TITLE:   GCN CIRCULAR
NUMBER:  23137
SUBJECT: Chandra X-ray observations of GW170817 at 1 year post-merger: increasingly rapid fading
DATE:    18/08/13 01:52:40 GMT
FROM:    Daryl Haggard at McGill U  <daryl.haggard@mcgill.ca>

Daryl Haggard, Melania Nynka, and J. J. Ruan (McGill/MSI) report:

We analyze new Chandra X-ray observations of neutron star merger 
GW170817 at 358.6 days post-merger, which now reveals fading at a t^-1.6 
rate.

Chandra obtained a 67.16 ks observation of GW170817 (ObsID: 21371, PI: 
Troja) on 10 August 2018, at 358.6 days post-merger. GW170817 is still 
clearly detected, and we measure a 0.5-8 keV count rate of 4.9 
(+0.9/-0.9) cts/s. We extract and fit the X-ray spectrum assuming an 
absorbed power-law spectral model, with fixed NH = 7.5e20 cm^-2. We 
measure an absorbed flux of f(0.3-8 keV) = 6.3 (+3.8/-2.1) x10^-15 
erg/s/cm^2 (90% c.l.) and photon index of Gamma = 1.6 (+1.3/-0.9), which 
corresponds to an unabsorbed luminosity of L(0.3-10 keV) = 1.7 
(+1.1/-0.5) x10^39 erg/s assuming a luminosity distance of 42.5 Mpc.

The previous Chandra observation at 260.0 days showed an absorbed flux 
of f(0.3-8 keV) = 1.09 (+0.24/-0.20) x10^-14 erg/s/cm^2 (Nynka et al. 
2018). A power-law fit of this previous 260.0 day flux to the new 358.6 
day flux reveals fading at a t^-1.6 rate. This is steeper than the 
t^-1.3 fading observed between 159.7 and 260.0 days post-merger, and 
thus the X-ray afterglow light curve is now fading more rapidly.