High energy gamma astronomy above 200 GeV

I| [gill I t'-,Yi",,illlh'&"![ik,lr!

PROCEEDINGS SUPPLEMENTS

Nuclear Physics B (Proc. Suppl.) 33A,B (1993) 93-112 North-Holland

High Energy Gamma Astronomy above 200 GeV E. Lorenz Max Planck Institute for Physics, Werner Heisenberg Institute Foehringer Ring 6, D 8000 Munich 40, FRG

The status of high energy gamma ray astronomy above 2x1011 eV will be reviewed. In this energy range earth bound detectors can be used. Recently new large area detectors with a significant increase in sensitivity became operational. We compare the performance and the preliminary results from the leading experiments. Also an outlook for the next generation detectors will be given. 1. INTRODUCTION The study of high energy (HE) cosmic gamma(7) radiation is an important sector of the general study of cosmic radiation (CR). Still, after an intense search during the last eighty years, the origin and the acceleration mechanisms of HE cosmic radiation is mostly unknown. Precise data from T ray astronomy might clarify many questions about its origin, are test data for existing models or might generate starting parameters for new models. 7's are not deflected by the weak galactic magnetic field like the much more abundant charged CR and point therefore back to their origin of production. Furthermore 7's might carry time information of the original processes like pulsar periods or episodic eruptions. Therefore ~ are an important diagnostic tool for the search for celestial accelerators or interstellar (galactic, extragalactic) gas or low density halo around stellar objects. The main physics motivations for 7 astronomy can be briefly summarized as: i) search for T point sources above the energy range which is accessible by satellites ii) detailed studies of the 7 energy spectra and temporal structure, mainly for understanding of the accelerator mechanism and stellar HE processes

iii) search for the diffuse 7 radiation from the galactic disc iv) search for isotropic 7 radiation according to process (1) and (2) v) is there a sizeable extragalactic contribution to the HE cosmic radiation In addition one plans to study some fundamental processes of HE T interaction with nuclei in the upper atmosphere like searches for non-classical behaviour of HE 7's or detailed studies of the HE T-hadron interaction. In the Kiel experiment [1] all the showers pointing to Cygnus X3 have been accompanied by a large number of muons. The muon content of the air showers corresponded to the content of normal hadronic cascades but was completely incompatible with the one expected in classical electromagnetic cascades. In the wake of the Kiel experiment models, based on an abnormal hadronic component of the 7, have been developed but the main difficulty is to formulate a process where the onset of the hadronic 'nature' completely supersedes the classical electromagnetic nature of the 7just above a threshold, which must be above the energy range of current accelerator experiments. It is expected that within the coming year T-hadron data up to 200 GeV centre of mass (CM) energy will be available from HERA. Anyhow, at first it is important to confirm the high muonic

0920--5632/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved.

94

E. Lorenz ~High energy gamma astronomy above 200 GeV

component in HE induced air showers from point sources. On the other hand it is obvious that both the search for an abnormal hadronic component of the 7or the proposed HE 7hadron studies are second generation experiments because a) high intensity sources must be known and b) more universal detectors and improved detection methods are needed because the search is currently based on the classical electromagnetic cascade structure of 7 induced air showers. A closely linked question to the source search with ys is the search for new long-living neutral particles. Their existence is highly speculative but observations might lead to evidence for deviations from the standard model and might be very important to give indicative guide-lines about what one might expect in experiments at the future accelerators LHC and SSC. These particles might also lead to the formation of cascades with high muon content. It is not in contradiction to observe these new particles in an energy range which is currently accessible by accelerators because the production might occur in interactions, say for example, at 1016 eV which is well above the energy range of current accelerators. Any time structure in such observations will set stringent upper limits on the mass of such new neutral particles. For completeness of the motivations I would like to mention that with the same type of detectors many other questions in HE cosmic ray physics can be studied like the structure of the energy spectrum around 1015 to 1016 eV or the coarse chemical composition above 1014 eV. The dominant process leading to the generation of HE 7s is the interaction of HE protons or nucleons with stellar or interstellar low density gas leading to meson production p+N->N*+ ..... +~0 (->77) (1) The highest chance for nuclear targets is normally in the vicinity of the accelerators, like, for example, the expanding gas clouds of supernovae. Therefore the majority of 7s will point back to 'point' sources. Other targets are for example the matter concentration in the central area of our galaxy or nearby large

10zw

r

i

i

1

i

i "6

,

i

i

T

1

1027

).i,,t (cm)

i 101]

I ~ t~ E~ (eV)

Centre 1~

10 zl

Figure 1.7 attenuation length as a function of energy, from ref [1] molecular clouds. One expects a diffuse 1' flux from the centre of our galaxy in the order of 10 -5 of the total CR flux from the same direction. Other rare processes for the generation of HE 7s are for example the inverse Compton effect or interactions with low energy photons like the 2.70 K background radiation or the infrared (visible photon) background. Two interesting processes are the reactions: 1~E >1014 eV)+7 (2.70K) -> e+e - ->--.7..p(E>1018 eV)+7(2.70K)-> N*->p+~0(->W) ->n+~ +

(2) (3) (4)

At sufficiently high energy the onset of muon- or pion pair production in (2) will become relevant. Reaction (2) reduces the trans-mission of the universe for HE ~'s and can limit the observation of extragalactic sources in certain energy bands. Fig 1 shows a calculation of the y attenuation due to interaction with the background photons from ref [2]. This process limits our observation distance to about the galactic centre at an energy around 1015 eV (= 1 MeV in the lq CM). 2. IMPORTANCE OF SATELLITE OBSERVATIONS AND COMBINED STUDIES

An important guide for the search for sources are satellite observations at lower energies because it is inconceivable that a HE

E. Lorenz ~High energy gamma astronomy above 200 GeV

95

Table1 EGRET detection of active galactic nuclei, from ref [4] OBJECT

I

b

=

I(> I00 MeY)

Type

x l O -6 crn -2 s-1

0202+149 4C15.05 0208-512 PKS 0235+164 0420-014 0528+134 0537-441 PKS 0716+714 0836+710 1101+384 M K N 421 1226+023 3C273 1253-055 3C279 1633+382 4C38.41 2230+114 CTA 102 2251+158 3C454.3

148 278 157 195 191 250 144 144 180 290 305 61 77 88

-44 42 -39 -33 -11 -31 28 34 85 04 57

42 -38

-38

1.003 0.852 0.915 0.894 0.894 2,17 0.031 0.158 0.538 1.81 1.037 0.859

0.3

4-

0.I

0.8 0.34 0,20 0.15 0.II 0.3 0.6 1.0

4i + 444-

0.I 0.09 0.04 0.04 0.03 0.1 4.9 0,1

to

4-

QSO Blazar Blazsr" Blazar QSO Blazar Blszar" QSO Blazar" Blazar Blazar Blazar Blazar Blazar

" weak emission lines (BL Lac Object)

