2021ApJ...921..101S

Radio magnetars are exotic sources noted for their diverse spectrotemporal phenomenology and pulse profile variations over weeks to months. Unusual for radio magnetars, the Galactic Center (GC) magnetar PSR J1745-2900 has been continually active since its discovery in 2013. We monitored the GC magnetar at 4-8 GHz for 6 hr in 2019 August-September using the Robert C. Byrd Green Bank Telescope. During our observations, the GC magnetar emitted a flat fluence spectrum over 5-8 GHz to within 2σ uncertainty. From our data, we estimate a 6.4 GHz period-averaged flux density, _6.4~(240±5)µJy. Tracking the temporal evolution of _6.4, we infer a gradual weakening of GC magnetar activity during 2016-2019 relative to that between 2013 and 2015.5. Typical single pulses detected in our study reveal marginally resolved subpulses with opposing spectral indices, a feature characteristic of radio magnetars but unseen in rotation-powered pulsars. However, unlike in fast radio bursts, these subpulses exhibit no perceptible radio frequency drifts. Throughout our observing span, ≃5 ms scattered pulses significantly jitter within two stable emission components of widths 220 ms and 140 ms, respectively, in the average pulse profile.