GCN Circular 22692

A. Hajela (Northwestern/CIERA), K. D. Alexander, T. Eftekhari
 (Harvard/CfA), R. Margutti, W. Fong (Northwestern/CIERA), E. Berger
(Harvard/CfA) report

"The Chandra X-ray Observatory started another round of follow-up
observations of GW170817. The first observation (ID 21080) was taken on May
03, 2018 at 10:41:26 UT (t ~ 259 d after merger) for a total exposure time
of 50.8 ks (PI Wilkes; program 19408644).   We find that an X-ray source is
detected at the location of GW170817 with a count-rate of (7.7 +/- 1.3)e-4
cts/s (0.5 - 8 keV) and 13.8-sigma significance. We modeled the spectrum
with an absorbed simple power-law and find a best-fitting photon index,
 Gamma = 1.4 +/- 0.3 and no evidence for intrinsic absorption. The galactic
neutral hydrogen column density in the direction of the transient is, N_H =
7.8E+20cm-2 (Kalberla et al., 2005). For these parameters, the unabsorbed
flux is (1.3 +/- 0.3)e-14 ergs cm-2 s-1 (0.3 - 10 keV).

A second observation (ID 21090) was acquired on May 05, 2018 at 01:25:30 UT
(~ 261 days after merger) for a total exposure time of 46.0 ks. GW170817 is
detected with 14.8-sigma significance and a count-rate of (8.3 +/-  1.4)e-4
cts/s (0.5 - 8 keV). The best-fitting photon-index is Gamma = 1.3 +/- 0.3.
The corresponding unabsorbed flux is (1.5 +0.5/-0.3) e-14 ergs cm-2 s-1
(0.3 - 10 keV).

The best fitting photon index from a joint spectral fit is Gamma = 1.43 +/-
0.15, consistent with the photon index measured at ~160 days. We conclude
that there is no evidence for the passage of the synchrotron cooling
frequency through the Chandra band.

The total 0.5-8 keV count-rate inferred from the two observations is
(8.0+/-0.9)e-4 cts/s.  The X-ray source shows significant fading compared
to the previous epoch at ~160 days (inferred rate of (1.50 +\-0.12)e-3
cts/s, Margutti et al., 2018). Applying a simple binomial test, we find
that the detected fading is significant at a level > 4 sigma. We can thus
reject the hypothesis of a random fluctuation of the X-ray count-rate, and
conclude that these Chandra observations provide the first statistically
significant evidence for a decaying X-ray emission from GW170817.

We thank the entire Chandra team for making these observations possible."