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Along with ¡ collisions in the infalling gas , producing § © according to Eq. 1, photoproduction
of pions can take place in the optically thick photon field around the core according to:
¡ ¥
¡ ¥
followed by neutrino production from § decay. If the photon density is very high relative to the
¢ ¡ ¥ ¡ ©
¡ ¥ ¡§ ©
photons:¢
proton density, these processes will dominate over collisions. Also, if the radiation is strong
enough in the disk, all photons will lose energy in interactions and all electrons
and photons cascade down to energies around 100 GeV by inverse Compton scattering and pair
production [19]. Neutrons with energies high enough from Eq. 11 can also produce mesons via
interactions with , contributing further down to neutrino (both
and
) production.
Proton energy loss by interaction with photons (Eq. 11) starts at a threshold -energy
¢ ¢¡
which ¥
yields a proton energy of TeV for the
£ UV
¡
bump [18]. The
lack of strong X-ray absorption [18] could indicate that the energy loss by reactions is
smaller, due to a lower proton density compared
¡ ¥ ¡ ¡
to photons. Energy loss by pair production
©
is also small, because of the low mass of the electron. It is however
the dominant process at proton energies between 30-3000 TeV [10]. Below a certain energy, the
proton interaction length with photons is larger than the radius of the optically thick region and
they can escape. Thus, neutrinos are expected to follow the proton energy spectrum down to that
¡
energy, estimated to ¥ [10].
Several AGN have been reported to emit GeV photons with a variability of the order of days
(flares), all of them radio-loud. Markarian 421 has even been observed to emit TeV photons [21].
These high photon energies could be explained by inverse Compton scattering of accelerated
electrons in the jets [10]. If the magnetic energy density above the
¡
disk
¥
is
§
larger
¡
than
the
¥ ¡ ¡
radia-
tion density, the photons produced in
can produce
© pairs via
interaction with the magnetic field. A cascade of such pair production and synchrotron radiation
would develop and could provide a spectrum extending up to
¡
100
¥
TeV
§
[19].
¥
Yet
another
possibility
is that those very high energy
photons could have a hadronic origin, from
decay in the jets, with subsequent electromagnetic cascading of the photons down to energies
for which the accelerating region is transparent.
¡ ¥
Since
¡§
this process would be accompanied by
, this model would be favored by the ¢
detection of high energy
neutrino observations
from AGN [10]. interactions could accompany , if the proton density is ¢ high enough
in the jets, and still be compatible with the time variability of the observations
[20]. Predictions
¢¡
for a detector with a threshold energy of 1 TeV vary between a few
up to several hundreds
of events/year integrated over all AGN, depending on the production model (a compilation
can be found in [20]).
The large uncertainty is due to the difficulty to put constraints on the model in order to produce
the observed photon fluxes at various wavelengths,
the
unknown fraction of luminosity participating
in nucleon interaction [10] and also to how strong the attenuation of photons by reactions
with extragalactic photons really is [21]. It is believed that only close AGN can be observed
at very high gamma energies [22], whereas observations of neutrinos could be made at much
higher energies. This prediction is based on the fact that the cross section for pair-production in
interactions off intergalactic infrared light is maximized at 1 TeV.
¢
¡§
¡§
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