(Suzaku Suzaku) observation of the high-inclination binary EXO 0748-676 in the hard state

• Published in: The Astrophysical journal 2016-06, Vol.823 (2)
• Main Authors: Zhang, Zhongli, Sakurai, Soki, Makishima, Kazuo, Nakazawa, Kazuhiro, Ono, Ko, Yamada, Shin’ya, Xu, Haiguang
• Format: Article
• Language: English
• Subjects: ACCRETION DISKS; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BINARY STARS; COMPARATIVE EVALUATIONS; COSMIC X-RAY SOURCES; DISTANCE; ECLIPSE; ELECTRON TEMPERATURE; INCLINATION; KEV RANGE; LUMINOSITY; MASS; NEUTRON STARS; RADIANT HEAT TRANSFER; X-RAY SPECTRA; X-RAY SPECTROSCOPY
• ISSN: 0004-637X; 1538-4357

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{sup 36} erg cm{sup −2} s{sup −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.