Xray astronomy

X-ray astronomy is a major observational branch of modern astronomy concerned with the detection and study of X-ray radiation emitted by celestial objects. Because X-rays are strongly absorbed by the Earth’s atmosphere, observations must be conducted from high altitudes or from space, using sounding rockets, balloons and specialised satellites. The field has revealed high-energy processes occurring in the Universe and has contributed to understanding compact objects, stellar evolution, supernova remnants and the interstellar and intergalactic medium.

Nature of X-Ray Emission

X-ray photons are typically generated in environments with extremely hot gas at temperatures ranging from about one million kelvin to several hundred million kelvin. Such conditions occur in stellar coronae, supernova remnants, neutron stars, black hole accretion discs and the hot plasma filling galaxy clusters. The existence of the ionised E-layer in the Earth’s upper atmosphere originally hinted at strong extraterrestrial X-ray sources long before they could be detected directly.
Because X-rays cannot penetrate the lower atmosphere, early astronomers relied on theoretical predictions suggesting that the Sun and other stars would be powerful X-ray emitters. These predictions were only verified after the development of high-altitude rockets and satellite platforms enabled direct detection above the atmosphere.

Early Discoveries and Solar Observations

Initial studies of solar X-rays began in the mid-twentieth century. The first confirmed measurements were obtained using V-2 sounding rockets launched from White Sands Proving Ground in the late 1940s. These pioneering missions established the presence of solar X-ray emission and demonstrated the feasibility of high-altitude observations.
Prior to direct detection, the hot and tenuous solar corona had been inferred from optical observations of highly ionised spectral lines in the late 1930s. Radio studies in the mid-1940s further revealed the existence of a radio-emitting corona, suggesting the Sun’s outer layers were far hotter than its surface.

The First Cosmic X-Ray Source

A major milestone occurred in 1962, when an Aerobee 150 sounding rocket detected Scorpius X-1, the first cosmic X-ray source beyond the Solar System. Its X-ray emission was found to be extraordinarily powerful—about ten thousand times greater than its optical output. Scorpius X-1 became the prototype for a class of compact, high-energy systems now known to include neutron stars and black hole binaries. This discovery launched a new era of high-energy astrophysics and earned Riccardo Giacconi the Nobel Prize in Physics in 2002.
Since the 1960s, thousands of X-ray sources have been detected across the sky, ranging from isolated stars and binaries to galaxies, quasars and diffuse emission in galaxy clusters. The intergalactic medium within clusters consists of hot plasma with temperatures of 100–1000 megakelvin, containing far more mass than the visible galaxies within the cluster.

Sounding Rockets

Sounding rockets played a critical role in early X-ray astronomy. Short suborbital flights in the upper atmosphere allowed detectors to collect data for a few minutes before returning to Earth. These missions were limited in duration and sky coverage, yet they provided essential initial results.
Notable achievements include:

• The 1949 V-2 flight that recorded solar X-rays using a Naval Research Laboratory detector.
• The 1962 Aerobee 150 flight that discovered Scorpius X-1.
• Subsequent sounding-rocket programmes that investigated diffuse X-ray backgrounds and tested improved focusing technologies.

While the rockets offered only moments of observation time, they were invaluable for technological development and early discoveries.

The Interstellar Medium and Diffuse X-Ray Emission

X-ray astronomy has significantly advanced understanding of the interstellar medium (ISM), which contains ions, atoms, molecules, dust grains, magnetic fields and cosmic rays. The ISM exhibits structure across all spatial scales and includes:

• The hot ionised medium (HIM), composed of coronal gas at temperatures of 10⁶–10⁷ K, emitting soft X-rays.
• Stellar wind bubbles and superbubbles formed by massive star clusters and supernova explosions.
• Regions of low density such as the Local Bubble, through which the Sun is currently travelling, and denser clouds like the Local Interstellar Cloud.

A NASA Black Brant rocket launched in 2008 carried the X-ray Quantum Calorimeter (XQC) to measure the diffuse emission between 0.07 and 1 keV, providing detailed information on the thermal structure of the ISM.

Balloon-Borne Observations

High-altitude scientific balloons can reach altitudes of around 40 kilometres, above more than 99.997% of the atmosphere. Although some X-ray bands remain inaccessible at these heights, balloons permit longer observation periods than rockets.
Key balloon achievements include:

• The 1964 detection of hard X-rays (15–60 keV) from the Crab Nebula using a scintillation counter.
• The High-Energy Focusing Telescope (HEFT), flown in 2005, which employed tungsten–silicon multilayer coatings to focus hard X-rays between 20 and 100 keV.
• The High-Resolution Gamma-Ray and Hard X-Ray Spectrometer (HIREGS), launched from Antarctica in 1991 and 1992, which collected high-energy data during circumpolar flights lasting approximately two weeks.

These missions demonstrated extended high-altitude observation capabilities and contributed to technological advances in hard X-ray optics and detectors.

Rockoons

Rockoons, hybrid systems combining balloons and rockets, were developed to achieve higher altitudes by igniting rockets above the densest layers of the atmosphere. After ascent by balloon, the rocket would separate and ignite automatically, saving fuel and allowing the payload to reach greater heights. Early work on rockoons by American researchers in the mid-twentieth century paved the way for later high-altitude observation platforms.

Evolution of Detection and Imaging Technologies

Over six decades, X-ray astronomy has progressed from crude detectors to highly sophisticated focusing telescopes capable of producing high-resolution images and spectra. Advances include:

• Grazing-incidence optics enabling telescopes to reflect X-rays at shallow angles.
• Multilayer coatings extending reflectivity to higher energies.
• Improved calorimeters and spectrometers measuring X-ray energies with high precision.

These innovations have enabled detailed imaging of compact objects, mapping of supernova remnants and studies of massive black hole accretion.

Scientific Impact

X-ray astronomy has become essential to astrophysics. It reveals processes invisible at optical wavelengths and provides insights into:

• Accretion onto neutron stars and black holes.
• Stellar winds and magnetic activity in stars.
• Shock heating in supernova remnants.
• The thermal content and structure of galaxy clusters.
• Large-scale behaviour of plasma in interstellar and intergalactic environments.