7 X-ray and $\gamma$-ray observations

The database contains X-ray light curves at 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100, 200 and 500keV and $\gamma$-ray light curves at 1MeV, 3MeV, 100MeV, 300MeV and 1GeV (see Table 5). These light curves were constructed with the spectral fit parameters given in the literature. All X-ray observations of 3C 273 until 1990 are summarized in Malaguti et al. (1994). We completed this list to the best of our knowledge with the more recent X-ray and $\gamma$-ray observations of 3C 273 found in the literature. The parameters we used were sometimes directly taken from Malaguti et al. (1994), but we often tried to find additional information in the original publications. We usually preferred to use the parameters from fits in which the absorbing hydrogen column density $N_{\mathrm{\,H}}$ was fixed to its galactic value ($\sim$1.810²⁰atomscm^-2), rather than left as a free parameter of the fit.

Apart from Malaguti et al. (1994), we used the spectral fit parameters from the following references. The European X-ray Observatory Satellite (EXOSAT) observations and most Ginga observations are from Turner et al. (1990). The Roentgen Observatory Satellite (ROSAT) observations are from Staubert et al. (1992), Bühler et al. (1995) and Leach et al. (1995). The CGRO observations are from Johnson et al. (1995), Lichti et al. (1995), McNaron-Brown et al. (1995) and von Montigny et al. (1997). The Advanced Satellite for Cosmology and Astrophysics (ASCA) observations are from Cappi et al. (1998), the SIGMA observations are from Churazov et al. (1994), the Extreme Ultraviolet Explorer (EUVE) observations are from Ramos et al. (1997) and the "Satellite per Astronomia X'' (SAX) observation is from Grandi et al. (1997).

  

Table 5: The X- and $\gamma$-ray light curves of 3C 273 in the database. The parameters are as in Table 1, with $\overline{F_{\nu}}$ and $\sigma_{\nu}$ expressed in Jy

From the spectral fit parameters we constructed the light curves as follows. For each observation, we first derived the mean photon energy $E_{\mathrm{\,mean}}$ in the energy range E₁-E₂ of the fit knowing the spectral index $\alpha$.We then calculated the flux normalization $F_{\mathrm{\,norm}}$ at $E_{\mathrm{\,mean}}$ and its uncertainty $\Delta\,F_{\mathrm{\,norm}}$.Finally, we obtained the flux densities F_E at the photon energies E of the light curves between E₁ and E₂ by the relation

$$F_E=F_{\mathrm{\,norm}}(E/E_{\mathrm{\,mean}})^{-\alpha} \quad \mbox{where} \quad E\in[\,E_1\,;\,E_2\,]\,.$$ (2)

The corresponding flux uncertainties $\Delta\,F_E$ were derived from both $\Delta\,F_{\mathrm{\,norm}}$ and $\Delta\,\alpha$ according to the equation As far as possible, we always converted the uncertainties from the literature into 1-$\sigma$ uncertainties (68% confidence level). $\Delta\,F_{\mathrm{\,norm}}$ and $\Delta\,\alpha$ were respectively assumed to be 5% of $F_{\mathrm{\,norm}}$ and 0.5, if the corresponding uncertainties were not found in the reference. If both upper and lower uncertainties are given by the authors, we fixed the uncertainty to the smaller value.

We also included in the database the Burst and Transient Source Experiment (BATSE) light curve of 3C 273 derived from Earth occultation data, which are made public on the WWW by the Compton Observatory Science Support Center (COSSC). The BATSE light curve was derived from the daily photon fluxes in the range 20-350 keV assuming a photon spectral index $\Gamma$ of 1.7. Its frequency corresponds to the mean photon energy in the range 20-350 keV with $\Gamma$=1.7. We used this value of $\Gamma$, because it is the value assumed when the occultation data are reduced. However, a $\Gamma$ of 1.6 would better correspond to the 1-200 keV SAX observation reported by Grandi et al. (1997). Since the hard X-ray spectral index of 3C 273 does not vary much, we show in Fig. 4 the general shape of the BATSE light curve extrapolated to 2keV with a constant $\Gamma$ of 1.6. This light curve is in fairly good agreement with the contemporary 2keV observations.

  

Figure 6: a and b. Average spectrum of 3C 273 constructed with the parameters $\overline{\nu}$, $\overline{F_{\nu}}$ and $\sigma_{\nu}$ given in Tables 1, 2 and 5 (points) and the average IUE spectrum (solid line). a) the $F_{\nu}$ representation; b) the $\nu\,F_{\nu}$representation. The dashed line is the contribution from the jet (3C273A) (see Sect. 2). The contribution from the host galaxy is suggested by a parabola in $\nu\,F_{\nu}$ (dotted line), which is normalized by the H and V band contributions (crosses) (see Sects. 4 and 5). The relatively small error-bars show that the variability of 3C 273 does not basically change its overall spectral shape