Quasi-simultaneous observations of 3C 273 by BeppoSAX and ASCA

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We report the results of quasi-simultaneous observations of the bright quasar 3C 273 using the BeppoSAX Low-Energy Concentrator Spectrometer (LECS) and the ASCA Solid-State Imaging Spectrometer (SIS). These observations are part of an intercalibration programme and demonstrate good agreement between both instruments in the common energy range 0.5-10 keV. In particular, the absorption feature discovered by BeppoSAX/LECS at 0.5 keV is consistently seen by ASCA/SIS. We present the most accurate measurement yet of the spectral shape of this absorption feature and provide constraints on its physical interpretation. A self-consistent warm absorber model in photoionization equilibrium only gives an adequate spectral fit if a very high velocity inflow is allowed for the warm absorber (v 0.36 c). The ionization parameter and the column density of the warm absorber are then 1 and 1.3e21 cm-2, respectively. If the differences in best-fit neutral absorption column density between the LECS and each SIS are taken as estimates of the systematic spectral uncertainties, then these are < 0.8e20 and < 2.e20 cm-2 (at 90 % confidence) for SIS0 and SIS1, respectively.