TITLE:   GCN CIRCULAR
NUMBER:  29019
SUBJECT: Chandra observations of GW170817 at ~3.3 years since merger (Observation 1 of 4)
DATE:    20/12/09 22:38:37 GMT
FROM:    Aprajita Hajela at Northwestern U  <AprajitaHajela2015@u.northwestern.edu>

A. Hajela, R. Margutti, K. D. Alexander (Northwestern U.), T. Laskar (U. of
Bath), D. Giannios (Purdue U.), V. A. Villar (Columbia U.) report:


We observed GW170817 with the Chandra X-ray Observatory (CXO) at t =1209
days (~3.3 years) after the binary neutron star merger. Here we report the
preliminary results from the analysis of the first epoch of data acquired
so far, for a total exposure time of 29.7  ks (out of a total of ~100 ks to
be completed over the next few days) corresponding to ObsID 22677 (PI
Margutti; program 21510449).

No X-ray emission is detected at the location of GW170817 and we infer a
3-sigma 0.5-8 keV count-rate upper limit of 1.0e-4 cts/s. Assuming a simple
power-law spectral model with a photon index Gamma =1.57 and no intrinsic
absorption NHint = 0 cm-2 (as found from previous CXO observations of
GW170817, e.g., Hajela et al. 2019), the corresponding 3-sigma absorbed
flux limit is Fx<1.6e-15 erg/cm2/s (0.3-10 keV), which is Lx< 3.5e+38 erg/s
at the distance of 40.7 Mpc (Cantiello et al., 2018).  The Galactic neutral
hydrogen column density in the direction of GW170817 is NH,gal = 7.8e+20
cm-2  (Kalberla et al., 2005).  As a comparison, the off-axis jet models
that best fit the earlier multi-band evolution of the afterglow predict a
luminosity of  Lx~1.5e+38 erg/s (0.3-10 keV) at the present epoch, which is
below the sensitivity of current CXO observations.  Further Chandra
observations planned in the next few days will probe deeper fluxes.

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