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

TL;DR

This study presents multi-frequency radio and X-ray observations of the Galactic Center magnetar SGR 1745−2900, revealing its flux evolution, spectral properties, and interstellar medium parameters during its initial outburst, with implications for magnetar emission models.

Contribution

First simultaneous wideband radio and X-ray observations of SGR 1745−2900, providing insights into its flux evolution, spectral index, and interstellar medium parameters during outburst.

Findings

Radio spectral index is significantly negative between 2 and 9 GHz.

Radio flux remained stable during the outburst, unlike other magnetars.

Dispersion measure appears frequency-dependent, inconsistent across bands.

Abstract

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.