History of X-ray Astronomy

The study of astronomical objects at the highest energies of X-rays and gamma-rays really began only in the early 1960's. Before then, we knew only that the Sun was an intense source in these wavebands. The Earth's atmosphere absorbs most X-rays and gamma-rays and, so, rocket flights which could lift scientific payloads above the Earth's atmosphere were necessary. The first rocket flight which successfully detected a cosmic source of X-ray emission was launched in 1962 by a group at American Science and Engineering (AS&E). The team of scientists on this project included Riccardo Giacconi, Herb Gursky, Frank Paolini, and Bruno Rossi. This rocket flight detected a very bright source they named "Scorpius X-1" ("Sco X-1" for short), because it was the first X-ray source found in the constellation Scorpius.

In the 1970s, dedicated X-ray astronomy satellites, such as Uhuru, Ariel 5, SAS-3, OSO-8, and HEAO-1, developed this field of science at an astounding pace. Scientists began to believe that X-rays from stellar sources in our Galaxy were primarily from a neutron star in a binary system with a normal star. In these "X-ray Binaries", the X-rays originate from material falling from the normal star to the neutron star in a process called accretion. The binary nature of the system allowed for measurements of mass of the neutron star. For other systems, the inferred mass of the degenerate object supported the idea of the existence of black holes, as they were too massive to be neutron stars. Some of the systems displayed a characteristic X-ray pulse, just as pulsars had been found to do in the radio regime, which allowed a determination of the spin rate of the neutron star. Finally, some of these galactic X-ray sources were found to be highly variable; in fact, some sources would appear in the sky, remain bright for a few weeks, and then fade again from view. Such sources are called "X-ray Transients". The inner regions of some galaxies were also found to emit X-rays. The X-ray emission from these "Active Galactic Nuclei" is believed to originate from ultra-relativistic gas near a very massive black hole at the galaxy's center. Lastly, a diffuse X-ray emission was found to exist all over the sky.

Today, the study of high-energy astrophysics continues to be carried out using data from a host of satellites past and present: the HEAO series, EXOSAT, Ginga, CGRO, RXTE, ROSAT, ASCA. Data from these satellites aid our further understanding of the nature of these sources and the mechanisms by which the X-rays and gamma-rays are emitted. Understanding these mechanisms can in turn shed light on the fundamental physics of our universe. By looking at the sky with X-ray and gamma-ray instruments, we gain unique, important information in our attempt to address questions such as "How did the Universe Begin, How does it Evolve, and What is its Fate?"

X-ray Telescopes

Prior to the introduction of imaging optics into X-ray astronomy, the most sensitive X-ray instruments consisted of collimated detectors with large collecting areas. A large collecting area was required in order to obtain a sufficiently strong signal from the relatively weak X-ray sources, in the presence of a large background signal. Placing a collimator in front of a large-area detector restricted the size of the sky from which a signal was collected at any time, and thus reduced the background signal when the detector was pointed at a source. For very bright X-ray sources, this approach is still adequate, and can still lead to major scientific advances (thus the rationale behind the recently launched Rossi X-Ray Timing Explorer with its array of large area proportional counters). There are practical limitations on how large an array of proportional counters or how restricted a collimator one can construct. Thus, a collimated detector cannot detect any of the many thousands of weak X-ray sources that comprise the background as seen by proportional counters.

One concept for increasing the ability to detect weaker sources is the use of an X-ray telescope to create an image of a portion of the X-ray sky. In much the same way as an optical telescope increases the ability of the human eye to see faint stars, an X-ray telescope can in principle concentrate the light from an X-ray star onto a small portion of an electronic eye. If that electronic eye is able to record the location where the X-ray signal impinges upon it, then the effective background signal from the sky is reduced dramatically to just that amount coincident with the source location. Equally important, such an "imaging detector" can view several X-ray emitting objects simultaneously, or can create pictures of regions from which diffuse X-ray emission arises. While the appeal of an imaging X-ray system is obvious, the means by which one actually constructs an X-ray telescope required many years to develop after the birth of X-ray astronomy. This is due primarily to the tricks one must employ in order to bring a beam of X-rays to a focus.

X-rays do not reflect off mirrors the same way that visible light does. Because of their high energy, x-ray photons penetrate into the mirror in much the same way that bullets slam into a wall. Likewise, just as bullets ricochet when they hit a wall at a grazing angle, so too will x-rays ricochet off mirrors (see diagram below). These properties mean that x-ray telescopes must be very different from optical telescopes.

The mirrors have to be precisely shaped and aligned nearly parallel to incoming x-rays. Thus they look more like barrels than the familiar dish shape of optical telescopes.

X-rays ricochet off mirrors Illustration of the barrel-shaped mirrors, which are aligned nearly parallel to incoming x-ray for Chandra (AXAF) Source: http://xrtpub.harvard.edu

X-ray detectors

"From the viewpoint of its instrumentation, X-ray astronomy possesses a certain moral simplicity. It is a perpetual battle of good versus evil; that is, of signal versus noise."

So...what would a perfect X-ray detector be like?

What would be the ideal detector for satellite-borne X-ray astronomy? It would possess high spatial resolution with a large useful area, excellent temporal resolution with the ability to handle large count rates, good energy resolution with unit quantum efficiency over a large bandwidth. Its output would be stable on timescales of years and its internal background of spurious signals would be negligibly low. It would be immune to damage by the in-orbit radiation environment and would require no consumables. It would be simple, rugged, and cheap to construct, light in weight and have a minimal power consumption. It would have no moving parts and a low output data rate.

Such a detector does not exist.

- taken from X-ray Detectors in Astronomy by G. W. Fraser.

The quest of X-ray astronomy is to be able to detect a weak source against a fairly strong background. Principally because of the relative weakness of the sources, integrating detectors (such as film) have not found a place in cosmic X-ray astronomy. Source detection is done on a photon-by-photon basis. A flux of one photon per square centimeter per second (in the 1-10 keV range) observed at Earth constitutes a bright cosmic X-ray source!

No matter which observational technique is used to view the X-ray sky, the final outcome of a measurement is determined by the properties of the electronic detector with which the collimated, focused, or diffracted X-ray photons eventually interact. There are several classes of these detectors, each with their strengths and weaknesses. They include

Targets of X-ray Astronomy Observations

Below are listed a number of topics which can be explored in greater detail. Some of these topics include links to science groups that are actively pursuing research in these fields.

X-ray Binaries
 X-ray Transients
Pulsars
 Black Holes
Dark Matter
 Diffuse Background
Cataclysmic Variables
 Active Galaxies
Supernovae
 Supernova Remnants
Gamma-ray Bursts
Source: http://imagine.gsfc.nasa.gov/docs/science/know_l2/history_xray.html