GW170817/GRB170817A: Preliminary results of Chandra monitoring

ATel #11243; E. Troja (UMD/GSFC), L. Piro (INAF/IAPS) report on behalf of a larger collaboration
on 29 Jan 2018; 20:51 UT
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Credential Certification: Eleonora Troja (eleonora.troja@nasa.gov)

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The Chandra X-ray Observatory re-observed the field of GW170817 starting on January 17th, 2018, and performed five short exposures observations as part of its on-going monitoring program (PI: Wilkes). Only three of these exposures (ObsID: 20936, 20938, 20939) are currently archived and available to the public. Here we report the preliminary findings from these observations. The X-ray afterglow is detected with high significance in all the exposures at an average count rate of 0.0016 cts/s in the 0.5-8.0 keV energy band. A preliminary inspection of the hardness ratio does not show any significant spectral variation. Therefore, we perform a spectral analysis using an absorbed power-law model with absorption column fixed at the Galactic value of 7.5E20 cm^-2 and a photon index Gamma=1.575 as derived from our broadband analysis (Troja et al. 2018, arXiv:1801.06516). We derive an unabsorbed X-ray flux of (3.2 +/- 0.3)E-14 erg/cm2/s in the 0.3-10 keV energy band. The quoted error is at the 68% confidence level. This new measurement is higher than the value measured by Chandra at ~110 days (~2.5E-14 erg/cm2/s, Troja et al. 2018), and higher than the value measured by XMM-Newton at ~135 days (D'Avanzo et al., 2018). The latest measurement is consistent with a rising afterglow with F~t^0.8, although, within the errors, a slow turn-over of the X-ray light curve cannot be excluded. Further analysis is on-going.