Hard X‐Ray Emission from Low‐Mass X‐Ray Binaries
Abstract
We report on Rossi X-Ray Timing Explorer observations of four type I X-ray bursters, namely, 1E 1724-3045, GS 1826-238, SLX 1735-269, and KS 1731-260. The first three were in a low state, with 1-200 keV X-ray luminosities in the range ~0.05-0.1 L Edd ( L Edd : Eddington luminosity for a neutron star, L Edd = 2.5 × 10 38 ergs s -1 ), whereas KS 1731-260 was in a high state, with luminosity ~0.35 L Edd . The low-state sources have very similar power spectra, displaying high-frequency noise up to ~200 Hz. For KS 1731-260, its power spectrum is dominated by noise at frequencies ≲20 Hz; in addition a quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1-200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature ( kT e ) around 25-30 keV. For KS 1731-260, the spectrum is also dominated by thermal Comptonization, but with a much lower kT e ~ 3 keV and no significant hard X-ray emission. With the exception of GS 1826-238, they each have an underlying soft component, carrying at most ~25% of the total 1-200 keV luminosity. For all sources, we have detected an iron Kα line at 6.4 keV (although it is weak and marginal in 1E 1724-3045). A reflection component is present in the spectra of GS 1826-238 and SLX 1735-269, and for both we find that the reflecting medium subtends only a small solid angle (Ω/2π ~ 0.15, 0.28). The origin of the line and the reflection component is most…
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