Discovery of a radio flare from GRB 990123

S. R. Kulkarni*, D. A. Frail, R. Sari, G. H. Moriarty-Schieven, D. S. Shepherd, P. Udomprasert, A. C.S. Readhead, J. S. Bloom, M. Feroci, E. Costa

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

107 Scopus citations


We report the discovery of a radio counterpart to GRB 990123. In contrast to previous well-studied radio afterglows that rise to peak flux on a timescale of a week and then decay over several weeks to months, the radio emission from this gamma-ray burst (GRB) was clearly detected 1 day after the burst, after which it rapidly faded away. The simplest interpretation of this "radio flare" is that it arises from the reverse shock. In the framework of the afterglow models discussed to date, a forward-shock origin for the flare is ruled out by our data. However, at late times, some radio afterglow emission (commensurate with the observed late-time optical emission and the optical afterglow) is expected from the forward shock. The relative faintness of the observed late-time radio emission provides an independent indication of a jetlike geometry in this GRB. We use the same radio observations to constrain two key parameters of the forward shock (the peak flux and peak frequency) to within a factor of 2. These values are inconsistent with the notion advocated by several authors that the prompt optical emission detected by the Robotic Optical Transient Search Experiment smoothly joins the optical afterglow emission. Finally, in hindsight, we now recognize another such radio flare, and this suggests that one out of eight GRBs has a detectable radio flare. This abundance, coupled with the reverse-shock interpretation, suggests that the radio flare phenomenon has the potential to shed new light on the physics of reverse shocks in GRBs.

Original languageAmerican English
Pages (from-to)L97-L100
JournalAstrophysical Journal
Issue number2 PART 2
StatePublished - 10 Sep 1999
Externally publishedYes

Bibliographical note

Funding Information:
8The JCMT is operated by the Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the UK, the Netherlands Organization for Scientific Research, and the National Research Council of Canada.

Funding Information:
1 California Institute of Technology, Owens Valley Radio Observatory, MS 105-24, Pasadena, CA 91125. 2National Radio Astronomy Observatory, P.O. Box O, 1003 Lopezville Road, Socorro, NM 87801. 3 California Institute of Technology, Theoretical Astrophysics, MS 103-33, Pasadena, CA 91125. 4Joint Astronomy Centre, 660 North A’ohoku Place, University Park, HI 96720. 5 Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V8X 4M6, Canada. 6Istituto di Astrofisica Spaziale CNR, via Fosso del Cavaliere, Roma, I-00133, Italy. 7 The VLA is a facility of the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc., under contract with the National Science Foundation.


  • Gamma rays: Bursts
  • Radio continuum: General
  • Shock waves


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