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We report on multi-frequency, wideband radio observations of the GalacticCenter magnetar (SGR 1745$-$2900) with the Green Bank Telescope for $\sim$100days immediately following its initial X-ray outburst in April 2013. We mademultiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us toexamine the magnetar's flux evolution, radio spectrum, and interstellar mediumparameters (such as the dispersion measure (DM), the scattering timescale andits index). During two epochs, we have simultaneous observations from theChandra X-ray Observatory, which permitted the absolute alignment of the radioand X-ray profiles. As with the two other radio magnetars with publishedalignments, the radio profile lies within the broad peak of the X-ray profile,preceding the X-ray profile maximum by $\sim$0.2 rotations. We also find thatthe radio spectral index $\gamma$ is significantly negative between $\sim$2 and9 GHz; during the final $\sim$30 days of our observations $\gamma \sim -1.4$,which is typical of canonical pulsars. The radio flux has not decreased duringthis outburst, whereas the long-term trends in the other radio magnetars showconcomitant fading of the radio and X-ray fluxes. Finally, our widebandmeasurements of the DMs taken in adjacent frequency bands in tandem arestochastically inconsistent with one another. Based on recent theoreticalpredictions, we consider the possibility that the dispersion measure isfrequency-dependent. Despite having several properties in common with the otherradio magnetars, such as $L_{\textrm{X,qui}}/L_{\textrm{rot}} \lesssim 1$, anincrease in the radio flux during the X-ray flux decay has not been observedthus far in other systems.

Simultaneous Multi-band Radio & X-ray Observations of the Galactic Center Magnetar SGR 1745$-$2900