Magnetic fields in AGNs and microquasars

Observations of AGNs and microquasars by ASCA, RXTE, Chandra and XMM-Newton indicate the existence of wide X-ray emission lines of heavy ionized elements in their spectra. The emission can arise in the inner parts of accretion discs where the effects of General Relativity (GR) must be counted, moreover such effects can dominate. We describe a procedure to estimate an upper limit of the magnetic fields in the regions where X-ray photons are emitted. We simulate typical profiles of the iron $K_\alpha$ line in the presence of a magnetic field and compare them with observational data. As an illustration we find $H < 10^{10} - 10^{11}$ Gs for Seyfert galaxy MCG--6--30--15. Using the perspective facilities of measurement devices (e.g. Constellation-X mission) a better resolution of the blue peak structure of iron $K_\alpha$ line will allow to find the value of the magnetic fields if the latter are high enough.