1 Introduction

One of the main observables of pulsar emission is its flux density ${S_{\nu }}$, measured at a given central frequency $\nu$ of the receiver bandwidth. Flux measurements are crucial for deriving, the so called, pulsar luminosity function and therefore the birth rate of the Galactic population of neutron stars. The first spectrum of a pulsar taken at 5 frequencies was published by Robinson et al. (1968). The flux density variations and spectra for frequencies between 0.15 and 1.6 GHz were later reported for a number of pulsars by McLean (1973). Spectra of 27 pulsars were published by Sieber (1973) who used pulse energy values obtained with the 100-m and 25-m telescopes of the Max-Planck-Institute for Radioastronomy (MPIfR) as well as other values published in the literature. He was the first to show a turn-over behaviour at low frequencies around 100 MHz and a break in spectrum at high frequencies (about 1 GHz). Sieber et al. (1975) published pulse shapes and their energies for 35 pulsars at 2.7 and 4.9 GHz, and for 7 pulsars at 10.7 GHz. Izvekova et al. (1981) and Slee et al. (1986) presented an analysis of flux measurements for pulsars at meter wavelengths ($\sim$ 80 MHz). A compilation of spectra of 45 pulsars over a wide frequency range was published for the first time by Malofeev et al. (1994). Seiradakis et al. (1995) published a collection of high frequency data on pulsar profiles and flux densities for 183 pulsars at 1.4 GHz, 46 pulsars at 4.85 GHz and 24 pulsars at 10.5 GHz. The catalogue of pulsar flux density measurements for 281 pulsars at frequencies ranging from 0.4 GHz to 1.6 GHz was published by Lorimer et al. (1995). The first spectra for four millisecond pulsars were published by Foster et al. (1991). Kramer et al. (1998) presented the results of flux density measurements for 23 millisecond pulsars at frequencies 1.4 and 1.7 GHz and an analysis of their spectra. Van Ommen et al. (1997) presented polarimetric data together with flux density measurements for a large number of southern pulsars at frequencies of 800 MHz and 950 MHz. Most recently, Toscano et al. (1998) have presented flux density measurements for southern millisecond and slow pulsars at frequencies between 0.4 GHz and 1.6 GHz, and Kramer et al. (1999) studied the emission properties of millisecond pulsars up 4.85 GHz. A first large sample of flux densities of weak pulsars at 4.85 GHz was published by Kijak et al. (1998).

It has been long believed that flux density spectra for most of the pulsars have been well described by a simple power law $S \propto \nu ^{\alpha }$with spectral index of $\sim {-1.6}$ (Sieber 1973; Lorimer et al. 1995). However, a considerable fraction of pulsars demonstrated spectra that required modelling by two power laws (Sieber 1973; Malofeev et al. 1994). These were commonly called the broken-type spectra. In this paper we show that the two power law spectra are rare exception and that majority of pulsars can be modelled by a single power law. Our analysis shows that only 10% of pulsars requires two power law spectra.

Recently, pulsar flux density measurements have been extended to the mm-wavelengths region, which provided information about this newly explored spectral region. These measurements suggested that spectrum flattens out or even turns up at very high frequencies (Kramer et al. 1996). In this paper we present a new and most complete compilation of spectra for 281 pulsars in the frequency range from 39 MHz to 43 GHz. The flux density measurements for frequencies from 300 MHz to 1.4 GHz were done at Jodrell Bank (Lorimer et al. 1995). For frequencies equal or above 1.4 GHz we have utilized flux measurements made by different authors at the Effelsberg Radiotelescope of the Max-Planck Institute for Radioastronomy (Sieber et al. 1975; Bartel et al. 1978; Sieber & Wielebinski 1987; Wielebinski et al. 1993; Seiradakis et al. 1995; Kramer et al. 1996; von Hoensbroech et al. 1997; Kramer et al. 1997; Kijak et al. 1998). We have also reduced a significant amount of the unpublished data available at the archives of the MPIfR. Most of these data is available throughout the European Pulsar Network Database (Lorimer et al. 1998). We have also included the data published by Izvekova et al. (1981) and Malofeev et al. (2000) containing observations made at low radio frequencies from 39 MHz to 102.5 MHz. Our own observations at 4.85 GHz made in Effelsberg in 1998 are also included.