2021/04/05 by A. Hajela, R. Margutti, J. S. Bright +34 · 1 voice
Physics and Astronomy · #astro-ph.HE
paper · pdf · doi:10.3847/2041-8213/ac504a
30 pages, 13 figures, Accepted for publication in ApJL
arxiv published 2021/04/05 · arxiv created 2022/03/05 · arxiv updated 2022/03/30
For the first ∼3 years after the binary neutron star merger event GW 170817 the radio and X-ray radiation has been dominated by emission from a structured relativistic off-axis jet propagating into a low-density medium with n < 0.01 \rmcm-3. We report on observational evidence for an excess of X-ray emission at δt>900 days after the merger. With Lx≈5× 1038 \rmerg s-1 at 1234 days, the recently detected X-ray emission represents a ≥ 3.2 σ (Gaussian equivalent) deviation from the universal post jet-break model that best fits the multi-wavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance ≥ 3.1 σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥ 3.7 σ. A lack of detectable 3 GHz radio emission suggests a harder broad-band spectrum than the jet afterglow. These properties are consistent with the emergence of a new emission component such as synchrotron radiation from a mildly relativistic shock generated by the expanding merger ejecta, i.e. a kilonova afterglow. In this context, we present a set of ab-initio numerical-relativity BNS merger simulations that show that an X-ray excess supports the presence of a high-velocity tail in the merger ejecta, and argues against the prompt collapse of the merger remnant into a black hole. Radiation from accretion processes on the compact-object remnant represents a viable alternative. Neither a kilonova afterglow nor accretion-powered emission have been observed before, as detections of BNS mergers at this phase of evolution are unprecedented.