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

Abstract

Utilizing an archived Suzaku data acquired on 2007 December 25 for 46 ks, Xray 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 gave 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 give a longer path for the seed photons when viewed from edge-on inclinations.