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

4. Discussion

We have presented the results of the Beppo SAX observation of 3C273 during the SVP. The overall continuum is well represented by a simple power-law from keV to 200 keV. A weak iron line is observed in the MECS spectrum. Below 1 keV, The LECS data show a soft excess and a feature in absorption.

A non-thermal origin for the hard X-ray/ -photons has been suggested so far. Arguments based on the observed -rays luminosity and rapid X-ray variability show that the radiation must be relativistically beamed (Lichti et al. 1995). Relativistic electrons associated with the jet upscatter lower energies photons located in the same jet (synchrotron radiation, Maraschi et al. 1992; Bloom & Marscher 1993) or/and coming from the accretion disk (Dermer et al. 1992) or from the Broad Line Region (Sikora et al. 1994). If this picture is a realistic representation of 3C273, the weak line (EW eV) can be interpreted as a typical iron line from accretion disk, diluted by a strong Doppler-enhanced continuum. An underlying 'Seyfert like' component is then expected to take part to the X-ray emission. The amount of such a contribution can be deduced by comparing the EW of the observed line with the typical EW ( eV) of the same line in Seyfert 1 galaxies (Nandra and Pounds 1994). It turns out to be at 6 keV.

We explored whether the Seyfert-like component can at least partially account for the observed soft excess. The LECS+MECS spectra were modeled with a double power-law plus a notch with fixed Galactic absorber. One power law slope was left to vary, the other one was fixed to the typical Seyfert 1 photon index (). The fit result is: , @ 6.4 keV =411/381 d.o.f.). The notch parameters are consistent with the ones in Table 2. We conclude that an unbeamed underlying component could account for the observed soft excess, although a residual contribution from the hard tail of the UV bump cannot be completely ruled out (Fig. 3).

Fig. 3. Spectrum (upper panel) and residuals (lower panel) when a double power-law model is applied to the spectrum of 3C273 in the 0.12-10  keV band.

We also report the detection of an absorption feature in the soft X-ray spectrum of 3C273. Such a feature can be modelled either with an absorption edge or with a notch. No discrimination between the two models can be made on statistical basis.

In the former case, the best-fit energy is consistent with the OIII-OV ionization stages at the source rest frame. We used Cloudy (version 90.01) to simulate absorption from gas characterized by a low ionization state, in order to reproduce the observed features. The ionizing continuum was described by a broken power law, as indicated by the BeppoSAX  data, with the softer part (spectral index ) extrapolated to lower energies to represent the UV continuum. The model can reproduce the observed feature, but, in addition, predicts strong absorption due to C (IV,V) and N (IV,VI) and HII below 0.5 keV, in completely disagreement with the excess emission present in the LECS data.

A possibility is that the absorption edge is produced by highly ionized oxygen (OVII, OVIII), as usually observed in Seyfert 1 galaxies. In this case, the gas is required to inflow () to explain the redshifted position of the feature

Alternatively, the notch structure might be due to highly ionized Oxygen (the most likely candidate being the OVIII Ly- ) as suggested for PKS2155-304 (Canizares & Kruper 1984; Madjeski et al. 1991, Giommi et al. 1997). In such a case, high-velocity material () within the jet might produce the blueshifted absorption through.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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