Brightening X-ray Emission from GW170817/GRB170817A: Further Evidence for an Outflow

Kavli Affiliate: Melania Nynka

| First 5 Authors: John J. Ruan, Melania Nynka, Daryl Haggard, Vicky Kalogera, Phil Evans

| Summary:

The origin of the X-ray emission from neutron star coalescence
GW170817/GRB170817A 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/GRB170817A has not been possible since a previous detection at 16 days
post-burst. We present new, deep Chandra observations of GW170817/GRB170817A 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 3.6 x 10^-15 erg/s/cm^2 at
16 days to 15.8 x 10^-15 erg/s/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/GRB170817A.

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