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Astron. Astrophys. 358, 169-176 (2000)

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1. Introduction

Although the majority of pulsar radio flux density spectra peak at frequencies around 400 MHz, during the 1980s it was realised that the sensitivity of pulsar surveys conducted at such frequencies becomes seriously compromised when searching along the Galactic plane. The reasons for this are twofold: (1) The system temperature becomes dominated by the sky background radiation. Typical 400-MHz sky background temperatures are [FORMULA] K in the direction of the Galactic centre, and [FORMULA] K along the Galactic plane (Haslam et al. 1982). (2) The observed pulse width can become much larger than the intrinsic width due to multi-path scattering and/or dispersion by free electrons in the interstellar medium. Both these effects lead to a net reduction in signal-to-noise ratio. In extreme cases of scattering and dispersion, the observed pulse width becomes comparable to the pulse period and the pulsar is no longer visible as a periodic radio source.

Fortunately, all these effects diminish strongly at a higher observing frequency: The brightness temperature of the radio continuum emission [FORMULA] at a given observing frequency [FORMULA] has a power law dependence [FORMULA] with a spectral index [FORMULA] (Lawson et al. 1987; Reich & Reich 1988). This means that the 408-MHz sky background temperatures quoted above are reduced by more than an order of magnitude for high frequency ([FORMULA]-GHz) surveys. Furthermore, pulse dispersion and scattering scale as [FORMULA] and [FORMULA] respectively (e.g. Manchester & Taylor 1977), for an observing frequency [FORMULA] and bandwidth [FORMULA].

Clifton & Lyne (1986) (see also Clifton et al. 1992) were the first to really demonstrate the worth of surveying at high frequencies. In a 1400-MHz survey of a thin strip of 200 deg2 along the Galactic plane, Clifton et al. found 40 new pulsars. All of these sources were missed by a previous 390-MHz survey (Stokes et al. 1985) which overlapped the same region. This was in spite of the fact that, after scaling the sensitivity limits for typical pulsar spectral indices, the Stokes et al. survey had twice the nominal sensitivity of the Clifton et al. survey. Johnston et al. (1992a) carried out a complementary survey of the southern Galactic plane using the Parkes radio telescope at 1520 MHz, finding 46 pulsars missed by previous lower frequency searches covering this region (Manchester et al. 1978).

The pulsars discovered in these two high frequency surveys are primarily young neutron stars that have not had time to move far from their birth places close to the Galactic plane. A large sample of such objects is desirable for studies of the birth and evolution of neutron stars and of the size of the neutron star population in the inner Galaxy (Johnston 1994). In addition, these surveys discovered several interesting binary pulsars including PSR B1259-63 - a 48-ms pulsar in a 3.4-yr orbit around a 10 [FORMULA] Be star (Johnston et al. 1992b).

Significant improvements in sensitivity have lead to renewed interest in Galactic plane searches. In particular Camilo et al. (2000a) report the discovery of over 400 pulsars in the first half of a new survey of the southern Galactic plane using the recently commissioned Parkes [FORMULA] 21-cm multibeam system. Their survey is some seven times more sensitive than the Clifton et al. and Johnston et al. surveys, and the new discoveries already include several binary pulsars (e.g. Lyne et al. 1999), as well as a large number of very distant, high dispersion measure, sources.

A preliminary account of the exciting results from the Parkes multibeam survey (Camilo et al. 1997) prompted us to utilise the large collecting area of the 100-m Effelsberg radio telescope to perform a new search along the northern Galactic plane. In this paper we report on a small survey carried out during 1998 to test the feasibility of future observations with a wide-bandwidth search system currently under development. This pilot search proved successful, discovering four new pulsars - the first ever found with this telescope. In Sect. 2 we describe in some detail the survey observations and data reduction techniques. In Sect. 3 we estimate the sensitivity of the survey. The results are presented in Sect. 4. These results, along with follow-up timing observations, are discussed in Sect. 5. Finally, in Sect. 6, we summarise the main conclusions from this work and their implications for future pulsar search experiments at Effelsberg.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000