For the first ∼3 yrs 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 cm-3. We report on observational evidence for an excess of X-ray emission at δt > 900 days after the merger. With Lx ≈ 5 × 1038 erg s-1 at 1234 days, the recently detected X-ray emission represents a ≥3.2σ (Gaussian equivalent) deviation from the universal post-jet-break model that best fits the multiwavelength afterglow at earlier times. In the context of JetFit afterglow models, current data represent a departure with statistical significance ≥3.1σ, depending on the fireball collimation, with the most realistic models showing excesses at the level of ≥3.7σ. A lack of detectable 3 GHz radio emission suggests a harder broadband 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 binary neutron star (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.
Bibliographical noteFunding Information:
We thank the referees for their constructive input on the earlier draft of the manuscript. A.H. is partially supported by a Future Investigators in NASA Earth and Space Science and Technology (FINESST) award No. 80NSSC19K1422. This research was supported in part by the National Science Foundation under grant No. AST-1909796 and AST-1944985, by NASA through Chandra Award No. G09-20058A, and through Space Telescope Science Institute program No. 15606. K.D.A. is supported by NASA through NASA Hubble Fellowship grant No. HST-HF2-51403.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. B.D.M. is supported by NSF grant AST-2002577 and NASA grants 80NSSC20K0909 and NNX17AK43G. A.K. acknowledges support from the Gordon and Betty Moore Foundation through grant GBMF5076. D.R. acknowledges support from the U.S. Department of Energy, Office of Science, Division of Nuclear Physics under award Nos. DE-SC0021177 and from the National Science Foundation under grant No. PHY-2011725. S.B. acknowledges support by the EU H2020 under ERC Starting grant No. BinGraSp-714626. L.S. acknowledges support from the Sloan Fellowship, the Cottrell Scholars Award, NASA 80NSSC18K1104 and NSF PHY-1903412. I.H. acknowledges support from the UK Science and Technology Facilities Council [ST/N000919/1] and the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation (NRF), an agency of the Department of Science and Innovation. B.M. is supported by NASA through NASA Hubble Fellowship grant No. HST-HF2-51412.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS5-26555. R.B.D. acknowledges support from National Science Foundation (NSF) under grant 1816694 and 2107932. V.A.V. is supported by the Simons Foundation through a Simons Junior Fellowship (#718240). M.N. is supported by a Royal Astronomical Society Research Fellowship and by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 948381). The Berger Time Domain group at Harvard is supported in part by NSF and NASA grants, as well as by the NSF under Cooperative Agreement PHY-2019786 (The NSFAI Institute for Artificial Intelligence and Fundamental Interactions http://iaifi.org/ ).
The scientific results reported in this article are based to a significant degree on observations made by the Chandra X-ray Observatory, and the data obtained from the Chandra Data Archive. Partial support for this work was provided by the National Aeronautics and Space Administration through Chandra Award No. GO1-22075X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.
© 2022. The Author(s). Published by the American Astronomical Society.
- Gamma-ray bursts
- Neutron stars
- Gravitational wave sources
- X-ray transient sources
- Radio transient sources