TITLE:   GCN CIRCULAR
NUMBER:  27414
SUBJECT: Chandra observations of GW170817 at 2.5 years since merger (All epochs 4 of 4)
DATE:    20/03/19 03:15:10 GMT
FROM:    Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>

A. Hajela, R. Margutti (Northwestern U.), T. Laskar (U. Bath), K. D.
Alexander, W. Fong (Northwestern U.), A. Kathirgamaraju (UC Berkeley),  D.
Giannios (Purdue U.), D. Coppejans, G. Terreran, A. Baldeschi, K. Paterson,
M. Stroh, P. K. Blanchard (Northwestern U.), E. Berger, T. Eftekhari, G.
Hosseinzadeh, S. Gomez, V. A. Villar (Harvard U), P. K. G. Williams
(Harvard U and AAS), M. Nicholl (U. Birmingham), R. Chornock (Ohio U.), P.
S. Cowperthwaite (Carnegie Observatories), A. MacFadyen (NYU), B. D.
Metzger, L. Sironi (Columbia U.), D. Radice (PSU) report:

We carried out Chandra X-ray observations to follow the evolution of
GW170817 at t ~ 935-941 days after the neutron star merger, for a total
exposure time of ~96 ks. The observations were taken in 4 distinct
exposures of ~ 24ks, ~16ks, ~19ks, and ~36 ks, respectively  (ObsID 21323,
23183, 23184 and 23185; PI Margutti; program 20500299, for which we waived
proprietary data rights). The observations were taken on March 09, 2020
through March 15, 2020.  We reported the results from our first observation
(ObsID 21323) in Hajela et al. (GCN 27357). Here we report on the results
from our entire data set of 96ks, using the most recent data products
distributed after a data-processing issue affecting ObsID 23185 was
resolved by the CXC-DS's SAP team. An earlier report can be found in Troja
et al., GCN 27411.

X-ray emission is detected at the location of GW170817 with a significance
of ~5 sigma with a total of ~7 source photons above background in the
merged file, which translates to a 0.5-8 keV count-rate of (7.2 +/- 2.7)e-5
cts/s. We perform a joint spectral analysis of all the observations using a
simple power-law spectral model with photon index Gamma ~ 1.6 and NHint = 0
cm-2, as found from the homogeneous modeling of all previous Chandra
observations (e.g. Hajela et al. 2019, ApJL, 886L, 17H). We freeze the
galactic NH = 7.8e+20 cm-2  (Kalberla et al., 2005) and derive an
unabsorbed flux of (2.3 +/- 0.9)e-15 erg/cm2/s  (0.3-10 keV energy range)
with 1-sigma uncertainties. The flux measurement derived here is slightly
in excess to, but consistent with the predictions from updated off-axis jet
models that best fitted the multi-wavelength afterglow until t~743 days. A
similar conclusion was initially reported by Hajela et al., GCN 27357, and
later by Troja et al., GCN 27411. An updated light-curve plot is linked
here:
https://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home

These observations might represent the last X-ray detection of the jet
afterglow of GW170817. Future Chandra observations are planned to be
acquired in December 2020 as part of our program.

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