Editing X-ray astronomy (section)

===Astronomical models===
[[File:NASA-2015IYL-MultiPix-ChandraXRayObservatory-20150122.jpg|thumb|right|Images released to celebrate the [[International Year of Light|International Year of Light 2015]]<br>([[Chandra X-Ray Observatory]]).]]
From the observed X-ray spectrum, combined with spectral emission results for other wavelength ranges, an astronomical model addressing the likely source of X-ray emission can be constructed. For example, with Scorpius X-1 the X-ray spectrum steeply drops off as X-ray energy increases up to 20 keV, which is likely for a thermal-plasma mechanism.<ref name=Morrison/> In addition, there is no radio emission, and the visible continuum is roughly what would be expected from a hot plasma fitting the observed X-ray flux.<ref name=Morrison/> The plasma could be a [[coronal cloud]] of a central object or a transient plasma, where the energy source is unknown, but could be related to the idea of a close binary.<ref name=Morrison/>

In the Crab Nebula X-ray spectrum there are three features that differ greatly from Scorpius X-1: its spectrum is much harder, its source diameter is in [[light-year]]s (ly)s, not [[astronomical unit]]s (AU), and its radio and optical synchrotron emission are strong.<ref name=Morrison/> Its overall X-ray luminosity rivals the optical emission and could be that of a nonthermal plasma. However, the Crab Nebula appears as an X-ray source that is a central freely expanding ball of dilute plasma, where the energy content is 100 times the total energy content of the large visible and radio portion, obtained from the unknown source.<ref name=Morrison/>

The [[Astrophysical X-ray source#X-ray dark stars|"Dividing Line"]] as [[giant star]]s evolve to become [[red giant]]s also coincides with the Wind and Coronal Dividing Lines.<ref name=Kashyap>{{Cite journal|author=Kashyap V|author2= Rosner R|author3=Harnden FR Jr.|author4=Maggio A|author5=Micela G|author6=Sciortino S |title=X-ray emission on hybrid stars: ROSAT observations of alpha Trianguli Australis and IOTA Aurigae |journal=Astrophys J|date=1994|volume=431 |page=402|doi=10.1086/174494 |bibcode=1994ApJ...431..402K}}</ref> To explain the drop in X-ray emission across these dividing lines, a number of models have been proposed:
# low transition region densities, leading to low emission in coronae,
# high-density wind extinction of coronal emission,
# only cool coronal loops become stable,
# changes in a magnetic field structure to that an open topology, leading to a decrease of magnetically confined plasma, or
# changes in the magnetic dynamo character, leading to the disappearance of stellar fields leaving only small-scale, turbulence-generated fields among red giants.<ref name=Kashyap/>