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 ∼ 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 × 10²⁰ 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 ∼ 20–30 eV), narrow and consistent with being produced by Fe Kα emission from neutral matter. Overall, the observations are qualitatively consistent with a variable, non-thermal X-ray continuum emission (a power law with Γ ∼ 1.6, possibly produced in the innermost regions of the radio-optical jet), plus underlying “Seyfert-like” features (a soft-excess and Fe Kα 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.