Suzaku Observation of the High-inclination Binary EXO 0748-676 in the Hard State

Abstract

Utilizing archived Suzaku data acquired on 2007 December 25 for 46 ks, the X-ray spectroscopic properties of the dipping and eclipsing low-mass X-ray binary EXO 0748-676 were studied. At an assumed distance of 7.1 kpc, the data provided a persistent unabsorbed luminosity of 3.4× {10}³⁶ erg cm^-2 s^-1 in 0.6-55 keV. The source was in a relatively bright low/hard state, wherein the 0.6-55 keV spectrum can be successfully explained by a “double-seed” Comptonization model incorporating a common corona with an electron temperature of ∼13 keV. The seed photons are thought to be supplied from both the neutron star surface and a cooler truncated disk. Compared to a sample of non-dipping, low-mass X-ray binaries in the low/hard state, the spectrum is subject to stronger Comptonization with a relatively larger Comptonizing y-parameter of ∼1.4 and a larger coronal optical depth of ∼5. This result, when attributed to the high inclination of EXO 0748-676, suggests that the Comptonizing corona may elongate along the disk plane and provide a longer path for the seed photons when viewed from edge-on inclinations.