X-ray

The term X-ray generally refers to electromagnetic radiation (EMR) with a wavelength within the range of 0.01-10 nm, lying between ultraviolet and gamma rays. Lower energy/frequency X-rays, toward the ultraviolet end, are termed soft and higher energy/frequency, toward the gamma rays end are termed hard. Intervening thresholds chosen to distinguish soft and hard are typically in the general region of 0.1-0.2 nm or 5-10 keV. Rather than by frequency or wavelength, X-rays are often cited by photon energy, in electron volts (eV). The equivalent to the above-cited range for X-rays is 124 eV to 124 keV, but astronomers often cite the line between X-rays and gamma rays as 100 keV: this difference is largely moot as in discussions of this EMR, a specific photon-energy range of interest is likely to be stated.

The phrase cosmic X-ray survey has been used for surveys of X-ray sources at cosmological distances, e.g., distant active galactic nuclei (AGNs).

There is not a consensus across all sciences and industries on the boundaries of EMR termed X-rays, especially between X-rays and gamma rays for which the distinction originally arose from what produced them: X-rays were from high-voltage vacuum tubes producing cathode rays, and gamma rays are a type of emission occurring during radioactive decay of radioactive substances such as radium. However, both these types of sources are producing EMR, and the respective wavelength ranges overlap, an example of the fact that mechanisms that produce EMR don't adhere to any particular absolute limits. In astronomical observation, any possible boundary would still be spanned by some instruments and some sources, but using some unambiguous classification-criteria is convenient even if it is arbitrary.

X-rays from space (like gamma rays) are virtually blocked by Earth's atmosphere, helping make Earth habitable, and X-ray telescopes are space-based. Among the telescopes/observatories:

• AstroSat.
• Chandra X-ray Observatory.
• Einstein Probe.
• HXMT.
• IXPE.
• MAXI.
• NICER.
• NuSTAR.
• Spektr-RG eROSITA (inactive).
• Suzaku.
• XMM-Newton.
• XRISM.

Also, Gamma-ray and UV observatories often have some X-ray capability, as well as solar observatories and some planetary probes. Some past X-ray telescopes/observatories:

• ALEXIS.
• Ariel 5.
• ASCA.
• BeppoSAX.
• EXOSAT.
• Ginga.
• GRANAT.
• HEAO-1 and HEAO-2.
• ROSAT.
• Rosetta.
• RXTE.
• Uhuru.
• Yohkoh.

Plans and concepts for future X-ray telescopes/observatories:

• Arcus.
• ATHENA.
• AXIS.
• CubIXSS.
• eXTP.
• Lynx.
• STROBE-X.
• STAR-X.