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

Abstract

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 (Γ ∼ 2.8) high-energy power law (detected up to energies of 50keV 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 ≲90eV. Stronger, broad lines (≈150eV EW, (≈150eV EW, σ≈ 1keV) 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 0.3Hz with a root-mean-square (rms) variability of ≈7per cent. At energies >5keV, the PSD shows evidence of a break at f>0.2Hz, possibly indicating an outer disc radius of ≲1000GMc² in this likely wind-fed system. Furthermore, the coherence function γ²(f), a measure of the degree of linear correlation between variability in the >5keV band and variability in the lower energy bands, is extremely low (≲50per cent). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.