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 \times 10 ^ { -15 } erg s ^ { -1 } cm ^ { -2 } at 16 days to 15.8 \times 10 ^ { -15 } 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/GRB170817A .