Iron emission line from low-mass x-ray binaries

By observing the x-ray spectra of ten low-mass binary x-ray sources, we detected a line emission feature around 6 - 7 keV from six x-ray sources, i.e., Sco X - 1, 4U 1608 - 52, GX 349 + 2, GX 5 - 1, Ser X - 1, and Cyg X - 2. Most of the line-center energies determined are consistent with their weighted average of 6.66 ± 0.05 keV, and the equivalent widths lie between 20 and 60 eV. For the remaining sources, i.e., 4U 1636 - 53, 4U 1735 - 44, NGC 6624, and 4U 1254 - 69, only a 90 % confidence upper limit of 20 - 30 eV was obtained for the equivalent widths. The 6.7-keV iron line observed is ascribed to the Kα radiative transitions of heliumlike ions of iron, due mainly to the recombination process taking place in the accretion disk coronae with temperatures of ∼ 106 K at radii of ∼ 109 cm. (author)