Simultaneous radio and X-ray observations of the magnetar Swift J1818.0−1607

Abstract

ABSTRACT Swift J1818.0−1607 is a radio-emitting magnetar that was discovered in X-ray outburst in 2020 March. Starting 4 d after this outburst, we began a nearly 5-month multifrequency 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. Over the course of the campaign, we find that the flux density increases substantially and the spectrum changes from uncharacteristically steep (α < −2.2) to the essentially flat (α ≈ 0) spectrum typical of radio-emitting magnetars. In addition to the expected profile evolution on time-scales 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 time-scales of seconds to minutes. For two of the radio observations, we also had accompanying X-ray observations using the Neutron Star Interior Composition Explorer 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.