UV and optical observations of X-ray sources in the Magellanic Clouds.

Abstract

Ultraviolet spectra of the Magellanic Cloud X-ray sources LMC X-4, SMC X-1 and SMC X-2 obtained by the IUE satellite are presented together with simultaneous optical photometry of SMC X-1 and SMC X-2. Short- and long-wavelength IUE spectra for LMC X-4 taken near X-ray eclipse provide no evidence for P Cygni-type emission, despite evidence for mass loss in optical spectra, and are found to be consistent with the model atmosphere of Kurucz (1979) with a temperature of 35,000 K, log surface gravity of 3.5, solar abundances and average galactic reddening with a B-V color index of 0.15. UV spectra for SMC X-1 at orbital phase 0.22 when compared with the Kurucz model atmosphere indicate a possible deviation of the reddening law from the galactic, while the optical light curves are found to be in agreement with previous results, with a V magnitude of 9.760 and B-V and U-B color indexes of 0.463 and -0.12, respectively. For the case of SMC X-2, UV observations reveal spectral variations increasing from 10 percent near 1500 A to 30 percent near 1900 A, while optical observations yield a V magnitude of 14.74, B-V index of -0.06 and U-B index of -1.04. Equivalent widths of the Si IV 1400 A and C IV 1550 A lines of the three sources are noted to differ significantly from those of Galactic stars of the same spectral type.