High-cadence observations and variable spin behaviour of magnetar Swift J1818.0-1607 after its outburst

TL;DR

This study presents high-cadence multi-frequency radio observations of magnetar Swift J1818.0-1607 post-outburst, revealing variable spin-down rates, a steep spectrum, and insights into its emission geometry and age estimation.

Contribution

First detailed high-cadence radio monitoring of Swift J1818.0-1607 revealing its spectral, polarization, and spin behavior variations after outburst.

Findings

Steep radio spectrum and simple pulse profile.

Large variation in spin-down rate with four timing events.

Estimated characteristic age around 500 years, larger than previous estimates.

Abstract

We report on multi-frequency radio observations of the new magnetar Swift J1818.0-1607, following it for more than one month with high cadence. The observations commenced less than 35 hours after its registered first outburst. We obtained timing, polarisation and spectral information. Swift J1818.0-1607 has an unusually steep spectrum for a radio emitting magnetar and also has a relatively narrow and simple pulse profile. The position angle swing of the polarisation is flat over the pulse profile, possibly suggesting that our line-of-sight grazes the edge of the emission beam. This may also explain the steep spectrum. The spin evolution shows large variation in the spin-down rate, associated with four distinct timing events over the course of our observations. Those events may be related to the appearance and disappearance of a second pulse component. The first timing event coincides with our actual observations, while we did not detect significant changes in the emission properties which could reveal further magnetospheric changes. Characteristic ages inferred from the timing measurements over the course of months vary by nearly an order of magnitude. A longer-term spin-down measurement over approximately 100 days suggests an characteristic age of about 500 years, larger than previously reported. Though Swift J1818.0-1607 could still be one of the youngest neutron stars (and magnetars) detected so far, we caution using the characteristic age as a true-age indicator given the caveats behind its calculation.