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arXiv · 1710.05431

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. V. Rising X-ray Emission from an Off-Axis Jet

Raffaella Margutti·E. Berger·W. Fong·C. Guidorzi·K. D. Alexander·B. D. Metzger·P. K. Blanchard·P. S. Cowperthwaite·R. Chornock·T. Efterkari·M. Nicholl·V. A. Villar·P. K. G. Williams·J. Annis·D. A. Brown·H. Y. Chen·Z. Doctor·J. A. Frieman·D. E. Holz·M. Sako·M. Soares-Santos
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Abstract

We report the discovery of rising X-ray emission from the binary neutron star (BNS) merger event GW170817. This is the first detection of X-ray emission from a gravitational-wave source. Observations acquired with the Chandra X-ray Observatory (CXO) at t~2.3 days post merger reveal no significant emission, with L_x<=3.2x10^38 erg/s (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching L_x\sim 9x10^39 erg/s 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 broad-band X-ray to radio observations are consistent with emission from a relativistic jet with kinetic energy E_k~10^49-10^50 erg, viewed off-axis with theta_obs~ 20-40 deg. Our models favor a circumbinary density n~ 0.0001-0.01 cm-3, depending on the value of the microphysical parameter epsilon_B=10^{-4}-10^{-2}. A central-engine origin of the X-ray emission is unlikely. Future X-ray observations at $t\gtrsim 100$ 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 gravitational wave information on GW170817 from Advanced LIGO/Virgo on the binary inclination.

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Raffaella Margutti, E. Berger, W. Fong, C. Guidorzi, K. D. Alexander, B. D. Metzger, P. K. Blanchard, P. S. Cowperthwaite, R. Chornock, T. Efterkari, M. Nicholl, V. A. Villar, P. K. G. Williams, J. Annis, D. A. Brown, H. Y. Chen, Z. Doctor, J. A. Frieman, D. E. Holz, M. Sako, M. Soares-Santos. 2017-10-16. The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. V. Rising X-ray Emission from an Off-Axis Jet. https://doi.org/10.3847/2041-8213%2Faa9057

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