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LIGO/Virgo GW170817: Brightening X-ray Emission from GW170817/GRB170817A/SSS17a

ATel #11041; Daryl Haggard (McGill/MSI), John J. Ruan (McGill/MSI), Melania Nynka (McGill/MSI), Vicky Kalogera (Northwestern/CIERA), Phil Evans (Leicester)
on 8 Dec 2017; 01:34 UT
Credential Certification: Daryl Haggard (daryl.haggard@mcgill.ca)

Subjects: X-ray, Gravitational Waves, Neutron Star

We have performed a detailed analysis of new Chandra X-ray observations of GW170817 obtained via Chandra Director's Discretionary Time (PI: Wilkes, Program Number 18408601). The X-ray counterpart to GW170817/GRB170817A/SSS17a is clearly detected in the new observations.

This program acquired two exposures of GRB170817A: (1) a 74.09 ks exposure (ObsID 20860) beginning at 2017 December 2.08 UT, approximately 108 days post-burst, and (2) a 24.74 ks exposure (ObsID 20861) beginning at 2017 December 6.45 UT, approximately 111 days post-burst. Since the two new exposures are close in time and the X-ray emission of GRB170817A is not expected to vary significantly over ~4 day timescales, we co-add the two data sets into one 98.83 ks exposure at 109.2 days post-burst.

We perform spectral extractions assuming an absorbed power-law spectral model with fixed NH = 7.5e20 cm^?2 and find that the X-ray flux of GRB170817A has an absorbed flux of f(0.3?8 keV) = 1.58e?14 +/- 0.14 erg s^?1 cm^?2 (Gamma = 1.62 +/- 0.27) at 109.2 days post-burst, which corresponds to an unabsorbed luminosity of L(0.3?10 keV) = 42.5e38 +/- 3.7 erg s^?1 (see also Troja et al. GCN 22201 and Margutti et al. GCN 22203/ATel #11037). This represents significant X-ray brightening compared to Chandra observations at 15.6 days post-burst, for which we find an absorbed flux of f(0.3?8 keV) = 0.36e?14 +/- 0.1 erg s^?1 cm^?2 (Gamma = 2.4 +/- 0.8) and an unabsorbed luminosity of L(0.3?10 keV) = 10.4e38 +/- 2.0 erg s^?1 (Haggard et al. 2017).

We also examine the three previously-detected X-ray sources CXOU J130948, CXOU 130946, and the host galaxy NGC 4993. The fluxes of CXOU 130946 and the host-galaxy NGC 4993 are consistent with our previous deep Chandra observations, while CXOU J130948 appears to be variable in X-rays (Margutti et al. 2017; Haggard et al. 2017).

The origin of the X-ray emission from the NS-NS coalescence GW170817/GRB170817A is an important diagnostic for all post-merger interpretations, and different scenarios predict distinct evolution in its X-ray light curve. These observations support scenarios in which the X-ray and radio emission share a common origin, i.e., the X-ray light curve is consistent with outflow models which may be either a cocoon shocked by the jet or dynamical ejecta from the merger. Further deep X-ray monitoring can place powerful constraints on the physical parameters of these models. The X-ray brightening strengthens the argument that simple top-hat jet models are not consistent with the latest observations. However, more advanced models of structured jets with off-axis viewing angles should be pursued and cannot yet be ruled out.

We thank Belinda Wilkes and the Chandra scheduling, data processing, and archive teams for making these observations possible.