The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. V. Rising X-Ray Emission from an Off-axis Jet

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 approximate to 2.3 days post-merger reveal no significant emission, with L-x less than or similar to 3.2 x 10(38) erg s(-1) (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L-x approximate to 9 x 10(39) erg s(-1) at approximate to 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 similar to 10(49-50) erg, viewed off-axis with theta(obs) similar to 20 degrees-40 degrees. Our models favor a circumbinary density n similar to 10(-4)-10(-2) cm(-3), depending on the value of the microphysical parameter epsilon(B) = 10(-4)-10(-2). A centralengine origin of the X-ray emission is unlikely. Future X-ray observations at t greater than or similar to 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 theta(obs) are testable with GW information on GW170817 from advanced LIGO/Virgo on the binary inclination.