THIS IS A PROTO-ARTICLE, UNDER CONSTRUCTION.
FURTHER DEVELOPMENT IS ENCOURAGED.
Introduction
High energy astrophysics is a branch of astrophysics that deals with phenomena which produce radiation at wavelengths shortward of the Lyman limit at 912 Angstroms. This includes the extreme ultraviolet, X-ray, and gamma-ray bands of the electromagnetic spectrum.
History
ObservingTechniques
Observatories
Data Archives
Galactic High Energy Astronomy
Extragalactic High Energy Astronomy
Beginning with the Einstein satellite in 1978, and continuing with the campaigns of the orbiting Chandra observatory (1999- ), observations of normal galaxies have revealed a variety of astrophysically interesting phenomena. These range from diffuse halos of hot gas from the death throes of massive stars to stars locked in binary systems with neutron stars and black holes. In all, the often time-varying spectra and luminosities of each individual galaxy's constituent X-ray sources serves as invaluable diagnostics to the system's star formation history, morphology, and evolution.
Diffuse Emission
In general, galaxies in the local universe (within about 50 million parsecs, or 163 million light years) are surrounded by diffuse ionized gas at a temperature of around 5-10 million Kelvin. This plasma is believed to be a hot component of the interstellar medium- the gas and dust between stars- that extends out beyond the galaxy and is continually heated by supernova explosions within the galactic disk. It is probable that for actively star-forming galaxies, this emission comes from violent, supersonic shocks from the fast, hot supernova winds colliding with the cool, ambient interstellar medium.
The emission varies from system to system, depending on how a galaxy is oriented to us, and also on how much material the supernovae ejecta may plow into and excite. This is contingent on the depth of the interstellar medium and the presence of tidal debris, or strewn leftovers from past galaxy interactions. In all cases, however, chemical abundances from high-resolution X-ray spectra indicate that heavy element-fusing supernova remnants are responsible for this extended emission.
Compact Stellar X-Ray Sources
Low Mass X-Ray Binaries (LMXBs)
These binaries, consisting of a compact object (neutron star or black hole) and a stellar companion around the mass of the sun, are found in normal galaxies of both “early” (bulge-dominated) and “late” (spiral, disk-dominated) morphological type. Given the predominance of low mass, older, redder stars in early types, LMXBs account for the vast majority of compact sources detected in X-rays. In all, they are responsible for 23-50% of all X-ray emission in these galaxies.
The spectra of these objects reveal obvious examples of accreting binaries, whereby material from the companion star is gravitationally drawn to the compact object. In some cases, around five percent of these binaries are located in orbiting globular clusters around the central galaxy. This subpopulation tends to be found in younger, redder clusters that are especially compact and orbit close to the galaxy.
High Mass X-Ray Binaries (HMXBs)
Unlike their low-mass counterparts, HMXBs are not the only tracers of X-ray activity in their host galaxies. Preferentially found in star-forming late-type galaxies, these binaries share their homes with LMXBs, supernova remnants, Wolf-Rayet protostars, and hot young stars that all shine prominently in X-rays. By examining the spectral properties of all these sources, however, it is possible to disentangle the contributions of all the different astrophysical populations.
The locations of HMXBs uniquely follow the star formation hotspots of a galaxy, which are often the leading edges of spiral arms, central starbursts in the galactic nuclei, and similar structures found in spiral, barred spiral and irregular galaxies.
Quasi-Soft and Super Soft Sources
These compact X-ray emitters are mixed with LMXBs and HMXBs in late type galaxies. Quasi-Soft Sources, characterized by accreting gas temperatures of a few million K, possess spectral signatures of binaries whose compact objects are neutron stars. Moreover, Super Soft Sources, whose disks are only a few 100,000 K, may have white dwarfs as their central objects.
Ultra-Luminous Sources (ULXs)
ULXs are akin to HMXBs in that they both tend to highlight active star-forming regions in their galaxies. However, ULXs are an estimated order of magnitude more luminous than the high mass binaries. Given that this innate brightness is past the normal limit of an accreting neutron star or 5 solar mass black hole, it is proposed that so-called “intermediate mass black holes” (of 100 solar masses or more) are responsible for the X-ray output. These black holes, sized between the remains of a single core-collapse supernova and the multi-million solar mass black holes at the center of galaxies, may have formed from the mergers of black holes formed in young star clusters or from the core collapse of large, primordial “Population III” stars.
