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 \upmu s 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 \pm 2 { rad m ^ { -2 } } and apparent dispersion measures that vary by 0.041 \pm 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 .