Iron K emission lines in the energy spectra of low-mass X-ray binaries observed with ASCA

ASCA archive data of 20 low-mass X-ray binaries (LMXBs) were analyzed to study the iron K emission lines. The LMXBs analyzed here include Z sources, atoll sources, dippers, bursters, and ADC (accretion disk corona) sources. We paid particular attention to reducing systematic errors for the estimation of line parameters, and the combination of SIS and GIS, which have different energy resolutions, was found to be very useful for this purpose. We detected significant iron lines from about one-half of the sources. The line center was 6.56 keV on average, and a finite width of similar to0.5 keV (FWHM) was obtained from six sources; these properties may be common to LMXBs. On the other hand, equivalent width (EW) of the lines showed large scatter among the sources, from less than 10 to 170 eV. These line parameters do not show any correlation with the source categories or luminosities. The iron K lines from LMXBs are likely produced through the radiative recombination of photoionized plasma. We discuss the origin of the line width, which may result from the combination of the line blending, Doppler broadening, and Compton scattering.