Discovery of recurring soft-to-hard state transitions in LMC X-3

We present the analysis of the approximately three-year long Rossi X-ray Timing Explorerd (RXTE) campaign for monitoring the canonical soft state black-hole candidates LMC X-1 and LMC X-3. In agreement with previous observations, we find that the spectra of both sources can be well-described by the sum of a multi-temperature disc blackbody and a power law. In contrast to LMC X-1, which does not exhibit any periodic spectral changes, we find that LMC X-3 exhibits strong spectral variability on time-scales of days to weeks. The variability pattern observed with the RXTE All Sky Monitor reveals that the variability is more complicated than the 99- or 198-d periodicity discussed by Cowley et al. For typical ASM count rates, the luminosity variations of LMC X-3 are due to changes of the phenomenological disc blackbody temperature, kT(in), between similar to1 to similar to1.2 keV. During episodes of especially low luminosity (ASM count rates less than or similar to0.6 counts s(-1); four such periods are discussed here), kT(in) strongly decreases until the disc component is undetectable, and the power law significantly hardens to a photon index of Gamma similar to1.8. These changes are consistent with state changes of LMC X-3 from the soft state to the canonical hard state of galactic black holes. We argue that the long-term variability of LMC X-3 might be owing to a wind-driven limit cycle, such as that discussed by Shields et al.