A Hard Tail in the Broadband Spectrum of the Dipper XB 1254−690

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 ~0.85 keV, a Comptonized spectrum with an electron temperature of ~2 keV, and bremsstrahlung at a temperature of ~20 keV. Adopting a distance of 10 kpc and taking into account a spectral hardening factor of ~1.7 (because of electron scattering which modifies the blackbody spectrum emitted by the disk), we estimated that the inner disk radius is Rin(cosi)1/2~11 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 ~19. The bremsstrahlung emission, contributing ~4% of the total luminosity, probably originates in an extended accretion disk corona with radius ~1010 cm. In this scenario, we calculated that the optical depth of this region is ~0.71 and its mean electron density is Ne~2.7×1014 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 ~1.27 keV with an optical depth of ~0.15. Its energy could correspond to the L edge of Fe XVII or K edge of Ne X or neutral Mg.