A good long look at the black hole candidates LMC X-1 and LMC X-3

LMC X-1 and LMC X-3 are the only known persistent stellar-mass black-hole candidates that have almost always shown spectra that are dominated by a soft, thermal component. We present here results from 170-ks-long Rossi X-ray Timing Explorer (RXTE) observations of these objects, taken in 1996 December, where their spectra can be described by a disc blackbody plus an additional soft (Gamma similar to2.8) high-energy power law (detected up to energies of 50 keV in LMC X-3). These observations, as well as archival Advanced Satellite for Cosmology and Astrophysics (ASCA) observations, constrain any narrow Fe line present in the spectra to have an equivalent width less than or similar to 90 eV. Stronger, broad lines (approximate to 150 eV EW, sigma approximate to1 keV) are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional to f(-1) between 10(-3) and 0.3 Hz with a root-mean-square (rms) variability of approximate to7 per cent. At energies >5 keV, the PSD shows evidence of a break at f >0.2 Hz, possibly indicating an outer disc radius of less than or similar to 1000 GMc(2) in this likely wind-fed system. Furthermore, the coherence function gamma (2)( f), a measure of the degree of linear correlation between variability in the >5 keV band and variability in the lower energy bands, is extremely low (less than or similar to 50 per cent). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.