David McKenna
We present the early results from 12 hours of data from a 40-hour observing campaign of the Crab Pulsar between 110 and 190 MHz using the Irish LOFAR station (I-LOFAR) at Birr Castle, 180 years on from the Crab Nebula’s discovery with the Great Leviathan Telescope of Birr, a stone’s throw from the station.
While scattering due to the nebula and ISM suppresses normal pulsed emission from the Crab pulsar at low frequencies, the large fractional bandwidth of I-LOFAR and ample observing time have allowed us to compile what we believe to be the largest catalogue of giant pulses at this frequency.
Based on this catalogue, we report a pulse fluence distribution with a power law fit higher than others at our frequency of −2.86±0.07, while the observed scattering power law fits appear to be Gaussian distributed in line with expectations, with a fit of −3.7±0.5.
Our observations are performed at radio frequencies, the same ones used to transmit FM radio or communicate with satellites. At these frequencies, we cannot resolve normal pulses from the Crab, but we can see “giant” pulses that have increased energies. We have spent 40 hours looking at the Crab pulsar with our telescope, and our results are based on the first 12 hours of processed data.
We found that the pulse energies fit a more gentle energy distribution that people work between 110 and 190 MHz, with results from the LOFAR core and MWA indicating a power law fit (N_{v,pulses }∝ I^{α}) close to −3.1, though we observe a fit of −2.86±0.07, which appears to decrease as more data is processed, trending towards the fit seen with the Lovell after 1,000,000 pulses of −2.4.
Performing analysis of the scattering time scales across our bandwidth using a thick screen pulse broadening function, we found the power law of scattering (𝛕_s ∝ 𝛎^α) across the bandwidth fits to a Guassian distribution of −3.7±0.5, in line with other observations at our frequencies and within error of the models for thin screens of electrons as the source of scattering (-4 to -4.4).
Our next steps are to perform an absolute flux calibration on the data and finish processing the remaining 28 hours of raw data. The software written as a part of this project has already been used to attempt to detect single pulses from other targets, such as rotating radio transients and the CHIME R3 periodic fast radio burst source.