A hard tail in the X-ray broadband spectrum of Circinus X-1 at the periastron: A peculiar Z source

We report on the spectral analysis of the peculiar source Cir X-1 observed by the BeppoSAX satellite when the X-ray source was near the periastron. A flare lasting similar to6 x 10(3) s is present at the beginning of the obse rvation. The luminosity during the persistent emission is 1 x 10(38) ergs s (-1), while during the flare it is 2 x 10(38) ergs s(-1). We produced broad band (0.1-100 keV) energy spectra during the flare and the persistent emiss ion. At low energies the continuum is well fitted by a model consisting of Comptonization of soft photons, with a temperature of similar to0.4 keV, by electrons at a temperature of similar to1 keV. After the flare, a power-la w component with photon index similar to3 is dominant at energies higher th an 10 keV. This component contributes similar to4% of the total luminosity. During the flare its addition is not statistically significant. An absorpt ion edge at similar to8.4 keV, with optical depth similar to1, correspondin g to the K edge of Fe XXIII-Fe XXV, and an iron emission line at 6.7 keV ar e also present. The iron-line energy is in agreement with the ionization le vel inferred from the absorption edge. The hydrogen column deduced from the absorption edge is similar to 10(24) cm(-2), 2 orders of magnitude larger than the low-energy absorption measured in this source. We calculated the r adius of the region originating the Comptonized seed photons, R-w similar t o 150 km. We propose a scenario where R-w (the Wien radius) is the inner di sk radius, a highly ionized torus surrounds the accretion disk, and a magne tosphere is present up to R-w. The absorption edge and the emission line co uld originate in the photoionized torus, while the Comptonized component or iginates in an inner region of the disk.