[vsnet-grb-info 26952] Chandra observations of GW170817 at ~3.3 years since merger (Full report)

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29055
SUBJECT: Chandra observations of GW170817 at ~3.3 years since merger (Full report)
DATE:    20/12/16 13:28:30 GMT
FROM:    Aprajita Hajela at Northwestern U  <AprajitaHajela2015 at u.northwestern.edu>

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

We report on the complete Chandra data set of observations of GW170817
acquired at t=1209-1213 days since merger (IDs 22677, 24887, 24888, and
24889; PI Margutti; program 21510449) for a total exposure time of ~91 ks.
Preliminary results from a subset of these observations have been reported
by Hajela et al., GCN 29041, GCN 29019; Troja et al., GCN 29038.

We confirm the presence of significant X-ray emission from GW170817
(significance of detection > 5 sigma, Gaussian equivalent) from the merged
observations, with a source count rate of (0.9 +/- 0.3) e-4 cps (0.5-8
keV). From a joint spectral fit with a simple power-law spectral 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), we estimate an
unabsorbed flux of 3.46(+1.06 -1.31) e-15 erg/cm2/s (0.3-10 keV, 1-sigma
uncertainties), corresponding to a luminosity of 6.9 (+2.1 -2.6) e+38 erg/s
at the distance of 40.7 Mpc (Cantiello et al., 2018). The observed emission
is a factor of ~2 brighter than the first X-ray non-detection of GW170817
at ~2 days after merger.

As previously reported in Hajela et al., GCN 29041, the inferred flux is in
excess to the extrapolation of the off-axis jet model that best fits the
broad-band afterglow observations at t<580 days (e. g., Hajela et al. 2019)
and might represent the emergence of a new component of emission. An
updated plot of the X-ray evolution of GW170817 can be found here:
https://sites.google.com/u.northwestern.edu/synchrotron-emission-from-gw17/home


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

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