TITLE:   GCN CIRCULAR
NUMBER:  27357
SUBJECT: Chandra observations of GW170817 ~ 2.5 years since merger (epoch 1 of 4)
DATE:    20/03/11 00:37:55 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 (Princeton) report:

We initiated a new round of observations of the NS merger GW170817 with the
Chandra X-ray Observatory (CXO) to continue following its evolution at ~
2.5 years after the merger. Four observations will be acquired between
March 9 and March 15, for a total exposure time of 100 ks. Here we report
on the first observation (ObsID 21323, PI Margutti; program 20500299),
which was acquired on March 09, 2020 at 22:04:48 UT (~935 days since
merger) with an exposure time of 24.29 ks.

X-ray emission is detected at the location of GW170817 with a significance
of ~ 3-sigma (Gaussian equivalent) and a 0.5-8 keV count-rate of  (1.2 +/-
0.7)e-04 c/s, and a total of ~ 2.8 net photons. Assuming a simple power-law
spectrum with photon index Gamma=1.6 and no intrinsic absorption, as found
in previous Chandra observations (e.g. Hajela et al. 2019, ApJL, 886L, 17H),
and following the same spectra-extraction procedure as in Hajela et al.
2019, the count-rate above translates into an unabsorbed flux of  ~ (3.2
+/- 1.9)e-15 erg/s/cm2 (0.3-10 keV). The Galactic neutral hydrogen column
density in the direction of the transient is N_H = 7.8e+20cm-2 (Kalberla et
al. 2005). This X-ray flux measurement is nominally above, yet
statistically consistent with the off-axis jet model predictions given the
large flux uncertainties. Further CXO observations comprising the remaining
exposure time of ~75 ks are scheduled in the next few days and will help
quantify the statistical evidence for the possible departure from the
off-axis jet model (which might represent the onset of the kilonova
afterglow).

An updated plot with the most recent results can be found at this link:
https://sites.google.com/u.northwestern.edu/xray-observations-of-gw170817/home


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