The 2008 October Swift detection of X-ray bursts/outburst from the transient SGR-like AXP 1E 1547.0-5408

TL;DR

This paper details the swift detection and analysis of the 2008 October outburst from the anomalous X-ray pulsar 1E 1547.0-5408, including timing, spectral evolution, and observational strategies over three weeks.

Contribution

First detailed multi-week observational study of the 2008 outburst from AXP 1E 1547.0-5408, revealing complex pulse shape evolution and spectral changes.

Findings

Pulse period and derivative measured with a positive second derivative.

Pulse fraction increased from 20% to 50%.

Spectral hardness decreased within one day post-outburst.

Abstract

We report on the detailed study of the 2008 October outburst from the anomalous X-ray pulsar (AXP) 1E 1547.0-5408 discovered through the Swift/Burst Alert Telescope (BAT) detection of SGR-like short X-ray bursts on 2008 October 3. The Swift/X-ray Telescope (XRT) started observing the source after less than 100 s since the BAT trigger, when the flux (about 6E-11 erg/cm^2/s in the 2-10 keV range) was >50 times higher than its quiescent level. Swift monitored the outbursting activity of 1E 1547.0-5408 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 period and period derivative, requires a positive Period second derivative term (spin-down increase). The time evolution of the pulse shape is complex and variable, with the pulsed fraction increasing from 20% to 50% 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 (BB). During the very initial phases of the outburst the spectrum is hard, with a PL photon index about 2 (or kT about 1.4 keV) which steepens to about 4 (or kT about 0.8 keV) 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 about 2E-11 erg/cm^2/s to the emission of 1E 1547.0-5408 in the 18-60 keV band.