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Astron. Astrophys. 354, 513-521 (2000)

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1. Introduction

Since its discovery as an extragalactic object in 1962 (Schmidt 1963), the quasar 3C 273 is one of the best studied Active Galactic Nuclei (AGN). With a redshift of 0.158 ([FORMULA]800 Mpc for H0 = 60 km/s/Mpc) it is relatively close, and by being bright in all wavelength regions from radio to [FORMULA]-ray energies, it is an excellent candidate for multiwavelength studies.

After eight years of operation, the EGRET experiment aboard CGRO has now detected [FORMULA]-ray emission from more than 70 AGN at energies above 100 MeV (e.g. Hartman et al. 1999). These observations have dramatically changed our picture of these sources. With the exception of Cen A (Sreekumar et al. 1999), all of them are identified with blazars (e.g. Mattox et al. 1997), the AGN subgroup consisting of either flat-spectrum radio quasars or BL Lacertae objects. Two remarkable [FORMULA]-ray characteristics of these sources are that 1) they are highly variable down to time scales of a day or even shorter, and 2) that during flaring states the [FORMULA]-ray luminosity can dominate their bolometric power.

The quasar 3C 273 is one of these blazar-type [FORMULA]-ray loud AGN. It was first detected at [FORMULA]-rays by the COS-B satellite at energies above 50 MeV (Swanenburg et al. 1978), and - until the launch of CGRO in 1991 - remained the only identified extragalactic point source at these energies. 3C 273 was redetected at [FORMULA]-ray energies by the EGRET experiment in 1991 (von Montigny et al. 1993). Analysing the first four years of EGRET data, von Montigny et al. (1997)found a time-variable [FORMULA]-ray flux, consisting of detections as well as non-detections in individual observational periods. Spectral variability was observed as well, showing the trend of spectral hardening with increasing flux. The third EGRET source catalogue (Hartman et al. 1999) lists 3C 273 with an average flux value of (15.4[FORMULA]1.8) [FORMULA] 10-8 ph cm-2 s-1 for energies above 100 MeV and the time period between April '91 and October '95.

3C 273 was first discovered to be an emitter of low-energy [FORMULA]-rays by COMPTEL in 1991 (Hermsen et al. 1993). The source is frequently detected in individual CGRO pointings (e.g. Collmar et al. 1999), however, non-detections occur as well indicating time variability of the MeV flux on time scales of months (Williams et al. 1995). In time-averaged analyses 3C 273 is detected very significantly and shows in the 0.75-30 MeV band a soft spectrum, i.e. photon index [FORMULA] in combined data (Collmar et al. 1996). However, combining contemporaneous high-energy data reveals that the MeV-band is a transition region for the spectrum of 3C 273 showing a turnover from a harder ([FORMULA]1.7) spectrum at hard X-ray energies to a softer one ([FORMULA]2.5) at high-energy ([FORMULA]100 MeV) [FORMULA]-rays (e.g. Lichti et al. 1995; von Montigny et al. 1997).

At hard ([FORMULA]50 keV) X-rays 3C 273 is always significantly detected by the Oriented Scintillation Spectrometer Experiment (OSSE) showing flux variations in the 50-150 keV band up to a factor of 8 during 5 years (McNaron-Brown et al. 1997). A power-law spectrum with a photon index of roughly 1.7 (Johnson et al. 1995) up to [FORMULA]1 MeV is typically observed. Above [FORMULA]1 MeV a spectral softening is found, consistent with the results of the multiwavelength campaigns. Recently, during the highest flux state as observed by OSSE, evidence for a low-energy spectral break at about 0.3 MeV was found (McNaron-Brown et al. 1997) suggesting an anticorrelation between flux and break energy.

In this paper we report on 7 weeks of continuous [FORMULA]-ray observations by the EGRET and COMPTEL experiments aboard CGRO in December 1996 and January 1997. In Sect. 2 we describe the observations and the data analyses, in Sect. 3 we give the results, and discuss their implications in Sect. 4. Finally, the conclusions are presented in Sect. 5.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000