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

TL;DR

This study analyzes Suzaku X-ray data of the high-inclination binary EXO 0748-676, revealing a bright low/hard state with strong Comptonization and suggesting the corona elongates along the disk plane due to the system's edge-on view.

Contribution

First detailed spectral analysis of EXO 0748-676 in the low/hard state using Suzaku data, highlighting the corona's geometry in a high-inclination system.

Findings

Spectrum explained by double-seed Comptonization model

Corona temperature around 13 keV, optical depth ~5

Enhanced Comptonization due to high inclination

Abstract

Utilizing an archived Suzaku data acquired on 2007 December 25 for 46 ks, 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 gave a persistent unabsorbed luminosity of $3.4\times10^{36}$ 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 $\sim13$ 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 $\sim1.4$ and a larger coronal optical depth of $\sim5$. 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.