(3) Interaction of high energy photons with matter - LUTH
(3) Interaction of high energy photons with matter - LUTH
(3) Interaction of high energy photons with matter - LUTH
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Photoelectric Absorption in the interstellar medium<br />
The K-edges provide the dominant source <strong>of</strong> opacity for X-rays<br />
traversing interstellar <strong>matter</strong>. The figure on the right shows the<br />
cross-section for standard cosmic abundances <strong>of</strong> the different<br />
elements.<br />
A practical interpolation formula can be used to describe the<br />
optical depth x for X-ray absorption in the interstellar medium:<br />
xℏ = 2×10 −26 <br />
where the optical depth is<br />
defined as:<br />
x =∫ xN H dl<br />
N H is the number density <strong>of</strong> hydrogen atoms (particles / m³ )<br />
optical density = fraction <strong>of</strong> radiation absorbed or scattered on<br />
a path dl : I<br />
I 0<br />
= e −<br />
∫N H dl<br />
ℏ <br />
−8/3<br />
∫NH dl<br />
1keV<br />
is the column density (particles / m² )<br />
(i.e. "grammage")<br />
A. Zech, Instrumentation in High Energy Astrophysics<br />
4
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