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Spectral evolution of circinus X-1 along its orbit
Full text linkWe report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in 1998 March along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from 2.5 x 10(38) ergs s(-1) at the periastron (orbital phase 0.01) to 1.5 x 10(38) ergs s(-1) at orbital phase 0.3. From the spectral analysis and the light curve, we argue that Cir X-1 shows three states along the orbital evolution. The first state is at the orbital phase interval 0.97-0.3 : the luminosity becomes super-Eddington, and a strong flaring activity is present. In this state a shock could form in the inner region of the system because of the super-Eddington accretion rate, producing an outflow of ionized matter whose observational signature could be the prominent absorption edge at similar to8.7 keV observed in the energy spectrum at these phases. In the second state, corresponding to the orbital phase interval between 0.3 and 0.7, the accretion rate is sub-Eddington, and we observe a weaker outflow, with a smaller hydrogen column: the absorption edge is now at similar to8.3 keV with an optical depth a factor of 2.5-6 smaller. The third state corresponds to the orbital phase interval 0.78-0.97. In this state the best-fit model to the spectrum requires the presence of a partial covering component, indicating that the emission from the compact object is partially absorbed by neutral matter, probably the atmosphere of the companion star and/or the accreting matter from the companion
A study of Cir X-1 broad band energy spectrum close to the periastron and the apoastron of its orbit.
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A hard tail in the broadband spectrum of the dipper XB 1254-690
No full textWe report on the results of spectral analysis of the dipping source XB 1254-690 observed by the BeppoSAX satellite. We find that the X-ray dips are not present during the BeppoSAX observation, in line with recent RXTE results. The broadband (0.1-100 keV) energy spectrum is well fitted by a three-component model consisting of a multicolor disk blackbody with an inner disk temperature of similar to0.85 keV, a Comptonized spectrum with an electron temperature of similar to2 keV, and bremsstrahlung at a temperature of similar to 20 keV. Adopting a distance of 10 kpc and taking into account a spectral hardening factor of similar to1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is R-in(cos i)(1/2) similar to 11 km, where i is the inclination angle of the system with respect to the line of sight. The Comptonized component could originate in a spherical corona or boundary layer, surrounding the neutron star, with optical depth similar to 19. The bremsstrahlung emission, contributing similar to4% of the total luminosity, probably originates in an extended accretion disk corona with radius similar to 10(10) cm. In this scenario, we calculated that the optical depth of this region is similar to0.71 and its mean electron density is N-e similar to 2.7 x 10(14) cm(-3). This last component might also be present in other low- mass X-ray binaries, but, because of its low intrinsic luminosity, it is not easily observable. We also find an absorption edge at similar to1.27 keV with an optical depth of similar to0.15. Its energy could correspond to the L edge of Fe XVII or K edge of Ne X or neutral Mg
Discovery of a Hard Component in the X-ray Spectrum of the Z Source GX 349+2 Observed by BeppoSAX
No full textHigh resolution and broad band spectra of low mass X-ray binaries: a comparison between black holes and neutron stars
No full textA common question about compact objects in high energy astrophysics is whether it is possible to distinguish black hole from neutron star systems with some other property that is not the mass of the compact object. Up to now a few characteristics have been found which are typical of neutron stars (like quasi periodic oscillations at kHz frequencies or type-I X-ray bursts), but in many respects black hole and neutron star systems show very similar behaviors. We present here a spectral study of low mass X-ray binaries containing neutron stars and show that these systems have spectral characteristics that are very similar to what is found for black hole systems. This implies that it is unlikely we can distinguish between black holes and neutron stars from their X-ray spectra, except for the fact that black hole systems show sometimes a more extreme behavior with respect to neutron star systems
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