{
  "body": "A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander\n(Northwestern U.), J. S. Bright (UC Berkeley), T. Laskar (Radboud U.), E.\nBerger (Harvard U.) report:\n\nChandra X-ray Observatory (CXO) observed GW170817 at t ~ 1575 days (~ 4.3\nyears) after the binary neutron star merger in four distinct exposures\n(ObsID 23869, 26223, 24336, 24337; PI Margutti; program 22510329). We\nreported the preliminary results from the first epoch (ObsID 23869) in GCN\n31187 (Hajela et al.). Here, we report results from all four observations\nacquired for a total exposure time of ~ 98ks.\n\nWe merged all four observations after reprocessing and correcting for the\nabsolute astrometry (as described in Hajela et al. 2021), and found 5 total\nphotons in 0.5-8 keV energy range in an 1��� region at the position of\nGW170817. This corresponds to a net 0.5-8 keV count-rate of ~4.3e-5 c/s,\nwhich translates to a detection of X-ray emission at the location of\nGW170817 with a statistical significance of  ~3.2 sigma (Gaussian\nequivalent). We joint-fitted all the four observations with an absorbed\npower law model with photon index fixed to ~ 1.6, no intrinsic absorption\n(NH,int = 0 cm-2 as found in e.g. Hajela et al. 2019), and Galactic neutral\nhydrogen column density NH,gal = 7.8e+20 cm-2  (Kalberla et al., 2005). The\nderived unabsorbed flux is ~1.3e-15 erg/cm2/s (0.3-10 keV).\n\nFrom a preliminary analysis we find that this measurement is in slight\nexcess to, but statistically consistent with, the expectations from the\noff-axis jets models of Hajela et al., 2021. However, we also note that,\nwhen compared to the Chandra observations of GW170817 obtained in March\n2020 (t=939.31 days since merger) and Dec 2020+Jan2021 (t=1234.11 days\nsince merger), and using a Binomial test for variability in the count phase\nspace, we find no statistically significant evidence of fading of the X-ray\nsource associated with GW170817 in the last ~650 days.\n\nWe thank the entire Chandra team for scheduling and executing these\nobservations.",
  "subject": "Chandra observation of GW170817 at ~1575 days since merger (all epochs)",
  "version": 2,
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>",
  "bibcode": "2021GCN.31231....1H",
  "circularId": 31231,
  "createdOn": 1639524036000,
  "email": "AprajitaHajela2015@u.northwestern.edu",
  "submitter": "Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>",
  "editedOn": 1777053048321,
  "eventId": "GW170817"
}