Chandra X-ray observations of GW170817 at 1 year post-merger: increasingly rapid fading
ATel #11945; Daryl Haggard, Melania Nynka, J. J. Ruan, (McGill/MSI)
on 13 Aug 2018; 01:28 UT
Credential Certification: John Ruan (jjr2125@gmail.com)
Subjects: X-ray, Gamma-Ray Burst, Gravitational Waves, Neutron Star, Transient
We report 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.