7 source does not show up at lower energies with increased flux.The satellite measurements extend presently up to 5 GeV like from SAS2, COS B and preliminary observations from the COMPTON gamma ray observatory (GRO). The main advantage of satellite observations are the much higher expected 7 fluxes and the highly efficient separation of 7's from the much more abundant charged cosmic background with charged particle veto counters. Also the angular resolution of the EGRET y detector on board of the GRO is nearly an order of magnitude better than that of the best earthbound HE 7 detectors. While COS B scanned basically a _ 200 band around the galactic equator [3], the GRO measurements will finally provide an all sky map with increased sensitivity and angular resolution. Already at this conference new observations of extragalactic sources have been presented by the GRO collaboration, see table 1 [4]. The GRO measurements will eventually extend up to 40 GeV; nevertheless there exists still a gap of about one order of magnitude between the highest GRO observations and the energy threshold of Cerenkov telescopes. It will be a challenge for the next generation of detectors to close or at least to narrow this gap. The

caveats of satellites are their limited life time, high cost and the limited angular observation range that might miss short term stellar activities. For example it was not possible to orient ROSAT towards CYGNUS X3 during the large radio burst around the 20th of January 91. It would be highly desirable to perform in the future combined observations with GRO and earthbound HE 7 detectors (mainly Cerenkov telescopes) on specific objects. Also it is conceivable that satellites can provide information on prolonged times of large activities of some objects; thus narrowing the time window for source searches with ground based detectors can help one to identify them in the HE range The same argument is also valid for phase data from satellite observations 3. SHORT OVERVIEW OF OBSERVATION TECHNIQUES I will only briefly mention the different observation techniques for HE 7s. The techniques can be grouped into two (three) classes. The most common detector is an array of scintillation counters that sample the charged particles of extensive air showers that reach the ground.

96

E. Lorenz ~High energy gamma astronomy above 200 GeV

''''1''''1''''1''''1''''1''''1''''1''''1'''0'1''''

12

10

o 0

o

\ 0 0

0

8

0 • 0

0 L.

Scintillator

signals

0

o

6

o

q

O

o

6

O

oocP-"

4 o

.,=i

L i-

o

2

o 0

0

•

o

"

o,oo°" o ~

"

OOo. •

o

o

oOo o

Cereakov

• • .e

•

..

"

,.

0

"

O

oO

o

°

"(3

- o O

o

qo~"

--;-;

.

"

o

~

-

o

o o o.o ~ o oOoO . " o od? u • "oo0 8= o

@

E

O

.

-~" •

"

.

sisnals •

_ - ~ -

•

•

-

111-

•

-

0 0

40

80 120 Distance of shower core

160

200 [m]

Figure 2. Simultaneous measurement of the shower front o f an extended air shower in the HEGRA scintillator matrix and the air Cerenkov matrix AIROBICC.

Because nearly all particles are moving with the speed of light one determines the direction from the arrival times in the array. The incident energy is determined from the NKG function which is fitted to the density of particles (more precisely to the individual pulse height data divided by the signal for a minimum ionising particle) in each hut. The shower tail consists mainly of electrons and a factor four (depends on the energy cut) more abundant low energy 7's. These 7's have a slightly inferior time spread and their detection behind thin lead converters can improve the angular resolution. The shower front has a conical structure [5] with an average slope of 14-15 nsec delay per 100 m distance from the shower core. The slope changes significantly from event to event. The advantage of scintillator arrays are their 24 hour sensitivity and angular acceptance of typically 1 sterad. This is important for the observation of transients or highly variable sources. In a subclass of the so-called EAS array detectors one intends to sustitute scintillation detectors by tracking detectors that allow one to determine the

incident cosmic particle direction from track analysis of the charged particles in the shower. Two new projects will be discussed in chapter 5. EAS scintillator arrays have very little to no ?/h separation power; but by adding muon counters one can improve the separation power because 7 induced showers contain at least an order of magnitude less muons than a hadronic shower. The muon flux is low, particularly in case of the primary being below 1014 eV; thus a large area muon detector with low electron punch-through is needed. Another factor which contributes very much to large fluctuations in the muon ratio is the variation of the altitude of the main shower activity. The second group of detectors are based on the observation of Cerenkov light which is generated by the charged cascade in the atmosphere. The Cerenkov light from HE ? showers illuminates at ground a disc of at least 100 m radius, therefore even single telescopes have a large acceptance area. The Cerenkov light disc of a 1014 eV shower has 4 orders of magnitude more photons than charged

E. Lorenz ~High energy gamma astronomy above 200 GeV

particles at ground level. The time spread of the photons is much smaller than that for the particles, also the cone slope is typically only 5 nsec/100 m distance from the shower axis vs 15 nsec in the particle disc of the charged particles. This is demonstrated in fig 2 which shows a simultaneous measurement in the HEGRA detector of a 1014 eV shower seen both by a 15 m grid scintillator array and a 30 m grid array of open photomultipliers. The difference in time jitter and cone slopes is well visible. The Cerenkov light is in first order proportional to the total energy loss of the charged particles in the atmosphere above the detector. Therefore one can view the combination of the atmosphere and the light sensors as a fully active calorimeter. In second order the relation of energy loss and Cerenkov light is not strictly proportional due to the 'running' threshold with altitude. Also, in terms of particle physicists, the calorimeter is not compensating, e.g. different particles generate different amount of Cerenkov light. In hadronic showers a substantial fraction of the energy is converted in nuclear excitation or heavy particles with momenta below threshold. Also a small fraction of the initial energy is transported away by neutrinos and penetrating muons. 7 showers in the 1011-1013 eV region generate by a factor 3 more light than hadron induced showers of the same energy. The classical Cerenkov detector consists of a telescope with one or multiple photomultipliers (pm) in the focal plane. From image analysis of a multi pm camera one can distinguish quite efficiently 7 showers from hadron showers. The leading instrument in the field, the Whipple telescope [7] achieves hadron suppression factors in the range of 100-200. The main disadvantage of Cerenkov detectors is their limitation to operate only during clear and moonless nights and the restriction to observe one source at a time. Nevertheless these limitations, when compared to scintillator arrays, are offset by the one order of magnitude lower threshold, the enrichment due to the factor 3 more light, the large 7/h separation power and the generally better angular resolution. Variants of the

97

Cerenkov detector concept consist of distributed sensors that sample the Cerenkov light disc at many places or view the shower from different positions. Examples will be discussed in a later chapter. The third class of detectors is based on the observation of scintillation light. Charged particles excite atmospheric Nitrogen which in turn de-excites by emitting fluorescence light. Above 1016 eV the amount of light is sufficiently large t o be detected by distant photomultipliers. This principle has been used by the FLY's EYE detector. As I am concentrating on the 1011-1014 eV energy range I will skip further details and refer the reader to ref [8] for the basic concept and detector description.

txlO°I lxiO'21

o

Ix1041 ixlO-6"~ o 'T <

IxlO"s

% o

> ixlO"I0

° 0

ixlO"12 <

IxlO14 ixlO "16' i x l 0 "111

%°

ixlOz°

olb *

ixlO"22.

