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X-ray Astronomy

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?"

* Show me a plot of all the High-Energy X-ray missions as a function of time
* Show me a plot of all the High-Energy Astrophysics X-ray missions as a function of energy

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
Black Holes
Dark Matter
Diffuse Background
Cataclysmic Variables
Active Galaxies
Supernova Remnants
Gamma-ray Bursts

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Nicholas White (Director), within the Exploration of the Universe Division (EUD) at NASA's Goddard Space Flight Center.

The Imagine Team
Project Leader: Dr. Jim Lochner
Curator:Meredith Bene
Responsible NASA Official:Phil Newman
All material on this site has been created and updated between 1997-2005.