[vsnet-grb-info 20278] GW170817/GRB170817A: Updated results from the full Chandra dataset

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22374
SUBJECT: GW170817/GRB170817A: Updated results from the full Chandra dataset
DATE:    18/01/30 13:00:12 GMT
FROM:    Eleonora Troja at GSFC  <eleonora.troja at nasa.gov>

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

We analyzed the full set of five observations of GW170817
performed by the Chandra X-ray Observatory between January 17th
and January 28th, 2018, i.e. ~153 and ~164 days after the merger.
A log of observations is reported below:

ObsID   Exposure [ks]     0.5-8.0 keV count rate [cts/s]

20936     31.75           0.0018 +/- 0.0002
20937     20.77           0.0014 +/- 0.0003
20938     15.86           0.0019 +/- 0.0003
20939     22.25           0.0011 +/- 0.0002
20945     14.22           0.0010 +/- 0.0003

The average net count-rate is 0.00148 +/- 0.00011 cts/s,
consistent with the value of 0.00145 +/- 0.00014 observed
at 110 days (Troja et al. 2018, arXiv:1801.06516).
The average spectrum, obtained by coadding the five exposures,
is well described by an absorbed power-law model with
N_H=7.5E20 cm^-2 and photon index Gamma=1.65+/-0.16 (68% c.l.),
consistent with the value derived from the broadband spectrum
at earlier times (Troja et al. 2018, arXiv:1801.06516).
Based on this new analysis, we estimate an unabsorbed X-ray flux
of (2.6 +/- 0.3)E-14 erg/cm2/s (68% c.l.) in the 0.3-10 keV band,
consistent with the X-ray flux measured at 110 days.

Our results do not support the claim of a decreasing X-ray flux,
as suggested by D'Avanzo et al. (2018, arXiv:1801.06164), and
are consistent either with a slowly rising afterglow or a slow
turn-over of the X-ray light curve expected when the afterglow
reaches its peak (e.g. Lazzati et al. 2017, arXiv:1712.03237;
Troja et al. 2018, arXiv:1801.06516).

We note that the X-ray afterglow displays a marginal level of
variability on timescales of a few days, being the count-rate
from the last two exposures (20939,20945) consistently lower.
The spectrum from these two observations is characterized by a
photon index Gamma= 1.9 +/- 0.3 (68% c. l.), slightly softer
than the value measured in the first three exposures (20936,20937,
and 20938) Gamma = 1.59+/-0.17  (68% c. l.), yet consistent within
the large uncertainties. The lower count-rate and soft spectral
shape could be indicative of the cooling frequency entering the
X-ray band, although the limited statistics prevent us to draw
any firm conclusion.