HEAD 2000, November 2000
Session 25. X-Ray Binaries - Spectroscopy
Display, Wednesday, November 8, 2000, 8:00am-6:00pm, Bora Bora Ballroom

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[25.11] A Hard Tail in Broad Band Spectrum of Cir X-1: a clue for a Z-source behavior

R. Iaria, T. Di Salvo, A. La Barbera (DSFA-University of Palermo), L. Burderi (Osservatorio di Roma), N. R. Robba (DSFA-University of Palermo)

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 x 103 s is present at the beginning of the observation. The luminosity during the persistent emission is 1 x 1038 erg s-1, while during the flare is 2 x 1038 erg s-1. The broad band (0.1--100 keV) energy spectrum 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, and a power-law component, with photon index ~3, dominant at energies higher than 10 keV. This component is present out the flare, contributing to ~4% of the total luminosity, while 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--XXV, and an iron emission line at 6.7 keV are also present. The iron line energy is in agreement with the ionization level of the absorbtion edge. The hydrogen column deduced from the absorption edge is ~1024 cm-2,two order of magnitude larger than the 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 is the inner disk radius, a highly ionized torus surrounds the accretion disk and a magnetospere is large up to RW. The absorption edge, the emission line could originate in the photoionized torus present in the outer edge of the accretion disk, while the comptonized component originates in an inner region of the disk.

The author(s) of this abstract have provided an email address for comments about the abstract: iaria@gifco.fisica.unipa.it

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