Origin of the Hard X-ray Emission from the Galactic Plane
Abstract
The Galactic plane is a strong emitter of hard x-rays (2 to 10 kiloelectron volts), and the emission forms a narrow continuous ridge. The currently known hard x-ray sources are far too few to explain the ridge x-ray emission, and the fundamental question of whether the ridge emission is ultimately resolved into numerous dimmer discrete sources or truly diffuse emission has not yet been settled. In order to obtain a decisive answer, using the Chandra X-ray Observatory, we carried out the deepest hard x-ray survey of a Galactic plane region that is devoid of known x-ray point sources. We detected at least 36 new hard x-ray point sources in addition to strong diffuse emission within a 17′ by 17′ field of view. The surface density of the point sources is comparable to that at high Galactic latitudes after the effects of Galactic absorption are considered. Therefore, most of these point sources are probably extragalactic, presumably active galaxies seen through the Galactic disk. The Galactic ridge hard x-ray emission is diffuse, which indicates omnipresence within the Galactic plane of a hot plasma, the energy density of which is more than one order of magnitude higher than any other substance in the interstellar space.
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We are grateful to F. E. Marshall, K. Mukai, and R. F. Mushotzky for helpful comments that improved an earlier version of this report. We also thank Y. Tanaka and other anonymous referees for useful comments. The observation was carried out under NASA's Chandra Guest Observer Program.
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Science
Volume 293 | Issue 5535
31 August 2001
31 August 2001
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Received: 18 June 2001
Accepted: 30 July 2001
Published in print: 31 August 2001
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