The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. V. rising X-ray emission from an off-axis jet

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

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Abstract

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× {10}38 {erg} {{{s}}}-1 (isotropic-equivalent). Continued monitoring revealed the presence of an X-ray source that brightened with time, reaching {L}x≈ 9× {10}38 {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}k̃ {10}49-50 {erg}, viewed off-axis with {θ }{obs}̃ 20^\circ {--}40^\circ . Our models favor a circumbinary density ñ {10}-4{--}{10}-2 {{cm}}-3, depending on the value of the microphysical parameter {∊ }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....
Original languageEnglish
Article numberL20
JournalAstrophysical Journal Letters
Volume848
Issue number2
DOIs
Publication statusPublished - 16 Oct 2017

Keywords

  • stars: neutron
  • gravitational waves
  • relativistic processes

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