ASCA Science Highlights: Active Galactic Nuclei
Follow the links to images and plotsThere is a compact source with extraordinary energy output at the center (nucleus) of many galaxies: these are called Active Galactic Nuclei (AGN). Most scientists agree that these are supermassive black holes accreting matter from the nearby gas clouds and stars. X-rays are emitted from near the black hole itself, around which a disk of hot gas is thought to orbit. Strong iron emission line is formed in the disk; the observed energies of the emission line photons are spread around due to Doppler shifts and relativistic effects. With the energy resolution of ASCA, scientists have succeeded in studying the disk very near these supermassive black holes.
- Evidence for ionized gas has been detected in ASCA data of many AGN. The variability of the absorption edges has been used to narrow down the location of the ionized gas.
- ASCA observations of distant quasars suggest that AGN can be a major contributor to the cosmic X-ray background.
- The shape of the iron emission line seen in the Seyfert galaxy, MCG-6-30-15, is exactly what is expected from relativistic effects of material circling a supermassive black hole. Also read this press release.
- The iron line in MCG-6-30-15 was observed to vary on a short timescale.
- The line profile is correlated with source luminosity
- According to the standard "unification" model, the accretion disk or the molecular torus that surrounds the central engine in Seyfert 2 type AGN are seen edge-on. The studies of the Seyfert 2 iron emission lines, however, suggest the situation may be more complicated.
This page created by Dr. Koji Mukai (USRA) at the U.S. ASCA Guest Observer Facility.
If you have any questions concerning ASCA, visit our Feedback form.