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 ~6 × 103 s is present at the beginning of the observation. The luminosity during the persistent emission is 1 × 1038 ergs s-1, while during the flare it is 2 × 1038 ergs s-1. We produced broadband (0.1-100 keV) energy spectra during the flare and the persistent emission. At low energies the continuum is well fitted by a model consisting of Comptonization of soft photons, with a temperature of ~0.4 keV, by electrons at a temperature of ~1 keV. After the flare, a power-law component with photon index ~3 is dominant at energies higher than 10 keV. This component contributes ~4% of the total luminosity. During the flare its addition is not statistically significant. An absorption edge at ~8.4 keV, with optical depth ~1, corresponding to the K edge of Fe XXIII-Fe XXV, and an iron emission line at 6.7 keV are also present. The iron-line energy is in agreement with the ionization level inferred from the absorption edge. The hydrogen column deduced from the absorption edge is ~1024 cm-2, 2 orders of magnitude larger than the low-energy absorption measured in this source. We calculated the radius of the region originating the Comptonized seed photons, RW ~ 150 km. We propose a scenario where RW (the Wien radius) is the inner disk radius, a highly ionized torus surrounds the accretion disk, and a magnetosphere is present up to RW. The absorption edge and the emission line could originate in the photoionized torus, while the Comptonized component originates in an inner region of the disk