Brightening X-Ray Emission from GW170817/GRB 170817A: Further Evidence for an Outflow

John J. Ruan, Melania Nynka, Daryl Haggard, Vicky Kalogera, Phil Evans

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Abstract

The origin of the X-ray emission from neutron star coalescence GW170817/GRB 170817A is a key diagnostic of the unsettled post-merger narrative, and different scenarios predict distinct evolution in its X-ray light curve. Due to its sky proximity to the Sun, sensitive X-ray monitoring of GW170817/GRB 170817A has not been possible since a previous detection at 16 days post-burst. We present new, deep Chandra observations of GW170817/GRB 170817A at 109 days post-burst, immediately after Sun constraints were lifted. The X-ray emission has brightened from a 0.3-8.0 keV flux of erg s-1 cm-2 at 16 days to erg s-1 cm-2 at 109 days, at a rate similar to the radio observations. This confirms that the X-ray and radio emission have a common origin. We show that the X-ray light curve is consistent with models of outflow afterglows, in which the outflow can be a cocoon shocked by the jet, dynamical ejecta from the merger, or an off-axis structured jet. Further deep X-ray monitoring can place powerful constraints on the physical parameters of these models, by both timing the passing of a synchrotron cooling break through the X-ray band and detecting the associated steepening of the X-ray photon index. Finally, the X-ray brightening strengthens the argument that simple off-axis top-hat jet models are not consistent with the latest observations of GW170817/GRB 170817A.

Original languageEnglish (US)
Article numberL4
JournalAstrophysical Journal Letters
Volume853
Issue number1
DOIs
StatePublished - Jan 20 2018

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Keywords

  • X-rays: binaries
  • gamma-ray burst: individual (GRB 170817A)
  • gravitational waves
  • stars: neutron

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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