A Hard Tail in the X‐Ray Broadband Spectrum of Circinus X‐1 at the Periastron: A Peculiar Z Source

Abstract

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 Ñare lasting D6 ] 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 Ñare it is 2 ] 1038 ergs s~1. We produced broadband (0.1È100 keV) energy spectra during the Ñare and the persistent emission. At low energies the continuum is well Ðtted by a model consisting of Com-ptonization of soft photons, with a temperature of D0.4 keV, by electrons at a temperature of D1 keV. After the Ñare, a power-law component with photon index D3 is dominant at energies higher than 10 keV. This component contributes D4% of the total luminosity. During the Ñare its addition is not statistically signiÐcant. An absorption edge at D8.4 keV, with optical depth D1, corresponding to the K edge of Fe XXIIIÈFe 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 D1024 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, km. We propose a scenario where (the Wien radius) is the inner disk radius, a R W D 150 R W highly ionized torus surrounds the accretion disk, and a magnetosphere is present up to The R W. absorption edge and the emission line could originate in the photoionized torus, while the Comptonized component originates in an inner region of the disk.