The history of X-ray astronomy begins in the 1920s, with interest in short wave communications for the U.S. Navy. This was soon followed by extensive study of the earth's ionosphere. By 1927, interest in the detection of X-ray and ultraviolet (UV) radiation at high altitudes inspired researchers to launch Goddard's rockets into the upper atmosphere to support theoretical studies and data gathering. The first successful rocket flight equipped with instrumentation able to detect solar ultraviolet radiation occurred in 1946. X-ray solar studies began in 1949. By 1973 a solar instrument package orbited on Skylab providing significant solar data.
In 1965 the Goddard Space Flight Center program in X-ray astronomy was initiated with a series of balloon-borne experiments. In the 1970s this was followed by high altitude sounding rocket experiments, and that was followed by orbiting (satellite) observatories.
The first rocket flight to successfully detect a cosmic source of X-ray emission was launched in 1962 by a group at American Science and Engineering (AS&E).
X-ray wavelengths reveal information about the bodies (sources) that emit them.
1920s to the 1940s The Naval Research Laboratory (NRL) opened in 1923. After E.O. Hulburt (1890-1982) arrived there in 1924 he studied physical optics. The NRL was conducting research on the properties of the ionosphere (Earth's reflecting layer) because of interest in short wave radio communications. Hubert (Hulburt ?) produced a series of mathematical descriptions of the ionosphere during the 1920s and 1930s. In 1927, at the Carnegie Institution of Washington, Hulburt, Gregory Breit and Merle Tuve explored the possibility of equipping Robert Goddard's rockets to explore the upper atmosphere. In 1929 Hulburt proposed an experimental program in which a rocket might be instrumented to explore the upper atmosphere. This proposal included detection of ultraviolet radiation and X rays at high altitudes.
Herbert Friedman began X-ray solar studies in 1949 and soon reported that the energy of "the solar X-ray spectrum ... is adequate to account for all of E-layer ionization." Thus one of Hulburt's original questions, the source and behavior of the radio-reflecting layer, began to find its answer in space research.
At the end of the 1930s other studies included the inference of an X-ray corona by optical methods and, in 1949, more direct evidence by detecting X-ray photons.
Because the Earth's atmosphere blocks X-rays at ground level, Wilhelm Röntgen's discovery had no effect on observational astronomy for the first 50 years. X-ray astronomy became possible only with the capability to use rockets that far exceeded the altitudes of balloons. In 1948 U.S. researchers used a German-made V-2 rocket to gather the first records of solar x-rays.
The NRL has placed instruments in rockets, satellites, Skylab, and Spacelab 2
Through the 1960s, 70s, 80s, and 90s, the sensitivity of detectors increased greatly during the 60 years of X-ray astronomy. In addition, the ability to focus X-rays has developed enormously—allowing the production of high-quality images.
1960s The study of astronomical objects at the highest energies of X-rays and gamma rays began in the early 1960s. Before then, scientists knew only that the Sun was an intense source in these wavebands. Earth's atmosphere absorbs most X-rays and gamma rays, so rocket flights that could lift scientific payloads above Earth's atmosphere were needed. The first rocket flight to successfully detect 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, Herbert Gursky, Frank Paolini, and Bruno Rossi. They used a small X-ray detector on board a rocket flight, with which they found a very bright source in the constellation of Scorpius. Hence, this source was later named Scorpius X-1.
1970s 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 hypothesized 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 traveling from the normal star to the neutron star in a process called accretion. The binary nature of the system allowed astronomers to measure the mass of the neutron star. For other systems, the inferred mass of the X-ray emitting object supported the idea of the existence of black holes, as they were too massive to be neutron stars. Other 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.
1980s to the present The study of X-ray astronomy continued to be carried out using data from a host of satellites that were active from the 1980s to the early 2000s: the HEAO Program, EXOSAT, Ginga, RXTE, ROSAT, ASCA, as well as BeppoSAX, which detected the first afterglow of a gamma-ray burst (GRB). Data from these satellites continues to 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 collect important information in our attempt to address questions such as how the universe began and how it evolves, and gain some insight into its eventual fate.Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center. All material on this site has been created and updated between 1997-2011.
Goddard Space Flight Center
Balloons In 1965, at the suggestion of Frank McDonald, Elihu Boldt initiated Goddard's program in X-ray astronomy with a series of balloon-borne experiments. At an early stage he was joined by Peter Serlemitsos, who had just completed his PhD space physics thesis on magnetospheric electrons, and by Guenter Riegler, a University of Maryland physics graduate student interested in doing his dissertation research in astrophysics.
