Skip to main content
New Swift-BAT/GUANO and IceCube Notice Types Available! See news and announcements

GCN Circular 22206

LIGO/Virgo G298048: Brightening X-ray Emission from GW170817/GRB170817A/SSS17a
2017-12-07T22:11:17Z (6 years ago)
Phil Evans at U of Leicester <>
Daryl Haggard, John J. Ruan, Melania Nynka (McGill/MSI), Vicky Kalogera (Northwestern/CIERA), and Phil Evans (Leicester) report:

We have performed a detailed analysis of new Chandra X-ray observations of GW170817 obtained via Chandra Director's Discretionary Time (PI: Wilkes, Program Number 18408601). The X-ray counterpart to GW170817/GRB170817A/SSS17a is clearly detected in the new observations.

This program acquired two exposures of GRB170817A: (1) a 74.09 ks exposure (ObsID 20860) beginning at 2017 December 2.08 UT, approximately 108 days post-burst, and (2) a 24.74 ks exposure (ObsID 20861) beginning at 2017 December 6.45 UT, approximately 111 days post-burst. Since the two new exposures are close in time and the X-ray emission of GRB170817A is not expected to vary significantly over ~4 day timescales, we co-add the two data sets into one 98.83 ks exposure at 109.2 days post-burst.

We perform spectral extractions assuming an absorbed power-law spectral model with fixed NH = 7.5e20 cm^���2 and find that the X-ray flux of GRB170817A has an absorbed flux of f(0.3���8 keV) = 1.58e���14 +/- 0.14 erg s^���1 cm^���2  (Gamma = 1.62 +/- 0.27) at 109.2 days post-burst, which corresponds to an unabsorbed luminosity of L(0.3���10 keV) = 42.5e38 +/- 3.7 erg s^���1 (see also Troja et al. GCN 22201 and Margutti et al. GCN 22203). This represents significant X-ray brightening compared to Chandra observations at 15.6 days post-burst, for which we find an absorbed flux of f(0.3���8 keV) = 0.36e���14 +/- 0.1 erg s^���1 cm^���2 (Gamma = 2.4 +/- 0.8) and an unabsorbed luminosity of L(0.3���10 keV) = 10.4e38 +/- 2.0 erg s^���1 (Haggard et al. 2017).

We also examine the three previously-detected X-ray sources CXOU J130948, CXOU 130946, and the host galaxy NGC 4993. The fluxes of CXOU 130946 and the host-galaxy NGC 4993 are consistent with our previous deep Chandra observations, while CXOU J130948 appears to be variable in X-rays (Margutti et al. 2017; Haggard et al. 2017).

The origin of the X-ray emission from the NS-NS coalescence GW170817/GRB170817A is an important diagnostic for all post-merger interpretations, and different scenarios predict distinct evolution in its X-ray light curve. These observations support scenarios in which the X-ray and radio emission share a common origin, i.e., the X-ray light curve is consistent with outflow models which may be either a cocoon shocked by the jet or dynamical ejecta from the merger. Further deep X-ray monitoring can place powerful constraints on the physical parameters of these models. The X-ray brightening strengthens the argument that simple top-hat jet models are not consistent with the latest observations. However, more advanced models of structured jets with off-axis viewing angles should be pursued and cannot yet be ruled out.

We thank Belinda Wilkes and the Chandra scheduling, data processing, and archive teams for making these observations possible.
Looking for U.S. government information and services? Visit