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Astron. Astrophys. 322, 846-856 (1997)

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1. Introduction

A long standing question in pulsar astronomy has been the location of the radio emission. Three places have been suggested; near the light cylinder (Smith 1969), perhaps located in regions of closed magnetic field lines (Gold 1968), and the open field line regions (Radhakrishnan & Cooke 1969). Nowadays, it is widely accepted (see e.g. Lyne & Smith 1990) that the radio emission originates from the open field line region well inside the light cylinder, whereas the very high energy emission (optical and above) probably arises in a quite different mechanism, much nearer to the light cylinder.

The concept of a radius-to-frequency mapping (RFM) i.e. the model that radio emission of a certain frequency, [FORMULA], is radiated at a particular distance, [FORMULA], above the neutron star surface was first thoroughly discussed by Cordes (1978). In fact RFM already exists in several theoretical models. Ruderman & Sutherland (1975), for instance, assumed that the radiation frequency is related to the local plasma frequency. The plasma frequency decreases with distance from the star as the particle density becomes smaller while the plasma flows along the spreading field lines. Therefore, lower frequency radiation is emitted as the plasma frequency decreases away from the star surface. Such a frequency dependence of the emission height can easily explain the observed narrowing of the profile width and component separation with increasing frequency (Sieber et al. 1975).

Such changes in the pulse profile widths are considered to be the best arguments in favour of the existence of a RFM, although they may also be interpreted differently (e.g. by as a dependence on the longitudinal separation to the magnetic axis, see Rickett & Cordes 1981). However, RFM fits in an attractive way to polar cap models where emission comes from the open field line region. Measurements of the profile widths at various frequencies can then be used to deduce the emission heights if dipolar field lines are assumed (Cordes 1978).

In addition to this geometrical method in order to place limits on the location of the emission region, timing data obtained at several frequencies can be used to look for delays in the expected arrival times of pulses caused by retardation, aberration effects or a possible magnetic sweep-back near the light cylinder. Blaskiewicz et al. (1991, hereafter BCW) used another independent method by deducing absolute emission altitudes from high quality polarimetry data. In their relativistic correction to the classical rotating-vector model of Radhakrishnan & Cooke (1969), they predicted a time delay between the centroids of the pulse profile and the corresponding polarisation position angle curve. BCW determined this delay for several profiles and determined emission altitudes for 0.408 GHz and 1.400 GHz emission. The same method was recently applied by Hoensbroech & Xilouris (1997) to data obtained between 1.400 GHz and 10.550 GHz. Both studies found a general good agreement with the results obtained from the geometrical method based on the profile width (cf. also Xilouris et al. 1996).

Interestingly, most of these studies were made at frequencies below 5 GHz, and rather little is known about the validity of a RFM at high frequencies (see Kramer et al. 1994, Xilouris et al. 1996, Hoensbroech & Xilouris 1997). This is of great importance, since emission observed at very high radio frequencies should originate from locations very close to the stellar surface, where the assumption of dipolar field lines might not hold true any longer but magnetic multipoles might become important (Wolszczan et al. 1980, BCW). In fact some observers have already claimed the detection of magnetic multipoles based on observations made at about 5 GHz (e.g. Davies et al. 1984, Kuzmin 1986).

The detection of pulsars at the highest radio frequencies ever (Wielebinski et al. 1993, Kramer et al. 1996), raises the question about the origin of the observed radiation. According to the canonical RFM it should be created closer to the neutron star surface than any other radio pulsar emission observed before. Here (paper I) and in a subsequent paper (paper II), we will try to place constraints on the size and location of the emission region by appling timing data obtained at various frequencies. In this paper, we will focus on the high frequency regime, i.e. on profiles observed between 1.4 GHz to 32 GHz with the MPIfR's 100m-radiotelescope in Effelsberg. A combination of the high frequency data with a corresponding low frequency data set observed with the 76m-Lovell telescope in Jodrell Bank will be presented in paper II.

The outline of this paper is as follows. We review in Sect. 2 all the important effects relevant for our analysis of the data. The details of our observations are given in Setc. 3. In Sect. 4 we explain the data analysis and present individual results in the Sect. 5. In Sect. 6 we will discuss the results obtained from the high frequency data set and compare them to previous results before finally conclusions are drawn.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998