THE PROJECT FOR A 17 m DIAMETER AIR CERENKOV TELESCOPE FOR GAMMA ASTRONOMY

E. Lorenz (for the MAGIC Telescope Design Group) Ma:z:-Planck-Institut fijr Physik, Fohringer Ring 6,

80805 Miinchen, Gennany

The 20-300 GeV energy range is up to now inaccessible to gamma ray astronomy. Here we report on a design of a 17 m air Cerenkov telescope, dubbed MAGIC telescope, which will have a threshold of 20 GeV, a high collection area of > 105 m2 and a high gamma/hadron separation power. The hardware investments are estimated to be 3.5 MS while it would take 2.5-3.5 years for the construction.

1 Introduction

Gamma ray (shortcut 1') astronomy is at present the only well established method to explore the relativistic universe. Between 100 KeV and 20 GeV research is mainly carried out by satellite borne detectors while at much higher energies ground based detectors, such as Cerenkov detec­tors, scintillator arrays or air fluorescence detectors can be used. Due to technical limitations the energy range between 20 GeV, the upper limit of contemporary 1' satellites, and 300 GeV, the lower limit of large air Cerenkov telescopes, is up to now inaccessible to observations. Satellites are flux limited due to their small collection area of typically 0.1 m2• Costs become prohibitively high for detectors above a few m2 area. On the other hand 20 GeV electromagnetic air showers contain still enough high momentum electrons above the Cerenkov threshold such that ACTs should be able to detect them provided the signal can be discriminated against the night sky background light (NSB). In this contribution we will report about a novel ATC design with the necessary increase in sensitivity. Further details of MAGIC can be found in ref. 1

2 Short Review of the Physics Goals

There are many physics motivations to carry out 1' astronomy in the up to now inaccessible en­ergy range. As the subject will go beyond the scope of this contribution and as furthermore most goals are already covered extensively in other contribution we will give only a short summary.

a) One of the main research targets would be to study active galactic nuclei (AGN) out to z :::::: 2.8. Beyond this distance currently known sources are unlikely to have sufficient power to generate a 1' flux detectable with state of the art detectors. Measurements of the energy flux between a few GeV and 1 TeV will allow one to set severe constraints on the existence and size of the (up to now unquantified) IR. background. Also one wants to know the source of the rapid flaring which has been observed in both Mkn 421 and Mkn 501 2 indicating that acceleration in these AGNs must take place over a rather limited volume.

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b) The second main activity would be the systematic study of possible galactic 'Y emitters such as supernova remnants (SNR), plerions, x-ray binaries etc. SNRs are considered to be the main sources of cosmic radiation up to about 1015 eV but no sources are observed up to now above 300 GeV, either due to the lack of sensitivity or a cut-off in the acceleration mechanism.

c) Testing of gamma ray bursts (GRB) in the new window. GRBs occur nearly daily but their origin is still a mystery. They are generated either close to our galaxy or at cosmological distances. EGRET reported recently a GRB associated with -ys up to ::::: 20 GeV 3. A telescope like MAGIC with a 105 - 106 times larger collection area should be able to provide an answer to the "galactic halo" vs. cosmological distance scenario by measuring the 'Y energy spectrum in the range where it could be affected by IR and 2.7°K absorption. Such studies need prompt position information of the GRBs, e.g. by BATSE or HETE. Also one must be able to position the telescope in a very short time ( < 1 min).

d) Search for exotics such as for the lightest supersymmetric particle (SUSY) etc.

Finally it should be mentioned that it is of utmost importance to overlap satellite and ground bases observations in order to have a cross calibration of the two energy domains when doing multiwavelength observations.

3 Steps to lower the threshold

Fig. 1 shows Monte Carlo (MC) simulations of the Cerenkov light flux at ground generated by air showers of different energy and particle nature (courtesy R. Ong, T. Tuemer) . The currently best ACT, the Whipple 10 m 0 telescope, has a sensitivity limit in terms of Cerenkov photons/m2 (300- 600 nm) of 35-45 photons/m2 at detector level, see table 1. This relatively high number is driven by the requirements of � 300 photoelectrons per image 4• Taking the present Whipple configuration as a yardstick the following improvements are considered:

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a) Increase of the mirror area from 74 to 234 m2, i.e. a collector of "" 17 m diameter. The actual MAGIC mirror will be octagonal.

