Probing the Accretion Scheme of the Dipping X-ray Binary 4U 1915-05 with Suzaku

TL;DR

This study analyzes Suzaku observations of the dipping low-mass X-ray binary 4U 1915-05, modeling its broadband spectrum to understand its accretion state and corona geometry, revealing a high-soft state with inclination-dependent Comptonization effects.

Contribution

It provides the first detailed broadband spectral modeling of 4U 1915-05 in a high-soft state, highlighting the impact of inclination on Comptonization and suggesting an oblate corona shape.

Findings

The spectrum is well described by an optically thick disk and neutron-star blackbody emission.

The source was in a high-soft state with a bolometric luminosity about 2% of Eddington.

Higher inclination correlates with stronger Comptonization effects.

Abstract

The dipping low-mass X-ray binary 4U 1915-05 was observed by Suzaku on 2007 November 8 for a net exposure of 39 ksec. It was detected by the XIS with a 0.8-10 keV signal rate of 9.84+\-0.01 cts/s per camera, and HXD-PIN with a 12-45 keV signal rate of 0.29+/-0.01 cts/s. 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 ~ 0.7 keV and a neutron-star blackbody emission with a temperature ~ 1.3 keV, on condition that the blackbody component, or possibly the disk emission too, is significantly Comptonized. This successful modeling 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 (~ 0.02 times the Eddington limit) is relatively low for the soft state, but within a tolerance, if considering the distance and inclination uncertainties. As a high-inclination binary, this source exhibited stronger Comptonization effect, with a larger Comptonizing y-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 y-parameter would not depend on the inclination if the corona were spherical.