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Why do the microstructures of the main pulse and the interpulse of the pulsar in the Crab nebula differ so dramatically?
Theoretical and Mathematical Physics, Vol. 202, #3, pp.390-398 (2020)

V.M. Kontorovich

Pulsars are magnetized neutron stars. They are not resolved by modern radio telescopes and are studied only by radiation coming from the regions of the magnetic poles. Because of the rotation, this narrow radiation is received as pulses. In a few pulsars whose magnetic axis is almost orthogonal to the rotation axis (the simplest hypothesis), pulses are observed from both poles: the (main) pulse and the interpulse. Such objects primarily include a pulsar in the Crab nebula, observed at many frequencies of the electromagnetic spectrum. In the remarkable work of Hankins and Eilek, a striking difference between the spectra of the main pulse and the interpulse in the Crab nebula in the centimeter wavelength range at microsecond resolution was found (surprising the authors: ?In traditional pulsar models... the MP and IP should be the same in their observable quantities (such as spectrum, time signature, or dispersion). We were?and remain?quite surprised that this turns out not to be the case in the Crab pulsar.? See T. H. Hankins and J. A. Eilek, ?Radio emission signatures in the Crab pulsar,? Astrophys. J., 670:693?701, 2007). In particular, a wide range of frequencies was observed in the spectra of the main pulse forming ?vertical structures,? while ?horizontal structures? with distinguished frequencies were observed in the spectra of the interpulse at the same frequencies. Such a difference, related to different radiation mechanisms (nonrelativistic electron emission in a longitudinal accelerating field for the main pulse and relativistic positron radiation due to the curvature of magnetic field lines for the interpulse), is explained by the change from the nonrelativistic to the relativistic mechanism as the frequency increases. Therefore, the frequencies at which the mechanism changes differ for the main pulse and the interpulse. The frequency of observation in the work of Hankins and Eilek is just between these frequencies with which the difference in the microstructure is connected.