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

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 ($>50\mathrm{\%}$) linearly polarized, with a lower degree ($<30\mathrm{\%}$) 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.