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

Abstract

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, kTᵢₙ, between ∼1 to ∼1.2keV. During episodes of especially low luminosity (ASM count rates ≲0.6countss⁻¹; four such periods are discussed here), kTᵢₙ strongly decreases until the disc component is undetectable, and the power law significantly hardens to a photon index of Γ ∼ 1.8. 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.