The blue-bump of 3C 273

Abstract

We present optical and ultraviolet observations of 3C 273 covering the whole life of the IUE satellite. We analyze the variability properties of the light curves, and find that two variable components, written V_\nuAA\ and S_\nuBB\ respectively, must contribute to the S_\nuB\ emission in this object. The V_\nuAA\ component produces most of the variability in the ultraviolet domain. A maximum time scale of variability of about 2 yr identical at all wavelengths is found. If discrete events produce this component, the event rate is ~ 3-30 yr(-1) . Assuming an isotropic emission, each event must liberate about 10(51) erg in the form of optical-to-ultraviolet radiation. The spectral properties of the V_\nuAA\ component suggest that reprocessing on a truncated disk, or partially-thick bremsstrahlung may be the emission mechanism. We find evidence of a lag of a few days between the light curves of the V_\nuAA\ component at optical and ultraviolet wavelengths. An irradiated geometrically-thick accretion disk model satisfies all the constraints presented here. Neither the variability properties, nor the spectral properties of the S_\nuBB\ component can be accurately measured. This component varies on very long time scales, and may completely dominate the historical light curve of 3C 273 in the optical domain. Combining knowledge from other wavelengths, we obtain several indications that this component could reveal the ``blazar'' 3C 273. In particular, the light curve of this component is very similar to the hard X-ray light curve.