The characteristics of millisecond pulsar emission. III. From low to high frequencies

In this paper we present the first observations of a large sample of millis econd pulsars at frequencies of 2.7 GHz (11 cm) and 4.9 GHz (6 cm). For alm ost all sources, these represent the first 11 cm observations ever. The new measurements more than double the number of millisecond pulsars studied at 6 cm. Our new flux measurements extend the known spectra for millisecond p ulsars to the highest frequencies to date. The coverage of more than a deca de of the radio spectrum allows us for the first time to search for spectra l breaks, as so often observed in normal pulsars around 1 GHz. The results suggest that, unlike normal pulsars, millisecond pulsar spectra can be larg ely described by a single power law. We align the observed millisecond puls ar profiles with data from lower frequencies to search for indications of d isturbed magnetic fields, and attempt to resolve questions that have been r aised in recent literature. Deviations from a dipolar magnetic held structu re are clot evident, and absolute timing across the wide frequency range wi th a single dispersion measure is possible. We seem to observe mainly unfil led emission beams, which must originate from a very compact region. The ex istence of nondipolar held components therefore cannot be excluded. A compa ct emission region is also suggested by a remarkably constant profile width or component separation over a very wide frequency range. This observed di fference from the emission properties of normal pulsars is highly significa nt. For a few sources, polarization data at 2.7 and 4.9 GHz could also be o btained that indicate that despite the typically larger degree of polarizat ion at lower frequencies, millisecond pulsars are weakly polarized or even unpolarized at frequencies above 3 GHz. The simultaneous decrease in degree of polarization and the constant profile width thus question proposals tha t link depolarization and decreasing profile width for normal pulsars to th e same propagation effect (i.e., birefringence). Comparing the properties o f core and conal-like profile components to those of normal pulsars, we fin d less significant patterns in their spectral evolution for the population of millisecond pulsars. Hence, we suggest that core and conal emission may be created by the same emission process. Given the small change in profile width, the indicated depolarization of the radiation, and the possible simp le flux density spectra, MSP emission properties tend to resemble those of normal pulsars, only shifted toward higher frequencies.