We report Chandra observations of GW170817 , the first neutron star–neutron star merger discovered by the joint LIGO-Virgo Collaboration , and the first direct detection of gravitational radiation associated with an electromagnetic counterpart , Fermi short \gamma -ray burst GRB 170817A .
The event occurred on 2017 August 17 and subsequent observations identified an optical counterpart , SSS17a , coincident with NGC 4993 ( \sim 10 ^ { \prime \prime } separation ) .
Early Chandra ( \Delta t \sim 2 days ) and Swift ( \Delta t \sim 1 - 3 days ) observations yielded non-detections at the optical position , but \sim 9 days post-trigger Chandra monitoring revealed an X-ray point source coincident with SSS17a .
We present two deep Chandra observations totaling \sim 95 ks , collected on 2017 September 01–02 ( \Delta t \sim 15 - 16 days ) .
We detect X-ray emission from SSS17a with L _ { 0.3 - 10 { keV } } = 2.6 ^ { +0.5 } _ { -0.4 } \times 10 ^ { 38 } erg s ^ { -1 } , and a power law spectrum of \Gamma = 2.4 \pm 0.8 .
We find that the X-ray light curve from a binary NS coalescence associated with this source is consistent with the afterglow from an off-axis short \gamma -ray burst , with a jet angled \gtrsim 23 ^ { \circ } from the line of sight .
This event marks both the first electromagnetic counterpart to a LIGO-Virgo gravitational-wave source and the first identification of an off-axis short GRB .
We also confirm extended X-ray emission from NGC 4993 ( L _ { 0.3 - 10 { keV } } \sim 9 \times 10 ^ { 38 } erg s ^ { -1 } ) consistent with its E/S0 galaxy classification , and report two new Chandra point sources in this field , CXOU J130948 and CXOU J130946 .