Studies of cosmic plasma using radioastron VLBI observations of giant pulses of the pulsar B0531+21

Аннотация

The structure of the interstellar plasma in the direction of the pulsar in the Crab Nebula is studied using several sets of space-VLBI observations obtained with networks of ground telescopes and the RadioAstron space antenna at 18 and 92 cm. Six observing sessions spanning two years are analyzed. Giant pulses are used to probe the cosmic plasma, making it possible to measure the scattering parameters without averaging. More than 4000 giant pulses were detected. The interferometer responses (visibility functions) on ground and ground–space baselines are analyzed. On the ground baselines, the visibility function as a function of delay is dominated by a narrow feature at zero delay with a width of δ_τ ~ 1/B, where B is the receiver bandwidth. This is typical for compact continuum sources. On the ground–space baselines, the visibility function contains a set of features superposed on each other and distributed within a certain interval of delays, which we identify with the scattering time for the interfering rays τ. The amplitude of the visibility function on ground baselines falls with increasing baseline; the scattering disk is partially resolved at 18 cmand fully resolved at 92 cm. Estimates of the scattering angle ? give 0.5–1.3mas at 18 cm and 14.0 mas at 92 cm. The measured values of ? and τ are compared to estimate the distance from the source to the effective scattering screen, which is found at various epochs to be located at distances from 0.33 to 0.96 of the distance from the observer to the pulsar, about 2 kpc. The screen is close to the Crab Nebula at epochs of strong scattering, confirming that scattering on inhomogeneities in the plasma in the vicinity of the nebula itself dominates at these epochs.