Proc. Neutrino Astrophysics - MPP Theory Group

Galactic cosmic-ray electrons (which account for about 1 % of the total cosmic-ray flux at
earth) are due to shock wave acceleration in SNRs. Perhaps this brings us near to a solution
of the historic question of the origin of the main hadronic part of cosmic rays: it had long been
speculated that all cosmic rays with energies below about 100 TeV are accelerated in SNRs.
The VHE upper limits on the SNR G78.2+2.1 shown as an example in Fig. 3 [9] present
a problem for this scenario, though. This SNR lies near a dense molecular cloud which
should act as a “target” for the accelerated cosmic-rays, leading to the prediction of a copious
production of VHE gamma-rays via the reaction p + p → π0 → 2 γ. It is seen that the
upper limits from various experiments shown in Fig. 3 are below theoretical predictions in
the mentioned scenario [9, 10]. This has recently led to a revival of speculations on an
extragalactical origin of the main part of hadronic cosmic rays [11].
The active galaxy Mkn 501 at a distance of about 100 Mpc from earth showed a spectacular
outburst of TeV radiation 1997 which was observed in detail by 5 different collaborations [12].
At the highest activity level in April 1997 the flux was about 40 times higher than during the
discovery of the source in 1995. Figure 4 shows the light curve determined by the HEGRA
telescope 1. This light curve is the most complete of all measured ones because data were also
taken during moonshine. The outburst was also observed in the X-ray and optical region and
these multiwavelength studies are especially valuable to discriminate between various models
for the origin of the TeV radiation and to find the reasons for its fluctuation. The measured
VHE spectra already allow interesting conclusions about the intensity of cosmological infrared
background radiation field [13]. Interactions of VHE photons with cosmological background
fields will allow a variety of cosmological studies unique to this wavelength range [14].
integral flux [cm -2 s -1 ]
10 -5
10 -6
10 -7
10 -8
10 -9
10 -10
10 -11
10 -12
10 -13
10 -14
10 -15
10 -5
EGRET 1995
Whipple 1997
HEGRA CT-System 1997
AIROBICC 1997
AIROBICC 1996
CASA-MIA 1995
CYGNUS 1995
10 -4
10 -3
10 -2
10 -1
theory prediction α=2.1
theory prediction α=2.3
1 10 10
energy [TeV]
2
10 3
Figure 3: Upper limits on VHE radiation from the SNR G78.2+2.1. Whipple and HEGRA
are Čerenkov telescopes, AIROBICC, CASA-MIA and CYGNUS are arrays. It now seems
likely that the detection of γ-rays from a satellite above 100 MeV (EGRET) is due to a pulsar.
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