The bright gamma-ray burst 991208: Tight constraints on afterglow models from observations of the early-time radio evolution

T. J. Galama*, M. Bremer, F. Bertoldi, K. M. Menten, U. Lisenfeld, D. S. Shepherd, B. Mason, F. Walter, G. G. Pooley, D. A. Frail, R. Sari, S. R. Kulkarni, E. Berger, J. S. Bloom, A. J. Castro-Tirado, J. Granot

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

25 Scopus citations


The millimeter wavelength emission from GRB 991208 is the second brightest ever detected, yielding a unique data set. We present here well-sampled spectra and light curves over more than two decades in frequency for a 2 week period. This data set has allowed us for the first time to trace the evolution of the characteristic synchrotron self-absorption frequency νa, peak frequency νm, and the peak flux density Fm; we obtain νa ∝ t-0.15±0.23, νm ∝ t-1.7±0.7, and Fm ∝ t-0.47±0.20. From the radio data we find that models of homogeneous or wind-generated ambient media with a spherically symmetric outflow can be ruled out. A model in which the relativistic outflow is collimated (a jet) can account for the observed evolution of the synchrotron parameters, the rapid decay at optical wavelengths, and the observed radio-to-optical spectral flux distributions that we present here, provided that the jet transition has not been fully completed in the first 2 weeks after the event. These observations provide additional evidence that rapidly decaying optical/X-ray afterglows are due to jets and that such transitions either develop very slowly or perhaps never reach the predicted asymptotic decay F(t) ∝ t-P.

Original languageAmerican English
Pages (from-to)L45-L49
JournalAstrophysical Journal
Issue number2 PART 2
StatePublished - 1 Oct 2000
Externally publishedYes

Bibliographical note

Funding Information:
11 The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.


  • Cosmology: observations
  • Gamma rays: bursts
  • Radio continuum: general


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