IL NUOVO CIMENTO
VOL. 19 C, N. 6
Novembre-Dicembre 1996
Development of the atmospheric Cherenkov imaging technique (*) M. F. CAWLEY St. Patrick's College - Maynoot~ County Kildare, Ireland
(ricevuto il 22 Maggio 1996; approvato il 28 Giuguo 1996)
Summary. -- Following a brief history of the atmospheric Cherenkov imaging technique and a summary of the current status of the field, we describe some of the potential developments of the method over the next few years. Due to constraints of space, we shall confine the discussion to single stand-alone telescopes--stereo imaging will not be discussed. PACS 96.40 - Cosmic rays. PACS 01.30.Cc - Conference proceedings.
1. - B r i e f h i s t o r y and c u r r e n t s t a t u s o f t h e t e c h n i q u e
The first detailed images of atmospheric Cherenkov flashes were obtained in the early 1960's using wide-field image intensifiers [1]. For various reasons, including long integration times and small apertures, the energy threshold of these early systems were of the order of 1014eV, and the approach was not suited to searches for point-sources of very high-energy gamma rays. A design for a Cherenkov imaging telescope suitable for VHE astronomy was proposed in 1977 [2], and this formed the basis for the first Whipple Collaboration imaging system. This was developed in stages over a number of years, and by 1985 it consisted of a cluster of 37 2-inch photomultiplier tubes at the focus of the 10 m reflector on Mt. Hopkins. The resolution of each pixel was 0.5~ with a 3.5~ field of view. This system detected a 9a effect from the Crab Nebula in 80 hours of data (with a similar amount of data taken on a control region) using the ~azwidth- analysis approach [3]. In 1989 the Whipple Collaboration system was upgraded to 109 pixels (0.25~ resolution), which led to the detection of a 20a effect from the Crab Nebula in a single season of observations (30 hours)[4]; improvements in off-line data analysis (the ,,Supercuts, approach) increased the significance of these observations to 34~[5]. This result clearly established the Cherenkov imaging technique as the most sensitive approach then available for TeV astronomy, and other
(*) Paper presented at the Special Session on ground-based gamma-ray astronomy of the XXIV International Cosmic-Ray Conference, Rome, August 28-September 8, 1995. 959
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groups began to develop similar systems. The first independent confirmation of the effectiveness of the technique came from the Cangaroo Collaboration, with the publication of a 12 o detection of PSR1706-44 [6] using a 220-pixel camera at the focus of a 3.8 m reflector. At present (October 1995), there are at least 6 installations worldwide using the imaging approach, with several more due to come on-line within the next 12 to 18 months. The Crab Nebula has been confirmed as a steady source of TeV radiation by several groups [4, 7-9] and is used as a ,,standard candle, for the calibration of new systems. PSR1706-44 has been seen by one group [6]. The most exciting development of the past few years has been the observation of TeV emission from a subclass of active galactic nuclei: Mkn421 has been observed by two groups ([10] and the Hegra Collaboration--oral presentation at the XXIV ICRC, Rome, 1995), and Mkn501 has been detected by the Whipple Collaboration [11]. Both of these sources share similar properties, and both are relatively close, with redshifts of 0.03. Both exhibit variability in TeV emission on timescales of days. Intensive study of these and other AGNs over the next few years promises to gain insight into the particle acceleration mechanisms operating within these sources, and may also provide an important probe of the intergalactic medium, yielding limits on the intergalactic infrared density [12]. The atmospheric Cherenkov imaging technique is now firmly established as one of the most promising avenues in TeV astronomy. In this paper we shall briefly discuss some of the directions in which the technique might be developed over the next few years.
