The complex X-ray spectrum of 3C 273: ASCA observations

The results obtained from 9 X-ray observations of 3C 273 performed by ASCA are presented (total exposure time similar to 160000 s). The analysis and interpretation of the data was complicated by the fact that 4 of these observations were used for the on-board calibration of the CCDs spectral response. The present standard analysis shows that, in agreement with official recommendations, a conservative systematic error (at low energies) of 2-3 x 10(20) cm(-2) must be assumed when analyzing ASCA SIS data. A soft-excess, with variable flux and/or shape, has been clearly detected as well as flux and spectral variability. An anti-correlation has been found between the spectral index and the flux in the 2-10 keV energy range. Fitting the data with the latest available calibration matrices, we also detect an emission line at 5.4-5.7 keV (6.3-6.6 keV in the quasar frame) in only the two observations with lowest fluxes where it is weak (EW similar to 20-30 eV), narrow and consistent with being produced by Fe K alpha emission from neutral matter. Overall, the observations are qualitatively consistent with a variable, non-thermal X-ray continuum emission (a power law with Gamma similar to 1.6, possibly produced in the innermost regions of the radio-optical jet), plus underlying "Seyfert-like" features (a soft-excess and Fe K alpha line emission). When the continuum (jet) emission is in a low state, the Seyfert-like spectral features (soft-excess, iron line, and possibly a steep power law plus reflection continuum) are more easily seen.