TITLE:   GCN CIRCULAR
NUMBER:  22372
SUBJECT: GW170817/GRB170817A: Preliminary results of Chandra monitoring
DATE:    18/01/29 20:42:49 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja@nasa.gov>

E. Troja (UMD/GSFC) and L. Piro (INAF/IAPS) report on behalf of
a larger collaboration:

The Chandra X-ray Observatory re-observed the field of GW170817
starting on January 17th, 2018, and performed five short exposures
observations as part of its on-going monitoring program (PI: Wilkes).

Only three of these exposures (ObsID: 20936, 20938, 20939) are currently
archived and available to the public. Here we report the preliminary 
findings
from these observations.

The X-ray afterglow is detected with high significance in all the exposures
at an average count rate of 0.0016 cts/s in the 0.5-8.0 keV energy band.
A preliminary inspection of the hardness ratio does not show any 
significant
spectral variation. Therefore, we perform a spectral analysis using an 
absorbed
power-law model with absorption column fixed at the Galactic value of
7.5E20 cm^-2 and a photon index Gamma=1.575 as derived from our broadband
analysis (Troja et al. 2018, arXiv:1801.06516).

We derive an unabsorbed X-ray flux of (3.2 +/- 0.3)E-14 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 the value measured by Chandra
at ~110 days (~2.5E-14 erg/cm2/s, Troja et al. 2018), and higher than the
value measured by XMM-Newton at ~135 days (D'Avanzo et al., 2018).

The latest measurement is consistent with a rising afterglow with F~t^0.8,
although, within the errors, a slow turn-over of the X-ray light curve
cannot be excluded.

Further analysis is on-going.