Millihertz quasi-periodic oscillations and broad iron line from LMC X-1

Abstract

We study the temporal and energy spectral characteristics of the persistent black hole Xray binary LMC X-1 using two XMM–Newton and a Suzaku observation. We report the discovery of low-frequency (∼26–29 m Hz) quasi-periodic oscillations (QPOs). We also report the variability of the broad iron Kα line studied earlier with Suzaku. The QPOs are found to be weak with fractional rms amplitude in the ∼1–2 per cent range and quality factor Q ∼ 2–10. They are accompanied by weak red noise or zero-centred Lorentzian components with rms variability at the ∼1–3 per cent level. The energy spectra consist of three varying components – multicolour disc blackbody (kTin ∼ 0.7–0.9 keV), high-energy power-law tail ( ∼ 2.4–3.3) and a broad iron line at 6.4–6.9 keV. The broad iron line, the QPO and the strong power-law component are not always present. The QPOs and the broad iron line appear to be clearly detected in the presence of a strong power-law component. The broad iron line is found to be weaker when the disc is likely truncated and absent when the power-law component almost vanished. These results suggest that the QPO and the broad iron line together can be used to probe the dynamics of the accretion disc and the corona. Key words: accretion, accretion discs – black hole physics – binaries: spectroscopic – stars: