UNUSUAL FREQUENCY-DEPENDENCE OF THE INTEGRATED PROFILE OF THE MILLISECOND PULSAR PSR J2145-0705

We report on observations of the recently discovered millisecond pulsa r PSR J2145-0705 (Bailes et al. 1994a) at 102 MHz, extending the frequ ency range of its study four times towards lower frequencies. The anal ysis of our measurements and higher frequency data of Bailes et al. 19 94b show an unusual frequency dependence of the integrated profile: th e separation between the components of the profile increases with freq uency contrary to a decrease inherent to most pulsars and the generall y adopted radius-to-frequency mapping model with a dipole magnetic fie ld. The spectrum of PSR J2145-0705 in the range from 100 to 1500 MHz c an be approximated by a power law F proportional to f(alpha), where al pha = -1.4. There is no evidence for a low-frequency turn-over. The sp ectrum of the trailing component is significantly steeper (alpha = -1. 9) than that of the leading one (alpha = -1.0). We interpret the anoma lous 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 spatia l variation of the energetic spectra of the emitting particles across the emission cone.