MWA and Parkes data confirm broadband nature of giant pulses
Crab pulsar is a relatively young pulsar situated in the central region of the Crab nebula. Its giant pulses are short duration radio bursts consisting of complex superpositions of nanosecond- and microsecond-scale bursts, occurring only at the main-pulse and the inter-pulse phases of the pulsar rotation. Their short duration implies broadband emission but although simultaneous observations had been performed in the past to ascertain the validity this assumption, the emission bandwidth was poorly determined – until recently.
In a new publication by CAASTRO student Samuel Oronsaye and the pulsar team at Curtin University, the researchers report on their simultaneous observations of the Crab pulsar giant pulses with the Murchison Widefield Array (MWA), operating at 193 MHz, and the CSIRO Parkes radio telescope, operating at 1382 MHz. In a single hour, the MWA detected 55 giant pulses while 2075 giant pulses were observed at Parkes. The authors estimated a power-law index of β = -3.35 ± 0.35 and -2:85 ± 0.05 for the giant pulse fluence (i.e. time integrated flux density) distribution observed at the MWA and Parkes, respectively, by using a new approach which removes any bias from the power-law index determination.
The team detected 51% of the MWA giant pulses at Parkes, with spectra indices in the range of -3.6 > α > -4.9. This range is much narrower than previously reported. Ideally, if the giant pulses were broadband, all pulses observed at the MWA frequency would also be observed at the Parkes frequency. The researchers performed a Monte Carlo analysis to investigate the less-than-100% correlation between the MWA and Parkes observations. Their analysis supports the initial assumption that the giant pulse emission in the Crab is intrinsically broadband, with the less-than-100% correlation being due to the relative sensitivities of the two instruments and the width of the spectral index distribution. These results are therefore consistent with the hypothesis that the spectral index of giant pulses is drawn from normal distribution of standard deviation 0.6 but with a mean that displays an evolution with frequency from -3.00 at 1382 MHz to -2.85 at 192 MHz.
S.I. Oronsaye, S.M. Ord, N.D.R. Bhat, S.E. Tremblay, S.J. McSweeney, S.J. Tingay, W. van Straten, A. Jameson et al. in ApJ (2015) “Simultaneous Observations of Giant Pulses from the Crab Pulsar, with the Murchison Widefield Array and Parkes Radio Telescope: Implications for the Giant Pulse Emission Mechanism”