X-rays are generally emitted by very high temperature gas (plasma), and in the astronomical context are in this state due to extreme gravity, magnetic fields, or explosive forces. X-Rays were first discovered by Wilhelm Conrad Röntgen, a German scientist who stumbled upon them while experimenting with vacuum tubes in the mid 1800s. He named them "X"-rays because it was an unknown radiation type at the time and the name has stuck with them, although occasionally they're referred to as Röntgen rays. X-Rays are a form of electromagnetic radiation with very high frequency (~10^17 to ~10^21 Hz) and short wavelength (~10^-8 to ~10^-12 m).
X-ray astronomy is a relatively young branch of astronomy, due to the fact that the Earth's atmosphere is effectively opaque to wavelengths at x-ray frequency. Because of this, it requires high-altitude balloons or orbiting observatories to collect enough x-ray photons to be of any use.
For comparison, trillions of photons in the visible frequency reach a detector of an optical telescope on Earth's surface every second, while it requires hours, or even days for even one x-ray photon to be observed by a detector on the Earth's surface.
Another consequence of the high energy x-ray photons is that the photons cannot be easily reflected and focused. This makes it impossible to resolve x-ray images using conventional telescope designs, however, x-rays can be deflected off a surface if the angle of incidence is very small. This grazing incidence property has lead to designs that involve complex nests of mirrors designed to gently coax the x-ray photons into focus. Previous to this type of design, x-ray observing was limited to detectors that were not much more complex than putting a geiger counter at the end of a tube and forming crude 'images' by counting the number of x-rays detected coming from the part of the sky it was aimed at.
Past X-Ray Observatories:
There has been a steady precession of x-ray observatories being launched since the 1970s by agencies like NASA and the ESA. A list of these, their technical specifications and mission objectives can be found at theHigh Energy Astrophysics Science Archive Research Centre
and European Space Agency Missions (look for X-ray). These past probes have been invaluable in our current understanding of the forementioned cosmic phenomena, but we'll focus on the two important current missions.
Present X-Ray Observatories:
The High-Throughput X-Ray Spectroscopy Mission. This is a large x-ray observatory put together by the European Space Agency launched in 1999. XMM-Newton is the most sensitive x-ray telescope that has ever been in operation, it utilises 15 EPIC CCDs capable of detecting tiny fluctuations of energy in the 0.5 to 10 keV band and resolving an image in between 40 and 80 milliseconds. The angular resolution isn't as fine, but comes in at a respectable 5 arcseconds (5/3600ths of a degree). This observatory also has the ability to monitor the UV spectrum simultaneously with x-ray spectrum and is expected to be operational until 2009.
The Advanced X-Ray Astrophysics Facility, renamed Chandra in honour of the Indian astrophysicist Subramanyan Chandrasekhar. This observatory was also launched in 1999 and has the finest angular resolution of any x-ray observatory ever operational at 0.5 arcseconds (10x better than XMM-Newton). Although it has a greater angular resolution than XMM, Chandra is not as sensitive.
Working together, XMM-Newton with its extreme sensitivity, and Chandra with its fine angular resolution are returning data with higher spatial and spectral resolutions, allowing us to produce more precise images (1,
and spectra of range of high-energy cosmic objects than ever before.
There is a myriad of proposed x-ray missions for the future, including Constellation-X, which would involve a network of multiple spacecraft with moderate resolution x-ray optics, the NGST, SWIFT, and XEUS. One of the more fascinating future missions is MAXIM (Micro-Arcsecond X-Ray Imaging Mission) which promises angular resolutions of an astounding 10^-7 arcseconds - 300000 times finer than the Hubble space telescope inferometry, and 20 million times finer than Chandra's resolution. This would potentially allow us to resolve the event horizon of supermassive black holes, the equivalent of resolving an object the size of a dinner plate on the surface of our Sun.
X-Ray astronomy is just starting to enter its prime. Challenging our previous notions of the behavior of accretion discs around black holes and other exotic objects and giving us insights into a multitude of high-energy events, it is a great subject for anyone interested in astrophysics, astronomy, or just big explosions.