We 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.
A hard tail in the broadband spectrum of the dipper XB 1254-690
BURDERI, LUCIANO;
2001-01-01
Abstract
We 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.
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