A study of the dipping low mass X-ray binary X 1624-490 from the broadband BeppoSAX observation

We present results of a study of the luminous dipping low mass X-ray binary X 1624-490 made using BeppoSAX. An interval of deep and rapidly varying dipping was included in the observation. The radial intensity profile of the source obtained using the MECS instruments revealed excesses in intensity above the instrument point spread function below ~5 keV demonstrating the presence of a dust-scattering halo. From modelling of the radial profile in several energy bands, halo intensity fractions rising to 30% in the lowest band 2.5-3.5 keV were obtained. From these data, the optical depth to dust scattering at 1 keV was found to be 2.4 ± 0.4. The non-dip spectrum of X 1624-490 in the energy band 1-100 keV is shown to be well- described by the emission model consisting of point-like blackbody radiation assumed to be from the neutron star plus extended Comptonized emission from an ADC. The blackbody temperature was 1.31 ± 0.07 keV and the Comptonized emission had photon power law index 2.0_-0.7^+0.5 and cut-off energy ~12 keV. The spectra of several dip levels were shown to contain an unabsorbed component below 5 keV. Good fits to the dip spectra were obtained by allowing the Comptonized emission to be progressively covered by an extended absorber while the blackbody was rapidly absorbed and a constant halo component accounted for dust scattering into the line-of- sight. It is shown that the unabsorbed component consists of the uncovered part of the Comptonized emission plus a halo contribution which in deepest dipping dominates the spectrum below 4.5 keV. From the dip ingress time, we have derived a diameter of the extended Comptonized emission region of 5.3 ± 0.8 × 10¹⁰ cm, consistent with a hot, X-ray emitting corona extending to ~50% of the accretion disk radius. The source luminosity for a distance of 15 kpc is 7.3 × 10³⁷ erg s^-1, an appreciable fraction of the Eddington limit making X 1624-490 the most luminous dipping LMXB. The half-height of the blackbody emitting region on the neutron star of 6.8 ± 1.8 km agrees with the half-height of the radiatively supported inner accretion disk of 6.3 ± 2.9 km, which together with similar agreement recently obtained for 13 other LMXB strongly supports the identification of the neutron star as the origin of the blackbody emission in LMXB. Finally, from RXTE ASM data, we derive an improved orbital period of 20.87±0.01 hr.