Possible signature of magnetic fields related to quasi-periodic oscillation observed in microquasars

Abstract: The study of quasi-periodic oscillations (QPOs) of X-ray flux observed in the stellar-mass black hole binaries can provide a powerful tool for testing of the phenomena occurring in strong gravity regime. The high frequency (HF) QPOs usually come in pairs of double peaks which have a frequency ratio close to $3:2$. In addition to HF QPOs, some sources display simultaneous existence of the low frequency (LF) QPOs in Fourier power spectra. We demonstrate that the explanation of these phenomena can be well related to the epicyclic oscillations of charged particles in accretion disks orbiting Kerr black holes immersed in external large-scale magnetic fields. Magnetized versions of the standard geodesic models of QPOs can explain the observationally fixed data from the three microquasars. We perform a successful fitting of the HF QPOs observed in three microquasars, GRS 1915+105, XTE 1550-564 and GRO 1655-40, containing black holes, for magnetised versions of both epicyclic rezonance and relativistic precession models and discuss the corresponding constraints of parameters of the model, which are the mass and spin of black hole and the parameter related to the external magnetic field. Assuming the main source of synchrotron radiation producing X-rays are the relativistic electrons, we estimate the magnetic field in the vicinity of the black hole in the three sources to be of order $10^{-5}$ Gs which can serve as possible signature of the Galactic magnetic field magnitude. For heavier particles (protons, ions) larger magnetic fields are necessary for fitting the data.