TITLE:   GCN CIRCULAR
NUMBER:  29041
SUBJECT: Potential X-ray KN afterglow from Chandra observations of GW170817 ~3.3 years since merger (3 Epochs of 4)
DATE:    20/12/14 17:55:09 GMT
FROM:    Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>

A. Hajela, R. Margutti, K. D. Alexander,  W. Fong  (Northwestern U.), T.
Laskar (Bath), D. L. Coppejans (Northwestern U.), V. A. Villar (Columbia
U.), D. Giannios (Purdue U.), T. Eftekhari (Harvard U.), L. Sironi
(Columbia), B. Margalit (Berkeley) report:

We observed GW170817 with the Chandra X-ray Observatory (CXO) at t ~
1209-1212 days (~3.3 years) after the binary neutron star merger. We report
here the preliminary results from the first three epochs acquired so far,
for a total exposure time of ~ 64 ks (out of a total of 100 ks requested;
we will report on the results from the complete data set in the next few
days). The observations were taken in three distinct exposures of ~30, ~18,
and ~17 ks, corresponding to ObsID 22677, 24888, and 24889, respectively (PI
Margutti; program 21510449). Preliminary results on these data have been
reported in Troja et al. (GCN 29038).


We find evidence for statistically significant X-ray emission at the
location of GW170817 in the merged file with a statistical significance of
>5 sigma (Gaussian equivalent) corresponding to a net count-rate of
(1.2+\- 0.4)e-04 (0.5 - 8 keV). We perform a joint spectral fit of all
three epochs with a simple power-law model using a photon index Gamma ~
1.6, no intrinsic absorption NHint = 0 cm-2 (as found in e.g. Hajela et al.
2019) and Galactic neutral hydrogen column density NH,gal = 7.8e+20
cm-2  (Kalberla
et al., 2005). With these spectral parameters we derive an unabsorbed flux
of 4.4 (+1.78, -1.50) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties),
corresponding to a luminosity of 8.7 (+3.5, -2.9)e+38 erg/s at the distance
of 40.7 Mpc (Cantiello et al., 2018).


These observations are consistent with a constant X-ray flux at t> 580
days, which deviates from the predictions of the off-axis jet afterglow
model (e.g. Hajela et al. 2019). The derived flux at 3.3 yrs after the
merger is in excess to the structured jet model that best described all the
observations up to t ~ 743 days after the merger and might represent the
first evidence for the emergence of the kilonova afterglow (
https://sites.google.com/u.northwestern.edu/synchrotron-emission-from-gw17/home
).


We thank the entire Chandra team for scheduling and executing these
observations.