X-ray Interferometry

For the astronomer, x-ray interferometry is the theory and practice of building dilute aperture telescopes for studying celestial x-ray sources. The short wavelengths and high surface brightness of x-ray sources will make the eventual scientific payoff very high, with direct imaging of the event horizons of black holes as the centerpiece. In this article we review the history of x-ray interferometry and discuss the recent technical developments toward astronomical applications. We present several mission concepts and show they are achievable with today's technology. Upon the launch of the Chandra Observatory, X-ray Astronomy matured. In under four decades the capabilities of x-ray observatories had developed to the level enjoyed by visible light observatories. Detection levels have improved by a factor of 10 10 (Giacconi, 2003) and x-ray sources with no detectable optical counterpart are routinely detected. The images returned from Chandra have only slightly lower resolution than those of the Hubble Space Telescope. Radio astronomers, relatively unhampered by the Earth's atmosphere, were the first to build truly giant observatories. They then pioneered radical new techniques to create true interferometric observations of the sky. While the first attempts at celestial interferometry in the visible band were almost a century ago, it is only in the last few years that there has been more than a minimal return. But it is in space, where there is no atmospheric interference, that the true potential of interferometry will be realized. It is thus unsurprising that X-ray Astronomers are now discussing interferometry. All these efforts have one goal in common, the push to higher resolution imaging. Spectroscopy, photometry, and polarimetry – the other astronomical disciplines – are limited mostly by lack of signal, encouraging astronomers to build ever larger and more expensive telescopes. But imaging needs primarily higher quality as opposed to quantity. X-ray interferometry has yet to reach fruition. It must be done in space and it has tight tolerances. Nobody has yet found a simple, low risk means for demonstrating the astronomical potential in space, so even the first proposed observatories are high priced and risky. This will doubtless delay the full realization of astronomical x-ray interferometry as a discipline. In this paper we review the state of x-ray interferometry as applied to astronomy. While x-ray interferometry is now within the scope of space science capabilities, there has not yet been an instrument launched. In this paper we will dwell primarily on the effort …