b) Use of photosensors with a GaAsP photocathode with about 45 % quantum efficiency ( QE) extended to 650 nm 5 . Low energy showers or showers at large zenith angles are stopped high up in the atmosphere and a sizeable fraction of the UV and blue light is lost due to Rayleigh scattering and 03 absorption. MC calculations show that a typical gain of 3 should be possible over conventional bialkali photocathodes. At large zenith angles the gain can increase to 8-10 at a transversed air mass of 5.

c) Selection of photosensors with basically 100 % photoelectron collection and a low excess noise factor close to 1 5. Contemporary photomultipliers (PM) have at most a collection efficiency of 80% and a typical excess noise factor of 1 .5-2. The envisaged hybrid pho­tomultiplier have basically 100 % collection efficiency and an excess noise factor below 1 . 1 .

d) Use of optimised light catchers, Winston cones, in front of the PM camera in order to funnel all the light onto the PM's sensitive area. This should result in a factor ;:::: 1 .6 gain over the current Whipple design. It should be mentioned that the Winston cone concentrators are already for some time in use in the HEGRA ACTs and in the recently commissioned CAT telescope 6.

e) As an ambitious goal we intend to lower the minimal number of photoelectrons/image to about 80. Limitations are given by the night sky background light (NSB), by the requirement of good ')'/hadron ('y/h) separation and good angular resolution. The impact of the NSB can only be reduced by narrowing the aperture time for the pulse height recording system counting the number of photoelectrons. One necessary precondition is that the timespread in the optical system is minimised. The following measures are planned. We intend to use an isochronous mirror profile, e.g. a gross parabolic mirror. and a ;:::: 250 MHz 8 bit F-ADC system. The time resolution for large pulses will be "" 1 nsec while it will be .:S: 8 nsec for single electrons. Other measures are to work with a very fine pixel size of 0.1° in order to minimise noise integration over areas being unnecessarily large compared to the small images of low energy showers. In essence the chosen telescope parameters should result in only 1 .5-3 NSB photoelectrons per pixel and aperture time of 8 nsec. It should be mentioned that it will be necessary to reduce stray background light by using a dark painted frame and Winston cones which will collect very little light coming from outside the mirror area, e.g. stray light from ground.

f) Some additional smaller improvements are foreseen such as minimisation of the obscura­tion by the camera support, antirefiex coating of the PM windows, minimisation of gaps between mirrors and non-reflecting patches on the mirrors etc. The improvements are estimated to add another factor 1 . 1 to 1 .2 in gain.

g) Two further goals are to operate the telescope in the presence of moon light with only a modest degradation and to have basically a full sky coverage while conserving a low threshold. In both cases a red extended sensor is of importance.

4 The Telescope

The telescope is modelled after a 1 7 m solar collector 7 from the DFVLR, Lampoldshausen, Germany. After initial studies the solar concentrator was found unsuitable for our purpose and a complete new design has been worked out using only some ideas from the original concept such

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Figure 2: A computer generated view of the 17 m 0 MAGIC telescope

as the alt. azimuth mount, the drive concept and the fixation of the focal plane instrumentation. The main mirror support dish is a three layer space frame made from carbon fiber-epoxy tubes for low weight and stiffness. A fundamental requirement is that the inertia of the telescope must be low in order to reposition it for GRB searches within 30-60 sec to any position on the sky. A finite element analysis of the frame shows that deformations can be held below 6 mm against the nominal curvature at any position for a frame weight of less than 5 tons. Fig. 2 shows a computer generated image of the telescope.

Both for optical and cost reasons the telescope has a tessellated mirror with a basic element size of 50 x 50 cm. The elements are lightweight sandwich aluminium panels with internal heating for prevention of dew and ice deposit. The elements are premachined with a spherical surface according to their position on the main dish (variation of the local focus between 17 and 18.6 m). A high quality reflecting surface is achieved by diamond turning with a surface roughness of < 10 nm. This technology allows for a very fast production of units with many different curvatures. A preproduction series showed high optical quality with a focal spot size of typically 6 mm diameter.

We plan to use a novel scheme for mirror adjustment and small corrections during telescope turning in order to counteract small residual deformations of the 17 m frame. Always four mirror elements will be preadjusted on a lightweight panel together with a switchable laser pointer. The panel can be tilted by two stepping motors under control of a videocamera that compares on demand the actual laser spot position on the camera cover with the nominal one. A first 1 : 1 prototype works successful.

