The dynamic magnetosphere of Swift J1818.0-1607

Radio-loud magnetars display a wide variety of radio-pulse phenomenology seldom seen among the population of rotation-powered pulsars. Spectropolarimetry of the radio pulses from these objects has the potential to place constraints on their magnetic topology and unveil clues about the magnetar radio emission mechanism. Here, we report on eight observations of the magnetar Swift J1818.0-1607 taken with the Parkes Ultra-Wideband Low receiver covering a wide frequency range from 0.7 to 4GHz over a period of 5 months. The magnetar exhibits significant temporal profile evolution over this period, including the emergence of a new profile component with an inverted spectrum, two distinct types of radio emission mode switching, detected during two separate observations, and the appearance and disappearance of multiple polarization modes. These various phenomena are likely a result of ongoing reconfiguration of the plasma content and electric currents within the magnetosphere. Geometric fits to the linearly polarized position angle indicate we are viewing the magnetar at an angle of similar to 99 degrees from the spin axis, and its magnetic and rotation axes are misaligned by similar to 112 degrees. While conducting these fits, we found the position angle swing had reversed direction on MJD 59062 compared to observations taken 15d earlier and 12d later. We speculate this phenomena may be evidence the radio emission from this magnetar originates from magnetic field lines associated with two co-located magnetic poles that are connected by a coronal loop.

Automated summary

Radio-loud magnetars, like Swift J1818.0-1607, exhibit significant temporal profile evolution, spectrum inversion, and polarization mode variations, possibly due to ongoing reconfiguration of plasma and electric currents within the magnetosphere. Geometric fits suggest that the magnetar is viewed at an angle of 99 degrees from its spin axis, with a misalignment of approximately 112 degrees between its magnetic and rotation axes. This might indicate that the radio emission from this magnetar originates from magnetic field lines associated with two co-located magnetic poles.