The Characteristics of Millisecond Pulsar Emission. I. Spectra, Pulse Shapes, and the Beaming Fraction

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Abstract

The extreme physical conditions in millisecond pulsar magnetospheres, as well as an evolutionary history that di ers from that of normal pulsars, raise the question whether these objects also di er in their radio emission properties. We have monitored a large sample of millisecond pulsars for a period of 3 yr using the 100 m E elsberg radio telescope in order to compare the radio emission properties of these two pulsar populations. Our sample comprises a homogeneous data set of very high quality. With some notable exceptions, our ndings suggest that the two groups of objects share many common properties. A comparison of the spectral indices between samples of normal and millisecond pulsars demonstrates that millisecond pulsar spectra are not signicantly di erent from those of normal pulsars. This is contrary to what has previously been thought. There is evidence, however, that millisecond pulsars are slightly less luminous and less efficient radio emitters than normal pulsars. We conrm recent suggestions that a diversity exists among the luminosities of millisecond pulsars, with the isolated millisecond pulsars being less luminous than the binary millisecond pulsars, implying that the di erent evolutionary history has an inuence on the emission properties. There are indications that old millisecond pulsars exhibit somewhat atter spectra than the presumably younger ones. We present evidence that, contrary to common belief, the millisecond pulsar proles are only marginally more complex than those found among the normal pulsar population. Moreover, the development of the proles with frequency is rather slow, suggesting very compact magnetospheres. The prole development seems to anticorrelate with the companion mass and the spin period, again suggesting that the amount of mass transfer in a binary system might directly inuence the emission properties. The angular radius of radio beams of millisecond pulsars does not follow the scaling predicted from a canonical pulsar model applicable for normal pulsars. Instead, they are systematically smaller, supporting the concept of a critical rotational period below which such a scaling ceases to exist. The smaller inferred luminosity and narrower emission beams will need to be considered in future calculations of the birthrate of the Galactic population.

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