Highlights of X-Ray Astronomy Mainly Based on ASCA Results

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I present three topics of X-ray astronomy to which ASCA^<1)> has made significant contributions. The first one is X-ray observations to explore sites of particle acceleration in our Galaxy: A discovery of clear evidence for electrons to be accelerated up to 〜100 TeV in SN1006 by ASCA, ^<2)> and an interesting problem on the energy source of the Galactic ridge X-ray emission are shown. The second topic is on ultra-luminous binary X-ray sources. ASCA observations of several point sources with X-ray luminosities of 10^<39-40> erg/s in nearby spiral galaxies showed that their X-ray spectra were commonly reproduced by a two-component-model composing of a so-called disk black-body component and a power-law component. This strongly suggests that these sources are binary sources containing black holes with the mass close to 100M⊙. An interesting result of spectral fits of the two-component-moel to the observed spectra is that the innermost radius of the disk seems to be so small as to indicate for the central objects to be Kerr black holes.^<3)> On the other hand, detailed analyses of X-ray spectrum of the Galactic superluminal source, GRS 1915 + 105 showed that the innermost radius of this source also becomes very small but that it does not always happen. In this source, the disk seems to change its state between a high temperature(small radius)state and a low temperature(large radius)state.^<4)> According to theoretical study of accretion disks, these two states can be considered to correspond to a so-called slim disk and a standard disk, respectively. The third topic is on a broad and skewed feature around 5-7 keV in spectra of Seyfert galaxies. It was first discovered from MCG-6-30-15^<5)> and is one of the most remarkable results with ASCA. The profile is well interpreted by the so-called "disk-line" model, in which the feature is considered to be iron fluorescent lines coming from an innermost region of arelatively cool disk and to be broadened and skewed by the relativistic motion of the matter as well as by the general relativistic effect in the vicinity of the central black hole.^<6)> However, analyses of long observations of MCG-6-30-15^<7)> and NGC4151^<8)> show that the time variability of the flux and profile of the "disk-line" feature rather prefer alternative interpretations to the "disk-line" model.
理論物理学刊行会の論文
2000-10-26

Highlights of X-Ray Astronomy Mainly Based on ASCA Results

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