Green Bank Telescope and Swift X-ray Telescope Observations of the Galactic Center Radio Magnetar SGR J1745-2900

TL;DR

This study presents multi-wavelength observations of the galactic center magnetar SGR J1745-2900, revealing two activity phases with distinct radio and X-ray behaviors, and unprecedented timing noise levels.

Contribution

It provides the first detailed multi-wavelength analysis of SGR J1745-2900's activity phases and timing noise, highlighting differences between radio and X-ray evolution.

Findings

Two main activity periods with different radio properties

Radio flux increased and became more variable during the second period

X-ray flux steadily decayed, independent of radio behavior

Abstract

We present results from eight months of Green Bank Telescope 8.7-GHz observations and nearly 18 months Swift X-ray telescope observations of the radio magnetar SGR J1745-2900, which is located 2.4" from Sgr A*. We tracked the magnetar's radio flux density, polarization properties, pulse profile evolution, rotation, and single-pulse behavior. We identified two main periods of activity in SGR J1745-2900. The first is characterized by approximately 5.5 months of relatively stable evolution in radio flux density, rotation, and profile shape, while in the second these properties varied substantially. Specifically, a third profile component emerged and the radio flux increased on average, but also became more variable. Bright single pulses are visible and are well described by a log-normal energy distribution at low to moderate energies, but with an excess at high energies. The 2-10 keV flux has decayed steadily since the initial X-ray outburst, in contrast with the radio flux. Our timing analysis includes Green Bank Telescope, Swift, and NuSTAR data. When we include the X-ray data in our analyses, we find that SGR J1745-2900 exhibited a level of timing noise unprecedented in a radio magnetar, though an analysis of only the radio data indicates timing noise at a level similar to that observed in other radio magnetars. We conclude that, while SGR J1745-2900 is similar to other radio magnetars in many regards, it differs by having experienced a period of relative stability in the radio band that now appears to have ended, while the X-ray properties have evolved independently.