TITLE:   GCN CIRCULAR
NUMBER:  32065
SUBJECT: Continued Chandra monitoring of GW170817 at ~4.8 yr post-merger
DATE:    22/05/18 19:48:10 GMT
FROM:    Brendan O'Connor at UMD  <oconnorb@umd.edu>

B. O'Connor (GWU/UMD), E. Troja (Tor Vergata),
on behalf of a larger collaboration:

The Chandra X-ray Observatory (CXO) carried out observations of GW170817
starting at 1671 d post-merger (ObsIDs: 25733, 25734; PI: Troja) for 59.4
ks,
an additional epoch was performed starting at 1734 d (ObsID: 25527) for
36.6 ks.
The data were re-processed and the astrometry corrected following Troja et
al.
2020, MNRAS, 4, 5643. In the 0.5-8 keV energy range we measure 2 counts
within
a 1" aperture centered on the X-ray position. A background contribution
of about 0.85 counts was estimated from adjacent source-free regions.

Following Kraft, Burrows, and Nousek (1991), we derive a 95% upper limit of
<5.5 source counts, yielding a rate of <6E-5 cts/s in the combined 95.8 ks.
This corresponds to a 0.3-10 keV flux limit of 1.3E-15 erg/cm2/s for a
power-law with photon index of 1.585. This limit is comparable to the value
measured at 1234 d (1.6E-15 erg/cm2/s; Troja et al. 2022, MNRAS, 510, 1902),
and does not show evidence for a rebrightening of the X-ray emission.

By combining this new epoch with the CXO observations acquired in
December 2021 at 1575 d (Hajela et al. 2021, GCN 31231), we obtain
a weak (3-sigma) detection with an average count-rate of 3E-05 cts/s,
and a 0.3-10 keV flux of ~0.6E-15 erg/cm2/s.

An updated light curve can be found at this link:
https://nora.users.roma2.infn.it/GW170817_May2022.jpg

We thank the Chandra staff for scheduling these observations.