A Deep Chandra X-Ray Study of Neutron Star Coalescence GW170817

<jats:title>Abstract</jats:title> <jats:p>We report <jats:italic>Chandra</jats:italic> 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, <jats:italic>Fermi</jats:italic> short <jats:italic>γ</jats:italic>-ray burst GRB 170817A. The event occurred on 2017 August 17 and subsequent observations identified an optical counterpart, SSS17a, coincident with NGC 4993 (∼10″ separation). Early <jats:italic>Chandra</jats:italic> (<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlaa8edeieqn1.gif" xlink:type="simple"/> </jats:inline-formula> days) and <jats:italic>Swift</jats:italic> (<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlaa8edeieqn2.gif" xlink:type="simple"/> </jats:inline-formula> days) observations yielded non-detections at the optical position, but ∼9 days post-trigger <jats:italic>Chandra</jats:italic> monitoring revealed an X-ray point source coincident with SSS17a. We present two deep <jats:italic>Chandra</jats:italic> observations totaling ∼95 ks, collected on 2017 September 01–02 (<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlaa8edeieqn3.gif" xlink:type="simple"/> </jats:inline-formula> days). We detect X-ray emission from SSS17a with <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlaa8edeieqn4.gif" xlink:type="simple"/> </jats:inline-formula> erg s<jats:sup>−1</jats:sup>, and a power law spectrum of <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlaa8edeieqn5.gif" xlink:type="simple"/> </jats:inline-formula>. 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 <jats:italic>γ</jats:italic>-ray burst, with a jet angled ≳23° 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 (<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjlaa8edeieqn6.gif" xlink:type="simple"/> </jats:inline-formula> erg s<jats:sup>−1</jats:sup>) consistent with its E/S0 galaxy classification, and report two new <jats:italic>Chandra</jats:italic> point sources in this field, CXOU J130948 and CXOU J130946.</jats:p>