% ixlO"~' lxlO~

(eV)

Figure 3 Energy spectrum of the cosmic radiation, compilation by S. Swordy [9]

98

E. Lorenz ~High ener~ gamma astronomy above 200 GeV

Fig 3 shows the differential energy spectrum of the CR from a compilation of Swordy [9]. The steep slope has important consequences for experiments. Detectors cannot be optimized for a large energy range and can at most span 3 decades. Due to the statistical nature of the shower development and the strong Zenith dependence of the observables the threshold of any detector is not well defined. On the other hand the bulk of the data is just above threshold. It is rather difficult to compare different experiments because many groups use different threshold definitions. A good quantity for specifying a detector would be the median energy which would be very helpful when comparing data from different experiments. Another important deficiency for the determination of the acceptance and the performance of a detector comes from the lack of test beams of well defined parameters. Nevertheless there is steady progress as it is possible to refine Monte Carlo(MC) simulations using input data from the new high energy accelerators. For example one expects soon data from HERA about the ~ interaction at 1013 eV lab energy. The maximum HERA CM energy of 270 GeV corresponds to a Lab energy of 39 TeV. 4. STATUS OF THE SOURCE SEARCH SUMMER 91

In the seventies and eighties many observations of HE 7 sources have been reported. The significances were quite marginal and exceeded seldom the 4.5 sigma level. The Step by step improvement of detectors did not raise the significance. After the observation of Cyg X3 by the Kiel experiment [1] some groups decided to build large area detectors with a quantum jump in sensitivity. The largest one is the CASA array [10] at Utah; the detector comprises a total of 2196 scintillation counters, grouped in units of four, and extends over an area of 25.104 m2. CASA was combined right from the start with large area muon counters, scintillators buried 3 m underground, for y/h discrimination. The experiment has an impressive trigger rate of 25 hz and posed new

challenges on the readout. Still, in general, all new experiments were aimed at improving the performance in the PeV region at the expense of a lower threshold. This was based on the models, that the source spectra would extend to the highest energies and that the prospects to enhance the observation due to stiffer source spectra than the background would be more favourable. The first results from the large new arrays were reported at the 22nd Int. Cosmic Ray Conference at Dublin. To the general disappointment, none of the previous observations could be confirmed and the new upper limits for sources like CYG X3, CYG X1, CRAB, HER XI.. were well below previous positive evidence. While for one or the other case the source might be inactive during the last years, it is highly improbable that all sources are temporarily quiet. Also, the old results raised problems to explain the total HE CR flux. The luminosity, as calculated from some of the old observations, would be sufficient to completely saturate the flux. On the other hand the new high statistics experiments approach the predictions made from straight extrapolations from lower energy observations. In some cases only an increase in sensitivity by a factor five will be a 'physical' measurement and will help to study cutoff energies of acceleration processes. Also some 7 burst observations were reported at Dublin but with significances below 4.5 sigmas. A controversial case was the observation of a burst of showers pointing back to Cyg X3 at around Jan 20,21 1991. The observations coincided with a large radio flare from Cyg X3 at the same time, therefore the probability for such a large random fluctuation is much more unlikely than for uncorrelated transient observations anywhere during a long measuring period. Two experiments reported positive evidence, the Soudan underground experiment [11] and HEGRA [12], while the two large arrays CASA [13] and Cygnus [14] observed no evidence. As there are no further data available in order to clarify the situation one has to wait for the next radio flare for possible clarification. On the other hand one can learn from these observation that transient

E. Lorenz ~High ener~ gamma astronomy above 200 GeV

99

Table 2 Results of the source searches from EAS arrays, as reported at the 22nd Int Cosmic Ray Conference, Dublin 1991; compilation from M. Samorski ARRAY

EPOCH

E [TeV]

AGASA

90-91

> 3 • l0 s

BAKSAN

85-87

200

CASA / UMC (49 det.) (49 det.) CYGNUS

EAS-TOP GREX

HEGRA

OOTY OHYA

SPICA

CYG X-3

RESULTS

PAPER

DC (3.5 a)

OG 4.3.20

CRAB, CYG X-3, HER X-1 + 6 objects

U.L. U.L.

4.3.13

burst U.L. U.L.

4.3.16 4.1.6 4.2.4

Febr. 23,89 90-91 90-91

100 100

CRAB CRAB (DC, pulsed) HER X-1 (DC, daily,/~)

89

100

CYG X-3 (DC, pulsed,/J)

U.L.

4.3.6

90-91

170

CYG X-13 (DC, pulsed daily, ~)

U.L.

4.3.7

89

100

~ = 20 -70 (DC, daily, ]J)

U.L.

4.6.3

90

100

6 = -10 °...+60 o (DC,/J)

U.L.

4.6.7

86-91

40

CRAB (DC, pulsed, daily)

U.L.

4.1.8

86-91

40

CRAB, CYG X-3, HER X-1 + 41 objects (daily)

86-91

40

4U00115+63 (pulsed)

U.L.

4.4.11

86-91

40

CRAB, CYG X-3, HER X-1 + 46 objects (DC)

U.L. U.L.

4.6.4

89-90

100

CRAB, CYG X-3, HER X-1

U.L.

4.3.9

+ 5 objects (DC, sporad.,pulsed)

U.L.

86-90

1000 CRAB, CYG X-3, HER X-1

86-90

1000

89-91 :]an. 21.91

KGF

OBJECTS

4.3.18

U.L.

+ 7 objects (DC) CYG X-3, HER X-1 + 8 objects (episodic, pulsed)

U.L.

50

CRAB, CYG X-3, HER X-1

U.L.

50

CYG X-I (DC) CYG X-3, (DC, pulsed)

4.3.17 4.6.2 4.3.4

"U.L. burst

87

I00

CRAB (p)

85-87

i00

Her X-I (/~)

DC (2.3 ~)

4.1.3 4.2.7

87-90 87-90

100 100

CYG X-3 (p) PSR 1957+20

DC (2.1 v)

4.3.14 4.6.2

87-90 March, 86 May, 89 88-91

250 250 I000 i000

CYG X-3 SCOR X-I (8 weeks) CRAB (3 days,/~) HER X-1 (pulsed, phase peak)

DC (5 a) burst (6.7 • I0 -s) DC (4.2 ¢,/~)

4.3.15 4.4.8 4.1.7 4.2.9

86-91

i000

CYG X-3 (pulsed, phase peak)

DC (4.1 a,/J)

4.3.23

89-90

1000

HER X-I

-

4.2.6

100

E. Lorenz ~High energy gamma astronomy above 200 GeV

Table 2 , continued ARRAY

EPOCH

E [TeV]

OBJECTS

RESULTS

PAPER

> CHACALTAYA

JANZOS

OG

86-89

30

sky survey (s-cut)

87-90

30

VELA X-I

U.L.

86-90

30

+ 6 objects (/~,h)

U.L.

