An Extraordinary Outburst of the Magnetar Swift J1822.3-1606

TL;DR

This paper analyzes the 2011 outburst of magnetar Swift J1822.3-1606, revealing similarities to giant flares and proposing a common triggering mechanism based on magnetospheric fireball dynamics.

Contribution

It provides detailed temporal analysis of the outburst, compares it with giant flares, and suggests a unified model for magnetar outbursts and giant flares.

Findings

Coherent spin period signal with a phase lag during outburst

Pulse profile evolves from complex to simple over a month

Energy release comparable to giant flare tails

Abstract

The 2011 outburst of Swift J1822.3--1606 was extraordinary; periodic modulations at the spin period of the underlying neutron star were clearly visible, remarkably similar to what is observed during the decaying tail of magnetar giant flares. We investigated the temporal characteristics of X-ray emission during the early phases of the outburst. We performed a periodicity search with the spectral hardness ratio (HR), and found a coherent signal near the spin period of the neutron star, but with a lag of about 3 radians. Therefore, the HR is strongly anti-correlated with the X-ray intensity, which is also seen in the giant flares. We studied time evolution of the pulse profile and found that it evolves from a complex morphology to a much simpler shape within about a month. Pulse profile simplification also takes place during the giant flares, but on a much shorter timescale of about few minutes. We found that the amount of energy emitted during the first 25 days of the outburst is comparable to what was detected in minutes during the decaying tail of giant flares. Based on these similarities, we suggest that the triggering mechanisms of the giant flares and the magnetar outbursts are likely the same. We propose that the trapped fireball that develops in the magnetosphere at the onset of the outburst radiates away efficiently in minutes in magnetars exhibiting giant flares, while in other magnetars, such as Swift J1822.3--1606, the efficiency of radiation of the fireball is not as high and, therefore, lasts much longer.