A study of the dipping low mass X-ray binary X 1624-490 from the broadbandBeppoSAX 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 di pping was included in the observation. The radial intensity profile of the sourer obtained using the MECS instruments revealed excesses in intensity a bove the instrument point spread function below similar to 5 keV demonstrat ing the presence of a dust-scattering halo. From modelling of the radial pr ofile in several energy bands, halo intensity fractions rising to 30% in th e 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 spect rum of X1624-490 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 emiss ion had photon power law index 2.0(-0.7)(+0.5) and cut-off energy similar t o 12 keV. The spectra of several dip levels were shown to contain an unabso rbed component below 5 keV. Good fits to the dip spectra were obtained by a llowing the Comptonized emission to be progressively covered by an extended absorber while the blackbody was rapidly absorbed and a constant halo comp onent accounted for dust scattering into the line-of-sight. It is shown tha t the unabsorbed component consists of the uncovered part of the Comptonize d 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 diamet er of the extended Comptonized emission region of 5.3 +/- 0.8 x 10(10) cm, consistent with a hot, X-ray emitting corona extending to similar to 50% of the accretion disk radius. The source luminosity for a distance of 15 kpc is 7.3 x 10(37) erg s(-1), an appreciable fraction of the Eddington limit m aking X 1624-490 the most luminous dipping LMXB. The half-height of the bla ckbody emitting region on the neutron star of 6.8 +/- 1.8 km agrees with th e half-height of the radiatively supported inner accretion disk of 6.3 +/- 2.9 km, which together with similar agreement recently obtained for 13 othe r LMXB strongly supports the identification of the neutron star as the orig in of the blackbody emission in LMXB. Finally, from RXTE ASM data, we deriv e an improved orbital period of 20.87 +/- 0.01 hr.