We present Swift observations of GRB 051109B, a soft long burst triggered by the Burst Alert Telescope (BAT). The soft photon index of the prompt emission suggest it is a X-ray Flash (XRF) or, at least, a X-ray Rich (XRR) burst. The X-ray lightcurve displays the canonical shape of many other GRBs, a double b roken power law with a small flare superimposed at ~T_0+1500 s, and its extrapolation to early times smoothly joins with the BAT lightcurve. On the basis of the derived optical to X-ray flux ratio, it cannot be classified as a dark burst.
Download