Editing X-ray astronomy (section)

==Major questions in X-ray astronomy==
As X-ray astronomy uses a major spectral probe to peer into the source, it is a valuable tool in efforts to understand many puzzles.

===Stellar magnetic fields===
Magnetic fields are ubiquitous among stars, yet we do not understand precisely why, nor have we fully understood the bewildering variety of plasma physical mechanisms that act in stellar environments.<ref name=Gudel/> Some stars, for example, seem to have magnetic fields, [[fossil stellar magnetic field]]s left over from their period of formation, while others seem to generate the field anew frequently.

===Extrasolar X-ray source astrometry===
{{main|Extrasolar X-ray source astrometry}}
With the initial detection of an extrasolar X-ray source, the first question usually asked is "What is the source?" An extensive search is often made in other wavelengths such as visible or radio for possible coincident objects. Many of the verified X-ray locations still do not have readily discernible sources. X-ray [[astrometry]] becomes a serious concern that results in ever greater demands for finer [[angular resolution]] and [[Radiance|spectral radiance]].

There are inherent difficulties in making X-ray/optical, X-ray/radio, and X-ray/X-ray identifications based solely on positional coincidents, especially with handicaps in making identifications, such as the large uncertainties in positional determinants made from balloons and rockets, poor source separation in the crowded region toward the galactic center, source variability, and the multiplicity of source nomenclature.<ref name=Thomas>{{Cite journal|author=Thomas RM|author2=Davison PJN |title=A comment on X-ray source identifications |journal=Proceedings of the Astronomical Society of Australia|date=1974 |volume=2 |issue=5 |page=290|doi=10.1017/S1323358000013953 |bibcode=1974PASA....2..290T|s2cid=116484669 }}</ref>

X‐ray source counterparts to stars can be identified by calculating the angular separation between source centroids and the position of the star. The maximum allowable separation is a compromise between a larger value to identify as many real matches as possible and a smaller value to minimize the probability of spurious matches. "An adopted matching criterion of 40" finds nearly all possible X‐ray source matches while keeping the probability of any spurious matches in the sample to 3%."<ref name=Gaidos>{{Cite journal|author=Gaidos EJ |title=Nearby Young Solar Analogs. I. Catalog and Stellar Characteristics |date=Nov 1998 |volume=110 |issue=753 |pages=1259–76 |doi=10.1086/316251 |journal=Publ. Astron. Soc. Pac. |bibcode=1998PASP..110.1259G|doi-access=free }}</ref>

===Solar X-ray astronomy===
{{Main|Solar X-ray astronomy}}
All of the detected X-ray sources at, around, or near the [[Sun]] appear to be associated with processes in the [[solar corona|corona]], which is its outer atmosphere.

====Coronal heating problem====
In the area of solar X-ray astronomy, there is the [[Solar corona#Coronal heating problem|coronal heating problem]]. The [[photosphere]] of the Sun has an effective temperature of 5,570 K<ref name=Massey>{{Cite journal|author=Massey P|author2= Silva DR|author3=Levesque EM|author4=Plez B|author5=Olsen KAG|author6=Clayton GC|author7=Meynet G|author8=Maeder A |title=Red Supergiants in the Andromeda Galaxy (M31) |journal=Astrophys J |volume=703|date=2009|issue=1 |page=420|doi=10.1088/0004-637X/703/1/420 |bibcode=2009ApJ...703..420M|arxiv = 0907.3767 |s2cid= 119293010}}</ref> yet its corona has an average temperature of 1–2 × 10<sup>6</sup> K.<ref name=Erdelyi>{{Cite journal|author=Erdèlyi R|author2=Ballai, I |title=Heating of the solar and stellar coronae: a review |date=2007|journal=Astron Nachr|volume=328|issue=8|page=726|doi=10.1002/asna.200710803|bibcode=2007AN....328..726E|doi-access=free}}</ref> However, the hottest regions are 8–20 × 10<sup>6</sup> K.<ref name=Erdelyi/> The high temperature of the corona shows that it is heated by something other than direct [[heat conduction]] from the photosphere.<ref name=Russell2001>{{Cite book|author=Russell CT |title=Space Weather (Geophysical Monograph) |date=2001 |publisher=[[American Geophysical Union]] |chapter=Solar wind and interplanetary magnetic filed: A tutorial |editor=Song, Paul |editor2=Singer, Howard J. |editor3-link=George Siscoe |editor3=Siscoe, George L. |isbn=978-0-87590-984-4|pages=73–88|url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-url=https://web.archive.org/web/20030828214153/http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-date=2003-08-28 |url-status=live}}</ref>