86-90

I00

VELA X-I

U.L.

+ 5 objects (/=,h)

U.L.

87-89

I00

BL 1

4.3.21

4.4.19

D C (3.4 ¢)

+ 5 objects BL

4.4.17

SPASE

90

50

SUGAR

68-79

2 • i0s

L M C X-4, 2A 1822-37.1

(Buckland Park)

68-79

2 • l0s

sky survey

TIBET

90-91

40

4.4.18

U.L.

1

-

4.4.22

+ 8 objects

YAKUTSK

74-91

5 • l0s

indications 4.4.20 -

C R A B , C Y G X-3, H E R X-I

U.L.

+ 46 objects (DC)

U.L.

C Y G X-3, H E R X-I (DC)

U.L.

4.6.8 4.6.6 4.3.19

Table 3 Results of the source searches from air Cerenkov telescopes, as reported at the 22nd Int Cosmic Ray Conference, Dublin 1991; compilation from M. Samorski TELESCOPE

EPOCH

E [TeV]

OBJECTS

RESULTS

> ASGAT

PAPER OG

90-91

0.5

CRAB

-

4.1.1

Jan. 2,89

2.8

CRAB

pulsed (0.3 %)

4.1.2

87-89

1.9

4U0115+83

U.L.

4.4.10

88-89

1.0

PSR 1957+20

-

4.5.1

90 89-91

1.0 2.0

PSR 1951+32 PSR 0355+54

pulsed ( 8.8 • 10 -8)

4.5.3 4.5.4

THEMISTOCLE

90-91

3.0

CRAB (prelimin.)

DC (4 ¢)

4.1.12

WHIPPLE

88-91

0.4

CRAB

DC (45.5 ¢)

4.1.4

PACHMARHI

+ 3 objects

U.L.

88-90

0.4

CYG X-3

U.L.

4.3.3

89-91

0.4

8 objects

U.L.

4.4.2

88-91

0.5

HER X-l, PSR 0355+54

U.L.

4.5.5

+ 7 objects

U.L.

101

E. Lorenz ~High energygamma astronomy above 200 GeV

Table 3 Continuation

FLY'S EYE II

87-91

70

CRAB, CYG X-3, HER X-I

U.L.

CYG X-1

U.L.

4.3.5

UTAH

88-90

I00

CRAB, CYG X-3, HER X-1

U.L.

4.3.10

JANZOS

88-89

1.0

CEN A, VELA X-I, CEN X-1

U.L.

4.4.7

CIR X-1

U.L.

88-90

30

SMC X-l, LMC, X-4

U.L.

+ 3 objects

U.L.

NARRABRI

POTCHEFSTR.

WOOMERA

4.4.15

91

0.3

VELA X-1

pulsed

4.4.4

90

0.4

AE AQUARII

pulsed

4.4.12

87-89

0.4

PSR 1855+09

pulsed

4.5.6

86-90 86-91

1.0 1.0

VELA X-1 CEN X-3

pulsed pulsed

4.4.3 4.4.6

88-90

1.0

AE AQUARII

pulsed

4.4.13

87-88

1.0

PSR 1855+09

pulsed

4.5.7

91

0.6

VELA X-l, CEN X-3

U.L.

4.4.5

searches might become more important because a) most sources in the GeV region are already highly variable and b) the general flux limits make steady state high intensity source highly improbable Table 2 summarizes the results on the source search of nearly all operational EAS arrays and table 3 the observations from Cerenkov detectors, respectively. The Dublin conference was not only a caesura for the EAS arrays but brought also the first solid evidence for TeV 7 emission from the CRAB with a significance of over 45 sigmas [15]. This observation by the Whipple group was the result of a much improved 7/h discrimination based on image analysis, the raw excess being at a 6.5 sigma level above the background. The prospects for finding sources should be higher for telescopes on the southern hemisphere where one has

access to the galactic centre which should be much richer in HE sources.Both the Durham groupat Narrabri and the Potchefstroom group reported on the 7 observation in the TeV energy range from VELA X1 [16], AE Aquarii [17] and PSR 1855+09 [18]. The significances of these observations were not too high but the independence of the measurements make the claims highly probable. In both cases nearly no 7/h discrimination was used but on the other hand the sensitivity was increased by phase analyses. In summary the Dublin conference showed a broad spectrum of activities for source searches. The many EAS arrays, see table 4 a, could not produce any solid evidence for sources while the much less numerous Cerenkov detectors, see table 5 a, could identify some sources in the 1 TeV region.

E. Lorenz ~High energy gamma astronomy above 200 GeV

102

Table 4 Operating EAS Array Detectors [19] Name

Location

Lat. (deg.)

Long. (deg.)

Depth gm/cm 2

Number of timing Detectors

Trigger Peak Rate sec "1Energy (TeV)

Baksan

Russia

43.42 N

42.67 E

833

10

1

300

140 E

1030

24

O.5

3OO

112.3 W

870

512

24

100

107.6 W

800

204

6

50

13.57 E

802

29

1.5

300

1.63 W

1016

36

.2

200

17.9 W

780

220

12

50

172 E

838

76

2

100

78.3 E

920

61

1

300

Buckland Australia 35 S Park CASA Dugway, 40.5 N USA Cygnus Los Alamos 39.5 N USA EASTOP Gran Sasso 42.45 N Italy GREX Havarah 54.0 N Park,GB HEGRA La Palma 28.8 N Canary Islands JANZOS New 41 S Zealand KGF India 12.95 N Liang Wang Ooty

China

24.7 N

102.9 E

733

37

?

?

India

11.42 N

76.71 E

773

30

1

200

SPASE

South Pole

90.0 S

All

695

16

1

100

SPICA

Akeno Japan Tibet

35.8 N

138.5 E

910

72

.1

500

30 N

91 E

606

49

20

10

1020

1264

few hz

= 200

> 4000

?

104,106

Yanbajing

Projects under construction or on proposal level KASCADE Germany

49 N

8E

EAS 1000

Russia

MILAGRO

USA

35.9 N

106.7 W

740

na

= khz

=1

ARGO

Italy

40.8 N

15.5 E

900

na

= 0.5 khz

=5

CRT

(Venezuala)

(8.8 N)

(70.9 W)

(600)

=360

= khz

=2

E. Lorenz/High energy gamma astronomy above 200 GeV

103

Table 5 Operating Cerenkov detectors Name

Location

Lat (deg)

Long (deg.)