Alternatively, the X-ray emission from these objects may be explained by invoking scattering of disk photons into a spectrum that mimics an IMBH, variations in the geometries of accretion disks surrounding ordinary HMXBs, or emission beamed in our direction as relativistic jets by accreting white dwarfs. In some cases, confusion of these sources with distant active galactic nuclei in the background is also a possibility.
In order to constrain the evolutionary models of this exotic stellar population, and therefore the star formation history and evolution of the galaxy as a whole, observations at other wavelengths are required. Ongoing studies of nearby galaxies include optical identification of possible stellar companions, accretion disks, and/or nebular surroundings, understanding their spatial distribution in star-forming galaxies, and searches for variability in ULX output as evidence for specific accretion dynamics.
References
- Fabian, A. C., Pounds, K. A., & Blandford, R. D. Frontiers of X-Ray Astronomy. Cambridge University Press; New York, NY, 2004.
- Fabbiano, G. 2006, Ann. Rev. A&A, 44, 323
- Fabbiano, G., & White, N. E. 2006, Compact Stellar X-Ray Sources, 475.
Preview Image
The Sombrero, also known as M104, is one of the largest galaxies in the nearby Virgo cluster, about 28 million light years from Earth. This Great Observatories view of the famous Sombrero galaxy was made using NASA's Chandra X-ray Observatory, Hubble Space Telescope and Spitzer Space Telescope. The main figure shows the combined image from the three telescopes, while the three inset images show the separate observatory views. (Source: NASA Chandra X-Ray Observatory - Credit: X-ray: NASA/UMass/Q.D.Wang et al.; Optical: NASA/STScI/AURA/Hubble Heritage; Infrared: NASA/JPL-Caltech/Univ. AZ/R.Kennicutt/SINGS Team.)
Citation
Corcoran, Michael, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "High Energy Astrophysics." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 23, 2007].
<http://www.cosmosportal.org/articles/view/135672/>
Are you absolutely sure you want to delete this article? This process cannot be undone and is permanent.
Yes, Delete This Article
Are you absolutely sure you want to remove this article? This process cannot be undone and is permanent.
Yes, Remove This Article
THIS IS A PROTO-ARTICLE, UNDER CONSTRUCTION.
FURTHER DEVELOPMENT IS ENCOURAGED.
Introduction
High energy astrophysics is a branch of astrophysics that deals with phenomena which produce radiation at wavelengths shortward of the Lyman limit at 912 Angstroms. This includes the extreme ultraviolet, X-ray, and gamma-ray bands of the electromagnetic spectrum.
History
ObservingTechniques
Observatories
Data Archives
Galactic High Energy Astronomy
Extragalactic High Energy Astronomy
Beginning with the Einstein satellite in 1978, and continuing with the campaigns of the orbiting Chandra observatory (1999- ), observations of normal galaxies have revealed a variety of astrophysically interesting phenomena. These range from diffuse halos of hot gas from the death throes of massive stars to stars locked in binary systems with neutron stars and black holes. In all, the often time-varying spectra and luminosities of each individual galaxy's constituent X-ray sources serves as invaluable diagnostics to the system's star formation history, morphology, and evolution.
Diffuse Emission
In general, galaxies in the local universe (within about 50 million parsecs, or 163 million light years) are surrounded by diffuse ionized gas at a temperature of around 5-10 million Kelvin. This plasma is believed to be a hot component of the interstellar medium- the gas and dust between stars- that extends out beyond the galaxy and is continually heated by supernova explosions within the galactic disk. It is probable that for actively star-forming galaxies, this emission comes from violent, supersonic shocks from the fast, hot supernova winds colliding with the cool, ambient interstellar medium.
The emission varies from system to system, depending on how a galaxy is oriented to us, and also on how much material the supernovae ejecta may plow into and excite. This is contingent on the depth of the interstellar medium and the presence of tidal debris, or strewn leftovers from past galaxy interactions. In all cases, however, chemical abundances from high-resolution X-ray spectra indicate that heavy element-fusing supernova remnants are responsible for this extended emission.
Compact Stellar X-Ray Sources
Low Mass X-Ray Binaries (LMXBs)
These binaries, consisting of a compact object (neutron star or black hole) and a stellar companion around the mass of the sun, are found in normal galaxies of both “early” (bulge-dominated) and “late” (spiral, disk-dominated) morphological type. Given the predominance of low mass, older, redder stars in early types, LMXBs account for the vast majority of compact sources detected in X-rays. In all, they are responsible for 23-50% of all X-ray emission in these galaxies.
