1. INTRODUCTION

3C 273 is the brightest quasar on the celestial sphere. This would suffice to qualify it to be the object of intense study over the years since the discovery of quasars in 1963. In addition, however, 3C 273 displays most if not all the phenomena that make Active Galactic Nuclei (AGN) such intriguing objects. Indeed, 3C 273 is a radio loud quasar that shows large flux variations at all wavelengths and at some epochs a non negligible polarization. 3C 273 has a small scale jet, the features of which move away from the core at velocities apparently larger than the speed of light and a radio structure that extends to large distances. Whether this makes 3C 273 an archetype of AGN in general or merely a very special case is left to the reader to assess. What is certain, though, is that the study of 3C 273 is relevant to all the AGN physics. The study of 3C 273 has therefore been very actively pursued over more than 3 decades.

This study has been further made easier by the position of 3C 273 very close to the celestial equator. This means that the object can be (and has been extensively) observed with the most appropriate instruments located as far North as Finland and as far South as Chile. This privileged position on the sky has the only disadvantage that the Sun comes close to 3C 273 on the celestial sphere during the year, hence hampering regularly the observations.

We will review in the next pages the early work on 3C 273, analyse then the existing data on the continuum and line emission including their variations. We will then discuss the jet which is observed at small and large distances from the core. The host galaxy and environment will also be described. Some of the efforts made to understand the physical nature of emission components and the quasar itself will be presented.

This review is written shortly after the switch off of the International Ultraviolet Explorer (IUE) satellite which has been one of the main research instruments for AGN in recent years. It is therefore timely, as it ought to be possible to summarize the knowledge we have including all the IUE data. Large sets of data in other wave bands have been obtained in a coordinated way while IUE was taking observations, as shown in Fig. 1. Some discussions of these data and in particular cross correlations between the light curves in different bands are included in the present review, although the work is still on-going. It will be seen that it is difficult to understand all these data in a consistent way.

Figure 1. The flux per logarithmic frequency internal . f is grey coded for all the frequencies and all the epochs since 1975 for which data available to us have been obtained. (From Türler et al. in preparation, see section 3). This figure shows the vast amount of data that have been gathered but illustrates also that most of the x epoch plane is not covered. A good understanding of the object would allow us to confidently extrapolate the observed data to fill the gaps. This understanding is not available yet.

The equatorial coordinates of 3C 273 are ₂₀₀₀ = 12 h 29 m 06.7 s, ₂₀₀₀ = + 02°03'08". The galactic coordinate of the quasar are l = 289.95 and b = +64.36. The V magnitude (average) is 12.9. 3C 273 is therefore a high galactic latitude object, a clear advantage to minimize the effects of gas and dust along the line of sight. The redshift of 3C 273 is z = 0.158 (Veron-Cetty & Veron 1995).