The telescope will have an f/d = 1 and a 3.6° 0 camera with a pixel size of 0.1° in the central region of 2° 0 and coarser pixels of 0.2° in the outer part (for cost reasons). As photondetector we intend to use novel hybrid PMs (INTEVAC) with a high QE {45%) red extended GaAsP photocathode combined with an avalanche diode as secondary amplification element 5. The device has an overall gain of typically 25 000, an excess noise factor of 1 .02 and is extremely fast. The device has a very good single electron response allowing to identify up to 6 separated photoelectrons. The hybrid PM has practically no ion feedback which plagues classical PMs in other telescopes. The first prototypes are now working, nevertheless still substantial changes are

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necessary to adopt it to our needs. At present the photosensor is considered as the critical path and it might be necessary to use a conventional PM camera from one of the HEGRA telescopes as start-up. Eventually we plan to use silicon avalanche photodiodes with about 80 % QE but major developments are needed in order to reduce the current noise from ::::: 20 photoelectrons to below 2 and to concentrate the light onto small diameter devices with the aid of so-called light traps. The development will take still a few years.

The camera will be connected to the electronics ground station by 100 m optical fibers working in the analog mode for the transfer of the fast PM signals. This solution is preferred over an installation of most of the DAQ electronics directly behind the photosensors in the camera head. The optical analog fibers allow for minimal weight of the camera, a comparable thin bundle of 500 fibers and much higher bandwidth compared to classical coax cables.

The PM signals will be digitised by <::: 250 MHz, 8-bit F-ADCs. This allows for noise min­imisation, good timing measurements, buffering for the multilevel trigger system and eventually to combine more telescopes in an efficient way. Large signals are still sufficiently resolved with the 8 bit FADC in the time over threshold mode. The DAQ will be configured for a sustained data taking rate of up to 1 KHz and up to 10 KHz in case of transients. The trigger system is still under study.

It should be mentioned that all the novel features are either already tested or in use in other research fields. We estimate a hardware price of about 5 million DM (coarse break-up: lm DM each for the frame, the mirrors, the camera sensors, the fiber readout with F-ADCs and 1 million DM for smaller items and contingency) and a construction time of 2.5-3.5 years.

5 Some Performance Data

MC simulations show that the telescope has a threshold (= maximum differential counting rate) of slightly below 20 GeV and a rather large collection area in zenith position plateauing to ::::: 105

m2 (at ::::: 100 GeV) when using a trigger area of 1 .5° 0 in the camera. Opening the trigger area to the full camera diameter increases the collection area to > 3 x 105 m2 for TeV signals. Fig. 3 shows the collection area as a function of energy while fig. 4 shows the differential rate for a CRAB-like gamma source and the charged cosmic background. The integral rates of 2.8 Hz for a CRAB-like source and 95 Hz for the general background are obtained for a simple trigger coincidence requirement of <::: 4 pixels with > 8 photoelectrons and do not yet make any use of -y/h separation. This rather favourable ratio reflects the fact that proton (and heavier ions) induced showers have a much higher Cerenkov threshold than pure electromagnetic showers, see fig. 1. -y /h separation will be low close above threshold but rises with energy to q-values of > 10, e.g. we expect to detect a > 5a signal from a CRAB-like source within a minute, but detailed studies are still ongoing.

We would like to comment further on two specific background sources. At GeV energies one is confronted with a sizeable cosmic electron background. Electrons will eventually form the irreducible background but a telescope such as MAGIC should allow to reject most electrons on the basis of ALPHA cuts and a so-called head-tail factor telling us if the shower head points towards the source or away. First estimates show that one can suppress the electron BG for point sources by at least a factor 20. Another critical BG in Cerenkov observations comes from deeply penetrating muons. The rate is high. Calculations show that MAGIC is, due to its very high sensitivity, in a unique position. MAGIC should see nearly all muons (/3 > f3c) up to about 120 m impact distance and also in most cases the parent hadron shower. Muons close to the telescope will produce large light flashes of many thousands of photoelectrons in rings or arc which are easily distinguishable from -y showers. At large impact distance muons form local light cluster images (due to optical distortions in a "hammer or hook" like shape of at least 300 photoelectrons for f3 = 1 muons) together with mostly a diffuse parent shower image which

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is not visible in contemporary telescopes. In essence the detection of muons will give another handle for 'Y /h separation but no quantitative number can yet be given.

