THE CHARACTERISTICS OF MILLISECOND PULSAR EMISSION - I - SPECTRA, PULSE SHAPES, AND THE BEAMING FRACTION

The extreme physical conditions in millisecond pulsar magnetospheres, as well as an evolutionary history that differs from that of normal pu lsars, raise the question whether these objects also differ in their r adio emission properties. We have monitored a large sample of millisec ond pulsars for a period of 3 yr using the 100 m Effelsberg radio tele scope in order to compare the radio emission properties of these two p ulsar populations. Our sample comprises a homogeneous data set of very high quality. With some notable exceptions, our findings suggest that the two groups of objects share many common properties. A comparison of the spectral indices between samples of normal and millisecond puls ars demonstrates that millisecond pulsar spectra are not significantly different 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 t han normal pulsars. We confirm recent suggestions that a diversity exi sts among the luminosities of millisecond pulsars, with the isolated m illisecond pulsars being less luminous than the binary millisecond pul sars, implying that the different evolutionary history has an influenc e on the emission properties. There are indications that old milliseco nd pulsars exhibit somewhat flatter spectra than the presumably younge r ones. We present evidence that, contrary to common belief, the milli second pulsar profiles are only marginally more complex than those fou nd among the normal pulsar population. Moreover, the development of th e profiles with frequency is rather slow, suggesting very compact magn etospheres. The profile development seems to anticorrelate with the co mpanion mass and the spin period, again suggesting that the amount of mass transfer in a binary system might directly influence the emission properties. The angular radius of radio beams of millisecond pulsars does not follow the scaling predicted from a canonical pulsar model ap plicable for normal pulsars. Instead, they are systematically smaller, supporting the concept of a critical rotational period below which su ch a scaling ceases to exist. The smaller inferred luminosity and narr ower emission beams will need to be considered in future calculations of the birthrate of the Galactic population.