Gamma-ray bursts are observed off-axis

Geoffrey Ryan, Hendrik Van Eerten, Andrew Macfadyen, Bin Bin Zhang

    Research output: Contribution to journalArticlepeer-review


    We constrain the jet opening angle and, for the first time, the off-axis observer angle for gamma-ray bursts in the Swift-XRT catalog by using the ScaleFit package to fit afterglow light curves directly to hydrodynamic simulations. The ScaleFit model uses scaling relations in the hydrodynamic and radiation equations to compute synthetic light curves directly from a set of high-resolution two-dimensional relativistic blast wave simulations. The data sample consists of all Swift-XRT afterglows from 2005 to 2012 with sufficient coverage and a known redshift, 226 bursts in total. We find that the jet half-opening angle varies widely but is commonly less than 0.1 rad. The distribution of the electron spectral index is also broad, with a median at 2.30. We find the observer angle to have a median value of 0.57 of the jet opening angle over our sample, which has profound consequences for the predicted rate of observed jet breaks and affects the beaming-corrected total energies of gamma-ray bursts.

    Original languageEnglish (US)
    Article number3 (15pp)
    JournalAstrophysical Journal
    Issue number1
    StatePublished - Jan 20 2015


    • gamma-ray burst: general
    • hydrodynamics
    • methods: data analysis
    • shock waves

    ASJC Scopus subject areas

    • Astronomy and Astrophysics
    • Space and Planetary Science


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