Unusual frequency dependence of the integrated profile of the millisecond pulsar PSR J2145-0705

Abstract

We report on observations of the recently discovered millisecond pulsar PSR J2145-0705 (Bailes et al. 1994a) at 102 MHz, extending the frequency range of its study four times towards lower frequencies. The analysis of our measurements and higher frequency data of Bailes et al. 1994b show an unusual frequency dependence of the integrated profile: the separation between the components of the profile increases with frequency contrary to a decrease inherent to most pulsars and the generally adopted radius-to-frequency mapping model with a dipole magnetic field. The spectrum of PSR J2145-0705 in the range from 100 to 1500 MHz can be approximated by a power law F ∝ fα, where α = -1.4. There is no evidence for a low-frequency turn-over. The spectrum of the trailing component is significantly steeper (α = -1.9) than that of the leading one (α = -1.0). We interpret the anomalous frequency dependence of the integrated profile as the effect of a quadrupole magnetic field. The observed difference in the spectra of the leading and trailing component is interpreted as due to the spatial variation of the energetic spectra of the emitting particles across the emission cone.