Magnetic fields in active galactic nuclei and microquasars

Observations of active galactic nuclei 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 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 the iron K-alpha line will allow us to find the value of the magnetic fields if the latter are high enough.