Energy Spectra of Low-Mass Binary X-Ray Sources Observed from Tenma

Abstract

Observations of X-ray spectra for four low-mass binary X-ray sources, Sco X-1, 4U 1608–522, GX 5–1, and GX 349+2 were performed with the gas scintillation proportional counters on board Tenma. Common to the four sources, the spectrum hardens with increasing intensity, and the difference of the spectra before and after an intensity increase shows invariably a blackbody spectrum of kT ≅ 2 keV. Every observed spectrum can be expressed by a sum of two spectral components: a hard component and a soft component, whose spectral shapes are fixed for each individual source. The hard component has a blackbody spectrum of kT ≅ 2 keV and its intensity varies largely with time. The soft component is best represented by a superposition of multicolor blackbody spectra expected from an optically-thick accretion disk, and its intensity remains faily stable. The spectral shapes of the both components are respectively similar to each other among the four sources, except for different bolometric flux ratio of the two components for the individual sources. The energy spectrum during the transient dips observed from GX 5–1 is in good agreement with that of the soft component, indicating that the 2-keV blackbody component has become almost absent. The 2-keV blackbody component and the soft component may be attributed to the emission from the neutron star surface and that from the optically thick accretion disk, respectively.