2022ApJ...931...67M

We present the long-term spectro-temporal evolution of the average radio emission properties of the magnetar XTE J1810-197 (PSR J1809-1943), following its most recent outburst in late 2018. We report the results from a 2.5 yr monitoring campaign with the upgraded Giant Metrewave Radio Telescope, carried out over the frequency range of 300-1450 MHz. Our observations show intriguing time variability in the average profile width, flux density, spectral index, and broadband spectral shape. While the average profile width appears to gradually decrease at later epochs, the flux density shows multiple episodes of radio rebrightening over the course of our monitoring. Our systematic monitoring observations reveal that the radio spectrum has steepened over time, resulting in evolution from a magnetar-like spectrum to a more pulsar-like spectrum. A more detailed analysis reveals that the radio spectrum has a turnover, and that this turnover shifts toward lower frequencies with time. We present the details of our analysis leading to these results, and discuss our findings in the context of magnetar radio emission mechanisms, as well as potential manifestations of the intervening medium. We also briefly discuss whether an evolving spectral turnover could be a ubiquitous property of radio magnetars.