{
  "bibcode": "2019GCN.25631....1H",
  "body": "A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U Bath),  D.\nCoppejans, G. Terreran, W. Fong, K. D. Alexander, A. Baldeschi, K.\nPaterson (Northwestern\nU.), E. Berger, P. K. Blanchard, T. Eftekhari, G. Hosseinzadeh, S. Gomez,\nV. A. Villar, P. K. G. Williams (Harvard U), M. Nicholl (U Edinburgh), R.\nChornock (Ohio U), P. S. Cowperthwaite (Carnegie Observatories), D.\nGiannios (Purdue U.), A. MacFadyen (NYU), A. Kathirgamaraju (UC Berkeley)\nreport:\n\nWe report on Chandra observations of GW170817 acquired ~2 yrs after the\nbinary neutron star merger. The Chandra X-ray Observatory started observing\nGW170817 August 27, 2019 at 14:16:40 UT (t ~ 740 d after merger) for an\nexposure time of 40.0 ks (PI Troja; program 20500691, observation ID 21372).\nAnother two epochs of Chandra observations were acquired on August 29, 2019\nat 13:46:01 UT (ID 22736) and on August 30, 2019 at 11:46:44 UT (ID 22737)\nfor an exposure time of 33.6 ks and 25.3 ks, respectively.\n\nA preliminary reduction shows that GW170817 is detected in the merged\nexposure with a ~3 sigma significance and a net count-rate of (1.03 +\\-\n0.33)e-4 c/s (0.5-8 keV). Assuming negligible intrinsic absorption and a\nGalactic neutral hydrogen column density N_h = 7.8E+20cm-2 (Kalberla et\nal., 2005), we infer a best-fit photon index Gamma=1.0+/-1.3 and an\nunabsorbed flux of  ~3e-15 erg/s/cm2 (0.3-10 keV). The photon index is\nloosely constrained by the observations and we therefore adopt the best-fit\nphoton index from the entire Chandra data set acquired in the first two\nyears of observations of GW170817 (Gamma=1.6) for the spectral calibration.\nWe find an unabsorbed flux of (2.9 +\\- 1.0)e-15 erg/s/cm2. These results\nare consistent within the uncertainties with the extrapolation of the\noff-axis structured jet model by Wu & MacFadyen (ApJ, 869, 55) at the\ncurrent epoch.\n\nWe thank the entire Chandra team for making these observations possible.",
  "circularId": 25631,
  "createdOn": 1567523847000,
  "email": "AprajitaHajela2015@u.northwestern.edu",
  "subject": "Chandra observations of GW170817 ~740-743 days since merger",
  "submitter": "Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>"
}