Transient narrowband radio bursts from 1E 1547.0-5408

Abstract

Radio-loud magnetars are well known for exhibiting rare and unusual radiative properties that are seldom seen in the wider pulsar population. Yet one form of emissive behavior that remains elusive among pulsars and magnetars is narrowband bursts of radio waves. Such emission is a hallmark of repeating sources of fast radio bursts (FRBs), intense radio flashes that originate from distant galaxies. Here, we report the detection of 84 narrowband radio bursts during observations of the magnetar 1E 1547.0-5408 by the Murriyang telescope one month after its 2009 outburst. All but six bursts appear temporally unresolved at millisecond timescales. They were confined to a transient profile component that appeared between 2009 February 23 to 25. This coincided with both dramatic changes in the magnetar line-of-sight magnetic-field geometry, and an emergent pulsed hard X-ray component detected by the Rossi X-ray Timing Explorer. The leading edge of the hard X-ray emission was phase-aligned with the narrowband component, indicating the bursts likely originated from pair cascades along closed magnetic field lines. Such closed-field emission could contribute to the lack of second-scale periodicity in repeating FRBs. Our characterization of the bursts suggests they may represent a low-energy analogue of the repeating FRB mechanism, further linking FRB progenitors to young, highly magnetized neutron stars.

Figures (8)

• Figure 1: A 1058 s duration Murriyang observation of the 2.07 s spin-period magnetar 1E 1547.0$-$5408 at 8356 MHz, showing the PA swing ($\Psi$) across the profile (panel a), polarization profile with total intensity in black, linear polarization in red and circular polarization in blue (panel b), and phase-resolved total intensity spectrum (panel c). The time-stamp in panel b indicates the UTC start time of the observation. Horizontal gaps in panel c indicate channels excised due to radio interference. Note the dynamic range of the bottom panel has been reduced to highlight low-intensity features.
• Figure 1: Time and frequency averaged polarization profiles detected by Murriyang between 2009 February 20 to March 1. The upper panel in each plot shows the linear polarization position angle ($\Psi$) after correcting for a Faraday rotation measure of $-1847.6$ rad m$^{-2}$ at infinite frequency, and the lower panel the total intensity emission (black), total linear polarization (red) and circular polarization (blue).
• Figure 2: Examples of sub-integrations containing only a single narrowband burst (top and middle) and those containing multiple bursts (bottom). The middle panels of each plot shows the burst dynamic spectrum. Upper panels the frequency-averaged profile of the bursts, while the red shaded region is the time-interval encompassing the burst. Right-hand panels show the same as the upper panels but for the burst spectra, where the orange traces indicate the median recovered fit to the spectrum.
• Figure 2: A pair of narrowband bursts recorded by the analogue filterbank signal processor that occurred within a single rotation of 1E 1547.0$-$5408. The fully frequency-averaged burst profile is shown in the top panel, while the dynamic spectrum of the bursts are shown in middle, and the time-averaged spectrum is presented in the right-hand panel.
• Figure 3: Distributions of the recovered burst widths ($\sigma_{t}$), spectral occupancy ($\Delta\nu$), central frequency ($\nu_{\rm cen}$) and fluence ($F$). The vertical dashed gray line indicates the $\sim$1 ms time resolution of the data. Shaded gray region indicates the observing band limits on center frequency measurements.