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Astron. Astrophys. 352, 489-494 (1999)

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1. Introduction

The frequency dependence of pulsar integrated profiles holds significant information on pulsar emission physics. In particular the narrowing of the profile width with increasing frequency, often observed at slow normal pulsars, has led many researchers to suggest that the magnetic field dominating the pulsar emission region has a purely dipolar structure. Furthermore, a radius-to-frequency mapping (RFM) is thought to exist in the magnetospheres of at least the normal pulsars. In this model the radio emission is closely related to the local plasma frequency, resulting in a stratification of the radio emission region. The high radio frequencies are emitted closer to the stellar surface while the lower frequencies are considered to emanate from further out. The integrated profile experiences a progressive narrowing as frequency increases (Komesaroff et al. 1970, Cordes 1978, Thorsett 1991). An alternative explanation attributes the observed low-frequency profile broadening to a propagation effect: the birefringence is assumed, with the whole spectrum emitted at the same altitude above the stellar surface. In this picture a RFM is not necessary (McKinnon 1997).

Millisecond pulsars are believed to be a special population of pulsars, which distinguish from normal pulsars by period, their first derivative, magnetic field strength, age and most importantly evolutionary history (Lyne & Smith 1990). One may expect that the radio emission characteristics of millisecond pulsars will be different from those of normal pulsars as well. However, it is only recently that comparative studies of the radio emission characteristics between millisecond and normal pulsars have appeared in the literature.

Manchester (1992) compared the pulse shape and polarization of two millisecond and two normal pulsars as well as the distribution of the observed radio luminosity. In this work Manchester concluded that the observed properties of the pulsed radio emission between the two pulsar populations are remarkably similar. Foster et al. (1991) studied the profile frequency dependence of four millisecond pulsars and concluded that in the frequency range between 425 and 3000 MHz no significant profile width narrowing is evident. Kuzmin & Losovsky (1996) performed 102 MHz observations of the millisecond pulsar PSR J2145-0705 and discovered an unusual frequency dependence of its profile. As shown by Kuzmin & Losovsky (1996) the width of the integrated profile of PSR J2157-0705 is nearly independent of frequency. Evidence is presented that the component separation of this pulsar profile even increases with frequency. Kramer et al. (1998) and Xilouris et al. (1998) presented a collection of 27 profiles of millisecond pulsars at 1.4 GHz including polarimetric observations. Comparing the frequency development of pulse profiles of normal and millisecond pulsars between 400 and 1400 MHz they showed that the development of the profiles of millisecond pulsars with frequency is rather slow, in stark contrast with what is known for normal pulsars.

These studies concentrate mainly on high radio frequencies, while there is a lack of data at lower radio frequencies. Low frequency radio observations are in particular difficult due to the scattering that the radio signals suffer as they propagate through the interstellar medium.

Kuzmin & Losovsky (1999) presented a collection of 20 millisecond pulsars at 102 MHz, which extends our knowledge of millisecond pulsar profiles to the lowest frequency where such observations have been performed so far. Based on these observations and higher frequency data we present in this work the results of an analysis of the frequency dependence of the integrated profile width for 12 millisecond pulsars in the frequency range between 102 [FORMULA] 1400 MHz. We compare our results with a sample of normal pulsars searching for similarities and differences between these two pulsar populations.

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999