page_title: Astrophysical X-ray source
section_title: Sun
hierarchical_section_title: Astrophysical X-ray source / X-ray emission from stars / Sun
caption_attribution_description: This image shows the Sun as viewed by the Soft X-Ray Telescope (SXT) onboard the orbiting Yohkoh satellite. The bright, loop-like structures are hot (millions of degrees) plasma confined by magnetic fields rooted in the solar interior. An image of the sun in visible light would show sunspots at the feet of many of these loops. The halo of gas extending well beyond the sun is called the corona. The darker regions at the North and South poles of the Sun are coronal holes, where the magnetic field lines are open to space and allow particles to escape.
context_page_description: Astrophysical X-ray sources are astronomical objects with physical properties which result in the emission of X-rays.
  There are a number of types of astrophysical objects which emit X-rays, from galaxy clusters, through black holes in active galactic nuclei to galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf, neutron star or black hole. Some solar system bodies emit X-rays, the most notable being the Moon, although most of the X-ray brightness of the Moon arises from reflected solar X-rays. A combination of many unresolved X-ray sources is thought to produce the observed X-ray background. The X-ray continuum can arise from bremsstrahlung, either magnetic or ordinary Coulomb, black-body radiation, synchrotron radiation, inverse Compton scattering of lower-energy photons by relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.
  Furthermore, celestial entities in space are discussed as celestial X-ray sources.
context_section_description: In the late 1930s, the presence of a very hot, tenuous gas surrounding the Sun was inferred indirectly from optical coronal lines of highly ionized species. In the mid-1940s radio observations revealed a radio corona around the Sun. After detecting X-ray photons from the Sun in the course of a rocket flight, T. Burnight wrote, "The sun is assumed to be the source of this radiation although radiation of wavelength shorter than 4 Å would not be expected from theoretical estimates of black body radiation from the solar corona." And, of course, people have seen the solar corona in scattered visible light during solar eclipses.
  While neutron stars and black holes are the quintessential point sources of X-rays, all main sequence stars are likely to have hot enough coronae to emit X-rays. A- or F-type stars have at most thin convection zones and thus produce little coronal activity.
  Similar solar cycle-related variations are observed in the flux of solar X-ray and UV or EUV radiation. Rotation is one of the primary determinants of the magnetic dynamo, but this point could not be demonstrated by observing the Sun: the Sun's magnetic activity is in fact strongly modulated (due to the 11-year magnetic spot cycle), but this effect is not directly dependent on the rotation period.
  Solar flares usually follow the solar cycle. CORONAS-F was launched on 31 July 2001 to coincide with the 23rd solar cycle maximum.
  The solar flare of 29 October 2003 apparently showed a significant degree of linear polarization (> 70% in channels E2 = 40–60 keV and E3 = 60–100 keV, but only about 50% in E1 = 20–40 keV) in hard X-rays, but other observations have generally only set upper limits.
  Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle, it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops populate both active and quiet regions of the solar surface. The Yohkoh Soft X-ray Telescope (SXT) observed X-rays in the 0.25–4.0 keV range, resolving solar features to 2.5 arc seconds with a temporal resolution of 0.5–2 seconds. SXT was sensitive to plasma in the 2–4 MK temperature range, making it an ideal observational platform to compare with data collected from TRACE coronal loops radiating in the EUV wavelengths.
  
  Variations of solar-flare emission in soft X-rays (10–130 nm) and EUV (26–34 nm) recorded on board CORONAS-F demonstrate for most flares observed by CORONAS-F in 2001–2003 UV radiation preceded X-ray emission by 1–10 min.
caption: The corona of the Sun as seen in the X-ray region of the electromagnetic spectrum on 8 May 1992 by the soft X-ray telescope on board the Yohkoh solar observatory spacecraft.
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