Probing the accretion scheme of the dipping X-ray binary 4U 1915-05 with Suzaku

The dipping low-mass X-ray binary 4U 1915-05 was observed by Suzaku on 2007 November 8 for a net exposure of 39 ks. It was detected by the XIS with a 0.8-10 keV signal rate of 9.84 +/- 0.01 cts s(-1) per camera, and HXD-PIN with a 12-45 keV signal rate of 0.29 +/- 0.01 cts s-1. After removing the periodic dips and an X-ray burst, the 0.8-45 keV continuum was successfully described by an optically thick disk emission with an inner-disk temperature similar to 0.7 keV and a neutron-star blackbody emission with temperature similar to 1.3 keV, on condition that the blackbody component, or possibly the disk emission too, is significantly Comptonized. This successful model is consistent with 4U 1915-05 being in a high-soft state in this observation, and implies that its broadband spectrum can be interpreted in the same scheme as for many non-dipping low-mass X-ray binaries in the soft state. Its bolometric luminosity (similar to 0.02 times the Eddington limit) is relatively low for the soft state, but within tolerance, if considering the distance and inclination uncertainties. As a high inclination binary, this source exhibited a stronger Comptonization effect, with a larger Comptonizing gamma-parameter, compared to low and medium inclination binaries. This suggests that the Comptonizing coronae of these objects in the soft state is in an oblate (rather than spherical) shape, extending along the accretion disk plane, because the gamma-parameter would not depend on the inclination if the corona were spherical.