Spectropolarimetric Analysis of FRB 181112 at Microsecond Resolution: Implications for Fast Radio Burst Emission Mechanism

Abstract

We have developed a new coherent dedispersion mode to study the emission of Fast Radio Bursts that trigger the voltage capture capability of the Australian SKA Pathfinder (ASKAP) interferometer. In principle the mode can probe emission timescales down to 3 ns with full polarimetric information preserved. Enabled by the new capability, here we present a spectropolarimetric analysis of FRB 181112 detected by ASKAP, localized to a galaxy at redshift 0.47. At microsecond time resolution the burst is resolved into four narrow pulses with a rise time of just 15 µs for the brightest. The pulses have a diversity of morphology, but do not show evidence for temporal broadening by turbulent plasma along the line of sight, nor is there any evidence for periodicity in their arrival times. The pulses are highly polarized (up to 95%), with the polarization position angle varying both between and within pulses. The pulses have apparent rotation measures that vary by 15 ± 2 rad m −2 and apparent dispersion measures that vary by 0.041 ± 0.004 pc cm −3 . Conversion between linear and circular polarization is observed across the brightest pulse. We conclude that the FRB 181112 pulses are most consistent with being a direct manifestation of the emission process or the result of propagation through a relativistic plasma close to the source. This demonstrates that our method, which facilitates high-time-resolution polarimetric observations of FRBs, can be used to study not only burst emission processes, but also a diversity of propagation effects present on the gigaparsec paths they traverse.

Scintillation Can Explain the Spectral Structure of the Bright Radio Burst from SGR 1935+2154

“…FRB 190611 shows time-varying polarization properties as well as an apparent change in dispersion measure between the two components (Day et al 2020). The apparent variations in dispersion measure, Faraday rotation measure, and polarization properties between the components of some FRBs (e.g., Cho et al 2020;Day et al 2020) may be interpreted as emission observed along different sightlines through a dense, magnetized, trans-relativistic plasma (Vedantham & Ravi 2019), possibly with several radial magnetic-field reversals (Gruzinov & Levin 2019). This scenario is consistent with our picture of significant spatial separations between emission sites for multiple-component FRBs.…”

Microsecond polarimetry of the repeating FRB 20180916B

“…In the case of FRB 20170827, the burst shows a single component of width ∼ 30 µs 15 , and similarly, FRB 20121102A produced a single burst of width ∼ 30 µs 16 . FRB 20181112A, also shows a single narrow component, but the results are limited by scattering at the 20 µs level 17 . In this work, we have demonstrated that not only does FRB 20180916B also exhibit short duration components similar to what has been seen in other FRBs (e.g.…”

“…Many FRBs show complex burst morphology and, to date, both repeaters and apparent non-repeaters have shown temporal structure, 'sub-bursts', as short as ∼ 20-30 µs [15][16][17] . In the case of FRB 20181112A 17 , the observations are limited by scattering.…”

What does FRB light-curve variability tell us about the emission mechanism?

“…For instance, FRB 170827 had an overall duration of ∼ 0.4 ms with strong variations on a timescale of ∼ 30µs (Farah et al 2018). Another FRB, 181112, had a pulse with a very rapid rise ∼ 10µs, followed by a significantly shallower decay ∼ 0.15 ms (Cho et al 2020). The data suggests that the variability timescale is intrinsic to the source, making it a diagnostic for the burst mechanism.…”