2017ApJ...848L..20M

We report the discovery of rising X-ray emission from the binary neutron star merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave (GW) source. Observations acquired with the Chandra X-ray Observatory (CXO) at t~2.3 days post-merger reveal no significant emission, with L_x 3.2×10³⁸ erg s^–1 (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L_x~9×10³⁹ erg s^–1 at ~15.1 days post-merger. We interpret these findings in the context of isotropic and collimated relativistic outflows (both on- and off-axis). We find that the broadband X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy E_k∼10^49–50 erg, viewed off-axis with θ_obs∼20^○–⁴⁰○^. Our models favor a circumbinary density n∼10^–4–10^–2 cm^–3, depending on the value of the microphysical parameter ε_B=10^–4–10^–2. A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at t 100 days, when the target will be observable again with the CXO, will provide additional constraints to solve the model degeneracies and test our predictions. Our inferences on θ_obs are testable with GW information on GW170817 from advanced LIGO/Virgo on the binary inclination.