Astrophys. J., 654, 403-412 (2007/January-1)
Swift XRT observations of the afterglow of XRF 050416A.
MANGANO V., LA PAROLA V., CUSUMANO G., MINEO T., MALESANI D., DYKS J., CAMPANA S., CAPALBI M., CHINCARINI G., GIOMMI P., MORETTI A., PERRI M., ROMANO P., TAGLIAFERRI G., BURROWS D.N., GEHRELS N., GODET O., HOLLAND S.T., KENNEA J.A., PAGE K.L., RACUSIN J.L., ROMING P.W.A. and ZHANG B.
Abstract (from CDS):
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 ∼6.8x1021/cm2. 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.
Gamma Rays: Bursts - X-rays: individual (XRF 050416A)
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