The result of a calculation of the expected number of muons in gamma ray initiated and cosmic ray initiated air showers using a realistic model of hadronic collisions in an effort to understand the available experimental results and to assess the feasibility of using the muon content of showers as a veto to reject cosmic ray initiated showers in ultra-high energy gamma ray astronomy are reported. The possibility of observing very-high energy gamma-ray sources by detecting narrow angle anisotropies in the high energy muon background radiation are considered

Edwards, P. G.

Protheroe, R. J.

English

NASA Technical Reports Server

235
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THE MOON CONTENT OF GAMMA-RAY SHOWERS
P.G. Edwards and R.J. Protheroe
DepartnJent of Physics, University of Adelaide
Adelaide, South Australia 5001
Abstract° We report the result of a calculation of the expected
number of muons in _'-rayinitiated and cosmic ray initiated air
showers using a realistic model of hadronic collisions in an
effort to understand the available experimental results and to
assess the feasibility of using the ntuon content of showers as a
veto to reject cosmic ray initiated showers in ultra-high energy
_-ray astronomy. We also co_isider the possibility of observing
very-high energy _-ray sources by detecting narrow angle
anisotropies in the high energy muon background radiation.
I. INTRODUCTION. With the recent observation of ultra-high energy (UHE)
_-rays from Cygnus X-3 (Samor_ki and Stamm, 1983a; Lloyd-Evans et al.,
1983), Vela X-1 (Protheroe et at., 1984) and LMC X-4 (Protheroe and Clay,
1985), together with the detection of excess air showers from the direction
of the Crab Nebula _Dzi'_uw:k_et al., 1983; Boone et al., 1984), it is
timely to e×amine the muon content of extensive air showers (EAS) initiated
by primary _-rays to investigate the possibility of" (a) using a "normal"
muon content to veto some fraction of cosmic ray (CR) showers; and (b)
detecting _'-raysources at very-high energies through observing narrow
angle anisotropies in the muon background radiation.
Measurements of the muon content were made in two of the recent
source observations, that of Cygnus X_3 by the Kiel group (Samorski and
Stature,1983b) and that of the Crab by the Lodz gr,-.up(Dzikowski et al.,
1983). Early predi,-tions of muons in _'-rayEAS (Karakula and Wdowczyk,
1963; Wdowczyk, 1965; Braun and Sitte, 1965) together with later work on
muons of photoproduction origin in CR EAS by McComb et al. 41979), had led
us to expect that at I0:=-I0 _m eV energies _'-rayinitiated EAS would have
a muon content about one tenth of that of proton-initiated EAS. The ratio
of muon number in the excess EAS to that in CR EAS was measured to be
somewhat higher in the two recerlt experiments, however: about 0.6 for the
,observation of the Crab and about 0.7 for the Cygnus X-3 observation. This
surprising result appeared to preclude the possibility of using a "normal"
muon content to veto CR events in UHE _'-rayastronomy.
A number of deep underground muon detectors are now being
commissioned to search for muons produced by the interaction of
extraterrestrial neutrinos and an estimate of the flux and neutrino light
• curve for Cygnus X-3 has recently been made by Gaisser and Stanev (1985).
If a significant number of muons are produceed in the atmosphere as
secondaries by Y-ray showers at very-high energies, it may also be worth
searching for narrow angle anisotropies in the sea-level muon background
• radiation as an alternative to the atmospheric Cerenkov technique.
Searches of this type were conducted some years ago (Allkofer et al., 1981)
although not specifically for _F-raysources.
To consider these questions we have recently performed a new
calculation of the muon content of _'-rayinitiated EAS using a realistic
model of high energy hadronic interactions. Details of the calculation are
given by Edwards et al. (1985).
https://ntrs.nasa.gov/search.jsp?R=19850027718 2020-03-20T17:05:10+00:00Z
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2. MUONS IN 6/IMI'IR-R/:/Y SHOWERS.
The integral energy -2 I I
spectra of muons we obtain
at an atmospheric depth of -3
1130 gcm -= are given in
Fig. 1 for average Y-ray and
proton initiated EAS for _6
primary energies in the _-. PROTON
range 10 == - 1017 eV and o-S '7 PRIMARIES
101== - 1017 eV _ \8
respectively. The >'L_ -6
atmospheric depth chosen is
appropriate to, _he }<iel (sea _x
_L- 7level) observation of ,.,
Cygnus X-3. In the Kiel and A 2 Y-RAY
PRIMARIES
Lodz experiments, the muon z -8
measurements were obtained
at fixed shower size rather cn_
0
than at fixed primary -- -9 3
energy. For a realistic
comparison, then, we have
-10
calculated the mean muon
number for showers of given 4
size by performing a -11
numerical integration over 0 I 2 3,_ 4 S 6
primary energy taking t0g10(E_/beV)
account of fluctuations in
Fig. I. Integral energy spectrum of muons in average proton
longitudinal development, and y-ray initiated EAS divided by primary energy Eo.
For primary CR a broken Numbers attached to the curves are Iog=o(Em/SeV).J
power law energy spectrum
was adopted with a
differential exponent of
-2.7 steepening to -3.1
above 3xI0 Is eV. For
_-rays, the spectrum adopted
had a differential exponent ,0_
of -2 and was cut off at
1017 eV. 131eresults are
given in Fig. 2 and show
that the ratio of the muon _ / .
number in y-ray initiated - ,0' /_av_
EAS to that in proton _- /
initiated EAS is about 0.1. _ /
If the primary composition _ / .
at I0 _= - 1017 eV energies /
is mixed ,-,ris enriched in _0' /
heavy nuclei, however, the //
