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 millisecond pulsars at frequencies of 2.7 GHz (11 cm) and 4.9 GHz (6 cm). For almost 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 pulsars to the highest frequencies to date. The coverage of more than a decade of the radio spectrum allows us for the first time to search for spectral breaks, as so often observed in normal pulsars around 1 GHz. The results suggest that, unlike normal pulsars, millisecond pulsar spectra can be largely described by a single power law. We align the observed millisecond pulsar profiles with data from lower frequencies to search for indications of disturbed magnetic fields, and attempt to resolve questions that have been raised in recent literature. Deviations from a dipolar magnetic field structure are not evident, and absolute timing across the wide frequency range with a single dispersion measure is possible. We seem to observe mainly unfilled emission beams, which must originate from a very compact region. The existence of nondipolar field components therefore cannot be excluded. A compact emission region is also suggested by a remarkably constant profile width or component separation over a very wide frequency range. This observed difference from the emission properties of normal pulsars is highly significant. For a few sources, polarization data at 2.7 and 4.9 GHz could also be obtained that indicate that despite the typically larger degree of polarization 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 that link depolarization and decreasing profile width for normal pulsars to the same propagation effect (i.e., birefringence). Comparing the properties of core and conal-like profile components to those of normal pulsars, we find 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 simple flux density spectra, MSP emission properties tend to resemble those of normal pulsars, only shifted toward higher frequencies.