The Early-Time Optical Properties of Gamma-Ray Burst Afterglows

A. Melandri, C.G. Mundell, S. Kobayashi, C. Guidorzi, A. Gomboc, I.A. Steele, R.J. Smith, D. Bersier, C.J. Mottram, D. Carter, M.F. Bode, P.T. O'Brien, N. Tanvir, E. Rol, R. Chapman

    Research output: Contribution to journalArticlepeer-review

    74 Citations (Scopus)


    We present a multiwavelength analysis of 63 gamma-ray bursts observed with the world's three largest robotic optical telescopes, the Liverpool and Faulkes Telescopes (North and South). Optical emission was detected for 24 GRBs with brightnesses ranging from R = 10 to 22 mag in the first 10 minutes after the burst. By comparing optical and X-ray light curves from t = 100 to ~106 seconds, we introduce four main classes, defined by the presence or absence of temporal breaks at optical and/or X-ray wavelengths. While 14/24 GRBs can be modeled with the forward-shock model, explaining the remaining 10 is very challenging in the standard framework even with the introduction of energy injection or an ambient density gradient. Early X-ray afterglows, even segments of light curves described by a power law, may be due to additional emission from the central engine. Thirty-nine GRBs in our sample were not detected and have deep upper limits (R < 22 mag) at early time. Of these, only 10 were identified by other facilities, primarily at near infrared wavelengths, resulting in a dark burst fraction of ~50%. Additional emission in the early-time X-ray afterglow due to late-time central engine activity may also explain some dark bursts by making the bursts brighter than expected in the X-ray band compared to the optical band.

    Original languageEnglish
    Pages (from-to)1209-1230
    JournalThe Astrophysical Journal
    Publication statusPublished - 2008


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