[vsnet-grb-info 21901] Further Chandra observations of GW170817 ~581-583 days since merger

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24000
SUBJECT: Further Chandra observations of GW170817 ~581-583 days since merger
DATE:    19/03/25 03:12:22 GMT
FROM:    Aprajita Hajela at Northwestern U  <AprajitaHajela2015 at u.northwestern.edu>

A. Hajela, R. Margutti, W. Fong , K. D. Alexander (Northwestern/CIERA), E.
Berger, P. Blanchard (Harvard), R. Chornock (Ohio U.), D. Coppejans
(Northwestern/CIERA), S. Gomez (Harvard), G. Hosseinzadeh  (Harvard), G.
Terreran (Northwestern/CIERA), A. Villar (Harvard) report:

“We initiated a series of  Chandra X-ray observations of GW170817 at t~581
days since merger, for a total exposure time of 100 ks. The results from
the first observation with the exposure time of ~36 ks is reported in
Hajela et al. (GCN 23987).

Here, we report on the remaining ~64ks of Chandra observations of GW170817
acquired starting on 22 March 2019 at 06:17 UT (obsID 22157, exposure time
of ~38 ks) and 23 March 2019 at 23:52 UT (obsID 22158, exposure time of ~25
ks, PIs Margutti and Fong). X-ray emission from GW170817 is detected with a
significance of >=3 sigma in each of the observations, with a total of ~6
and ~3 photons from the source in obsID 22157 and obsID 22158, respectively
(0.5-8 keV energy range).

Assuming a simple power-law spectral model with photon index Gamma = 1.6
and NHint = 0, as found in previous Chandra observations (e.g., Margutti et
al., 2018; Alexander et al., 2018), we find an unabsorbed flux of (2.7 +\-
1.2)e-15 erg/s/cm2 and (6.3+\- 3.5)e-15 erg/s/cm2 for ObsID 22157 and
22158, respectively (0.3-10 keV). The Galactic neutral hydrogen column
density in the direction of the transient is NH = 7.8e+20cm-2 (Kalberla et
al., 2005).

>From a joint spectral fit of the three epochs of data (obsID 21322, 22157,
and 22158) we find that the photon index is loosely constrained to
Gamma=1.6 +/- 0.8 and there is no evidence for intrinsic absorption.  For
Gamma=1.6, and NHint=0, the joint fit of the entire 100ks of Chandra
observations indicates an unabsorbed flux of (3.6 +/- 0.9)e-15 erg/s/cm2 (t~582
days since merger, 0.3-10 keV). We confirm that this flux measurement is
consistent with the expectations from the fast decay of the emission from
the jet (e.g., Margutti et al., 2018; Alexander et al., 2018; Wu et al.,
2018).  All fluxes reported are in the 0.3-10 keV energy range with 1-sigma
uncertainties.

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