The Spin-down of Swift J1822.3-1606: A New Galactic Magnetar

TL;DR

This paper reports the discovery and initial analysis of Swift J1822.3-1606, a magnetar with the second lowest magnetic field among known magnetars, based on X-ray timing and flux decay observations.

Contribution

The study provides the first detailed timing and flux decay analysis of Swift J1822.3-1606, confirming it as a new magnetar with unique magnetic field properties.

Findings

Measured spin period of 8.43771963 seconds.

Inferred magnetic field strength of 5.1×10^13 G.

Pulsed X-ray flux decays exponentially with a 16.4-day time constant.

Abstract

On 2011 July 14, a new magnetar candidate, Swift J1822.3-1606, was identified via a rate trigger on the Swift/Burst Alert Telescope. Here we present an initial analysis of the X-ray properties of the source, using data from the Rossi X-ray Timing Explorer, Swift, and the Chandra X-ray Observatory, spanning 2011 July 16--September 22. We measure a precise spin period of P=8.43771963(5) s and a spin-down rate of 2.97(28)E-13, at MJD 55761.0, corresponding to an inferred surface dipole magnetic field strength of B=5.1E13 G, the second lowest thus far measured for a magnetar, though similar to 1E~2259+586 as well as to several high-magnetic field radio pulsars. We show that the pulsed X-ray flux decay in the 2--10 keV band is best fit by an exponential with a time constant of 16.4+/-0.3 days. After increasing from ~35% during the first week after the onset of the outburst, the pulsed fraction in the 2--10 keV band remained constant at ~45. We argue that these properties confirm this source to be a new member of the class of objects known as magnetars.