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GW170817/GRB170817A: Updated results from the full Chandra dataset

ATel #11245; E. Troja (UMD/GSFC) and L. Piro (INAF/IAPS) report on behalf of a larger collaboration
on 30 Jan 2018; 13:12 UT
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Credential Certification: Eleonora Troja (eleonora.troja@nasa.gov)

Subjects: X-ray, Gamma-Ray Burst, Gravitational Waves

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 0.00145 +/- 0.00014 observed at 110 days. 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.