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating.<ref name=Erdelyi/> The first is [[wave]] heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.<ref name=Erdelyi/> These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat.<ref name=Alfven>{{Cite journal|author=Alfvén H |title=Magneto-hydrodynamic waves, and the heating of the solar corona |bibcode=1947MNRAS.107..211A|journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=107|issue=2 |page=211 |date=1947 |doi=10.1093/mnras/107.2.211|doi-access=free }}</ref> The other is [[magnetic field|magnetic]] heating, in which magnetic energy is continuously built up by photospheric motion and released through [[magnetic reconnection]] in the form of large [[solar flare]]s and myriad similar but smaller events—[[nanoflares]].<ref name=Parker2>{{Cite journal|author=Parker EN |title=Nanoflares and the solar X-ray corona|journal=Astrophys J|volume=330 |page=474 |date=1988 |doi=10.1086/166485 |bibcode=1988ApJ...330..474P}}</ref>

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except [[Alfvén wave]]s have been found to dissipate or refract before reaching the corona.<ref name=Sturrock>{{Cite journal|author=Sturrock PA|author2=Uchida Y |title=Coronal heating by stochastic magnetic pumping|journal=Astrophys J|volume=246|page=331 |date=1981 |doi=10.1086/158926 |bibcode=1981ApJ...246..331S|hdl=2060/19800019786|hdl-access=free}}</ref> In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.<ref name=Erdelyi/>

====Coronal mass ejection====
A [[coronal mass ejection]] (CME) is an ejected plasma consisting primarily of electrons and [[proton]]s (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entraining coronal closed magnetic field regions. Evolution of these closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the CME.<ref name=Gopalswamy>{{Cite journal|author=Gopalswamy N|author2=Mikic Z|author3=Maia D|author4=Alexander D|author5=Cremades H|author6=Kaufmann P|author7=Tripathi D|author8= Wang YM |title=The pre-CME Sun |journal=Space Science Reviews|date=2006 |volume=123|issue=1–3 |page=303|doi=10.1007/s11214-006-9020-2|bibcode = 2006SSRv..123..303G |s2cid=119043472 }}</ref> Small-scale energetic signatures such as plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs.

The soft X-ray sigmoid (an S-shaped intensity of soft X-rays) is an observational manifestation of the connection between coronal structure and CME production.<ref name=Gopalswamy/> "Relating the sigmoids at X-ray (and other) wavelengths to magnetic structures and current systems in the solar atmosphere is the key to understanding their relationship to CMEs."<ref name=Gopalswamy/>

The first detection of a Coronal mass ejection (CME) as such was made on December 1, 1971, by R. Tousey of the US Naval Research Laboratory using [[OSO 7]].<ref name=Howard>{{cite web|title=R.A.Howard, A Historical Perspective on Coronal Mass Ejections |url=http://hesperia.gsfc.nasa.gov/summerschool/lectures/vourlidas/AV_intro2CMEs/additional%20material/corona_history.pdf |archive-url=https://web.archive.org/web/20061012002045/http://hesperia.gsfc.nasa.gov/summerschool/lectures/vourlidas/AV_intro2CMEs/additional%20material/corona_history.pdf |archive-date=2006-10-12 |url-status=live}}</ref> Earlier observations of '''coronal transients''' or even phenomena observed visually during [[solar eclipses]] are now understood as essentially the same thing.

The largest geomagnetic perturbation, resulting presumably from a "prehistoric" CME, coincided with the first-observed solar flare, in 1859. The flare was observed visually by [[Richard Christopher Carrington]] and the [[geomagnetic storm]] was observed with the recording magnetograph at [[Kew Gardens]]. The same instrument recorded a '''crotchet''', an instantaneous perturbation of the Earth's ionosphere by ionizing soft X-rays. This could not easily be understood at the time because it predated the discovery of X-rays (by [[Wilhelm Conrad Roentgen|Roentgen]]) and the recognition of the [[ionosphere]] (by [[Arthur Edwin Kennelly|Kennelly]] and [[Heaviside]]).