Pulsars can have a steep radio spectrum at frequencies around 1 GHz and can be highly polarised (e.g. Manchester & Taylor 1977). Synthesis maps can therefore be used to select pulsar candidates on the basis of their steep spectrum and/or high degree of polarisation. These sources are later observed with a high time resolution instrument to search for pulsations. At least two pulsars, PSR B1937+21 (Backer et al. 1982) and PSR J0218+4232 (Navarro et al. 1995), have been found in this way, while they were missed in regular pulsar surveys because their pulses were smeared out in the detection process due to their small period and their high dispersion measure.
The pulsar population found in this way, may supplement the presently known population, since this method has totally different selection effects. These effects can be investigated by studying the spectral indices and polarisation degrees of known pulsars in these continuum observations. The spectral indices are strongly influenced by scintillation. This may cause the flux density of a pulsar to vary by more than 100 percent on time scales of minutes (diffractive scintillation) to days (refractive scintillation).
In this Paper I search for detections of pulsars in the Westerbork Northern Sky Survey (WENSS), a survey performed at 325 MHz. I compare my results with those from Kaplan et al. (1998) and Han & Tian (1999), who did similar analyses with data from the 1400 MHz NRAO VLA Sky Survey (NVSS). Section 2 describes the WENSS and Sect. 3 describes how the pulsar catalog was correlated with the WENSS source catalog. In Sect. 4 the positions of the non-detected pulsars are searched for flux densities above three times the local noise level. Section 5 discusses the remaining non-correlations. Section 6 combines the WENSS results with the NVSS correlation studies to determine the spectral indices and compares these with values reported in other literature. Finally, in Sect. 7 the role of scintillation is discussed.
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