Multifrequency Radio Observations of the Magnetar Swift J1818.0--1607

TL;DR

This study presents multi-frequency radio observations of the magnetar Swift J1818.0--1607, revealing a gigahertz-peaked spectrum, high linear polarization, and complex pulse profile features over several months post-outburst.

Contribution

First detailed multi-frequency radio analysis of Swift J1818.0--1607, highlighting its gigahertz-peaked spectrum and polarization properties, expanding understanding of magnetar radio emission.

Findings

Spectrum peaks at 5.4 GHz with steep decay above 9 GHz.

Radio emission is highly linearly polarized (>50%).

Pulse profiles show a central component with magnetar-like features.

Abstract

We report on Green Bank Telescope observations of the radio magnetar Swift J1818.0--1607 between 820 MHz and 35 GHz, taken from six to nine months after its 2020 March outburst. We obtained multi-hour observations at six frequencies, recording polarimetric, spectral, and single-pulse information. The spectrum peaks at a frequency of $5.4 \pm 0.6$ GHz, making Swift J1818.0--1607 one of many radio magnetars which exhibit a gigahertz-peaked spectrum (GPS). The radio flux decays steeply above the peak frequency, with in-band spectral indices $\alpha < -2.3 $ above 9 GHz. The emission is highly ($> 50\%$) linearly polarized, with a lower degree ($< 30\%$) of circular polarization which can change handedness between single pulses. Across the frequency range of our observations, the time-integrated radio profiles share a common shape: a narrow ``pulsar-like'' central component flanked by ``magnetar-like'' components comprised of bright, spiky subpulses. The outer profile components exhibit larger degrees of flux modulation and flatter spectral indices when compared to the central pulse component.