Mirror(s) #PM's #, area(m2)

ASGAT

Pyrenees France

42 N

2E

7, 7x30

7x7

NOITGEDACHT MKI

Potchefstr. SAU

26.9 S

27.2 E

4; 21

4

Durham MARK V

Narrabri Australia

30 S

149 E

3; 3x9

3x6

PACHMARHI

India

22.28 N

78.26 E

8+10

18

TH EM ISTOCLE

Pyrenees France

42 N

2E

18; 18x0.5 18

UMC

Dugway USA

40.2 N

112.8 W

4; 4x0.1

4

WHIPPLE

Mt Hopkins

31.6 N

110.9W

1; 50

109

31.6 E

136.5 E

1; 11

450

Projects under construction CANGAROO

Womera Austalia

BIGRAT

~w

"

3+1 ;30+40 3x(37+8)

GRANITE

Mt Hopkins USA

31.6 E

110.9 W

1; 55

HEGRA

La Palma Canary Islands

29 N

17E

1+4;5+4x9 34+ 4x128

AIROBICC

Bw

t~

37

49, 49x.12 49

NOITGEDACHT MK II

Potchefstr. SAU

26.9 S

27.2 E

+6; 6x6.8

+6x2

YEREVAN

Byurakan Armenia

40.2 N

44.5 E

1; 5

37

104

E. Lorenz ~High ener~ ganona astronomy above 200 GeV

5. TEST OF THE ANGULAR RESOLUTION OF EAS ARRAYS FROM THE OBSCURATION OF THE CR FLUX BY THE MOON AND THE SUN A key parameter for the sensitivity in HE 7 astronomy is, besides the 7/h discrimination, the angular resolution of EAS detectors. The lack of celestial test beams make the determination difficult. The standard method is to split the array into two interleaved subarrays, the so-called chequer board approach, and to compare the directions. Particularly for small showers, the bulk of the data, will yield only coarse results because of poor sampling and inevitably larger grid spacing. Also a fundamental parameter, the correct orientation of the array, cannot be checked. Some time ago G. W. Clark [20] proposed to use the obscuration of cosmic rays by the moon and the sun to determine the resolution and the correct alignment. This method has been demonstrated at first by the CYGNUS collaboration [21] and is now used by the large arrays. Fig 4 shows a recent result from HEGRA [6] for the moon+sun 'shadow'. 7

o

Table 6 compares the latest results from the large new EAS arrays for cO 63, (063 being the angle where 63 % of all data are contained in a cone with a radius O). It should be pointed out that below about 50 TeV corrections due to the magnetic field of the earth, respectively of the sun have to be applied (actually the solar magnetic field is not precisely known, detailed shower studies might be a method to determine the large distance solar magnetic field). The obscuration study is only applicable for large arrays because it will take at least a few years to amass a sufficiently large sample of events. A typical resolution of c e 63 of 0.7-1° of the current large arrays is a clear progress over the 1.4-4° resolution of a typical EAS array in the seventies and eighties. In conclusion one will judge in the future the performance of all EAS arrays (with exception of the polar ones) by their ability to observe the shadow of the sun and the moon. Table 6. Angular resolution for some EAS arrays, as determined from the obscuration of the CR by the moon and the sun Experiment

~ e63 (0)

E median (TeV)

ref

CYGNUS

1.05

50

[21]

CASA

1.25

120

[22]

EAS TOP

0.8

=200

[23]

HEGRA

0.7

70

[24]

3400

ii 0'1

3200 ta,,,l

3000 2800 2600 2400

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Distance from sun/moan center 13

Figure 40bscuration of the CR by the moon and the sun as observed with the HEGRA detector [24]

For completeness I would like to mention another powerful method for the calibration of the angular resolution. One can combine an EAS array with air Cerenkov detectors, see for example ref [48]. Recently, the HEGRA EAS array has been complemented by a matrix of large bare pms that sample the Cerenkov light disc of showers. In a single night about 105 showers can be recorded simultaneously. The

E. Lorenz ~High energy gamma astronomy above 200 GeV

lower threshold of the Cerenkov array and the much better angular resolution allow one to calibrate with high statistics the EAS array within a very short time; also one can determine precisely the angular resolution as a function of the shower size N e. 6. RECENT OBSERVATIONS

During the last 12 month since the Dublin conference some important new observations have been made: i) observation of the first extragalactic 1i source, MK 421, by the Whipple observatory ii) observation of ~s above 1013 eV from the Crab by Themistocle iii) identification of the Geminga pulsar at 1012 eV from the analysis of old data by the Durham group. The by far most important recent discovery is the observation of HE 7s from the BL-LAC MK421 by the Whipple observatory [25]. MK 421 is one of the extragalactic objects observed by the GRO in the MeV and GeV energy range [4]. In this range MK 421 was one of the least brightest sources, but the energy spectrum was found to be relatively stiff with a spectral index slightly below 2. MK 421 has a distance of = 125 mpsec and a redshift of z = 0.03. The GRO measurement showed a significant time variability. The Whipple collaboration observed MK 421 in spring 92 during a program to search for a series of potential extragalactic sources. The observation, a 6.3 sigma effect, was based on a total "on source" observation time of only 7.5 hours and about the same 'off source' time, respectively. These times were summed over shorter measuring periods spread over 3 month. Fig 5 shows the results from the image analysis using the orientation angle alpha; for comparison data from the Crab analysis are added. The MK 421 observation shows an excess of 136 events over a background of 166 events from the off source data after applying some image filter cuts. The very preliminary analysis confirmed the time variability of the source. The observed flux

105

above 500 GeV was about one third of the Crab flux, thus a luminosity of -- 1043 erg/sec has been calculated for a source distance of 125 mpsec. The group was also able to determine a coarse energy spectrum. The data in the lower energy range are consistent with the extrapolation from the GRO data, on the other hand there are very few events in the energy bins above 1.5 TeV. It is of utmost importance to confirm the observation of HE from MK 421 and to determine the energy spectrum with high precision. The steep drop in the spectrum, if confirmed, can be evidence for y attenuation due to interaction with the infrared background analogous to the predicted 7 attenuation at higher energies due to the interaction with the 2.7 ° K background radiation. The infrared background is not known, only upper limits exist. If one observes in high statistics experiments HE ~s between 10 and 100 TeV one can set new upper limits on the extragalactic infrared background, while on the other hand the non-observation would not unambiguously prove a sizeable infrared background because the acceleration mechanism might just have an energy limit around a few tens of TeV. Nevertheless it can be concluded that the study of other active galactic nuclei with much higher sensitivity might be an efficient tool to measure the infrared background in the nearby part of the universe. Another conclusion from the observation is that one should further lower the energy threshold for the search for more distant AGNs. Yet another conclusion can be drawn from the observation: 7s are only secondaries from HE charged particle interactions, therefore extragalactic HE charged cosmic radiation must exists. This seems to be trivial, but on the other hand we have no proof that the observed CR have any extragalactic component. The second important result of this year has been reported by the Themistocle collaboration [26]. This group studied air showers from the direction of the Crab with a prototype Cerenkov array detector. The detector consists of 18 minitelescopes which are distributed over an area of = 5x104 m 2 at

106

E. Lorenz ~High energy gamma astronomy above 200 GeV

'°I 160

't'''1'''1'

'i'

a) Mrk 421 On Source

120

Off Source ................ I

,=

80

P o

60

0

, 1 !

