50 Tev Hegra Sources and Infrared Radiation

The recent observations of 50 TeV gamma radiation by HEGRA have the potential of determining the extragalactic flux of infrared radiation. The fact that radiation is observed in the range between 30 and 100 TeV sets an upper limit on the infrared flux, while a cutoff at Eγ ≈ 50 TeV fixes this flux with a good accuracy. If the intrinsic radiation is produced due to interaction of high energy protons with gas or low-energy target photons, then an accompaning high-energy neutrino flux is unavoidable. We calculate this flux and underground muon flux produced by it. The muon flux is dominated by muons with energies about 1 TeV and can be marginally detected by a 1 km detector like an expanded AMANDA. The detection by HEGRA [1] of sources of gamma radiation with energy about 50 TeV, if confirmed, is a remarkable observation which hopefully is the first step in ultra high energy gamma astronomy. Besides its own importance, this radiation is an excellent tracer of infrared fluxes in intergalactic space. The existence of an upper limit of the observed energies of photons is interpreted in [1] as absorption on diffuse infrared radiation (IR). This assumption looks reasonable due to the following argument. Observation by CASA [2] has not resulted in detection of these sources [3]. This detector has much higher flux sensitivity than HEGRA, but also higher energy threshold (Eth = 100 TeV), which is sharp and makes ineffective the observations of gamma ray sources below about 50 TeV [4]. Therefore the cutoff of the spectrum at 50 TeV must be very sharp. The known acceleration mechanisms and in particular shock acceleration cannot provide such a sharp cutoff, while absorption naturally results in an exponential cutoff. On the other hand, the observation of 50 TeV gamma-radiation from the HEGRA sources, which are typically at a distance greater than 200 Mpc, implies that the flux of diffuse IR radiation is lower than was theoretically estimated (see [5, 6] and references therein). It is interesting to note that recently the EASTOP collaboration analyzed their data on Mk 421, which was detected by HEGRA with 3.8σ excess over background. At energy higher than 40 TeV EASTOP collaboration established [8] the 90% c.l. upper limit 1.2 · 10 cms close to the flux observed by HEGRA. Within the statistical errors there is no serious contradiction between these two measurements. In our analysis we concentrate on the source 0116+319 detected by HEGRA at 5.7σ level with flux 1.4 · 10 cms at E > 50 TeV. We shall discuss some consequences of the HEGRA observations. One of them is the deriviation of the flux of IR radiation consistent with these observations. If the intrinsic flux has its origin in the interaction of high energy protons with gas or target photons then an accompanying high energy neutrino flux is unavoidable. We shall also discuss the detectability of this flux. Let us parametrize the density of IR photons in intergalactic space, n(ε), as n(ε) = n0 ε0 ( ε ε0 ) −ν , (1) where ε is the energy of the IR photons and the normalization value is fixed at ε0 = 1 · 10 −2 eV. One can formally use Eq.(1) for an unlimited range of energies because at ε ≤ 3 · 10 eV the microwave radiation dominates while at ε ≥ 0.3 eV optical radiation does. The probability of absorption (the inverse absorption length) of high energy photons with energy Eγ is given by [9]: dW dl = 4 E γ ∫