We report on multi-frequency , wideband radio observations of the Galactic Center magnetar ( SGR 1745 - 2900 ) with the Green Bank Telescope for \sim 100 days immediately following its initial X-ray outburst in April 2013 .
We made multiple simultaneous observations at 1.5 , 2.0 , and 8.9 GHz , allowing us to examine the magnetar 's flux evolution , radio spectrum , and interstellar medium parameters ( such as the dispersion measure ( DM ) , the scattering timescale and its index ) .
During two epochs , we have simultaneous observations from the Chandra X-ray Observatory , which permitted the absolute alignment of the radio and X-ray profiles .
As with the two other radio magnetars with published alignments , 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 that the radio spectral index \gamma is significantly negative between \sim 2 and 9 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 during this outburst , whereas the long-term trends in the other radio magnetars show concomitant fading of the radio and X-ray fluxes .
Finally , our wideband measurements of the DMs taken in adjacent frequency bands in tandem are stochastically inconsistent with one another .
Based on recent theoretical predictions , we consider the possibility that the dispersion measure is frequency-dependent .
Despite having several properties in common with the other radio magnetars , such as L _ { \textrm { X,qui } } / L _ { \textrm { rot } } \lesssim 1 , an increase in the radio flux during the X-ray flux decay has not been observed thus far in other systems .