, 1 , F , ! 2O

o 1200

i

! ,

40

I ,

,. , ]

60

I1

80

j

b) Crab Nebula 1000

On-source Off-source ................

8OO

600

z

•.

400

.;

.:- ."'~..-~.- ~

::':"~--'i

:":

.:..?

200

0t

,

!

,

t.

=. r ,.

. t , ,.

!

I , , ,

20 40 60 Orientation Angle: zdpha (degrees)

I

,

80

Figure 5 On and Off-sourc orientation angle distributions for Mk 421 and the Crab; for showers that passe d the socalled supercuts

the Themis site, an abandoned solar power plant in the French Pyrenees. The detector has an excellent angular resolution of 2.4 mrad, very likely the best one of all the currently running detectors. Therefore a high sensitivity for the search of point sources is achieved even without further 7/h discrimination. The collaboration reported at the recent Cerenkov telescope workshop at Paris an 8.6 sigma excess (the second highest excess reported up to now) of ~s from the direction of Crab. Fig 6 shows the preliminary spectrum together with the Whipple data and a new observation from the ASGAT collaboration [27]. The data are distributed over an energy range between 2 and 20 TeV and follow a power law except for the highest 2 data point at 15 TeV where an indication of a downturn in the spectrum is seen. Hopefully it will be possible to extend the detailed studies to the energy where the large arrays can produce viable data in the near future. On the other hand, if the downturn is real, then the prospects of observing the Crab with the large EAS arrays is very much diminishing. An exception might be the Yanbajing array at 4000 m altitude in Tibet with a 20 TeV threshold. All the satellite observations of Crab show up to their energy limit of = 5 GeV a pulsed 7 emission while the HE data, starting at = 0.4 TeV, show no more the pulsar structure. This and the spectral shape (with a free Emax cutoff parameter between 10 TeV and 1 PeV) has been predicted by the so-called S.S.C model of Dejager and Harding [28]. The wide energy range, the high flux and the change from pulsed to unpulsed emission make the Crab an interesting object for accelerator models. The observation of up to 20 TeV 7s identifies the Crab as the most energetic observed source of cosmic radiation. At the recent Cerenkov telescope workshop at Palaiseau also results from ASGAT [27] have been reported on the 7 observation from the Crab ( = 700 GeV energy, 5.6 sigmas) with a 7 telescope Cerenkov detector, again installed at the Themis site. The multiple evidence of 7s in the energy range from 400 GeV to 20 TeV make the Crab

E. Lorenz ~High ozergyganmza astronomy above 200 Gev

a calibration source for Cerenkov detectors on the northern hemisphere. The flux is both sufficiently high and steady in order to make studies of the sensitivity, the angular resolution and the efficiency for the y/h discrimination of new instruments.

107

result but demonstrates also the importance of the phase analysis as an additional parameter in pulsar source searches and the importance of the collaboration between satellites and/or radio telescopes. IO0

= W-t0

,,0"11

WIVI~ a

a,bservator y

~i'qll"~ :.,:..~ . TMEMiSTOCLE " _ • %..

"t, E

"¢"Z,~..,

10-12

"~.

"',...

=-

6O r.

E • ~. 10"U"

~

,'~.UM( .,,. ~..~. -,,. ,~. '% -~. . L& 11111

.,,,.

"C

-,, %...~

Gamma Ray Energy (eV)

Figure 6 The HE 7 spectrum from the CRAB.

( 00

The third new result, the observation of TeV 7s from Geminga, has been presented at this conference by the Durham group [29]. Actually the data were already recorded in October 83 at the Dugway site, but at that time it was not known that Geminga is a pulsar and the bare data did not show the existence of a 7 source. Recently the ROSAT collaboration reported the discovery of 1Eo630+17 as an X ray pulsar with a 237 ms period [30]. Subsequently the Geminga pulsar has been confirmed by EGRET [31] and by the analysis of archive data from COS B [32] using the ROSAT period. The Durham group reanalysed their old data which are concentrated around an energy of 1 TeV. After appropriate folding of the ephemeris and the epoch of the main pulse they observed the same light curve as the recent EGRET data. Fig 7 shows the light curve from the EGRET data plotted at the phase indicated by the COS B data [33], and the corresponding light curve from the Durham analysis. The group determined also a luminosity of > 1 TeV 7s of 6x1030 erg s-1. When combining their data with the observation of EGRET in the 100 MeV range, they determined a spectral index of 2.2 between 100 MeV and 1 TeV. The Durham observation is not only an interesting physics

o2

o.4

o.6

o Is

l .o

Pnase

1800 Energy > 1000 GeV

1750 c~

E Z 1700 L_J

1650 0.0

0.5 1.0 Phase Figure 7 Geminga light curves for data from the EGRET and from the analysis of archive data in the TeV range recorded by the Durham group 7. W H A T CAN W E E X P E C T IN THE NEXT FEW YEARS

The recent results have triggered many activities to improve the detectors and to

108

E. Lorenz ~High energy gamma astronomy above 200 GeV

propose new ones. The main driving forces are the (a) lowering of the energy threshold in order to observe more sources, to close the gap between satellites and earthbound detectors and to make observations in the energy range where 7 interaction with the infrared or the 2.70K background might be observable, and (b) improvement of the sensitivity by improving the 7/h separation. Third in priority is the increase in sensitivity by increasing the detector area. The increase in detector area is normally set by the financial resources while the lowering of the threshold or a better 7/h separation requires new detector ideas. Only two new detectors based on scintillator arrays are currently under construction, the Karlsruhe KASCADE array [34] and the EAS 1000 array [35]. KASKADE is fairly advanced and will begin soon taking data with a subset of the array and around 1994 with the full array. The main aim of the experiment is the study of the chemical composition of the CR by a large hadron calorimenter but the surrounding scintillator and muon array detector is also well suited for 7 astronomy. The expected performance is in the range of the currently running large scintillator array, therefore its main contribution will be in the search for transients. The main deficiency of KASCADE is its low altitude of nearly sea-level; also the installation of Cerenkov counters is hampered by light pollution and poor weather conditions. EAS 1000 aims for much higher energies. The detector will be spread over 1000 km 2 and has a graded density. The high density central area is also suited for TeV y astronomy. The construction of such a large detector will proceed step by step and will take many years to complete. Two new array projects, MELAGO [36] and ARGO at SINGAO [37], have reached decision level. Both detectors aim for a lower detection threshold and for improved 7/h separation by having a close to 100% active detector coverage in the central region. MELAGRO consists basically of a large water pool of 5000 m2 area equipped with layers of large diameter photomultipliers that observe

