Partially Absorbed Comptonization Spectrum from the Nearly Edge-on Source X 1822-371

Abstract

We report the results of a spectral analysis over the range 0.1-200 keV performed on the dipping source X 1822-371 observed by BeppoSAX. We find the best fit to the continuum using a partially covered Comptonization model, due to scattering off soft seed photons by electrons at a temperature of ~4.8 keV, without the presence of any soft blackbody emission. The equivalent hydrogen column obtained for the absorbed component is ~4.5 10^{22} cm^{-2}, an order of magnitude larger than the Galactic absorption for this source, and the covering fraction is ~71%. Because the inclination angle of X 1822-371 to the line of sight is ~85^\circ, this model gives a reasonable scenario for the source: the Comptonized spectrum could come from an extended accretion disk corona (ADC), probably the only region that can be directly observed due to the high inclination. The excess of matter producing the partial covering could be close to the equatorial plane of the system, above the outer disk, occulting the emission from the inner disk and the inner part of the ADC. An iron emission line is also present at ~6.5 keV with an equivalent width of ~150 eV. We argue that this strong iron line cannot be explained as reflection of the Comptonized spectrum by the accretion disk. It is probably produced in the ADC. An emission line at ~1.9 keV (with an equivalent width of ~54 eV) and an absorption edge at \~8.7 keV (with an optical depth of ~0.1) are also required to fit this spectrum. These features are probably produced by highly ionized iron (Fe XXIV) present in the outer part of the ADC, where the plasma density is \~10^{11}-10^{12} cm^{-3} and ionized plasma is present.