The Diffuse X-Ray Background

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The observation in 1963 that detected the first extra-solar x-rays discovered two things: the bright x-ray source Sco X-1 and a faint glow coming from all directions of the sky. In the three intervening decades there have been many observations of this diffuse emission over a wide range of x-ray energies. The origin of the x-rays depends on the energy observed.

During the 1970's researchers at the University of Wisconsin, lead by Dr. William Kraushaar, used a series of sounding rocket flights to map the diffuse x-ray emission from the sky in several bands. Some of these maps are presented below (McCammon et al. 1983, ApJ, 269, 107).

University of Wisconsin Sky Survey map of the B-band (0.13-0.188 keV) x-ray intensity. This map and the ones below are Aitoff projections in galactic coordinates with the galactic center in the center of the map. Color variation tracks detector count rate variations with blue representing low rates and red high rates. The highest rates are about a factor of three brighter than the lowest rates.

University of Wisconsin Sky Survey map of the C-band (0.16-0.284 keV) x-ray intensity. Note the similarity with the B-Band map above.

The x-ray intensity in these two bands is dominated by emission from the local interstellar medium of our galaxy. This emission is thermal emission from a hot, partially ionized plasma. If one assumes that the plasma is in collisional equilibrium, the B/C ratio implies a temperature of one million degrees Kelvin. More recent observations using the ROSAT satellite to detect absorption of x-rays caused by neutral material have demonstrated that a fraction of the emission in these bands comes from regions beyond the local interstellar medium.

University of Wisconsin Sky Survey map of the M-band (0.5-0.9 keV) x-ray intensity. Color variation tracks detector count rate variations with blue representing low rates and red high rates. Most of the sky is fairly uniform in this band. The notable exception to the uniformity is the large circular region near the center of the map; this region is coincident with "Loop I", a nearby supernova remnant/stellar wind bubble. The rest of the background is a mixture of galactic and extragalactic emission; however, the details of the relative contributions is not well understood.

University of Wisconsin Sky Survey map of the 2-6 keV band x-ray intensity. Color variation tracks detector count rate variations with blue representing low rates and red high rates. The sky is fairly uniform in this band. The intensity in this bad is dominated by extragalactic emission; although, some emission from "Loop I" is still present.

A more recent survey with higher spatial resolution was performed with the position sensitive proportional counter on ROSAT. Information for this survey is available here.

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Dr. Michael Juda
Harvard-Smithsonian Center for Astrophysics
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Cambridge, MA 02138, USA
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