6 Comparison with other Projects

At present quite a few new ACT projects for 'Y astronomy below 300 GeV are under considera­tion, mostly based on "10 m class" telescopes with conventional PMs. Most favoured concepts are arrays where one hopes to lower the threshold and to improve on the angular resolution, the energy resolution and the -y/h separation by means of so-called stereo observation (see contri­butions from the VERITAS collaboration and the Heidelberg array proposal to this conference) Detailed comparisons are difficult to make, partly because of other experimental goals. Con­ventional 10 m telescopes have about a factor 15 fewer photoelectrons per image compared to MAGIC, see table l. On statistical arguments it is difficult to see how in the low energy domain this deficiency can be compensated by coincidence observations. Even the best array concept advocates a threshold being a factor 3 higher than that of MAGIC. The price for the array's improved 'Y /h separation is a restriction to smaller collection areas, e.g. for the Heidelberg basic four telescope cell to 104 m2 compared to the 105 m2 of MAGIC. The factor 10 in collection area and 15 in photoelectron yield/image compensates for some lower precision in angular and energy resolution on individual showers .. Large zenith angle observations with an even larger collection area are one of the main strengths of MAGIC, while efficient large zenith angle stereo observation would require to reposition the telescopes more apart. Also the new photosensors would allow for observation in the presence of moon light while sacrifying less in threshold than conventional PMs, e.g. even at 80 % moon illumination the MAGIC threshold would rise to ;::; 50 GeV which is still below the threshold of a 10 m classical telescope array under best optical condition.

Eventually the comparison has to be made on a price/performance relation because un­doubtedly a large array of N telescopes should allways be more sensitive on the simple square root N argument.

The main difference compared to some recently proposed solar array detectors are a much smaller collection area, the lack of image analysis power (requiring frequent on/off source mea­surements), and severe limitations to achieve a large sky coverage for the latter ones while

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Table 1: Light flux sensitivity, thresholds and area of MAGIC and some contemporary telescopes

I Telescope I minimal photon flux I threshold I collector (m2) I comments I AIROBICC 4000 photons/m' 12 TeV 50 x 0.125 wide angle detector IHEGRA CTI 220 " 1.5 TeV 5 � 100 photoelects./image HEGRA CT3-6 150 " 700 GeV 8.4 � 100 " CAT 35 (?) " 300 GeV (?) 18 � 30 " WHIPPLE 35 " 300 GeV 74 � 300 n

WHIPPLE 98 16 n 100 GeV 74 � 100 "

VERITAS 16/ tel.? " 60 GeV ? 9 x 74 MAGIC 1 .1 n 20 GeV 234 > 100 "

thresholds should be in the range of 40-60 GeV. The main advantage of solar arrays is their ready availability.

7 Conclusion

In summary it seems to be possible to construct a telescope with a low threshold of 20 GeV, high "f/h separation power, a large collection area, nearly full sky coverage and potential to measure in the presence of moon light. The telescope has the potential to discover and study a few hundred new "I sources. Compared to satellite borne g detectors the collection area is at least a factor 105 times larger while the investment would be only in the range of 1-5 %. Also the telescope could be build in much shorter time.

Acknowledgements

Herewith I would like to thank my colleagues from the MAGIC design group for provision of the information and many fruitful discussions. Also I would like to mention that part of the development work for component studies has been supported by the German BMBF and the Spanish CICYT. Furthermore I thank Edeltraud Haag and Ina Holl for the preparation of the report.

References

1 . S. Bradbury et al. : The design for the 17 m diameter MAGIC telescope. To be published as Max-Planck-Institut Report

2. T. Weekes et al.: Observation of gamma-ray sources at energies > 300 GeV. Submitted to A&A

3. K. Hurley et al.; Nature 372 (1994) 652 4. In 1998 the Whipple Group plans to install a new camera and to improve the readout elec­

tronics; therefore a lower threshold of, say, 100 photoelectrons/image is expected resulting in about 120 GeV "I threshold.

5. S. Bradbury et al.: Test of a new INTEVAC intensified photocell for the use in air Cerenkov telescopes. Contr. paper First Conf. : New Dev. in Photodetection, BEAUNE96, 24-28 .6.1996

6. M. Punch et al.; Proc. Int. Workshop: Towards a Major Atmospheric Cerenkov Telescope III. ed. T. Kifune , Universal Academic Press, Tokyo, p. 215 (1993)

7. Schlaich, Bergermann und Partner: Solar Power Plant with a Membrane Concave Mirror. Company report. Hohenzollernstr. 1, D-70178 Stuttgart.

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