intrinsic absorption. The galactic\nneutral hydrogen column density in the direction of the transient is, N_H =\n7.8E+20cm-2 (Kalberla et al., 2005). For these parameters, the unabsorbed\nflux is (1.3 +/- 0.3)e-14 ergs cm-2 s-1 (0.3 - 10 keV).\n\nA second observation (ID 21090) was acquired on May 05, 2018 at 01:25:30 UT\n(~ 261 days after merger) for a total exposure time of 46.0 ks. GW170817 is\ndetected with 14.8-sigma significance and a count-rate of (8.3 +/-  1.4)e-4\ncts/s (0.5 - 8 keV). The best-fitting photon-index is Gamma = 1.3 +/- 0.3.\nThe corresponding unabsorbed flux is (1.5 +0.5/-0.3) e-14 ergs cm-2 s-1\n(0.3 - 10 keV).\n\nThe best fitting photon index from a joint spectral fit is Gamma = 1.43 +/-\n0.15, consistent with the photon index measured at ~160 days. We conclude\nthat there is no evidence for the passage of the synchrotron cooling\nfrequency through the Chandra band.\n\nThe total 0.5-8 keV count-rate inferred from the two observations is\n(8.0+/-0.9)e-4 cts/s.  The X-ray source shows significant fading compared\nto the previous epoch at ~160 days (inferred rate of (1.50 +\\-0.12)e-3\ncts/s, Margutti et al., 2018). Applying a simple binomial test, we find\nthat the detected fading is significant at a level > 4 sigma. We can thus\nreject the hypothesis of a random fluctuation of the X-ray count-rate, and\nconclude that these Chandra observations provide the first statistically\nsignificant evidence for a decaying X-ray emission from GW170817.\n\nWe thank the entire Chandra team for making these observations possible.\"",
  "circularId": 22692,
  "createdOn": 1525743864000,
  "email": "kalexander@cfa.harvard.edu",
  "subject": "Chandra observations of GW170817 260 days since merger: first statistically significant evidence for an X-ray decay",
  "submitter": "Kate Alexander at Harvard  <kalexander@cfa.harvard.edu>"
}