From 1965 to 1972 there were over a dozen balloon-borne experiments (mostly from New Mexico), including the first such to take place from Australia (1966), one in which hard X-ray emission was discovered (albeit with crude angular resolution) from a region towards the Galactic Center whose centroid is located among subsequently identified sources GX1+4, GX3+1, and GX5-1. A balloon-borne experiment in 1968 was based on the multi-anode multi-layer xenon gas proportional chamber that had recently been developed in our lab and represented the first use of such a high performance instrument for X-ray astronomy.
Due to the attenuation of soft X-rays by the residual atmosphere at balloon altitudes these early experiments were restricted to energies above ~20 keV. Observations down to lower energies were begun with a series of high altitude sounding rocket experiments; by this stage Steve Holt had already joined the program. A 1972 rocket-borne observation of Cas A, the youngest supernova remnant in our galaxy, yielded the first detection of an X-ray spectral line, iron K-line emission at ~7 keV.
Rockets
The figure to the right shows 15-second samples of the raw counts (per 20.48ms) observed in a 1973 sounding-rocket-borne exposure to three of the X-ray brightest binary sources in the Milky Way Galaxy: Her X-1 (1.7 days), Cyg X-3 (0.2 day), and Cyg X-1 (5.6 days). The 1.24 second pulsar period associated with Her X-1 is immediately evident from the data, while the rate profile for Cyg X-3 is completely consistent with the statistical fluctuations in counts expected for a source that is constant, at least for the 15s duration of the exposure shown. The Cyg X-1 data, on the other hand, clearly exhibit the chaotic "shot noise" behavior characteristic of this black-hole candidate and also provided preliminary evidence for the additional feature of millisecond "burst" sub-structure, noted for the first time in this observation. The sharp cut-off at ~24 keV in the flat spectrum observed for Her X-1 in this exposure provided the first reported evidence for radiative transfer effects to be associated with a highly magnetized plasma near the surface of a neutron star. The black-body spectral component observed for Cyg X-3 during this experiment gave strong evidence that this emission is from the immediate vicinity of a compact object the size of a neutron star.
An observation of Cyg X-3 a year later with the same instrument yielded an optically thin thermal spectrum for this source and provided the first evidence for strong spectral iron K-line emission from an X-ray binary.
Orbiting observatories
Our large area PCA (Proportional Counter Array) on the current RXTE (Rossi X-ray Timing Explorer) mission genuinely reflects the heritage of our sounding rocket program. RXTE continues to provide very valuable data as it enters the second decade of successful operation. Goddard's ASM (All-Sky Monitor) pin-hole X-ray camera on Ariel-5 (1974-1980) was the first X-ray astronomy experiment to use imaging proportional counters (albeit one-dimensional); it provided information on transient sources and the long-term behavior of several bright objects. Jean Swank joined the program in time for the beginning of our OSO-8 experiment (1975-1978), the first broadband (2-40 keV) orbiting observatory based on multi-anode multi-layer proportional chambers, one that showed the power of X-ray spectroscopy; for example, it established that iron K-line emission is a ubiquitous feature of clusters of galaxies.
The HEAO-1 A2 full-sky cosmic X-ray experiment (1977-1979) provided the most comprehensive data (still the most definitive) on the cosmic X-ray background broadband spectrum and large-scale structure, and a much used complete sample of the brightest extragalactic sources; it posed the challenging "spectral paradox" just now being unraveled with new results on evolution (from deep surveys) and on individual source spectra extending into the gamma-ray band. The SSS (Solid State Spectrometer) at the focus of the HEAO-2 Einstein Observatory (1978-1981) grazing incidence telescope was the first high spectral resolution non-dispersive spectrometer to be used for X-ray astronomy, here for energies up to ~3 keV, limited by the telescope optics.
By the use of conical foil optics, developed in our lab, the response of a grazing incidence X-ray telescope was extended to 12 keV, amply covering the crucial iron K-band of emission. A cooled Si(Li) solid state detector was used at the focus of such a telescope for the BBXRT (Broad Band X-Ray Telescope) on the Astro-1 shuttle mission (STS-35) on Columbia in December 1990, the first broadband (0.3-12keV) X-ray observatory to use focusing optics.
In collaboration with X-ray astronomers in Japan, Goddard supplied conical foil X-ray optics have been used for the joint Japanese and American ASCA mission (1993-2000). It was the first broadband imaging observatory using CCD non-dispersive spectrometers.