Swift XRT observations of the afterglow of XRF 050416a

Swift discovered XRF 050416A with the Burst Alert Telescope and began observing it with its narrow-field instruments only 64.5 s after the burst onset. Its very soft spectrum classifies this event as an X-ray flash. The afterglow X-ray emission was monitored up to 74 days after the burst. The X-ray light curve initially decays very fast (decay slope alpha similar to 2.4), subsequently flattens (alpha similar to 0.44), and eventually steepens again (alpha similar to 0.88), similar to many X-ray afterglows. The first and second phases end similar to 172 and similar to 1450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index Gamma similar to 3.0 during the initial steep decay, to a harder emission with Gamma similar to 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of similar to 6.8 x 10(21) cm(-2). The consistency of the initial photon index with the high-energy BAT photon index suggests that the initial fast decaying phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give a possible description of the XRF 050416A X-ray afterglow for an opening angle larger than a few tens of degrees, although numerical simulations show that the late-time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored.