1999/04/30 by Z. G. Dai, T. Lu · 3 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Gamma-ray bursts and supernovae #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/312127
published as Astrophys.J. 519 (1999) L155-L158 · 12 pages, LaTeX, published in ApJ Letters
openalex publication_date 1999/07/10 · arxiv created 1999/08/20 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/01
Recent observations show that the temporal decay of the R -band afterglow from GRB 990123 steepened about 2.5 days after the burst. We here propose a possible explanation for such a steepening: a shock expanding in a dense medium has undergone the transition from a relativistic phase to a nonrelativistic phase. We find that this model is consistent with the observations if the medium density is about 3 × 10 6 cm -3 . By fitting our model to the observed optical and X-ray afterglow quantitatively, we further infer the electron and magnetic energy fractions of the shocked medium and find that the two parameters are about 0.1 and 2 × 10 -8 , respectively. The former parameter is near the equipartition value, while the latter is about 6 orders of magnitude smaller than inferred from the GRB 970508 afterglow. We also discuss possibilities that the dense medium can be produced.