Illustration: A supermassive black hole at the center of a galaxy.

Material falling (accreting) onto a black hole in a binary system, or at the center of a galaxy, forms an accretion disk about the event horizon, with the inner regions heated by the release of gravitational energy to temperatures of tens of millions of degrees. X-ray radiation is the dominant energy output. It is only observations in the X-ray band that can study the inner accretion disk close to the event horizon. The detection of X-ray emission was one of the key reasons that black holes were first identified and it remains today a fundemental signature of accreting black holes. Recent results from the Japanese-US ASCA mission have revealed a relativistically broadened iron line feature that comes from so close to the event horizon that a gravitationally redshift is observed. This is 10,000 times closer into the black hole than can be imaged by HST. The Constellation-X mission to be launched in 2008 is optimized to study the iron K line feature discovered by ASCA and will determine the black hole mass and spin for a large number of systems. This will provide an indirect measure of the properties of the region within a few black hole radii of the event horizon.

Image Credit: CXC/A. Hobart.

Constellation-X is the first step towards the quest in the SEU program to obtain a direct image of a black hole. While it may seem contradictory to image an object from which light cannot escape, the black hole can be seen in silouette against hot material as it spirals in close to the event horizon. Such direct imaging of a black hole would revolutionize our understanding of these enigmatic objects. We would directly be able to observe light from the accretion disk bending around the black hole, i.e., the distortion of space-time by the intense field of the black hole. This would enable direct tests of General Relativity in the strong gravity limit. Last, but not least, the SEU quest to image a black hole would capture the public imagination.

The best candidate black hole to observe is the nearby active galaxy M87. This is believed to harbor a 100 million solar-mass black hole at a distance of order 1 million parsecs. An angular scale of micro arc-seconds is required to resolve the event horizon of the supermassive black hole in M87 (the currently known stellar mass black holes in our galaxy have an angular scale several orders of magnitude less than this and would be even more challenging to observe). Thus, to directly image a black hole will require a Micro-Arcsecond X-ray Imaging Mission (MAXIM). This mission would have 100,000 times better angular resolution than the current capability (as defined by the Chandra X-ray optic).

The objectives of the MAXIM study are to

• define conceptual designs and explore alternate approaches,
• understand the technology requirements and fundemental limitations, and
• establish a technology roadmap.

The study will integrate the results of both past and ongoing related studies into the above analysis and concept development.

To achieve MAXIM will require a quantum leap forward in technology. A diffraction limited X-ray optic with a size of meters can yield an angular resolution of milli-arcseconds, which would be sufficient to resolve coronal structures on nearby stars but falls far short of that required to image the black hole in M87. To do that requires X-ray interferometry. The short 1-10 Angstrom wavelength of X-rays means that the required baselines are 100 to 1,000 times smaller than those in the optical and infrared bands, where observational interferometry is now being implemented. To reach micro-arcsecond resolution would require a 2-arm interferometer with grazing incidence diffraction limited optics having a 100 m baseline. Probably the most challenging aspect to this problem will be to maintain a precise knowledge of the path difference required to reconstruct the fringe pattern. There is some ongoing work at the University of Colorado (PI: Webster Cash) in developing diffraction limited optics. But no studies exist as to the feasibility and technology requirements to realize an orbiting X-ray interferometer. This is because the technology challenges are severe and this program maybe at least 25-50 years in the future. Given the recent break throughs in establishing the existance of black holes and the tremendous scientific potential of directly imaging a black hole, the time seems ripe to study in more detail the possibility of using MAXIM to directly image the black hole in M87. It is worth noting that the capabilities of MAXIM would be such a huge leap forward that it would have an enormous impact in all areas of astronomy, not only the study of black holes.