Black hole X-ray binaries LMC X-1 and X-3: observations confront spectral models

We present a comprehensive spectral analysis of black hole X-ray binaries, LMC X-1 and X-3, based on BeppoSAX observations. We test both the multicolour disc plus power-law (MCD+PL) model and a newly developed Monte Carlo simulation-based model for a Comptonized MCD (CMCD) with either a spherical or a slab-like corona, by comparing the inferred parameters with independent direct measurements. While all models give an adequate description of the spectra, we find a significant discrepancy between the MCD+PL inferred X-ray-absorbing gas column density and the absorption-edge measurement based on dispersed X-ray spectra. The MCD+PL fits to the LMC X-1 spectra also require a change in the inner disc radius during the BeppoSAX observation, which may be due to the unphysical effects inherited in the model. In contrast, the CMCD model with the spherical corona gives predictions of both the disc inclination angle and the absorption that are consistent with the direct measurements, and only slightly underpredicts the black hole mass of LMC X-3. The model explains the spectral state evolution of LMC X-1 within the BeppoSAX observation as a change in the accretion rate, which leads to an increase in both the inner disc temperature and the Comptonization opacity. On the other hand, the CMCD model with the slab-like corona is more problematic in the test and is thus not recommended.