2 Radio observations  

We grouped together all radio measurements into 17 light curves from 15MHz to 37GHz (see Table 1). The 8.0GHz light curve obtained with the 26m radio telescope of the University of Michigan Radio Astronomy Observatory (UMRAO) is the most complete light curve of 3C 273 obtained by a single observatory during more than 30years (see Fig. 2). It is therefore given alone in the database, whereas other observations around 8GHz are included in the 10GHz light curve. The UMRAO monitoring of 3C 273 at 14.5GHz and 4.8GHz started respectively in 1974 and in 1978. The 5GHz and the 15GHz light curves contain mainly these observations. Details on the instrumentation and the calibration used at the UMRAO are given by Aller et al. (1985) together with the data obtained until 1984. We do not include here the polarization observations, which are publicly available at the UMRAO Database Interface on the WWW at http://www.astro.lsa.umich.edu/obs/radiotel/umrao.html.

Shorter wavelengths radio observations at 22.2 and 36.8GHz were performed since 1980 both with the 14m telescope of the Metsähovi Radio Observatory, Finland and with the 22m telescope of the Crimean Astrophysical Observatory, Ukraine. The 22GHz and 37GHz light curves are very well sampled since 1986 except for a gap in the summer of 1994 due to the replacement of the Metsähovi antenna (see Fig. 2). The observations during 1980-85 and during 1985-90 are respectively published in Salonen et al. (1987) and in Teräsranta et al. (1992), together with details on the measurement methods and the calibrations. Calibrations at 22GHz were usually performed with the nearby source 3C274 (VirgoA, M87), whose flux was taken to be 21.7Jy. Since there might have been a recent outburst in 3C274, the 22GHz data from Metsähovi presented here are only calibrated with the primary calibrator DR21. We therefore have the same calibration procedure at 22 and 37GHz. The differences between the two calibrations are generally within the uncertainties.

Daily observations of 3C 273 were performed by the Green Bank Interferometer (GBI) at 2.7GHz and at 8.1GHz from 1979 to 1988. This huge data set was published by Waltman et al. (1991). Additional observations carried out with new receivers at 2.25GHz and at 8.3GHz from 1989 to 1994 are also included in the database. The GBI light curves display brightness dips, which occur when the sun is too close to 3C 273 on the sky. Since the GBI is an interferometer, it does not measure the total flux of an extended source like 3C 273. It is therefore difficult to compare the GBI measurements with single dish telescope observations. For clarity, the GBI data are stored in separate files.

Other repeated radio observations from the literature were added to the database. Observations at 2.7, 4.75 and 10.55GHz from the 100m telescope at Effelsberg, Germany reported in von Montigny et al. (1997) were added to the 2.5GHz, the 5GHz and the 10GHz light curves respectively. The 5GHz and 10GHz light curves also contain earlier observations at 6.6GHz and at 10.6GHz from the 46m telescope of the Algonquin Radio Observatory (Medd et al. 1972; Andrew et al. 1978). Observations at 7.8, 7.9 and 15.5GHz from the 37m antenna of the Haystack Radio Observatory are included in the 10GHz and in the 15GHz light curves (Allen & Barrett 1966; Dent & Kapitzky 1976; Dent & Kojoian 1972; Dent et al. 1974). We also added to the 22GHz and 37GHz light curves the 22 and 44GHz observations from the 13.7m Itapetinga radio telescope, Brazil (Botti & Abraham 1988) and the 24GHz observations from the UMRAO (Haddock et al. 1987). Finally, we included in the 37GHz light curve a few earlier observations at 31.4GHz made with the 11m antenna of the National Radio Astronomical Observatory (NRAO) at Kitt Peak (Dent & Hobbs 1973).

We did not include all the isolated observations from early radio catalogues. We added only the flux measurements reported by Kühr et al. (1981), since they are all recalibrated to the scale of Baars et al. (1977), and the total flux densities (core plus jet) reported by Conway et al. (1993). At very low frequency (<30MHz), we added the observations from Braude et al. (1979), but multiplied by the scaling factor of 1.23 used for other objects by Kühr et al. (1981). In the 100-1000 MHz range other isolated observations are from Artyukh (1984), Dennison et al. (1981), Fanti et al. (1979, 1981), Fisher & Erickson (1980) and Hunstead (1972). Above 1GHz, some isolated radio observations were found in Jones et al. (1981), Landau et al. (1983) and Lichti et al. (1995). All these data were included in the respective light curves.

The radio light curves of 3C 273 at 8.0GHz, 15GHz and 37GHz are shown in Fig. 2. The contribution from the jet (3C273A) was derived from Fig. A1 of Conway et al. (1993). It is a broken power law with a spectral index $\alpha_{\mathrm{\,jet}}$ of 0.67 below 735MHz (29.0Jy) and of 0.85 at higher frequencies. The flux density of 3C273A declines strongly in the infrared-to-optical domain with $\alpha_{\mathrm{\,jet}}$$\sim$4, as shown in Fig. 4f of Meisenheimer et al. (1989). Figure 6 shows that the jet component is dominant below $\sim$1GHz, whereas it becomes negligible (<5% of $\overline{F_{\nu}}$) above 22GHz.

  

Figure 1: Time versus frequency distribution of all observations of 3C 273 presently in the database. This figure summarizes more than 30years of observations in the spectral range from 10MHz to 1GeV, covering 16 orders of magnitude in frequency. The relative intensity of the observed flux (log($\nu\,F_{\nu}$)) is given by a greyscale

  

Figure 2: Four characteristic light curves of the radio-to-millimetre behaviour of 3C 273 shown on a same scale from 1965 to 1998. The dashed line is the contribution from the jet (3C273A) (see Sect. 2)

  

Table 1: The radio and mm/submm light curves of 3C 273 in the database. The light curves are characterized by the date range (Epoch) between the first and the last observation, the number N of observations, the mean frequency $\overline{\nu}$ in Hz, the mean flux density $\overline{F_{\nu}}$ in Jy and the dispersion $\sigma_{\nu}$ in Jy