$4-8\mathrm{GHz}$ Spectro-temporal Emission from the Galactic Center Magnetar $\mathrm{PSR}J1745-2900$

Abstract

Radio magnetars are exotic sources noted for their diverse spectro-temporal phenomenology and pulse profile variations over weeks to months. Unusual for radio magnetars, the Galactic Center (GC) magnetar $\mathrm{PSR}J1745-2900$ has been continually active since its discovery in 2013. We monitored the GC magnetar at $4-8\mathrm{GHz}$ for 6 hours in August$-$September 2019 using the Robert C. Byrd Green Bank Telescope. During our observations, the GC magnetar emitted a flat fluence spectrum over $5-8\mathrm{GHz}$ to within $2\sigma$ uncertainty. From our data, we estimate a $6.4\mathrm{GHz}$ period-averaged flux density, ${\overline{S}}_{6.4}\approx (240\pm 5)\mu$Jy. Tracking the temporal evolution of ${\overline{S}}_{6.4}$, we infer a gradual weakening of GC magnetar activity during $2016-2019$ relative to that between $2013-2015.5$. Typical single pulses detected in our study reveal marginally resolved sub-pulses with opposing spectral indices, a feature characteristic of radio magnetars but unseen in rotation-powered pulsars. However, unlike in fast radio bursts, these sub-pulses exhibit no perceptible radio frequency drifts. Throughout our observing span, $\simeq 5\mathrm{ms}$ scattered pulses significantly jitter within two stable emission components of widths, $220\mathrm{ms}$ and $140\mathrm{ms}$, respectively, in the average pulse profile.

20 pages, 13 figures, published in ApJ

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