X-RAY SPECTRAL STUDY OF 2 X-RAY BINARIES - 4U 1957-624(11 AND 2S 1543)

The X-ray observations of two binary systems, 4U 1957+11 and 2S 1543-6 24, by the EXOSAT satellite are analyzed. The continuum spectra are be st fitted by a superposition of a blackbody spectrum (kT similar to 1 keV) and emission from a Comptonized region, consistent with the spect ra of low-mass X-ray binary systems. The Comptonization parameters fou nd for 4U 1957+11 are observed to change significantly from kT(e) > 12 .5 keV and tau < 4.5 in the higher intensity state to kT(e) similar to 1.3 keV and tau similar to 22 in the lower intensity state. The value s of kT(e) and tau derived for 2S 1543-624 are limited to be > 4 keV a nd < 4.8, respectively. A broad line ascribed to be due to the fluores ence of iron is detected in both the objects with an equivalent width of similar to 60-200 eV. The blackbody radius is similar to 3.3 km for 2S 1543-624 and similar to 4.9 km in the case of 4U 1957+11 in its hi gher intensity state. In an observation of 4U 1957+11 with similar to 20% lower intensity, the blackbody component was not detected. Some po ssible interpretations of the results are discussed, and an estimate o f similar to 10(8) G is derived for the magnetic field on the surface of the neutron star in 4U 1957+11.