Quasi-periodic oscillations and energy spectra from the two brightest Ultra-Luminous X-ray sources in M82

Ultra-Luminous X-ray sources (ULXs) are thought to be accreting black holes (BHs) that might host intermediate-mass black holes (IMBH), proposed to exist by theoretical studies, even though a firm detection (as a class) is still missing. The brightest ULX in M82 (M82 X–1) is probably one of the best candidates to host an IMBH. In this work, we analysed the data of the recent release of observations obtained from M82 X–1 taken by XMM–Newton. We performed a study of the timing and spectral properties of the source. We report on the detection of ≈(46 ± 2) mHz quasi-periodic oscillations (QPOs) in the power density spectra of two observations. A comparison of the frequency of these high-frequency QPOs with previous detections supports the 1:2:3 frequency distribution as suggested in other studies. We discuss the implications if the ≈(46 ± 2) mHz QPO detected in M82 X–1 is the fundamental harmonic, in analogy with the high-frequency QPOs observed in BH binaries. For one of the observations, we have detected for the first time a QPO at 8 mHz (albeit at a low significance), which coincides with a hardening of the spectrum. We suggest that the QPO is a millihertz QPO originating from the close-by transient ULX M82 X–2, with analogies to the low-frequency QPOs observed in BH binaries.