![]() | Annu. Rev. Astron. Astrophys. 2001. 39:
249-307 Copyright © 2001 by Annual Reviews. All rights reserved |
4.4. Interaction with High Energy
-Rays
As discussed in Section 3.7, the CIB strongly
attenuates
-rays from
1-100 TeV. Since the cross section for the
-
reaction varies
strongly with energy near the peak, ~ 50% of the total cross section for
17 TeV photons arises from interactions with 40 to 80 µm
background photons
(Guy et al. 2000).
If
I
= 28 nW
m-2 sr-1 at 60 µm, as claimed by
Finkbeiner et
al. (2000),
then the mean free path for 17 TeV photons would be ~ 14 Mpc. This is
considerably smaller than the distance to Mrk 501
(
160 Mpc for h
= 0.65), which would imply that the intrinsic 17 TeV flux from Mrk 501
is larger than the observed flux by a factor of exp(~ 12)
105. This
leads to an excessive power output for that galaxy, a situation
described by
Protheroe & Meyer
(2000)
as "an infrared background-TeV
-ray crisis."
Several possible solutions have been suggested to resolve this "crisis". (a) Harwit et al. (1999) suggested that the actual flux of 10 to 20 TeV photons could be much lower than inferred from the observations. This could occur if lower-energy photons were to arrive coherently, simulating the effect of single 10 to 20 TeV photons with the sum of their energies. (b) Kifune (1999) noted that because the photon energy-momentum relation violates Lorentz invariance in quantum gravity scenarios (Amelino-Camelia et al. 1998, and references therein), the energy threshold for electron pair production can be raised, thus reducing the opacity of the universe to TeV photons by orders of magnitude. (c) The most obvious solution to the crisis is, of course, to assume that most of the 60 and 100 µm radiation tentatively identified as the CIB by Finkbeiner et al. (2000) actually arises from foreground sources.
The opacity of the universe to TeV
-rays as a
function of
-ray energy
and source redshift has been calculated using conventional physics by
several authors
(Primack et al. 1999,
Salamon & Stecker
1998,
Biller et al. 1998).
Photons arriving from distant
-ray
sources traverse different intergalactic radiation fields at different
redshifts. Primack et al. used semianalytical models
(Section 5.2.3) to calculate the
evolution of the EBL with redshift, whereas Salamon & Stecker
calculated these evolutionary effects in the framework of the
Fall et al. (1996)
cosmic chemical evolution model
(Section 5.2.4). For local sources,
z << 1, the opacity can be calculated from the local EBL
(Coppi & Aharonian
1999,
Protheroe & Meyer
2000).
Figure 6 presents the opacity of the local
universe for TeV
-rays
implied by the EBL measurements summarized in
Figure 5.
Figure 6 suggests that the uncertainties in the
EBL are sufficiently large that the detection of 10 to 20 TeV
-rays does
not yet constitute a crisis and that 30 TeV sources could potentially be
visible up to a distance of ~ 100 h-1 Mpc.
![]() |
Figure 6. Limits for the TeV
|