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

ABSTRA C T 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 softOG , 2:8U 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 &90 eV. Stronger, broad lines (<150 eV EW, s < 1 keVU are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional to f 21 between 10 23 and 0.3 Hz with a rootmean-square (rms) variability of <7 per cent. At energies .5 keV, the PSD shows evidence of a break at f . 0: 2H z; possibly indicating an outer disc radius of &1000 GM/c 2 in this likely wind-fed system. Furthermore, the coherence function g 2 O fU; 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 (&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.