X-ray observations of nearby active galaxies

Optical image of the nearby active galaxy NGC 4051, showing the bright active nucleus embedded in the host spiral galaxy: © George Seitz/Adam Block/NOAO/AURA/NSF

Acvtive galaxies are those that have detectable non-stellar emission from their nuclei: first discovered in the 1940s, we now recognise that these are powered by accretion onto a supermassive black hole, with masses of millions or billions times the mass of our Sun. Black hole accretion produces emission right across the electromagnetic spectrum, from radio wavelengths to the highest gamma ray energies. The inner regions of the accretion process, close to the black hole, are too small for us to be able to image with high spatial resolution, given the distances to even the nearest active galactic nuclei (AGN). The highest spatial resolution can be obtained using Very Long Baseline Interferometry at radio wavelengths, but to probe the accretion process at other parts of the electromagnetic spectrum we need to use less direct methods.

At X-ray energies we can potentially probe to within a few tens of Schwarzschild radii of the black hole, and we have two tools to understand the physics. X-ray spectroscopy allows us to model the physical emission mechanisms, and in particular the reprocessing that arises through absorption and scattering of X-rays by copious amounts of circumnuclear material. The latest observations reveal fast outflows, up to one-third the speed of light, and demonstrate the complexity of the accretion process in black hole systems. Long observations with an X-ray telescope also allow us to study the complex variations exhibited by the X-ray emission, where rapid fluctuations on timescales as short as 1000 seconds are observed. Such rapid fluctuations confirm that we are seeing emission from very compact regions, with sizes comparable to those of the Earth's solar system. Recently, time delays have been discovered between X-ray variations at differing X-ray energies, and we now believe that this is due to energy-dependent scattering of X-rays from gas clouds orbiting or outflowing from the black hole.

Owing to absorption by the Earth's atmosphere, X-ray telescopes have to be placed in space. In the past decade, key advances have been made by the XMM-Newton, Suzaku and Chandra orbiting X-ray observatories. New missions have recently been, or are about to be, launched: NuSTAR and ASTRO-H, with the European Space Agency's Athena X-ray mission on the horizon.