The X - ray afterglow of GRB 000926 observed by BeppoSAX and Chandra : a mildly collimated fireball in a dense medium ?

We present X-ray observations of the afterglow of GRB000926 performed around and after the break observed in the optical light curve two days after the burst. The steep X-ray light curve observed around the break confirms the presence of this feature in X-rays. However, the spectral and temporal properties are not consistent with a standard jet scenario based on synchrotron emission, requiring a more complicated model. We find that X-ray and optical data are compatible with a moderately collimated fireball (with opening angle θ ≈ 25) expanding in a dense medium (n ≈ 4 × 10 cm). This produces two breaks in the light curve. The first, at t ≈ 2 days, is due to jet behavior. The second, around 5 days, is attributed to the transition of the fireball to a non-relativistic expansion. This transition predicts aflattening of the light curve, that explains the late X-ray measurement in excess above the extrapolation of the simple jet scenario, and is also consistent with optical data. Subject headings: gamma rays: bursts