ratio would be less than /
0.1. From these results
then it would appear that
the muon component of EAS _0 _°_ _0° _o'
could usefully be employed sJ,o_,,z__
to veto against CR initiated rig.2. Average muon number (>i GeV) at fixed shower size
EAS in UHE y-ray astronomy, at an atmospheric depth of 1130 g cm-= in Y-ray and nucleus
initiated EAS. (Reproduced from Edwards et aZ., 1985).
237
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3. GROUND LEUEL MUON FLU;(. The flu:,;of atmospheric muons due to _-rays
from Cygnus X-3 has been calculated by convolving the muon energy spectra
in _'-rayshowers (Fig. I) with the S-ray energy spectrum of Cygnus X-3.
For this we took an E-= differential photon spectrum normalised to the
integral flu× above 3×10 =m eV reported by Lloyd-Ev\ans et al. (1983):
N(E) = 4.5xlO-4(E/GeV) -= (photons m-= s-= GeV-I). (1)
The resulting integral r,,uonspectrum is shown in Fig. 3 assuming the S-ray
spectrum of equation (1)
continues to a cut-off -5 _ _ _ _ i I
energy of: (a) i02_ eV; (b) -6 _ c¥_Husx-3 _I
Whether this muon flu× can F. 7
be seen significantly above
the background for a given
exposure (area x time) _ -8_ X 1depends on the accuracy with
recorded reflect the _'-ray -Iarrival directions. The
angular uncertainty is
likely to be made up of two _ -11 _ \_ _
parts for high energy muons: -Iz(i) uncertainty in muon
track reconstruction in the
detector; and (ii) angular -13
• 1 2 3 _ 5 6
spread due to transverse logs (E_/IGeV)
momentum imparted to parent
pions. For the background Fig. 3. Integral flux of atmospheric secondary muons due to
muon intensity summarised by gamma-rays from Cygnus X-3. The three curves given
correspond to different assumptions about the high energy
Allkofer et al. (19717, and cut-off energy in the _-ray spectrum (see text).
assuming that _-rays are
present over 1/100th of the 1°s _
orbital period, the exposure _ 3o ,o
required to detect muons due _ xx_%//_/i
" to _-rays from Cygnus X-3 -_ \\
with 99% confidence is shown -_I°_- \x 3
in Fig. 4 for various _ \\
detector track resolutions. _ \
4. DISCUSSION. From the _ I0_
present calculations the
.. Kiel and Lodz results are T_
inconsistent with the excess _ lOZlo° J a ,JEAS detected by thes group I0' _0_ 10_
being due to _-rays. For MUONENER6Y Ell (OeV) '
* the nearer of the two
sources, the Crab, Dzikowski Fig. 4. Minimum exposure required as function of muon
et el. (1983) have already energy threshold and muon track reconstruction accuracy (the
numbers attached to the curves). A lower limit is imposed
suggested the possibility by the angular spread of muons due to tranverse momentum
that the ex,:ess EAS are due (assumedto be:e _ <p_>c/E_) and by Poissonstatistics
either to neutrons and is indicated by the broken line.
238
HE4.5-7
or protons, energetics arguments strongly favouring the latter alternative.
The present calculations would also favour the excess EAS being due to
protons or light nuclei: a ratio of 0.7 could easily be obtained if the
excess events are due to protons and light nuclei from the Crab Supernova
and if the galactic CR are of mixed composition at these energies (i.e. the
CR's produced during the Crab supernova had a somewhat lighter composition
than average). For Cygnus X-3, protons or nuclei can be ruled out because
the excess EAS are observed in phase with the orbital motion of the system
and it appears fairly certain that the excess EAS from Cygnus X-3 are
indeed due to Y-rays. The observed ratio of muons in these EAS to that in
CR EAS of 0.7 is then very difficult to explain. We conclude then that the
discrepancy is likely to be due either to a systematic effect in the
experiment (e.g. array triggering biasses, etc.) or alternatively, may
indicate that the nature of hadronic interactions at I0:4 - 10I_ eV
energies differs from ou_ current expectations. If the first possibility
turns out to be correct, then the observation of a "normal" muon content in
an EAS could be used to veto CR initiated EAS in UHE N-ray astronomy. A
similar conclusion has been reached independently by Stanev et al. (1985).
We turn now to the possibility of observing N-ray sources through
detecting atmospheric muons. From Fig. 4 the best energy range to examine
appears to be that above 100 GeV for which a detector with a 0.1° track
reconstruction accuracy would require an exceptionally large exposure, in
excess of 300 m=-years when the source was in the field of view, in order
to detect a significant excess from Cygnus X-3. While technically such an
experiment may be feasible, there appear to be few (if any) advantages of
such a system over over more conventional methods of very-high energy N-ray
astronomy (i.e. the atmospheric Cerenkov technique).
_cknowledgements. This research is supported by the Australian Research
Grants Scheme. P.G. Edwards acknowledges receipt of a Commonwealth
Postgraduate Research Award.
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Allkofer OC et al. 1971 Phys. Lett., 36B, 425
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Boone Jet al. 1984 Ap. J. 285, 264
Braun 0 and Sitte K 1965 Proc. 9th ICRC (London) 2, 712
Dzikowski T et al. 1983 J. Phys. G:Nucl. Phys. 9, 459
Edwards PG, Protheroe RJ and Rawinski E 1985 J. Phys G: Nucl. Phys.
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Gaisser TK and Stanev T 1985 Bartol preprint BA-85-12
Karakula S and Wdowczyk J 1963 Acta Phys. Polonica 24, 231
Lloyd-Evans Jet al. 1983 Nature 305, 784
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The muon content of gamma-ray showers

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