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Astron. Astrophys. 325, L17-L20 (1997)

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1. Introduction

3C273 is a bright, relatively nearby ([FORMULA]) quasar and therefore has been very well studied at all wavelengths from radio to [FORMULA] -rays. It is a core-jet radio source characterized by a compact nucleus and a jet showing superluminal motion. Multifrequency campaigns have recently shown that at least 3 maxima characterize the Spectral Energy Distribution (SED) of this object, the emission peaking in the IR, in the UV and in the [FORMULA] -rays (Lichti et al. 1995, von Montigny et al. 1997). The complexity of the spectrum indicates that several physical mechanisms contribute to the continuum. It is generally believed that synchrotron emission dominates the spectrum from radio to IR frequencies (Robson et al. 1993), thermal emission is responsible for the UV bump (Ulrich et al. 1988) and inverse Compton emission produces the observed spectrum from a few keV up to GeV energies (von Montigny et al. 1997 and references therein). In the 2-10 keV region, 3C273 is represented by a power-law which extends up to [FORMULA] MeV, where a break has been recently detected by OSSE (McNaron-Brown et al. 1995). The presence of a reflection component, similar to that observed in Seyfert galaxies, was suggested by GINGA data (Williams et al. 1992), but has never been confirmed. On the contrary, the iron line at 6.4 keV (rest frame) detected by GINGA, when the source was at a very low flux level, was later confirmed by ASCA (Cappi & Matsuoka 1996). At low energies, EXOSAT and ROSAT found evidence of a strong excess, usually parameterized by power-law or thermal models (Turner et al. 1990; Leach et al. 1995; Laor et al. 1994). Here we will show that the low-energy spectrum of 3C273 is actually more complex than implied by previous observations. Below 1 keV, the BeppoSAX   data show a clear feature in absorption and a soft component. This is the first time that an absorption feature at [FORMULA] keV (observer frame) is unambiguously detected in the 3C273 spectrum.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998