Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996rftu.proc..623p&link_type=abstract
International Conference on X-ray Astronomy and Astrophysics: Röntgenstrahlung from the Universe, p. 623 - 626
Astronomy and Astrophysics
Astrophysics
1
Interstellar Dust: X Rays, Interstellar Dust: Scattering, Supernova Remnants: X Rays, Supernova Remnants: Scattering
Scientific paper
The interaction of X-rays with interstellar dust grains leads to small angle scattering. X-ray sources behind sufficiently dense dust columns are therefore surrounded by haloes of faint and diffuse X-ray emission. Since these X-ray sources are also highly absorbed, X-ray observations offer the unique opportunity to measure directly the gas to dust ratio along a given line of sight. A surprisingly good correlation exists between the simultaneously measured dust and hydrogen column densities, indicating that gas and dust must be to a large extent cospatial. For the X-ray sources with known optical counterparts, the visual extinction correlates well with the X-ray derived dust scattering optical depth τsca:τsca = 0.087×Av(mag)×E(keV)-2 and NH[cm-2/Av] = 1.79×1021. The strong correlation of all three quantities can be used, for instance, to distinguish quantitatively between interstellar and local absorption. In particular the author finds that for Cas A the optical and the X-ray scattering are too low for the observed absorption indicating that the remnant is embedded in a dense but dust-free environment. From the correlation between scattering and absorption, some properties of the dust grains can be derived: if, as commonly assumed, all heavy elements are depleted, grains must be "fluffy" or porous with voids up to 70%.
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