Cerenkov light generated in the water. Muons are detected in a layer at 7 m depth where the electromagnetic showers have died out. The water pool detector will be surrounded by a classical scintillator array. ARGO uses a large carpet of 120x120 m2 of resistive plate counters providing both timing and tracking. Layers of limited streamer tubes track muons that pass a thick concrete absorber. Due to the large active area an improved angular resolution is expected in both detectors which should have a threshold close to 1-2x1012 eV. The muon detection area will be much larger than present detectors; on the other hand it should be noted that by lowering the threshold the y/h separation becomes less and less effective because of the reduced muon content of air showers and the large statistical fluctuations. Finally I would like to mention the CRT project [38]. This detector will use high precision tracking chambers and the incident direction is determined by analysing a large sample of low momentum tracks in the shower tail. The combination of a new technology, a large active area coverage of 5%, about 10 times larger than current detectors, and a proposed high altitude of = 4000 m above sea level should result in a threshold close to 1012 eV and a very good angular resolution of 0.20.3 degree. Prototype detectors for the CRT project will be tested soon. Some of the essential parameters of the proposed new detectors are added in table 4. In summary one can expect in the next years at most half an order of magnitude decrease in energy threshold and a modest improvement in angular resolution. The largest detectors will collect up to a few billion events which should result in a factor 2-8 increase in sensitivity in the steady state source search.The addition of new EAS arrays at different locations might very likely increase more the prospects for observing transients then improving the observation of steady state sources. On the other hand very likely more than half of the detectors in table 4 will cease operation because they are no longer competitive. Remarkably, there are no new

E. Lorenz ~High energy ganuna astronomy above 200 GeV

state of the art detectors proposed for the southern hemisphere where the prospects for observing galactic sources are higher. One of the main challenges for better EAS arrays remains the development of very cheap muon detectors in the price range of, say, 100 $/m2 including shielding because a high 7/h discrimination will be essential and up to now muon detectors are the only proven tools. Water pool detectors seem to be most promising. The development of Cerenkov detectors is more diversified. In the near future we expect quite a few new or improved telescopes to take data. Table 5 lists also these projects. The very impressive and pioneering results from the image analysis by the Whipple collaboration has triggered other groups to convert their running telescopes from no or modest image read-out to high resolution cameras. Recently industry has developed the first prototypes of a multianode photomultiplier with 100 pixels housed in a single glass envelope [39]. We will very likely see cameras up to 500 pixels in the near future. In parallel the optical quality of the large collection mirror has to improve. The present mirror diameter of the second Whipple telescope of 11 m is difficult to surpass because of exponentially rising costs and a degradation of time resolution. The prospects of converting solar plants like the SOLAR ONE project [40] have to be seen with some caution as the wide separation of mirrors results in large signal spreads making fast coincidence measurements difficult. Hand in hand with the technical progress in image observation goes the development of efficient algorithms for ~,/h discrimination and better simulations. The new fast and cheap RISC workstations allow one to generate Monte Carlo simulated air showers taking all the details of the light generation, light losses and detector performances into account and to amass a large statistics of events in reasonable time. One of the areas for more fundamental work is the precise understanding and modeling of hadronic showers.

109

A further improvement in 7/h separation can be expected by viewing with two or more telescopes the air shower from different locations. Recently the Whipple telescope has been complemented by a second 11 m telescope [41]. On preparation are the JapanAustralian project Cangaroo [42], and the HEGRA project [43] comprising 5 telescopes. The most ambitious project is the Japanese "Telescope Array" [44] of 2 distant sets of 25 telescopes in a 5x5 matrix with 50-60 m grid spacing. This project is presently on the level of a feasibility study. The concept of wave front sampling of the Cerenkov light disc at many points is persued by other groups. This technique has been pioneered by the Durham group [45] using three separate mirrors on a single stand and fast coincidence technique. In the latest design, like the ASGAT detector [49] or the Noitgedacht MK II detector [46], currently under construction, fast coincidence signals from multiple telescopes with a spacing of a few hundred meters are used. Two groups, the Themistocle collaboration [47] and the Hegra collaboration are pushing the multiple wavefront sampling to the extreme. The Themistocle collaboration has demonstrated that even with their prototype detector of 18 distributed minitelescopes (0.5 m2 mirror area, 20 mrad stop, <.5 nsec timing resolution) a high angular resolution of 2.5 mrad can be achieved. This excellent resolution was sufficient to observe the Crab even without any further 7/h discrimination. After enlarging the detector to the proposed number of more than 300 cells they predict a further improvement in angular resolution to sub mrad values and in turn an increase in sensitivity by the same factor. Further increases in sensitivity are expected from 7/h discrimination by analyzing the radial light pattern. The Hegra collaboration follows up an old concept to observe the Cerenkov disc by bare photomultipliers. The AIROBICC detector [48] consists of a 7x7 matrix (30 m spacing, 0.8 nsec timing resolution) of stationary large diameter pms. The light collection area is augmented by socalled Winston cones to 0.12 m2 per station. A

110

E. Lorenz ~High energy gamma astronomy above 200 GeV

7 threshold of 1013 eV is predicted. The main aim of the experiment is to search for transients over a large solid angle of 1 sterad. AIROBICC is implemented in the HEGRA EAS array. Multiple informations on a shower should be obtained like Cerenkov light pattern and electron density and muon flux at ground. The combined analysis should give another quantum step in 7/h separation. New ideas and concepts, based on the observation of the UV light below 300 nm, are persued by the ARTEMIS-Whipple [50] and by the CLUE collaboration [51]. Grindlay [52] proposed already in 1971 the measurement of the UV component as a means to discriminate 7 showers from hadronic showers. Experiments have been performed for example by Zyskin et al. [53] by combining UV-visible light measurements with rather modest increase in discrimination power. The new efforts concentrate to observe the UV component only between 200 and 300 nm. In this spectral range any light from above the atmospheric ozone layer, like stellar-, moon-, sun- or ionospheric fluorescence light, is blocked. It is therefore possible to perform measurements during moon shine or perhaps during day-time. The measurements are nevertheless handicapped by Rayleigh- and Mie scattering and reduced atmospheric transmission due to traces of ozone. The ARTEMIS-WHIPPLE collaboration intends to equip the Whipple telescope with a camera of solar blind pms. The CLUE collaboration plans to set up a matrix of 64 telescopes (2 m mirrors, 50 m spacing). The heart of the detectors will be a photosensitive proportional chamber with a 256 pixel pad readout, equivalent to the so-called fast RICH detectors. The photosensitive gas TMAE, has an upper spectral cutoff around 230 nm.

i)

at least five 7 sources have been observed with a significance of c > 4.5 ii) The observation of the Crab has been confirmed by many groups and Crab is presently the most energetic identified source iii) Crab has a sufficiently high and steady 7 flux in order to serve as a "standard candle" for Cerenkov telescopes on the northern hemisphere iv) The first extragalactic source has been observed v) EAS arrays lag behind Cerenkov telescopes because of a generally higher energy threshold, poorer angular resolution and lower 7/hadron separation, although they have a larger angular acceptance and 24 hour uptime. vi) Currently, rapid evolution of the detection techniques takes place; key issues for further progress will be the improvement in 7/hadron separation and the lowering of the energy threshold of future detectors vii) The collaboration with satellites for 7 observation is important. HE 7 astronomy has now come to age and we can expect further progress in the coming years. Not only new sources should be found but also detailed studies of these should give us deeper insight into the production of HE cosmic radiation. The general trend in the experiments parallels the developments in high energy particle physics, both in complexity and costs. The inevitable concentration will require large and presumably international collaborations The very interesting searches for diffuse or isotropic HE 7 radiation or the search for an abnormal hadronic component of HE 7s will remain a challenge to the end of this century. 9. ACKNOWLEDGMENTS

