ALMA and GMRT Constraints on the Off-axis Gamma-Ray Burst 170817A from the Binary Neutron Star Merger GW170817

S. Kim, S. Schulze, L. Resmi, J. González-López, A. B. Higgins, C. H. Ishwara-Chandra, F. E. Bauer, I. De Gregorio-Monsalvo, M. De Pasquale, A. De Ugarte Postigo, D. A. Kann, S. Martín, S. R. Oates, R. L.C. Starling, N. R. Tanvir, J. Buchner, S. Campana, Z. Cano, S. Covino, A. S. FruchterJ. P.U. Fynbo, D. H. Hartmann, J. Hjorth, P. Jakobsson, A. J. Levan, D. Malesani, M. J. Michałowski, B. Milvang-Jensen, K. Misra, P. T. O'Brien, R. Sánchez-Ramírez, C. C. Thöne, D. J. Watson, K. Wiersema

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35 Citations (Scopus)


Binary neutron-star mergers (BNSMs) are among the most readily detectable gravitational-wave (GW) sources with the Laser Interferometer Gravitational-wave Observatory (LIGO). They are also thought to produce short γ-ray bursts (SGRBs) and kilonovae that are powered by r-process nuclei. Detecting these phenomena simultaneously would provide an unprecedented view of the physics during and after the merger of two compact objects. Such a Rosetta Stone event was detected by LIGO/Virgo on 2017 August 17 at a distance of ∼44 Mpc. We monitored the position of the BNSM with Atacama Large Millimeter/submillimeter Array (ALMA) at 338.5 GHz and the Giant Metrewave Radio Telescope (GMRT) at 1.4 GHz, from 1.4 to 44 days after the merger. Our observations rule out any afterglow more luminous than 3 × 1026 erg s-1 Hz-1 in these bands, probing >2-4 dex fainter than previous SGRB limits. We match these limits, in conjunction with public data announcing the appearance of X-ray and radio emission in the weeks after the GW event, to templates of off-axis afterglows. Our broadband modeling suggests that GW170817 was accompanied by an SGRB and that the γ-ray burst (GRB) jet, powered by EAG,iso ∼ 1050erg, had a half-opening angle of ∼20°, and was misaligned by ∼40° from our line of sight. The data are also consistent with a more collimated jet: EAG,iso ∼ 1051 erg, θ1/2, jet ∼ 5°, θobs ∼ 17°. This is the most conclusive detection of an off-axis GRB afterglow and the first associated with a BNSM-GW event to date. We use the viewing angle estimates to infer the initial bulk Lorentz factor and true energy release of the burst.

Original languageEnglish
Article numberL21
JournalAstrophysical Journal Letters
Issue number2
Publication statusPublished - 1 Dec 2017

Bibliographical note

Funding Information:
We thank the referee for a careful reading of the manuscript and for helpful comments that improved this paper. We gratefully acknowledge support from: FONDECYT grant 3130488 (SK), FONDECYT grant 3160439 (J.B.), We observed the position of AT2017gfo with ALMA and GMRT at 338.5 and 1.4 GHz, respectively, from 1.4 days to 44 days after the merger, our objective being to constrain the GRB afterglow component. The afterglow evaded detection at all epochs. Our radio and sub-mm observations allow us to place a CONICYT grants Basal-CATA PFB-06/2007 (S.K., F.E.B., J.B.), FONDECYT Regular 1141218 (F.E.B., J.G.-L.), and Programa de Astronomia FONDO ALMA 2016 31160033 (J.G.-L.); the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS (F.E.B., J.B.); the Science and Technology Facilities Council (A.B.H., R.L.C.S.); the Spanish research project AYA 2014-58381-P (C.C.T., A.d.U.P., D.A.K.); the Ramón y Cajal fellowship RyC-2012-09975 (A.d.U.P.); the Ramón y Cajal fellowship RyC-2012-09984 (C.C.T.); the 2016 BBVA Foundation Grant for Researchers and Cultural Creators (A.d.U.P.); the Juan de la Cierva Incorporación fellowship IJCI-2015-26153 (D.A.K.); a VILLUM FONDEN Investigator grant (project number 16599; J.H.); the National Science Centre, Poland through the POLONEZ grant 2015/19/P/ST9/ 04010 (M.J.M.); the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 665778 (M.J.M.); the Leverhulme Trust Early Career Fellowship (S.R.O.); S.C. acknowledge partial funding from ASI-INAF grant I/004/11/3; R.S.-R. acknowledges support from ASI (Italian Space Agency) through the contract No. 2015-046-R.0 and from European Union Horizon 2020 Programme under the AHEAD project (grant agreement No. 654215).

Funding Information:
Development of the Boxfit code was supported in part by NASA through grant NNX10AF62G issued through the Astrophysics Theory Program and by the NSF through grant AST-1009863. Simulations for BOXFIT version 2 have been carried out in part on the computing facilities of the Computational Center for Particle and Astrophysics (C2PAP) of the research cooperation “Excellence Cluster Universe” in Garching, Germany. Facilities: ALMA, GMRT. Software: AIPS, Boxfit, CASA, MultiNest, PyMultiNest.

Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.

Other keywords

  • gamma-ray burst: individual (GRB 170817A)
  • gravitational waves


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