SIMULTANEOUS MULTI-BAND RADIO AND X-RAY OBSERVATIONS OF THE GALACTIC CENTER MAGNETAR SGR 1745–2900

Abstract

We report on multi-frequency, wideband radio observations of the Galactic\nCenter magnetar (SGR 1745$-$2900) with the Green Bank Telescope for $\\sim$100\ndays immediately following its initial X-ray outburst in April 2013. We made\nmultiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to\nexamine the magnetar's flux evolution, radio spectrum, and interstellar medium\nparameters (such as the dispersion measure (DM), the scattering timescale and\nits index). During two epochs, we have simultaneous observations from the\nChandra X-ray Observatory, which permitted the absolute alignment of the radio\nand X-ray profiles. As with the two other radio magnetars with published\nalignments, the radio profile lies within the broad peak of the X-ray profile,\npreceding the X-ray profile maximum by $\\sim$0.2 rotations. We also find that\nthe radio spectral index $\\gamma$ is significantly negative between $\\sim$2 and\n9 GHz; during the final $\\sim$30 days of our observations $\\gamma \\sim -1.4$,\nwhich is typical of canonical pulsars. The radio flux has not decreased during\nthis outburst, whereas the long-term trends in the other radio magnetars show\nconcomitant fading of the radio and X-ray fluxes. Finally, our wideband\nmeasurements of the DMs taken in adjacent frequency bands in tandem are\nstochastically inconsistent with one another. Based on recent theoretical\npredictions, we consider the possibility that the dispersion measure is\nfrequency-dependent. Despite having several properties in common with the other\nradio magnetars, such as $L_{\\textrm{X,qui}}/L_{\\textrm{rot}} \\lesssim 1$, an\nincrease in the radio flux during the X-ray flux decay has not been observed\nthus far in other systems.\n