LIGO/Virgo GW170817: Chandra X-ray brightening of the counterpart 108 days since merger

ATel #11037; R. Margutti, W. Fong (Northwestern), T. Eftekhari, K. Alexander, E. Berger (Harvard), R. Chornock (Ohio University)
on 7 Dec 2017; 18:00 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Raffaella Margutti (rmargutti@cfa.harvard.edu)

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The Chandra X-ray Observatory (CXO) started observing GW170817 on 2017 December 3 at 01:38:45UT for a total of 74.09 ksec (obs ID 20860, PI Wilkes). At the position of the electromagnetic counterpart, an X-ray source is clearly detected with a significance of ~33-sigma (RA = 13:09:48.090, Dec = -23:22:52.87, J2000), with a net count-rate of 1.47e-3 cps (0.5-8 keV).

The CXO observed the field for an additional 24.74 ksec starting on 2017 December 6 at 10:43:31UT (obs ID 20861, PI Wilkes). The X-ray source is still detected with a significance of ~15-sigma with a net count-rate of 1.41e-3 cps (0.5-8 keV).

The joint spectrum can be fit with an absorbed power-law spectral model with photon index Gamma= 1.50 +\-0.16 (1 sigma c.l.). We find no evidence for intrinsic neutral hydrogen absorption in addition to the Galactic value (NH_MW=0.0784e22 cm-2, Kalberla et al., 2005) and place a 3 sigma upper limit of NHint<0.7e22 cm-2. These properties are consistent with the X-ray spectral properties of GW170817 at t<15 days inferred by Margutti et al., 2017 and Troja et al., 2017.

Based on our best fitting spectral parameters, the 0.3-10 keV unabsorbed flux is 2.3e-14 erg/s/cm^2, indicating a substantial brightening of the X-ray source during the last ~90 days, in agreement with the results from Troja et al., GCN 22201. A comparison with radio observations acquired 93 days since merger (Mooley et al., 2017) indicates a spectral slope beta~-0.6 of the Fnu ~nu^-beta spectrum, similar to the radio-to-Xray spectrum of GW170817 before Sun block (Alexander et al., 2017; Margutti et al., 2017; Troja et al., 2017; Haggard et al., 2017; Hallinan et al., 2017; Mooley et al., 2017). This result suggests negligible spectral evolution of GW170817 in the last 90 days, and that the radio and X-ray emission continue to represent the same emission component.

Based on this spectrum, we expect GW170817 to be detectable at optical-NIR wavelengths at approximately 26.5 AB magnitude.

We thank Belinda Wilkes and the entire CXO team for approving these DDT requests and making these observations possible.