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 GRBs, but the reason for which their prompt emission peaks at lower photon energies, $E_p$, is still under debate. Although many different models have been invoked in order to explain the lower $E_p$ values, 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 class. 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 $\theta_{obs}$ from the jet axis of $(\theta_{obs}-\theta_0)\sim 0.15\theta_0$ and $\sim \theta_0$, respectively, where $\theta_0\sim 3$ deg is the jet half-opening angle. This suggests that GRBs, XRGRBs and XRFs are intrinsically similar relativistic jets viewed from different angles, corresponding to $\gamma(\theta_{obs}-\theta_0)$ of less than 1, between 1 and a few, and more than a few, respectively, where $\gamma$ is the Lorentz factor. Future observations with 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.Comment: some references added, small typos corrected, and the important role of HETE II emphasize