XMM-Newton observations of SGR 1806-20 over seven years following the 2004 Giant Flare

TL;DR

This study analyzes 8 years of XMM-Newton observations of magnetar SGR 1806-20 post-2004 giant flare, revealing increased torques, spectral hardening, and exponential flux decay, indicating complex magnetic and seismic activity.

Contribution

It provides a detailed long-term observational analysis linking spectral and timing behaviors of SGR 1806-20 after a giant flare, highlighting mechanisms of magnetic twist and seismic activity.

Findings

Torque increased significantly after the flare.

Spectral fluxes increased and spectra hardened post-flare.

Flux decayed exponentially over 1.5 years, not reaching quiescence until 3.5 years later.

Abstract

We report on the study of 14 XMM-Newton observations of the magnetar SGR 1806-20 spread over a period of 8 years, starting in 2003 and extending to 2011. We find that in mid 2005, a year and a half after a giant flare (GF), the torques on the star increased to the largest value yet seen, with a long term average rate between 2005 and 2011 of $\lvert\dot{\nu}\rvert\approx1.35\times10^{-11}$ Hz s$^{-1}$, an order of magnitude larger than its historical level measured in 1995. The pulse morphology of the source is complex in the observations following the GF, while its pulsed-fraction remained constant at about $7\%$ in all observations. Spectrally, the combination of a black-body (BB) and power-law (PL) components is an excellent fit to all observations. The BB and PL fluxes increased by a factor of 2.5 and 4, respectively, while the spectra hardened, in concordance with the 2004 major outburst that preceded the GF. The fluxes decayed exponentially back to quiescence with a characteristic time-scale of $\tau\sim1.5$ yrs, although they did not reach a constant value until at least 3.5 years later (2009). The long-term timing and spectral behavior of the source point to a decoupling between the mechanisms responsible for their respective behavior. We argue that low level seismic activity causing small twists in the open field lines can explain the long lasting large torques on the star, while the spectral behavior is due to a twist imparted onto closed field lines after the 2004 large outburst.