Editing X-ray background

The observed '''X-ray background''' is thought to result from, below 0.3 [[Electronvolt|keV]], galactic [[X-ray]] emission, the galactic X-ray background, and, above 0.3 keV, from a combination of numerous [[X-ray]] point sources outside of the [[Milky Way]], the '''cosmic X-ray background''' (CXB,<ref>{{Cite journal |last=Luca |first=A. De |last2=Molendi |first2=S. |date=2004-06-01 |title=The 2–8 keV cosmic X-ray background spectrum as observed with XMM-Newton |url=https://www.aanda.org/articles/aa/abs/2004/21/aa0421/aa0421.html |journal=Astronomy & Astrophysics |language=en |volume=419 |issue=3 |pages=837–848 |doi=10.1051/0004-6361:20034421 |issn=0004-6361|arxiv=astro-ph/0311538 }}</ref> XRB,<ref>{{Cite journal |last=Gilli |first=R. |last2=Comastri |first2=A. |last3=Hasinger |first3=G. |date=2007-02-01 |title=The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era |url=https://www.aanda.org/articles/aa/abs/2007/07/aa6334-06/aa6334-06.html |journal=Astronomy & Astrophysics |language=en |volume=463 |issue=1 |pages=79–96 |doi=10.1051/0004-6361:20066334 |issn=0004-6361}}</ref> CXRB<ref>{{Cite journal |last=Moretti |first=A. |last2=Pagani |first2=C. |last3=Cusumano |first3=G. |last4=Campana |first4=S. |last5=Perri |first5=M. |last6=Abbey |first6=A. |last7=Ajello |first7=M. |last8=Beardmore |first8=A. P. |last9=Burrows |first9=D. |last10=Chincarini |first10=G. |last11=Godet |first11=O. |last12=Guidorzi |first12=C. |last13=Hill |first13=J. E. |last14=Kennea |first14=J. |last15=Nousek |first15=J. |date=2009-01-01 |title=A new measurement of the cosmic X-ray background |url=https://www.aanda.org/articles/aa/abs/2009/02/aa11197-08/aa11197-08.html |journal=Astronomy & Astrophysics |language=en |volume=493 |issue=2 |pages=501–509 |doi=10.1051/0004-6361:200811197 |issn=0004-6361|arxiv=0811.1444 }}</ref>).  

== History ==
In 1962, in a project led by [[Riccardo Giacconi]], [[Herbert Gursky]], Frank R. Paolini, and [[Bruno Rossi|Bruno B. Rossi]], Geiger counters mounted on a rocket detected a "diffuse background radiation" of X-ray flux spanning all directions covered by the rocket's rotation. This X-ray background flux was understood to consist primarily of X-rays with energy of "several tens of keV". Analysis showed that the flux originate from outside of the solar system, but it was unclear whether such sources are galactic or extragalactic.<ref>{{Cite journal |last=Giacconi |first=Riccardo |last2=Gursky |first2=Herbert |last3=Paolini |first3=Frank R. |last4=Rossi |first4=Bruno B. |date=1962-12-01 |title=Evidence for x Rays From Sources Outside the Solar System |url=https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.9.439 |journal=Physical Review Letters |volume=9 |issue=11 |pages=439–443 |doi=10.1103/PhysRevLett.9.439|url-access=subscription }}</ref>
[[File:Figure_of_detected_X-ray_flux_in_all_directions_from_Giacconi+1962.png|thumb|400x400px|Figure from Giacconi+1962 showing X-ray flux detected in all directions of the Geiger counter's rotation]]
This background X-ray flux discovered by Giacconi et al. was later characterized as an isotropic background of hard X-rays (with energies greater than 2 keV) of extragalactic origins,<ref>{{Cite journal |last=Tanaka |first=Y. |last2=Bleeker |first2=J. A. M. |date=1977-11-01 |title=The diffuse soft X-ray sky |url=https://link.springer.com/article/10.1007/BF02431836 |journal=Space Science Reviews |language=en |volume=20 |issue=6 |pages=815–888 |doi=10.1007/BF02431836 |issn=1572-9672|url-access=subscription }}</ref> while other rocket-based studies detected an anisotropic X-ray flux of energies around 0.2 keV, believed to be emitted by sources within the Milky Way.<ref>{{Cite journal |last=Bowyer |first=C. S. |last2=Field |first2=G. B. |last3=Mack |first3=J. E. |date=1968-01-06 |title=Detection of an Anisotropic Soft X-ray Background Flux |url=https://www.nature.com/articles/217032a0 |journal=Nature |language=en |volume=217 |issue=5123 |pages=32–34 |doi=10.1038/217032a0 |issn=0028-0836|url-access=subscription }}</ref> 

