At the centre of some galaxies is an engine confined to a very small volume of space, but capable of out-shining the entire host galaxy of stars many times. These engines are known as active galactic nuclei (AGN), and it is believed that their energy is produced by a supermassive black hole which is accreting near-by matter. The X-ray group at MPE has a strong history in the study of AGN and related phenomena.
Narrow-Line Seyfert 1(NLS1) galaxies form a special branch of the AGN phenomenon. They are believed to be young AGN which are accreting matter close to or in excess of the upper-most limit determined by their mass. As a class, in the X-rays NLS1 exhibit a strong soft-excess in their spectrum and show more extreme variability than their broad-line counterparts of similar luminosities. With the high-energy sensitivity of XMM-Newton came the discovery of sharp spectral drops above about 7 keV. The features could be explained by either partial-covering of the X-ray source by a dense, patchy absorber, or by a relativistically blurred iron emission line which is enhanced by light-bending close to the black hole.
Fluorescent emission by iron in the accretion disc can be detected in the X-ray spectra of some AGN. The redshifted, broadened, and asymmetric profile of the line suggests emission close to the supermassive black hole. Understanding this feature is of fundamental importance as it ultimately reveals information about the accretion processes close to the black hole, the black hole mass, and even black hole spin. Though expected to occur naturally in most low-luminosity type-1 AGN, very few convincing examples have have been found. Utilising the high sensitivity of XMM-Newton, evidence for relativistic iron lines has been found in the spectra of some unexpected sources, such as quasars and type-2 AGN. Interestingly, some surprising profiles have also emerged (e.g. in ESO113-G010; bottom panel) which could be explained by short-lived hotspot on the accretion disc. Shown below are the X-ray spectra of the quasar Q0056-363 (left panel), the narrow-line quasar PG1402+261 (right panel), and the Seyfert 1.8 ESO113-G010 (bottom panel).
Imaging spectroscopy performed with the Chandra X-ray Observatory led to the discovery of two active black holes in the nucleus of the luminous infrared galaxy NGC 6240. This is the first positive identification of an active binary black hole at the center of a galaxy. This discovery shows that massive black holes can grow through mergers in the centers of galaxies, and that these events may be detectable with future space-borne gravitational wave observatories. A search for further binary black holes in luminous infrared galaxies using Chandra is presently ongoing.
Some of the most fundamental issues regarding ULIRGs are the nature of their power source (strong starburst and/or a dust-enshrouded AGN), and their relative contribution to the total energy budget. The capability of X-rays to penetrate high absorbing material makes X-ray observations essential in attempting to understand the physical processes at work in ULIRGs. The XMM-Newton observations of the ULIRG NGC 6240 reveal three distinct narrow iron lines (E = 6.4, 6.7, and 7 keV). The broad-band spectrum is successfully described by emission from three different plasma components located at different locations, and a highly absorbed power-law component.
Various aspects of ionized absorbers were studied at MPE during the last decade, including photoionization modeling of the ionized gas, detailed study of dusty warm absorbers, the role of warm absorbers in high-redshift quasars and in Narrow-line Seyfert 1 galaxies, the connection of warm absorbers with optical-UV emission and absorption-line regions in AGN, and the detection and detailed analysis of narrow absorption and emission lines which became possible with the high-resolution spectroscopic capabilities of XMM-Newton and Chandra.
Giant-amplitude X-ray flares from the centres of optically non-active galaxies were discovered with ROSAT and followed-up with XMM-Newton, Chandra, and HST. These events represent the highest amplitudes of variability ever recorded among galaxies. They are interpreted as flares from stars which are tidally disrupted by the supermassive black holes at the centers of these galaxies; a process long predicted by theory (e.g., Rees, 1988, Nature 333, 523). These observations open up a new window to search for and study black holes at the centers of otherwise non-active galaxies and are of great interest in the context of black hole growth and galaxy evolution.