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Diffuse X-rays in Galaxies

المؤلف:  James Binney and Michael Merrifield

المصدر:  GALACTIC ASTRONOMY

الجزء والصفحة:  p-579

2026-11-01

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The distribution of diffuse X-radiation in a band centred on 0.2 keV is strongly concentrated towards the Galactic poles. More generally, in this band the X-ray flux in each direction is strongly anti-correlated with the strength of the 21-cm line in that direction. A natural interpretation of this finding is that the X-rays come from outside the relatively thin layer to which neutral hydrogen is confined, and at low latitudes are absorbed by cool gas on their way to us. If this interpretation were correct, X-rays of lower energy, which are more strongly absorbed by cool gas, would be even more strongly concentrated to high latitudes. Actually the distribution of lower-energy X-rays is remarkably similar to that observed for the band centred on 0.2 keV.

These findings led Snowden et al. (1990) to hypothesize that the Sun sits in a 'local bubble' of hot X-ray emitting gas that is embedded in the layer of cool gas. In this model, the X-ray and 21-cm fluxes are anticorrelated because the X-ray flux is small and the 21-cm flux is large in directions in which we are near the bubble's edge, while in directions in which we are far from the bubble's edge the X-ray flux is large and the 21-cm flux is small. Subsequently, the ROSAT satellite demonstrated that at least part of the correlation between X-ray and 21-cm fluxes is generated by absorption, so the situation is now rather confused.

A major problem that dogs attempts to study the ISM with X-rays is that of determining how much of the observed diffuse flux is of extragalactic origin. It is known that a large part of the high-latitude soft X-ray flux derives from unresolved quasars and Seyfert galaxies (Hasinger et al. 1993).