Wavelet analysis of MCG-6-30-15 and NGC 4051: a possible discovery of QPOs in 2:1 and 3:2 resonance

Abstract

Following our previous work of Lachowicz & Czerny (2005), we explore further the application of the continuous wavelet transform to X-ray astronomical signals. Using the public archive of the XMM-Newton satellite, we analyze all available EPIC-pn observations for nearby Seyfert 1 galaxies MCG-6-30-15 and NGC 4051. We confine our analysis to 0.002-0.007 Hz frequency band in which, on the way of theoretically motivated premises, some quasi-periodic oscillations (QPOs) are expected to be found. We find that indeed wavelet power histogram analysis reveals such QPOs centered at two frequencies of ~2.5E-3 Hz and 4-6E-3 Hz, respectively. We show that these quasi-periodic features can be disentangled from the Poisson noise contamination level what is hardly to achieve with the standard Fourier analysis. Interestingly, we find some of them to be in 2:1 or 3:2 ratio. If real, our finding may be considered as a link between QPOs observed in AGN and kHz QPOs seen in X-ray binary systems.