Variability in GRB afterglows and GRB 021004

Ehud Nakar*, Tsvi Piran, Jonathan Granot

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


We present general analytic expressions for GRB afterglow light curves arising from a variable external density profile and/or a variable energy in the blast wave. The former could arise from a clumpy ISM or a variable stellar wind; The latter could arise from refreshed shocks or from an angular dependent jet structure (patchy shell). Both scenarios would lead to a variable light curve. Our formalism enables us to invert the observed light curve and obtain possible density or energy profiles. The optical afterglow of GRB 021004 was detected 537 s AB (after the burst) [GCN (2002) 1564]. Extensive follow up observations revealed a significant temporal variability. We apply our formalism to the R-band light curve of GRB 021004 and we find that several models provide a good fit to the data. We consider the patchy shell model with p=2.2 as the most likely explanation. According to this model our line of sight was towards a 'cold spot' that has lead to a relativity low γ-ray flux and an initially weak afterglow (while the X-ray afterglow flux after a day was above average). Observations above the cooling frequency, νc, could provide the best way to distinguish between our different models.

Original languageEnglish
Pages (from-to)495-505
Number of pages11
JournalNew Astronomy
Issue number5
StatePublished - Jul 2003
Externally publishedYes


  • Gamma rays: Bursts


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