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 6 to 9 months after its 2020 March outburst. We obtained multihour observations at six frequencies, recording polarimetric, spectral, and single-pulse information. The spectrum peaks at a frequency of 5.4 ± 0.6 GHz, making Swift J1818.0–1607 one of many radio magnetars that exhibit a gigahertz-peaked spectrum. The radio flux decays steeply above the peak frequency, with in-band spectral indices α < −2.3 above 9 GHz. The emission is highly (>50%) linearly polarized, with a lower degree (<30%) of circular polarization that 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 when compared to the central pulse component.