The broadband counterpart of the short GRB 200522A at z = 0.5536: a luminous kilonova or a collimated outflow with a reverse shock?

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Authors

  • W. Fong
  • T. Laskar
  • J. Rastinejad
  • A. Rouco Escorial
  • G. Schroeder
  • J. Barnes
  • C. D. Kilpatrick
  • K. Paterson
  • E. Berger
  • B. D. Metzger
  • Y. Dong
  • A. E. Nugent
  • R. Strausbaugh
  • P. K. Blanchard
  • A. Goyal
  • A. Cucchiara
  • G. Terreran
  • K. D. Alexander
  • T. Eftekhari
  • C. Fryer
  • B. Margalit
  • R. Margutti

Colleges, School and Institutes

Abstract

We present the discovery of the radio afterglow and near-infrared (NIR) counterpart of the Swift short gamma-ray burst (GRB) GRB 200522A, located at a small projected offset of ≈1 kpc from the center of a young, star-forming host galaxy at z = 0.5536. The radio and X-ray luminosities of the afterglow are consistent with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by our Hubble Space Telescope observations at a rest-frame time of ≈2.3 days, has a luminosity of ≈(1.3–1.7) × 1042 erg s−1. This is substantially lower than on-axis short GRB afterglow detections but is a factor of ≈8–17 more luminous than the kilonova of GW170817 and significantly more luminous than any kilonova candidate for which comparable observations exist. The combination of the counterpart's color (i − y = −0.08 ± 0.21; rest frame) and luminosity cannot be explained by standard radioactive heating alone. We present two scenarios to interpret the broadband behavior of GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially boosted by magnetar energy deposition), or forward and reverse shocks from a ≈14°, relativistic (Γ≳ 10) jet. Models that include a combination of enhanced radioactive heating rates, low-lanthanide mass fractions, or additional sources of heating from late-time central engine activity may provide viable alternate explanations. If a stable magnetar was indeed produced in GRB 200522A, we predict that late-time radio emission will be detectable starting ≈0.3–6 yr after the burst for a deposited energy of ≈1053 erg. Counterparts of similar luminosity to GRB 200522A associated with gravitational wave events will be detectable with current optical searches to ≈250 Mpc.

Details

Original languageEnglish
Article number127
Number of pages26
JournalThe Astrophysical Journal
Volume906
Issue number2
Publication statusPublished - 15 Jan 2021