2. - H a r d w a r e
developments
2"1. F i n e r r e s o l u t i o n . - Several groups are preparing to move towards a pixel resolution of order 0.1 ~ (CAT, Cangaroo, Whipple Granite III). Evidence from simulations indicates that the Cherenkov flash contains useful structure down to at least this resolution [13], and improvement in sensitivity with increased resolution has already been verified (the Whipple Collaboration 0.5~ resolution camera detected 5a from the Crab Nebula in 25 hours; the 0.25 ~ resolution camera detected 5a from the Crab Nebula in 0.5 hours; the Cangaroo 0.18 ~ resolution camera yields a-plots with narrower gamma-ray peaks than other lower~resolution cameras). There are many advantages in moving towards higher resolution. The most obvious advantage, alluded to above, is the increase in sensitivity arising simply from the improved ability to accurately determine the pointing angle (,~alpha-) of the image major axis. There is also evidence from simulations that higher-resolution cameras will perform better at lower energies [14]. Finer resolution will reveal structure of the images in more detail, leading to improved pattern recognition capabilities, and better discrimination of image classes; for example, the correlation of the ratio of width to length against distance for gamma rays could be exploited (gamma-ray images should become narrower and more elongated as distance from the source location increases--this correlation is not presently exploited in approaches such as ~,Supercuts-). Other image properties, such as the predicted asymmetrical ,,comet- shape, should be more clearly revealed. Arcs and rings due to local muons would be better resolved, and more readily distinguished from potential gamma-ray images at lower energies. Apart from obtaining improved images of the Cherenkov flashes, higher-resolution cameras would also resolve individual
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star positions to a much improved degree, giving a direct means of monitoring the pointing and tracking performance of the telescope. As regards possible drawbacks associated with higher-resolution systems, it is clear that 0.1~ pixel resolution would place considerable demands on the design and performance of the telescope mount and optics. To reduce the effects of off-axis aberrations, it may be desirable to move towards slower f numbers (e.g., in the CAT project, it is proposed to use f/1.25, compared with f / 0 . 7 for the Whipple Collaboration 10 m reflector). Another possible disadvantage of moving to higher resolution is that the signal-to-noise ratio in individual pixels scales with the square root of the pixel area, and therefore decreases as the pixel size decreases. This argument was used in a recent simulation-based study to conclude that the optimal pixel size for TeV astronomy imaging systems was in the range 0.20-0.3 ~ [15]. This conclusion is only valid, however, if one relies solely on the signal/noise ratio within individual pixels during image cleaning. In reality, use is made of ,topological, considerations--clusters of adjacent pixels showing higher signal/noise indicating the presence of a genuine signal. An extreme situation would be in the use of an image intensifier system with near-perfect resolution, capable of resolving individual photon positions; use of the signal/noise per pixel argument would lead to the conclusion that a pixel hit by a Cherenkov photon is indistinguishable from a pixel hit by a NSB photon. In reality, the Cherenkov image would be clearly distinguishable--and cleanable--in such a system due to the clustering of pixels registering Cherenkov photons.
3~
2"2. Wide field of view. - Fields of view of current ACI systems are typically of order ~ There are several proposals to move to larger FOVs: Hegra: 4.7~ - CAT: 4.8~ Granite III: 6~ - TACTIC: 6~ - Shalom 7.2~ - Telescope Array Project: 10~ -
-
Some of the advantages accrued from large FOVs would be: reduce the role of ,edge effects- for gamma-ray collection area (due to images partially falling outside the FOV); this would lead to improved sensitivity (fiat background regions in the a-plots) and better energy resolution and spectral determination, increased gamma-ray collection area (for sources with hard spectra), - facilitate sky surveys, facilitate searches for burst sources, - better for diffuse sources (e.g. SNRs, etc.), - better overlap in stereo systems, - better chance of detecting sources with large positional uncertainties (some of the weaker unidentified Egret sources have positional uncertainties of 0.5~ -
-
The drawbacks of large FOVs are similar to those associated with high-resolution cameras: greater cost and complexity, and greater demands on the mirror optics and off-axis performance. If non-parabolic figures are used to improve the latter, then advantages due to isochronicity are lost--the time profile of the Cherenkov pulse becomes elongated.