With the improved angular resolution and longer observation times made possible by newly launched X-ray satellite missions since the 1970s, the nature of both components of the X-ray background became better understood. Observations from the [[Einstein Observatory]] revealed that the cosmic X-ray background (CXB) originates from discrete distant sources,<ref>{{Cite journal |last=Giacconi |first=R. |last2=Bechtold |first2=J. |last3=Branduardi |first3=G. |last4=Forman |first4=W. |last5=Henry |first5=J. P. |last6=Jones |first6=C. |last7=Kellogg |first7=E. |last8=van der Laan |first8=H. |last9=Liller |first9=W. |last10=Marshall |first10=H. |last11=Murray |first11=S. S. |last12=Pye |first12=J. |last13=Schreier |first13=E. |last14=Sargent |first14=W. L. W. |last15=Seward |first15=F. |date=1979 |title=A high-sensitivity X-ray survey using the Einstein Observatory and the discrete source contribution to the extragalactic X-ray background. |url=https://ui.adsabs.harvard.edu/abs/1979ApJ...234L...1G/abstract |journal=The Astrophysical Journal |language=en |volume=234 |pages=L1–L7 |doi=10.1086/183099 |issn=0004-637X|hdl=1887/6390 |hdl-access=free }}</ref> which were hypothesized and later confirmed to be [[Active galactic nucleus|active galactic nuclei]] (AGN).<ref>{{Cite journal |last=Setti |first=G. |last2=Woltjer |first2=L. |date=1973 |title=Extragalactic X-Ray Sources and Their Contribution to the Diffuse Background |url=https://ui.adsabs.harvard.edu/abs/1973IAUS...55..208S |journal=X- and Gamma-Ray Astronomy |language=en |volume=55 |pages=208 |issn=1743-9221}}</ref> The galactic component of the X-ray background was surveyed by [[ROSAT]], which revealed resolved structures of the galactic X-ray-emitting sources.<ref>{{Cite journal |last=Snowden |first=S. L. |last2=Freyberg |first2=M. J. |last3=Plucinsky |first3=P. P. |last4=Schmitt |first4=J. H. M. M. |last5=Truemper |first5=J. |last6=Voges |first6=W. |last7=Edgar |first7=R. J. |last8=McCammon |first8=D. |last9=Sanders |first9=W. T. |date=1995 |title=First Maps of the Soft X-Ray Diffuse Background from the ROSAT XRT/PSPC All-Sky Survey |url=https://ui.adsabs.harvard.edu/abs/1995ApJ...454..643S/abstract |journal=The Astrophysical Journal |language=en |volume=454 |pages=643 |doi=10.1086/176517 |issn=0004-637X}}</ref>

The following generation of X-ray missions, including [[Chandra X-ray Observatory|Chandra]] and [[XMM-Newton]], further propelled the understanding of the cosmic X-ray background. Observations show that 94.3% of CXB emissions in soft X-rays (between 0.5-2 keV) and 88.8% of cosmic X-ray background emissions in hard X-rays (between 2-10 keV) could be attributed to resolved point sources, such as AGN, [[Quasar|quasars]], and [[Bremsstrahlung|thermal bremsstrahlung]] emissions from [[Galaxy cluster|galaxy clusters]].<ref>{{Cite journal |last=Moretti |first=A. |last2=Campana |first2=S. |last3=Lazzati |first3=D. |last4=Tagliaferri |first4=G. |date=2003-05-10 |title=The Resolved Fraction of the Cosmic X‐Ray Background |url=https://iopscience.iop.org/article/10.1086/374335 |journal=The Astrophysical Journal |language=en |volume=588 |issue=2 |pages=696–703 |doi=10.1086/374335 |issn=0004-637X|arxiv=astro-ph/0301555 }}</ref>

==References==
<references />

==See also==
*[[X-ray astronomy]]
*[[Wilkinson Microwave Anisotropy Probe]]
*[[S150 Galactic X-Ray Mapping]]

[[Category:X-ray astronomy]]
[[Category:Astronomical X-ray sources|*X-ray background]]
[[Category:Observational astronomy]]
[[Category:Cosmic background radiation]]