Probing cosmic plasma with giant radio pulses

VLBI observations of the Crab pulsar with the 64-m radio telescope at Kalyazin (Russia) and the 46-m radio telescope of the Algonquin Radio Observatory (Canada) at 2.2 GHz and single-dish observations of the millisecond pulsar B1937+21 with the GBT (USA) at 2.1 GHz were conducted to probe the interstellar medium and study the properties of giant pulses. The VLBI data were processed with a dedicated software correlator, which allowed us to obtain the visibility of single giant pulses. Two frequency scales of 50 and 450 kHz were found in the diffraction spectra of giant pulses from the Crab pulsar. The location of the scattering region was estimated to be close to the outer edge of the nebula. No correlation was found between the power spectra of giant pulses at left- and right-hand circular polarization. We explain this lack of correlation through the influence of the strong magnetic field on circularly polarized emission in the region close to the Crab pulsar. Combining the measurement of the decorrelation bandwidth with that of the scattering time of giant pulses for B1937+21, we found three frequency scales of 1.7, 3.8, and 16.5 MHz. The scattering time of giant pulses of B1937+21 at 2.1 GHz was found to be 40+-4 ns. We obtained an upper limit of the intrinsic width of giant pulses from B1937+21 of less than 8 ns. The frequency dependences of the scattering times for the Crab pulsar and PSR B1937+21 were found to be different. They are characterized by exponents of -3.5 and -4.2, respectively. We attribute the difference to the large influence of scattering in the Crab nebula.