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2"3. I n t e l l i g e n t triggers. - Typical ACI systems at present use a simple trigger which demands temporal coincidence between two or more pixels within the imaging array (e.g., for the Whipple Collaboration system, the trigger demands that at least 2 of the inner 91 pixels exceed a threshold of 40 p.e.). A less frequent form of trigger uses pixels on ancillary dishes to determine whether or not the information in the main imaging array should be recorded (Durham Mark VI [16]; TACTIC [17]). As the number of pixels is increased due to moves towards higher resolution and/or wider FOVs, the rate of accidental triggers will increase (by a factor of about 4 for a doubling of the number of pixels for 2-fold coincidences). This rate of accidental triggers may be reduced by demanding that the triggering pixels occur within a particular region or ,,sector- of the camera (CAT [18]) or through the use of a more ,,intelligent, trigger which might, for example, demand that the triggering pixels be adjacent. It has been proposed that such a trigger system could be implemented using programmable gate arrays or hardware neural networks, and could result in a 30% reduction in energy threshold in existing systems (by permitting the lowering of discriminator thresholds while maintaining the same rate of accidental triggers)[19]. More sophisticated hardware triggering--e.g., selection of events pointing towards the source location--could also be envisaged. 2"4. E l e c t r o n i c s - o t h e r p o s s i b i l i t i es . - The traditional approach to recording a Cherenkov image is the use the trigger pulse to gate a bank of charge-to-time converters for a preset time interval (typically 10 to 30 ns). Time-to-digital converters may also be used on each channel to select photons due to the Cherenkov flash [20]. As systems progress to many hundreds of pixels, the charge-to-time approach becomes prohibitively expensive, and alternative avenues will be worth exploring. High-speed flash ADCs, such as those used in the newer generations of digitizing oscilloscopes, will become cost-competitive in the near future, and will offer the additional possibility of recording in detail the shape of the pulse (which may yield additional discrimination information). Use of FADCs also offers the possibility of multiplexing several channels into a single digitizer by using analog delays, thus reducing costs by a considerable factor. In addition to pursuing different strategies in future signal processing systems, we should also briefly mention here that new technology photodetectors may offer considerable improvements in sensitivity and resolution for future Cherenkov imaging telescopes (e.g. [21]). Hybrid PMTs (combination of PMT technology with solid-state devices) using PIN diodes or avalanche photodiodes may yield a doubling in quantum efficiency over present PMTs, and very high-resolution solid-state photodetectors (again based around the avalanche photodiode) are under development.
REFERENCES [1] HILL D. A. and PORTER N. A., Nature, 191 (1961) 690. [2] WEEKES T. C. and TURVER K. E., Proceedings of the XII ESLAB Symposiun, Frascati, 1977 (ESA SP-124), p. 279. [3] WEEKES T. C. et al., Astrophys. J., 342 (1989) 379. [4] VACANTI G. et al., Astrophys. J., 377 (1991) 467. [5] PUNCS M. et al., Proc. XXII ICRC, Dublin, 1 (1991) 464.
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[6] KIFUNE T. et al., Astrophys. J., 438 (1995) L91. [7] BAIbLON P. et al., Proc. XXII ICRC, Dublin, 1 (1991) 220. [8] GORET P. et al., Astron. Astrophys., 227 (1993) 401. [9] KRENNRICH F. et al., Proc. XXIII ICRC, Calgary, 1 (1993) 251. [10] PUNCH M. et al., Nature, 358 (1992) 477. [11] QUINN J. et al., IAU Circ. 7168 (1995). [12] STECKER F. W. et al., Astrophys. J., 415 (1993) L71. [13] HILLASA. M., Proceedings of the Workshop on VHE Gamma-Ray Astronomy, Crimea, 1989, edited by A. A. STEPANIAN, D. J. FEGAN and M. F. CAWLEY,p. 134. [14] TANIMORI T. et al., Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector, III, edited by T. KIFUNE (Universal Academy Press, Inc., Tokyo) 1994, p, 311. [15] ZYSKIN Yu. L. et al., J. Phys. G, 20 (1994) 1851. [16] BOWDEN C. C. G. et al., Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector, H, edited by R. C. LAMB (Iowa State University, Ames, Ia.) 1993, p. 230. [17] BRAT C. L., et al., Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector, H, edited by R. C. LAMB (Iowa State University, Ames, Ia.) 1993, p. 101. [18] DEGRANGE B. et al., Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector, H, edited by R. C. LAMB (Iowa State University, Ames, Ia.) 1993, p. 235. [19] BUCKLEYJ. H., Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector, III, edited by T. KIFUNE (Universal Academy Press, Inc., Tokyo) 1994, p. 221. [20] TAMURAT. et al., Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector, III, edited by T. KIFUNE (Universal Academy Press, Inc., Tokyo) 1994, p. 179. [21] LORENZ E., Proceedings of the International Workshop Towards a Major Atmospheric Cherenkov Detector, III, edited by T. KIFUNE (Universal Academy Press, Inc., Tokyo) 1994, p. 341.
