X-ray flare in XRF 050406:: evidence for prolonged engine activity

We present observations of XRF 050406, the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at t(peak) similar to 210s after the BAT trigger, a flux variation of delta F/F similar to 6 in a very short time delta t/t(peak) << 1 was observed. Its measured fluence in the 0.2-10 keV band was similar to 1.4 x 10(-8) erg cm(-2), which corresponds to 1-15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210 s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of similar to 0.5 after similar to 4400 s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (similar to 10(-7) erg cm(-2) in the 15-350 keV band, E-iso similar to 10(51) erg), a soft spectrum (photon index 2.65), no significant flux above similar to 50 keV and a peak energy E-p < 15 keV. XRF 050406 is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.