The 2008 October Swift detection of X-ray bursts/outburst from the transient SGR-like AXP 1E1547.0-5408

We report on the detailed study of the 2008 October outburst from the anomalous X-ray pulsar 1E1547.0-5408 discovered through the Swift/Burst Alert Telescope (BAT) detection of soft gamma-ray repeater like short X-ray bursts on 2008 October 3. The Swift/X-ray Telescope (XRT) started observing the source after less than 100s since the BAT trigger, when the flux (~6 × 10-11 erg cm-2 s-1 in the 2-10 keV range) was >50 times higher than its quiescent level. Swift monitored the outbursting activity of 1E1547.0-5408 on a daily basis for approximately three weeks. This strategy allowed us to find a phase-coherent solution for the source pulsations after the burst, which, besides P and , requires a positive term (spin-down increase). The time evolution of the pulse shape is complex and variable, with the pulsed fraction increasing from 20 to 50 per cent within the Swift observational window. The XRT spectra can be fitted well by means of a single component, either a power law (PL) or a blackbody. During the very initial phases of the outburst the spectrum is hard, with a PL photon index Γ ~ 2 (or kT ~ 1.4keV), which steepens to Γ ~ 4 (or kT ~ 0.8keV) within one day from the BAT trigger, though the two components are likely present simultaneously during the first-day spectra. An INTEGRAL observation carried out five days after the trigger provided an upper limit of ~2 × 10-11 erg cm-2 s-1 to the emission of 1E1547.0-5408 in the 18-60keV band.