A High Time-resolution Study of the Millisecond Pulsar J2241-5236 at Frequencies below 300 MHz

One of the major challenges for pulsar timing array (PTA) experiments is the mitigation of the effects of the turbulent interstellar medium (ISM) from timing data. These can potentially lead to measurable delays and/or distortions in the pulse profiles and scale strongly with the inverse of the radio frequency. Low-frequency observations are therefore highly appealing for characterizing them. However, in order to achieve the necessary time resolution to resolve profile features of short-period millisecond pulsars, phase-coherent dedispersion is essential, especially at frequencies below 300 MHz. We present the lowest-frequency (80-220 MHz), coherently dedispersed detections of one of the most promising pulsars for current and future PTAs, PSR J2241-5236, using our new beamformer software for the MWA's voltage capture system, which reconstructs the time series at a much higher time resolution of similar to 1 mu s by resynthesizing the recorded voltage data at 10 kHz/100 mu s native resolutions. Our data reveal a dual-precursor type feature in the pulse profile that is either faint or absent in high-frequency observations from Parkes. The resultant high-fidelity detections have enabled dispersion measure determinations with very high precision, of the order of (2-6) x 10(-6) pc cm(-3), owing to the microsecond level timing achievable for this pulsar at the MWA's low frequencies. This underscores the usefulness of low-frequency observations for probing the ISM toward PTA pulsars and informing optimal observing strategies for PTA experiments.