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 α ~ 2.4), subsequently flattens (α ~ 0.44), and eventually steepens again (α ~ 0.88), similar to many X-ray afterglows. The first and second phases end ~172 and ~1450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index Γ ~ 3.0 during the initial steep decay, to a harder emission with Γ ~ 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of ~6.8 × 1021 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.