{
  "bibcode": "2020GCN.27357....1H",
  "body": "A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U. Bath), K. D.\nAlexander, W. Fong (Northwestern U.), A. Kathirgamaraju (UC Berkeley),  D.\nGiannios (Purdue U.), D. Coppejans, G. Terreran, A. Baldeschi, K. Paterson,\nM. Stroh, P. K. Blanchard (Northwestern U.), E. Berger, T. Eftekhari, G.\nHosseinzadeh, S. Gomez, V. A. Villar (Harvard U), P. K. G. Williams\n(Harvard U and AAS), M. Nicholl (U. Birmingham), R. Chornock (Ohio U.), P.\nS. Cowperthwaite (Carnegie Observatories), A. MacFadyen (NYU), B. D.\nMetzger, L. Sironi (Columbia U.), D. Radice (Princeton) report:\n\nWe initiated a new round of observations of the NS merger GW170817 with the\nChandra X-ray Observatory (CXO) to continue following its evolution at ~\n2.5 years after the merger. Four observations will be acquired between\nMarch 9 and March 15, for a total exposure time of 100 ks. Here we report\non the first observation (ObsID 21323, PI Margutti; program 20500299),\nwhich was acquired on March 09, 2020 at 22:04:48 UT (~935 days since\nmerger) with an exposure time of 24.29 ks.\n\nX-ray emission is detected at the location of GW170817 with a significance\nof ~ 3-sigma (Gaussian equivalent) and a 0.5-8 keV count-rate of  (1.2 +/-\n0.7)e-04 c/s, and a total of ~ 2.8 net photons. Assuming a simple power-law\nspectrum with photon index Gamma=1.6 and no intrinsic absorption, as found\nin previous Chandra observations (e.g. Hajela et al. 2019, ApJL, 886L, 17H),\nand following the same spectra-extraction procedure as in Hajela et al.\n2019, the count-rate above translates into an unabsorbed flux of  ~ (3.2\n+/- 1.9)e-15 erg/s/cm2 (0.3-10 keV). The Galactic neutral hydrogen column\ndensity in the direction of the transient is N_H = 7.8e+20cm-2 (Kalberla et\nal. 2005). This X-ray flux measurement is nominally above, yet\nstatistically consistent with the off-axis jet model predictions given the\nlarge flux uncertainties. Further CXO observations comprising the remaining\nexposure time of ~75 ks are scheduled in the next few days and will help\nquantify the statistical evidence for the possible departure from the\noff-axis jet model (which might represent the onset of the kilonova\nafterglow).\n\nAn updated plot with the most recent results can be found at this link:\nhttps://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home\n\n\nWe thank the entire Chandra team for making these observations possible.",
  "circularId": 27357,
  "createdOn": 1583887075000,
  "email": "AprajitaHajela2015@u.northwestern.edu",
  "subject": "Chandra observations of GW170817 ~ 2.5 years since merger (epoch 1 of 4)",
  "submitter": "Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>"
}