{
  "bibcode": "2017GCN.22203....1M",
  "body": "R. Margutti, W. Fong (Northwestern), T. Eftekhari, K. Alexander, E. Berger\n(Harvard), R. Chornock (Ohio University) report:\n\n\n���The Chandra X-ray Observatory (CXO) started observing GW170817 on 2017\nDecember 3 at 01:38:45UT for a total of 74.09 ksec (obs ID 20860, PI\nWilkes). At the position of the electromagnetic counterpart, an X-ray\nsource is clearly detected with a significance of ~33-sigma (RA =\n13:09:48.090, Dec = -23:22:52.87, J2000), with a net count-rate of 1.47e-3\ncps (0.5-8 keV).\n\nThe CXO observed the field for an additional 24.74 ksec starting on 2017\nDecember 6 at 10:43:31UT (obs ID 20861, PI Wilkes). The X-ray source is\nstill detected with a significance of ~15-sigma with a net count-rate of\n1.41e-3 cps (0.5-8 keV).\n\nThe joint spectrum can be fit with an absorbed power-law spectral model\nwith photon index Gamma= 1.50 +\\-0.16 (1 sigma c.l.). We find no evidence\nfor intrinsic neutral hydrogen absorption in addition to the Galactic value\n(NH_MW=0.0784e22 cm-2, Kalberla et al., 2005) and place a 3 sigma upper\nlimit of NHint<0.7e22 cm-2. These properties are consistent with the X-ray\nspectral properties of GW170817 at t<15 days inferred by Margutti et al.,\n2017 and Troja et al., 2017.\n\nBased on our best fitting spectral parameters, the 0.3-10 keV unabsorbed\nflux is\n\n2.3e-14 erg/s/cm^2, indicating a substantial brightening of the X-ray\nsource during the last ~90 days, in agreement with the results from Troja\net al., GCN 22201. A comparison with radio observations acquired 93 days\nsince merger (Mooley et al., 2017)  indicates a spectral slope beta~-0.6 of\nthe Fnu ~nu^-beta spectrum, similar to the radio-to-Xray spectrum of\nGW170817 before Sun block (Alexander et al., 2017; Margutti et al., 2017;\nTroja et al., 2017; Haggard et al., 2017; Hallinan et al., 2017; Mooley et\nal., 2017). This result suggests negligible spectral evolution of GW170817\nin the last 90 days, and that the radio and X-ray emission continue to\nrepresent the same emission component.\n\nBased on this spectrum, we expect GW170817 to be detectable at optical-NIR\nwavelengths at approximately 26.5 AB magnitude.\n\nWe thank Belinda Wilkes and the entire CXO team for approving these DDT\nrequests and making these observations possible.���",
  "circularId": 22203,
  "createdOn": 1512669844000,
  "email": "rafmargutti@gmail.com",
  "subject": "LIGO/Virgo GW170817: Chandra X-ray brightening of the counterpart 108 days since merger",
  "submitter": "Raffaella Margutti at Northwestern U  <rafmargutti@gmail.com>",
  "eventId": "LIGO/Virgo GW170817"
}