8. SUMMARY AND CONCLUSIONS After many years of frustrating searches 1' astronomy has now solid evidence for some HE 1' sources. The findings and the reasons for it can be summarized as:

I would like to thank P. Grieder for the invitation to this meeting. Also I would like to thank my colleagues I. Holl, A.Karle, J. Fernandez-Macarron and M. Merck for assistance in preparing this manuscript. I am

E. Lorenz / High energy ganm~a astronomy above 200 GeV

very grateful to M. Samorski who provided me with compilations of some of the recent Dublin conference results. 10. REFERENCES

1. M. Samorski and W. Stamm; Ap. J. 268 (1983) 117 2. J. Wdowczyk et A. W. Wolfendale. J. Phys. A. Vol. 5 (1972) 1419 3. I.A. Grenier et al.; Proceedings High Energy Gamma-Ray astronomy. Ann Arbor 1990, American Inst. of Physics Conference proceedings 220 (1991) 3 4. G. Kanbach: EGRET status report. Contribution Workshop: Towards a Major Atmospheric Cerenkov Detector. Palaiseau, France 11-12.6. (1992) 5. M.V.S. Rao, : Results from the KGF air shower array. ECRC Balatonfuret (1989). 6. A. Karle, HEGRA collaboration, private information 7. M.F. Cawley et al.;Exp. Astr. 1, (1990) 447 8. R.M. Baltrusaitis et al.; Nucl. Inst. Methods A240 (1985) 410 9. S. Swordy: Cosmic ray composition. Contribution to: Workshop: Towards a Major Atmospheric Cerenkov Detector. Palaiseau, France 11-12.6. (1992) 10. R. A. Ong et al.; Nuc. Phys. B (Proc. SuppI.)14A (1990) 273 11. M. A. Thomson et al.; Proc. 22nd ICRC Dublin 4 (1991) 603 12. M. Merck et al.; Proc. 22nd ICRC Dublin 2 (1991) 261 13. R. A. Ong et al.; Proc. 22nd ICRC Dublin 2 (1991) 273 14. D. E. Alexandreas et al.; Proc. 22nd ICRC Dublin ?,, (1991) 301 15. M. J. Lang et al.; Proc. 22nd ICRC Dublin 2 (1991) 204 16. C. C. G. Bowden et al.; Proc. 22nd ICRC Dublin 2 (1991) 332 and R. A. North et al.; Proc. 22nd ICRC Dublin 2 (1991) 672 17. C. C. G. Bowden et al.; Proc. 22nd ICRC Dublin 2 (1991) 356 and P. J. Meintjes et al.; Proc. 22nd ICRC Dublin 2 (1991) 360

111

18. C. C. G. Bowden et al.; Proc. 22nd ICRC Dublin 2 (1991) 396 19. J. A. Goodman.; Cosnews- Cosmic Ray News Bulletin 29 (1991) 10 20. G. W. Clark; Phys. Rev. 108 (1957) 450 21. D. E. Alexandreas et al.; Proc. 22nd ICRC Dublin 2 (1991) 672 22. B. E. Fick et al.; Proc. 22nd ICRC Dublin 2 (1991) 728 23. P. L. Ghia : The EAS-Top detector at Gran Sasso: recent results. Proceedings XXVIth Rencontre de Modond Ed. Frontieres (1991) 217 24. M. Merck, HEGRA, Private information 25. M. Punch et al.; Nature, 358 (1992) 447 26. G. Fontaine : Gamma Ray Detection of the Crab Source in the Multi-TeV range. Workshop: Towards a Major Atmospheric Cerenkov Detector. Palaiseau, France 1112.6. (1992) 27. D. C. Lamb : DC sources:status.Workshop: Towards a Major Atmospheric Cerenkov Detector. Palaiseau, France 11-12.6. (1992) 28. O. C. Dejager and A. K. Harding; Proc. 22nd ICRC Dublin i (1991)571 29. C. C. G. Bowden et al.: TeV gamma rays from Geminga. Contributed paper to this conference 30. J. P. Helpern and S. S. Holt; Nature 357 (1992) 222 31. D. L. Bertsch et al.; Nature 357 (1992) 306 32. G. F. Bigzami and P. A. Gavavea; Nature 357 (1992) 287 33. W. Hermsen et al.; IAU Circular 5541 (1992) 34. P. Doll et al.: The Karlsruhe Cosmic Ray Projekt KASCADE. KFK Report 4686 (1990) Kernforschungszentrum Karlsruhe 35. G. B. Khistiansen et al. 21st ICRC, Adelaide 10 (1990) 282 36. C. Sinnis : MILAGRO. Workshop: Towards a Major Atmospheric Cerenkov Detector. Palaiseau, France 11-12. 6. (1992) 37. M. Abbrescia et al. :Letter of intent for "ARGO at SINGAO" submitted to INFNFeb. 1992 38. J. Heintze et al.; Nuc. Inst. Methods A277 (1989) 29

112

E. Lorenz / High energy gamma astronomy above 200 GeV

39. Hamamatsu Photonics 40. O.T. Turner et al.; Proc. 21st ICRC Adelaide 4 (1990) 238 41. C. F. Akerlof et al.; Nuc. Phys. B, 14A (1991) 237 42. S. Ebisuzakai et al.; Proc. 22nd ICRC Dublin 2 (1991) 607 43. V. Haustein et al.: The HEGRA PROJECT, an update to the proposal. (1989) unpublished 44. M. Teshima et al. Nuc. Phys B Proceedings Supplements 28B (1992) 169 45. R. Brazier et al.; Prec. 21st ICRC Adelaide 4 (1990) 274 46. G. Brink et al.; Prec. 22nd ICRC Dublin 2 (1991) 622 47.G. Fontaine et al.; Nuc. Phys. B 14B (1990) 79 48. M. Bott-Bodenhausen et al.; Nuc. Inst. Methods A315 (1992) 236 49. P. Goret et al.; Proc. 22nd ICRC Dublin 2 (1991) 91 50. Nuc. Phys. B 14B (1990) 223 51. M. Cresti et al.; CLUE proposal 52. J.E. Grindlay, Nuovo Cim. 2B, N1 (1971) 119 53. Y. L. Zyskin et al.; Proc. 20th ICRC Moscow 2 (1987) 342

1) The definition of high energy is somewhat arbitrary. Normally one defines the energy range between 1011 .1014 eV as very high energy (VHE) and the range above 1014 eV as ultra high energy (UHE). For convenience I will use HE for the energy range above 2x1011 eV throughout this paper.