The spectra of these objects reveal obvious examples of accreting binaries, whereby material from the companion star is gravitationally drawn to the compact object. In some cases, around five percent of these binaries are located in orbiting globular clusters around the central galaxy. This subpopulation tends to be found in younger, redder clusters that are especially compact and orbit close to the galaxy.
High Mass X-Ray Binaries (HMXBs)
Unlike their low-mass counterparts, HMXBs are not the only tracers of X-ray activity in their host galaxies. Preferentially found in star-forming late-type galaxies, these binaries share their homes with LMXBs, supernova remnants, Wolf-Rayet protostars, and hot young stars that all shine prominently in X-rays. By examining the spectral properties of all these sources, however, it is possible to disentangle the contributions of all the different astrophysical populations.
The locations of HMXBs uniquely follow the star formation hotspots of a galaxy, which are often the leading edges of spiral arms, central starbursts in the galactic nuclei, and similar structures found in spiral, barred spiral and irregular galaxies.
Quasi-Soft and Super Soft Sources
These compact X-ray emitters are mixed with LMXBs and HMXBs in late type galaxies. Quasi-Soft Sources, characterized by accreting gas temperatures of a few million K, possess spectral signatures of binaries whose compact objects are neutron stars. Moreover, Super Soft Sources, whose disks are only a few 100,000 K, may have white dwarfs as their central objects.
Ultra-Luminous Sources (ULXs)
ULXs are akin to HMXBs in that they both tend to highlight active star-forming regions in their galaxies. However, ULXs are an estimated order of magnitude more luminous than the high mass binaries. Given that this innate brightness is past the normal limit of an accreting neutron star or 5 solar mass black hole, it is proposed that so-called “intermediate mass black holes” (of 100 solar masses or more) are responsible for the X-ray output. These black holes, sized between the remains of a single core-collapse supernova and the multi-million solar mass black holes at the center of galaxies, may have formed from the mergers of black holes formed in young star clusters or from the core collapse of large, primordial “Population III” stars.
Alternatively, the X-ray emission from these objects may be explained by invoking scattering of disk photons into a spectrum that mimics an IMBH, variations in the geometries of accretion disks surrounding ordinary HMXBs, or emission beamed in our direction as relativistic jets by accreting white dwarfs. In some cases, confusion of these sources with distant active galactic nuclei in the background is also a possibility.
In order to constrain the evolutionary models of this exotic stellar population, and therefore the star formation history and evolution of the galaxy as a whole, observations at other wavelengths are required. Ongoing studies of nearby galaxies include optical identification of possible stellar companions, accretion disks, and/or nebular surroundings, understanding their spatial distribution in star-forming galaxies, and searches for variability in ULX output as evidence for specific accretion dynamics.
References
- Fabian, A. C., Pounds, K. A., & Blandford, R. D. Frontiers of X-Ray Astronomy. Cambridge University Press; New York, NY, 2004.
- Fabbiano, G. 2006, Ann. Rev. A&A, 44, 323
- Fabbiano, G., & White, N. E. 2006, Compact Stellar X-Ray Sources, 475.
Preview Image
The Sombrero, also known as M104, is one of the largest galaxies in the nearby Virgo cluster, about 28 million light years from Earth. This Great Observatories view of the famous Sombrero galaxy was made using NASA's Chandra X-ray Observatory, Hubble Space Telescope and Spitzer Space Telescope. The main figure shows the combined image from the three telescopes, while the three inset images show the separate observatory views. (Source: NASA Chandra X-Ray Observatory - Credit: X-ray: NASA/UMass/Q.D.Wang et al.; Optical: NASA/STScI/AURA/Hubble Heritage; Infrared: NASA/JPL-Caltech/Univ. AZ/R.Kennicutt/SINGS Team.)
Citation
Corcoran, Michael, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "High Energy Astrophysics." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 23, 2007].
<http://www.cosmosportal.org/articles/view/135672/>
Are you absolutely sure you want to delete this article? This process cannot be undone and is permanent.
Yes, Delete This Article
Are you absolutely sure you want to remove this article? This process cannot be undone and is permanent.
Yes, Remove This Article
Contact Us
Send to a Friend
Comments
There are no comments.