X-ray Astrophysics Research

These technologies include microcalorimeters, solid state detectors, proportional counters, and lightweight replicated optics. Measurement techniques include imaging (mapping spatial structures), spectroscopy (deriving temperatures, densities, chemical abundances, ionization states) and timing (probing dynamical phenomena such as accretion flows, oscillations, accretion disk instabilities, and magnetic field configurations). The X-ray astronomy group at the X-Ray Astrophysics Laboratory is actively engaged in instrument development and observational research programs. The close association of scientists involved with instrument development, data analysis and data archiving provides a stimulating atmosphere that allows calibration and other issues to be identified and worked out.

Various observational programs provide a continuous flow of new results, which stimulate the requirements for new missions and instrument development. Current and approved missions are ASCA, the X-ray Calorimeter (XQC) sounding rocket program, RXTE and IXO (formerly Constellation-X). Missions under study for the future are MAXIM and Generation-X.

Specific research topics currently include:

Supernova Remnants - The violent explosion of a massive star at the end of its life is called a supernova. A supernova is one of the most energetic events in the universe, and causes a single star to briefly outshine the entire galaxy in which it is located. Supernova remnants are the dramatic objects produced by these violent explosions. The tenuous gas in the interior of the supernova remnant glows at X-ray wavelengths, and X-ray observations are a valuable source of information on the interactions between the explosion and the surrounding gas.

X-ray Binaries - X-ray binaries contain a collapsed, compact star at the end of its evolution (a neutron star or black hole), accreting material from a more normal star as the two orbit around their common center of mass. Often, this material forms a disk around the compact object. We see a wide range of dramatic phenomena from such systems, including: pulsations from rapidly-spinning neutron stars, intense bursts from thermonuclear burning, eclipses, and dips due to occulting material on the disk edge. X-ray Spectroscopy and fast timing studies reveal a wealth of information about the physical processes taking place in these complex systems.

Active Galactic Nuclei - Active galaxies contain a core (or nucleus) of emission that is embedded in an otherwise typical galaxy. This core may be highly variable and bright compared to the rest of the galaxy. X-rays penetrate outward from the core and provide scientists with unique insights into the physical processes occurring there. At the very center of the galaxy core lies a supermassive black hole. Dense material accretes onto the black hole releasing large amounts of gravitational energy. X-rays coming from close to the black hole are gravitationally redshifted, introducing a characteristic distortion in spectral features, such as the relativistically broadened iron K fluorescence line.

Galaxy clusters - Most galaxies in the Universe do not exist in isolation but are gravitationally bound with other galaxies. Small associations, called groups, may have a few dozen galaxies which extend over a million light years. Larger and rarer associations may have thousands of galaxies which extend tens of millions of light years. X-ray emission arises from hot (10-100 million degrees) intracluster gas trapped in the potential well of the cluster. It is thought that the metals in clusters were formed by stars in elliptical galaxies and were driven out into the intracluster medium by supernovae winds. The total mass of clusters appears to be larger (by factors of 10-30) than can be accounted for by the visible matter in the galaxies and gas. Hence, clusters are believed to contain dark matter, in addition to the baryonic matter (the "ordinary" matter in the stars and gas). The properties of dark matter are not well understood and its presence is only detected through gravitational influence.

The Soft X-ray Diffuse Background - The cosmic soft X-ray diffuse background (SXRB, with energies of ~0.1 to 1.5 keV) is a source of unique information on the local interstellar medium, the Galactic halo, general Galactic structure, and cosmological evolution. The nature of the SXRB varies considerably over its energy range. At the lowest energies, 0.1 - 0.3, nearly all of the SXRB originates as thermal emission from a hot (~10⁶ K) plasma. One component of the plasma is contained within a hot bubble in the disk of the Galaxy which surrounds the Sun and extends from ~50 pc to ~200 pc in different directions (this region is known as the Local Hot Bubble). There is also an extensive distribution of this plasma in the halo of our Galaxy. Between 0.5 and 1 keV, both extragalactic discrete sources and Galactic emission from hot plasma contribute to the observed flux.

X-ray Observations of Eta Carinae - Extremely massive stars play a role in chemical enrichment and galactic evolution. They mark the end of their lives as supernovae explosions in which a single supernova can equal the entire radiant output of a galaxy. Some members of this class have been suggested to produce the "hypernovae". The energy emitted in a "hypernova" is perhaps equivalent to the radiant energy output of an entire universe of galaxies. Such extraordinary explosions require stellar precursors of unusually large mass, and so should be rare. The Milky Way possesses one possible member of this class, the massive, luminous, and relatively nearby star, Eta Carinae. Eta Carinae is unstable, and is surrounded by ejecta from an eruption in the mid-19th century. X-rays are produced as the ejecta expands into the circumstellar medium near the star at speeds of 100-1000 km/s.

The Wide Angle ROSAT Pointed Survey - The Wide Angle ROSAT Pointed Survey (WARPS) is an international collaboration to compile and investigate the properties of an x-ray selected sample of galaxy clusters out to high redshifts and down to low x-ray luminosities. Large samples of clusters are essential to studying the evolution of large-scale structure in the universe. The sample is compiled from PSPC pointed observations of non-cluster targets. WARPS is the only x-ray cluster survey which has tested the efficacy of its approach by optically imaging all x-ray sources lacking counterparts on sky survey plates. This test is crucial to the completeness of the sample. Searching for clusters with x-ray observations is efficient and also provides information about the cluster gravitational potential.

The WGA Catalog of ROSAT Point Sources - A catalog being generated from the ROSAT PSPC archive.