{
  "version": 2,
  "createdOn": 1638972345000,
  "editedOn": 1777053046813,
  "eventId": "GW170817",
  "bibcode": "2021GCN.31187....1H",
  "body": "A. Hajela (Northwestern U.), R. Margutti (UC Berkeley), K. D. Alexander\n(Northwestern U.), T. Laskar (Radboud U.), R. Chornock (UC Berkeley), E.\nBerger (Harvard), V. Ashley Villar (PSU)  report:\n\nWe have acquired X-ray observations of GW170817 with the Chandra X-ray\nObservatory (CXO) at t ~ 1573 days (~ 4.3 years) after the binary neutron\nstar merger. We report here the preliminary results from the first epoch\n(ObsID 23869; PI Margutti; program 22510329), for a total exposure time of\n28.5 ks (out of a total of 100 ks requested; we will report on the results\nfrom the complete data set in the next few days).\n\nAfter aligning the images to a common astrometric solution as described in\nHajela et al., 2021, we find evidence for X-ray emission at the location of\nGW170817 with a statistical significance of  3.8 sigma (Gaussian\nequivalent) corresponding to a net count-rate of  (1.31 +/- 0.72)e-04 c/s\n(0.5 - 8 keV). Assuming a simple absorbed power-law spectral model with a\nphoton index Gamma ~ 1.6, no intrinsic absorption (NH,int = 0 cm-2 as found\nin e.g. Hajela et al. 2019) and Galactic neutral hydrogen column density\nNH,gal = 7.8e+20 cm-2  (Kalberla et al., 2005), we derive an unabsorbed\nflux of ~ 1.7e-15 erg/cm2/s (0.3-10 keV), corresponding to a luminosity of\n~3.4e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018).\n\nBased on the extrapolation of the jet afterglow models calculated with\nJetFit (gammaB=12) and the universal post jet break models with start time\ntstart=196 days as calculated in Hajela et al., 2021, and the observed\nbackground, the expected number of photons within a 1��� region at the\nlocation of GW170817  is ~0.55 (0.5-8 keV, exposure time of 28.5 ks). The\nobserved number of counts after Xspec filtering corresponds to a ~10%\nchance fluctuation probability for universal post jet break models (~12%\nfor JetFit). For this choice of tstart and gammaB,  the total probability\nof observing at least as many counts as those collected for GW170817 at\nt>900 days based on jet afterglow models as a result of a chance\nfluctuation is ~6e-5 (~ 4.0 sigma Gaussian equivalent, for the universal\npost jet break model) and ~3e-5 (~ 4.2 sigma Gaussian equivalent for\nJetFit). The remaining ~75 ks of planned CXO observations will further\nconstrain the statistical evidence of an excess of X-ray emission in\nGW170817 with respect to jet afterglow models as reported in Hajela et al.,\n2021.\n\nWe thank the entire Chandra team for scheduling and executing these\nobservations.",
  "submitter": "Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>",
  "circularId": 31187,
  "subject": "Chandra observation of GW170817 at ~1573 days since merger (Epoch 1 of 4)",
  "email": "AprajitaHajela2015@u.northwestern.edu",
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>"
}