Afterglow Observations Shed New Light on the Nature of X‐Ray Flashes

Abstract

X-ray flashes (XRFs) and X-ray-rich gamma-ray bursts (XRGRBs) share many observational characteristics with long-duration (>~2 s) GRBs, but the reason for which the spectral energy distribution of their prompt emission peaks at lower photon energies Ep is still a subject of debate. Although many different models have been invoked in order to explain the lower values of Ep, their implications for the afterglow emission were not considered in most cases, mainly because observations of XRF afterglows have become available only recently. Here we examine the predictions of the various XRF models for the afterglow emission and test them against the observations of XRF 030723 and XRGRB 041006, the events with the best monitored afterglow light curves in their respective classes. We show that most existing XRF models are hard to reconcile with the observed afterglow light curves, which are very flat at early times. Such light curves are, however, naturally produced by a roughly uniform jet with relatively sharp edges that is viewed off-axis (i.e., from outside of the jet aperture). This type of model self-consistently accommodates both the observed prompt emission and the afterglow light curves of XRGRB 041006 and XRF 030723, implying viewing angles θobs from the jet axis of (θobs-θ0)~0.15θ0 and (θobs-θ0)~θ0, respectively, where θ0~3deg is the half-opening angle of the jet. This suggests that GRBs, XRGRBs, and XRFs are intrinsically similar relativistic jets viewed from different angles. It is then natural to identify GRBs with γ(θobs-θ0) ~a few, where γ is the Lorentz factor of the outflow near the edge of the jet, from which most of the observed prompt emission arises. Future observations with HETE-2 and Swift could help test this unification scheme in which GRBs, XRGRBs, and XRFs share the same basic physics and differ only by their orientation relative to our line of sight.