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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

Hajela, A. and Margutti, R. and Bright, J. S. and Alexander, K. D. and Metzger, B. D. and Nedora, V. and Kathirgamaraju, A. and Margalit, B. and Radice, D. and Guidorzi, C. and Berger, E. and MacFadyen, A. and Giannios, D. and Chornock, R. and Heywood, I. and Sironi, L. and Gottlieb, O. and Coppejans, D. and Laskar, T. and Cendes, Y. and Barniol Duran, R. and Eftekhari, T. and Fong, W. and McDowell, A. and Nicholl, M. and Xie, X. and Zrake, J. and Bernuzzi, S. and Broekgaarden, F. S. and Kilpatrick, C. D. and Terreran, G. and Villar, V. A. and Blanchard, P. K. and Gomez, S. and Hosseinzadeh, G. and Matthews, D. J. and Rastinejad, J. C. (2022) 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. Astrophysical Journal Letters, 927 (1). Art. No. L17. ISSN 2041-8205. doi:10.3847/2041-8213/ac504a.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hajela, A.0000-0003-2349-101X
Margutti, R.0000-0003-4768-7586
Bright, J. S.0000-0002-7735-5796
Alexander, K. D.0000-0002-8297-2473
Metzger, B. D.0000-0002-4670-7509
Nedora, V.0000-0002-5196-2029
Kathirgamaraju, A.0000-0002-8560-692X
Margalit, B.0000-0001-8405-2649
Radice, D.0000-0001-6982-1008
Guidorzi, C.0000-0001-6869-0835
Berger, E.0000-0002-9392-9681
MacFadyen, A.0000-0002-0106-9013
Giannios, D.0000-0003-1503-2446
Chornock, R.0000-0002-7706-5668
Heywood, I.0000-0001-6864-5057
Sironi, L.0000-0002-1227-2754
Gottlieb, O.0000-0003-3115-2456
Coppejans, D.0000-0001-5126-6237
Laskar, T.0000-0003-1792-2338
Cendes, Y.0000-0001-7007-6295
Barniol Duran, R.0000-0002-5565-4824
Eftekhari, T.0000-0003-0307-9984
Fong, W.0000-0002-7374-935X
McDowell, A.0000-0002-7857-4798
Nicholl, M.0000-0002-2555-3192
Xie, X.0000-0002-2798-6880
Zrake, J.0000-0002-1895-6516
Bernuzzi, S.0000-0002-2334-0935
Broekgaarden, F. S.0000-0002-4421-4962
Kilpatrick, C. D.0000-0002-5740-7747
Terreran, G.0000-0003-0794-5982
Villar, V. A.0000-0002-5814-4061
Blanchard, P. K.0000-0003-0526-2248
Gomez, S.0000-0001-6395-6702
Hosseinzadeh, G.0000-0002-0832-2974
Rastinejad, J. C.0000-0002-9267-6213
Alternate Title:The emergence of a new source of X-rays from the binary neutron star merger GW170817
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 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.
Funding AgencyGrant Number
NASA Earth and Space Science Fellowship80NSSC19K1422
Space Telescope Science Institute15606
Gordon and Betty Moore FoundationGBMF5076
Department of Energy (DOE)DE-SC0021177
European Research Council (ERC)714626
Alfred P. Sloan FoundationUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Science and Technology Facilities Council (STFC)ST/N000919/1
South African Radio Astronomy Observatory (SARAO)UNSPECIFIED
National Research Foundation (South Africa)UNSPECIFIED
Department of Science and Innovation (South Africa)UNSPECIFIED
NASA Hubble FellowshipHST-HF2-51412.001-A
Simons Foundation718240
Royal Astronomical SocietyUNSPECIFIED
European Research Council (ERC)948381
Subject Keywords:Gamma-ray bursts; Neutron stars; Gravitational wave sources; X-ray transient sources; Radio transient sources
Issue or Number:1
Classification Code:Unified Astronomy Thesaurus concepts: Gamma-ray bursts (629); Neutron stars (1108); Gravitational wave sources (677); X-ray transient sources (1852); Radio transient sources (2008)
Record Number:CaltechAUTHORS:20220414-26957000
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Official Citation:A. Hajela et al 2022 ApJL 927 L17
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:114318
Deposited By: George Porter
Deposited On:15 Apr 2022 17:24
Last Modified:15 Apr 2022 17:24

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