- #1
Luther_Harkon
- 2
- 0
Homework Statement
Intergallactic ionized gas produces X ray photons via the bremsstrahlung process
[tex]
e^-\ p\rightarrow e^-\ p\ \gamma
[/tex]
In contrast to photons produced in a star (see previous exercises) these X ray photons escape from the galaxy cluster. Indeed, the largest galaxy clusters contain a few thousand galaxies with a total mass ∼ [tex]10^{14} M_\odot[/tex] and a radius of several Mpc.
Note. take [tex]n_{e^-}=n_p\sim\Omega_B\rho_c/m_p[/tex]
Homework Equations
Show that the mean free path of a photon is [tex]\lambda_\gamma=1/(n\sigma)\gg1Mpc[/tex], where [tex]\sigma[/tex] can be approximated by the Thomson cross-section of photons on electrons. Why not on photons?
The Attempt at a Solution
I had no problem in showing that the free mean path of photons , when given the conditions shown here, is much greater than 1 Mpc. But I am quite confused on why can't be the free mean path computed by the Thomson cross section on protons. Any clue on that?
Thanks!
Last edited: