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

Abstract

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.