CaltechAUTHORS
Published March 1, 2022 | Version Accepted Version + Published
Journal Article Open

Evidence for X-Ray Emission in Excess to the Jet-afterglow Decay 3.5 yr after the Binary Neutron Star Merger GW 170817: A New Emission Component

Creators

  • 1. ROR icon Northwestern University
  • 2. ROR icon University of California, Berkeley
  • 3. ROR icon Columbia University
  • 4. ROR icon Max Planck Institute for Gravitational Physics
  • 5. ROR icon University of Potsdam
  • 6. ROR icon California Institute of Technology
  • 7. ROR icon Pennsylvania State University
  • 8. ROR icon University of Ferrara
  • 9. ROR icon INFN Sezione di Ferrara
  • 10. ROR icon Harvard-Smithsonian Center for Astrophysics
  • 11. ROR icon New York University
  • 12. ROR icon Purdue University West Lafayette
  • 13. ROR icon University of Oxford
  • 14. ROR icon Rhodes University
  • 15. ROR icon South African Radio Astronomy Observatory
  • 16. ROR icon University of Warwick
  • 17. ROR icon Radboud University Nijmegen
  • 18. ROR icon California State University, Sacramento
  • 19. ROR icon University of Birmingham
  • 20. ROR icon University of Southampton
  • 21. ROR icon Clemson University
  • 22. ROR icon Friedrich Schiller University Jena
  • 23. ROR icon Las Cumbres Observatory Global Telescope Network
  • 24. ROR icon University of California, Santa Barbara
  • 25. ROR icon Space Telescope Science Institute
  • 26. ROR icon University of Arizona

Abstract

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 −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.

Additional Information

© 2022. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Received 2021 April 30; revised 2021 December 28; accepted 2022 January 25; published 2022 March 8. 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.

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Published - Hajela_2022_ApJL_927_L17.pdf

Accepted Version - 2104.02070.pdf

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Additional details

Additional titles

Alternative title
The emergence of a new source of X-rays from the binary neutron star merger GW170817

Identifiers

Eprint ID
114318
Resolver ID
CaltechAUTHORS:20220414-26957000

Related works

Describes
https://arxiv.org/abs/2104.02070 (URL)

Funding

NASA Earth and Space Science Fellowship
80NSSC19K1422
NSF
AST-1909796
NSF
AST-1944985
NASA
G09-20058A
Space Telescope Science Institute
15606
NASA
HST-HF2-51403.001-A
NASA
NAS5-26555
NSF
AST-2002577
NASA
80NSSC20K0909
NASA
NNX17AK43G
Gordon and Betty Moore Foundation
GBMF5076
Department of Energy (DOE)
DE-SC0021177
NSF
PHY-2011725
European Research Council (ERC)
714626
Alfred P. Sloan Foundation
Cottrell Scholar of Research Corporation
NASA
80NSSC18K1104
NSF
PHY-1903412
Science and Technology Facilities Council (STFC)
ST/N000919/1
South African Radio Astronomy Observatory (SARAO)
National Research Foundation (South Africa)
Department of Science and Innovation (South Africa)
NASA Hubble Fellowship
HST-HF2-51412.001-A
NSF
AST-1816694
NSF
AST-2107932
Simons Foundation
718240
Royal Astronomical Society
European Research Council (ERC)
948381
NSF
PHY-2019786

Dates

Created
2022-04-15
Created from EPrint's datestamp field
Updated
2022-04-15
Created from EPrint's last_modified field