Simultaneous Radio and X-ray Observations of the Magnetar Swift J1818.0-1607

Swift J1818.0−1607 is a radio-emitting magnetar that was discovered in X-ray outburst in March 2020. Starting four days after this outburst, we began a nearly five month multi-frequency observing campaign at 2.2, 8.4, and 32 GHz using telescopes in the NASA Deep Space Network. Using a dual-frequency observing mode, we were able to observe Swift J1818.0−1607 simultaneously at either 2.2 and 8.4 GHz or 8.4 and 32 GHz. We find that the flux density increases substantially and the spectrum changes from uncharacteristically steep (alpha < −2.2) to the essentially flat (alpha \approx 0) spectrum typical of radio-emitting magnetars. In addition to the expected profile evolution on timescales of days to months, we find that Swift J1818.0−1607 also exhibits mode switching where the pulse profile changes between two distinct shapes on timescales of seconds to minutes. For two of the radio observations, we also had accompanying X-ray observations using the Neutron Star Interior Composition Explorer (NICER) telescope that occurred on the same day. We find a near anti-alignment (0.40 phase cycles) between the peaks of the radio and X-ray pulse profiles, which is most likely explained by an intrinsic misalignment between the X-ray and radio emitting regions.