{
  "bibcode": "2018GCN.22371....1H",
  "body": "Daryl Haggard, Melania Nynka, John J. Ruan (McGill/MSI), Phil Evans \n(Leicester), and Vicky Kalogera (Northwestern/CIERA) report:\n\nWe have obtained new X-ray observations of GW170817 via Chandra \nDirector's Discretionary Time (PI: Wilkes, Program Number 19408607). The \nX-ray counterpart to GW170817/GRB170817A/SSS17a continues to be detected \nand the X-ray emission continues to brighten approximately 156 days \nafter the neutron star merger. This contradicts previous claims of the \nX-ray fading of GW170817 in XMM-Newton observations at 135 days \n(D'Avanzo et al. 2018, arXiv: 1801.06164).\n\nThis new Chandra program acquired several exposures of GRB170817A:\n\nObsID,  ExpTime,  StartDate,           Days Post-Burst\n-----   -------   ---------            ---------------\n20936   31.75 ks  2018-01-17 21:55:17  153.5 days\n20938   15.86 ks  2018-01-21 13:45:18  157.1 days\n20939   22.25 ks  2018-01-24 08:18:48  159.9 days\n\nSince these new Chandra exposures are close in time and the X-ray \nemission of GRB170817A is not expected to vary significantly over <10 \nday timescales, we co-add these three data sets into one 69.86 ks \nexposure at 156.4 days post-burst.\n\nWe perform spectral extractions assuming an absorbed power-law spectral \nmodel with fixed NH = 7.5e20 cm^���2 and find that the X-ray flux of \nGRB170817A has an absorbed flux of f(0.3���8 keV) = 1.93(+0.39/-0.32)e���14 \nerg s^���1 cm^���2  (with Gamma ~ 1.67) at 156.4 days post-burst, which \ncorresponds to an unabsorbed luminosity of L(0.3���10 keV) = \n5.23(+1.30/-0.95)e39 erg s^���1 (assuming a luminosity distance of 42.5 Mpc).\n\nThis represents continued X-ray brightening compared to Chandra \nobservations at 15.6 and 109.2 days post-burst, for which we find an \nabsorbed flux of f(0.3���8 keV) = 0.36(+0.1/-0.07)e���14 erg s^���1 cm^���2 \n(with Gamma = 2.4 +/- 0.8, unabsorbed L(0.3���10 keV) = 10.4(+2.0/-1.6)e38 \nerg s^���1; Haggard et al. 2017) and f(0.3���8 keV) = 1.58(+0.14/-0.13)e���14 \nerg s^���1 cm^���2 (with Gamma = 1.6 +/- 0.3, unabsorbed L(0.3���10 keV) = \n42.5(+3.7/-3.5)e38 erg s^���1; Ruan et al. 2018), respectively.\n\nOur findings here contradict recent reports of dimming in the X-ray flux \nfrom XMM Newton at 135 days (D'Avanzo et al. 2018), which was reported \nto be 2.1(+0.7/-0.5)e-15 erg s^���1 cm^���2 (0.3-10 keV unabsorbed). \nHowever, this reported flux value is a typo, and should be \n2.1(+0.7/-0.5)e-14 erg s^���1 cm^���2 (D���Avanzo 2018, private \ncommunication). Taking this corrected flux and rescaling to a 0.3-8 keV \nabsorbed flux for comparison to the previous Chandra measurements above \ngives 1.67(+0.87/-0.64)e-14 erg s^���1 cm^���2. Thus, the recent X-ray data \nat 15.6 days (Chandra), 109.2 days (Chandra), 135 days (XMM), and 156.4 \ndays (Chandra) are all consistent with continued X-ray brightening. We \nprovide a light curve table summarizing these measurements:\n\nDays,  Telescope,  Flux* (0.3-8 abs),     Ref\n----   ---------   ----------------       ---\n15.6   Chandra     0.36(+0.10/-0.07)e���14  Haggard et al. (2017)\n109.2  Chandra     1.58(+0.14/-0.13)e���14  Ruan et al. (2018)\t\t\n135    XMM         1.67(+0.87/-0.64)e-14  D'Avanzo et al. (2018)\t\n156.4  Chandra     1.93(+0.39/-0.32)e-14  This work\n\n*Flux units: erg s^���1 cm^���2; all uncertainties are 90% confidence interval\n\nCurrent post-merger models suggest that the origin of the X-rays could \nbe afterglow emission from either a mildly-relativistic cocoon or a \nstructured jet. For a cocoon, the continued rise of X-ray emission \nsuggest that the cocoon has not yet reached a deceleration phase. For a \nstructured jet, the rising X-ray emission suggest emission from the jet \ncore has not yet entered the observed line of sight. Continued \nmonitoring of GW170817 will be critical for discriminating between these \nand other models.\n\nNote that another ~30 ks of Chandra observations during this same time \ninterval are forthcoming.\n\nWe thank Belinda Wilkes and the Chandra scheduling, data processing, and \narchive teams for making these observations possible.",
  "circularId": 22371,
  "createdOn": 1517253682000,
  "email": "daryl.haggard@mcgill.ca",
  "subject": "LIGO/Virgo GW170817: Chandra X-ray Emission Continues to Rise ~156 Days Post-Merger",
  "submitter": "Daryl Haggard at McGill U  <daryl.haggard@mcgill.ca>",
  "eventId": "LIGO/Virgo GW170817"
}