The unusually soft X-ray spectrum of LMC X-3.

Spectral studies of LMC X-3 using the HEAO 1 A-2 experiment and the Einstein Solid State Spectrometer show that the X-ray spectrum is extremely soft relative to the vast majority of galactic accretion-driven X-ray sources, but similar to that of black hole candidates in their 'high state'. Rapid approximately 1 s flickering comparable to that seen from Cyg X-1 in its low state was not detected. In the 1977-1979 interval LMC X-3 was for most (if not all) of its time in a 'high state'. The fact that several other steady and transient X-ray sources also have similarly soft X-ray spectra and should be regarded as potential black hole candidates is noted. The spectra of these sources are not well described by any one simple model.