GRB 991216 joins the jet set: Discovery and monitoring of its optical afterglow

J. P. Halpern, R. Uglesich, N. Mirabal, S. Kassin, J. Thorstensen, W. C. Keel, A. Diercks, J. S. Bloom, F. Harrison, J. Mattox, M. Eracleous

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The optical light curve of the energetic γ-ray burst GRB 991216 is consistent with jetlike behavior in which a power-law decay steepens from t-1.22±0.04 at early times to t'1-53*0-05 in a gradual transition at around 2 days. The derivation of the late-time decay slope takes into account the constant contribution of a host or intervening galaxy, which was measured 110 days after the event at R = 24.56 ± 0.14, although the light curve deviates from a single power law whether or not a constant term is included. The early-time spectral energy distribution of the afterglow can be described as Fv ∝ v-0.74±0.05 or flatter between optical and X-ray, which, together with the slow initial decay, is characteristic of standard adiabatic evolution in a uniformly dense medium. Assuming that a reported absorption-line redshift of 1.02 is correct, the apparent isotropic energy of 6.7 × 1053 ergs is reduced by a factor of ≈ 200 in the jet model, and the initial half-opening angle is ≈6°. GRB 991216 is the third good example of a jetlike afterglow (following GRB 990123 and GRB 990510), supporting a trend in which the apparently most energetic γ-ray events have the narrowest collimation and a uniform interstellar medium environment. This, plus the absence of evidence for supernovae associated with jetlike afterglows, suggests that these events may originate from a progenitor in which angular momentum plays an important role but a massive stellar envelope or wind does not, e.g., in the coalescence of a compact binary.

Original languageEnglish (US)
Pages (from-to)697-703
Number of pages7
JournalAstrophysical Journal
Issue number2 PART 1
StatePublished - Nov 10 2000

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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