Variability in GRB afterglows and GRB 021004

Ehud Nakar, Tsvi Piran, Jonathan Granot

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

Bibliographical note

Funding Information:
J.G. thanks the Hebrew University for hospitality while this research was done. This work was partially supported by the Horwitz foundation (E.N.) and by the Institute for Advanced Study, funds for natural sciences (J.G.).


  • Gamma rays: Bursts


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