[vsnet-grb-info 25311] GW170817: Continued X-ray emission detected with Chandra at 940 days post-merger

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27411
SUBJECT: GW170817: Continued X-ray emission detected with Chandra at 940 days post-merger
DATE:    20/03/18 17:59:10 GMT
FROM:    Eleonora Troja at NASA/GSFC/UMD  <eleonora at umd.edu>

E. Troja (UMD/NASA/GSFC), L. Piro (INAF/IAPS), G. Ryan (UMD), H. van Eerten
(Bath U.), B. Zhang (UNLV) ob behalf of a larger collaboration

The Chandra X-ray Observatory re-observed the field of GW170817 between
March 9th and March 15th, 2020,and performed four short exposures
observations (ObsIDs: 21323, 23183, 23184, 23185)
for a total of 96 ks, as part of its on-going monitoring program. At the
afterglow position, an X-ray source is detected with significance >4.5
sigma
at an average count rate of (8+/-0.4)e-05 cts/s in the 0.5-8.0 keV energy
band. By using an absorbed power-law model with absorption column fixed at
the Galactic value of 7.5E20 cm^-2 and a photon index Gamma=1.585, as
derived from our broadband analysis, we estimate an unabsorbed X-ray flux
of (1.1 +/- 0.6)E-15 erg/cm2/s in the 0.3-10 keV energy band. The quoted
error is at the 68% confidence level.
This new measurement is higher than earlier predictions of a structured
jet, based on a full year of data (Troja E., et al., 2019, MNRAS, 489,
1919),
yet still consistent with this model within the uncertainties.

Additional components of emission, such as continued energy injection from
a long-lived neutron star (Piro L., et al., 2019, MNRAS, 483, 1912) or
afterglow from the sub-relativistic ejecta (Kathirgamaraju A., Giannios D.,
Beniamini P., 2019, MNRAS, 487, 3914), cannot be excluded at this time.

Further analysis is on-going.