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

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.

💡 Research Summary

On 3 October 2008 the Swift Burst Alert Telescope (BAT) triggered on a series of short, soft‑gamma‑repeater‑like bursts originating from the transient anomalous X‑ray pulsar (AXP) 1E 1547.0‑5408. Within less than 100 seconds the Swift X‑ray Telescope (XRT) began pointed observations, catching the source at a 2–10 keV flux of ≈6 × 10⁻¹¹ erg cm⁻² s⁻¹ – more than fifty times its quiescent level. Swift then monitored the outburst daily for roughly three weeks, accumulating a dense set of timing and spectral data that allowed the authors to construct a phase‑coherent timing solution extending from the burst epoch onward.

The timing analysis revealed the expected spin period (P ≈ 2.069 s) and a positive period derivative ( (\dot{P}) ≈ 2.3 × 10⁻¹¹ s s⁻¹), but also required a significant positive second derivative ( (\ddot{P}) ≈ 1.1 × 10⁻¹⁹ s s⁻²). This indicates an accelerating spin‑down torque, a behaviour that is not explained by simple magnetic dipole braking alone. The authors discuss plausible mechanisms such as a rapid increase in magnetospheric twist, enhanced particle wind, or a re‑configuration of the external magnetic field that can temporarily boost the torque.

Pulse profiles evolved dramatically throughout the campaign. Early observations showed a complex, multi‑peaked shape with a modest pulsed fraction of ~20 %. As the outburst progressed the profile simplified to a single‑peak morphology and the pulsed fraction rose to ~50 %. This evolution suggests a shrinking or migrating hot‑spot on the neutron‑star surface, or a change in viewing geometry caused by magnetospheric restructuring.

Spectrally, the source was initially hard. Within the first half‑day the XRT spectrum could be fitted equally well by a power‑law (PL) with photon index Γ ≈ 2 or a blackbody (BB) with temperature kT ≈ 1.4 keV. By the end of the first day the spectrum had softened substantially, requiring either Γ ≈ 4 or kT ≈ 0.8 keV. The authors argue that both components were likely present simultaneously during the first day, and that the rapid softening reflects cooling of a magnetically heated crust and/or the decay of a magnetospheric twist that initially supplied high‑energy photons.

An INTEGRAL observation performed five days after the BAT trigger placed a 3σ upper limit of ≈2 × 10⁻¹¹ erg cm⁻² s⁻¹ in the 18–60 keV band, confirming that the hard X‑ray emission faded quickly after the initial burst phase.

When compared with other transient AXPs such as XTE J1810‑197 and CXOU J1647‑4552, 1E 1547 exhibits the same suite of phenomena: a large, rapid flux increase, a temporary acceleration of spin‑down, spectral softening on a timescale of hours to days, and a growing pulsed fraction. These commonalities support the magnetar model in which a sudden injection of magnetic energy (e.g., a crustal fracture) creates a twisted magnetosphere, drives a particle wind that enhances torque, and heats the surface. The twist then untwists on a timescale of days, producing the observed spectral evolution and torque relaxation.

In summary, the Swift rapid‑response capability allowed the authors to capture the earliest phases of an AXP outburst, providing a uniquely complete data set that links burst activity, timing irregularities, pulse‑profile changes, and spectral evolution. The findings reinforce the view that magnetar outbursts are governed by coupled magnetospheric and crustal processes, and they highlight the importance of immediate, high‑cadence X‑ray monitoring for unraveling the physics of these extreme neutron stars.