TITLE:   GCN CIRCULAR
NUMBER:  22693
SUBJECT: Chandra observations of GW170817 reveal a fading afterglow
DATE:    18/05/08 03:59:58 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja@nasa.gov>

E. Troja (UMD/GSFC), L. Piro (IAPS/INAF), G. Ryan (UMD) report on
behalf of a larger collaboration:


The Chandra X-ray Observatory re-observed the field of GW170817
starting on May 3rd, 2018 as part of its on-going monitoring
program (PI: Wilkes). Observations were split into two exposures
of 50.7 ks and 46 ks, respectively.

The X-ray afterglow is significantly detected at an average level
of (8.5 +/- 1.4)E-04 cts/s, well below the value (14.8 +/- 1.1)E-04 cts/s
measured  at ~160 d after the merger (Troja & Piro, GCN Circ. 22374;
Troja et al. 2018). The spectrum is well described by an absorbed
power-law model with photon index 1.5 +/- 0.2, consistent with the
value of 1.575 derived at earlier times from the broadband spectrum.
By using this value we derive an unabsorbed X-ray flux of
(1.4 +/- 0.2)E-14 erg/cm2/s at 260 days after the merger.

A comparison with the afterglow models derived in Troja et al. (2018)
is shown at this link:
https://www.astro.umd.edu/~gsryan/GW170817/xray.png

This new X-ray observation supports earlier claims (D'Avanzo et al. 2018;
Dobie et al. 2018) that the afterglow entered its post-peak decay phase.
However, no clear distinction between the competing models is possible.
Further observations are critical to probe the structure of the
relativistic outflow.