The Outburst Decay of the Low Magnetic Field Magnetar SWIFT J1822.3-1606: Phase-resolved Analysis and Evidence for a Variable Cyclotron Feature

TL;DR

This study analyzes the spectral and timing evolution of the low-magnetic field magnetar SWIFT J1822.3-1606 during its outburst decay, revealing a variable cyclotron feature indicating complex magnetic field structures.

Contribution

It provides the first phase-resolved spectral analysis of this magnetar, identifying a variable proton cyclotron line linked to small-scale magnetic field components.

Findings

Detection of a phase and energy-variable spectral feature.

Evidence for a strong, localized magnetic field component.

Insights into the emission regions on the magnetar's surface.

Abstract

We study the timing and spectral properties of the low-magnetic field, transient magnetar SWIFT J1822.3-1606 as it approached quiescence. We coherently phase-connect the observations over a time-span of ~500 days since the discovery of SWIFT J1822.3-1606 following the Swift-BAT trigger on 2011 July 14, and carried out a detailed pulse phase spectroscopy along the outburst decay. We follow the spectral evolution of different pulse phase intervals and find a phase and energy-variable spectral feature, which we interpret as proton cyclotron resonant scattering of soft photon from currents circulating in a strong (10^14 G) small-scale component of the magnetic field near the neutron star surface, superimposed to the much weaker (~3 x 10^13 G) magnetic dipole field. We discuss also the implications of the pulse-resolved spectral analysis for the emission regions on the surface of the cooling magnetar.