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 GRB 000926, performed aro und and after the break observed in the optical light curve 2 days after th e 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 pro perties are not consistent with a standard jet scenario based on synchrotro n emission, requiring a more complicated model. We find that X-ray and opti cal data are compatible with a moderately collimated fireball (with opening angle theta approximate to 25 degrees) expanding in a dense medium (n appr oximate to 4 x 10(4) cm(-1)). This produces two breaks in the light curve, The first, at t approximate to 2 days, is due to jet behavior. The second, around 5 days, is attributed to the transition of the fireball to a nonrela tivistic expansion. This transition predicts a flattening of the light curv e, which explains the late X-ray measurement in excess above the extrapolat ion of the simple jet scenario, and is also consistent with optical data.