Maier, G

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29th International Cosmic Ray Conference Pune (2005) 5, 391394Monte Carlo studies of possible upgrades of current arrays of imagingCherenkov telescopesGernot Maier and Johannes KnappUniversity of Leeds, LS2 9JT Leeds, United KingdomPresenter: Gernot Maier (gm@ast.leeds.ac.uk), uki-maier-G-abs1-og27-posterAt present several second generation imaging Cherenkov telescope systems are in operation (CANGOROO [1],HESS [2]) or currently under construction (MAGIC [3], VERITAS [4]). All are based on the idea of a telescopearray with 2-4 telescopes operating together. The main advantages of these systems compared to stand-alonetelescopes are superior rejection of background events from primary muons or cosmic rays showers, angularresolution of   0.1or better, energy resolution  20%, and significant sensitivity in the energy range below 100GeV. We discuss in this paper four different scenarios for future upgrades of these experiments: the addition oftelescopes, and the replacements of the cameras by systems with either higher pixel densities, larger fields ofview, or new high quantum efficiency photodetectors. The analysis is based on Monte Carlo simulations, theVERITAS design is used as a representative example for the different IACT systems. Direction reconstructionaccuracies, off-axis acceptance, energy thresholds, and  -ray detection rates are compared.1. IntroductionThe rapid development of gamma-ray astronomy in the energy range between 150 GeV and 1 TeV is welldocumented by the contributions to this subject area in these proceedings. The number of detected galactic andextragalactic sources in the field has been almost doubled over the last two years and flux sensitivities below1% of the flux of the Crab Nebula have been achieved. In 2006, four systems of imaging Cherenkov telescopeslocated in both hemispheres will be operational and further increase the catalog of high energy gamma raysources. The history of the Whipple telescope shows that these systems will likely undergo several upgradesover the next ten years.The goal of all upgrades is to achieve higher flux sensitivity for time resolved studies of variable objects(like AGNs, GRBs,..), to extend the energy range into the 10-100 GeV range, and to improve the angularreconstruction accuracy. The interest in lower energies is motivated by the large number of detected gamma-ray sources in the sub-GeV region by the EGRET experiment, the steep energy spectra of most sources, and theextended  -ray horizon. Higher angular accuracy is motivated by the study of extended sources and the highphysics potential of spatially resolved imaging of gamma-ray sources. The large extension of sources like RXJ1713.7-3946 (   arcmin [2]) is motivation for considering cameras with large field of views. These camerashelp are well suited for an efficient sky survey and the detection of serendipitous sources. Additionally, animprovement of the detection of high energies events is expected.The different configurations of the telescope arrays examined are summarized in Table 1. The basic shape ofthe array is a hexagon with a side length of 80 m and one telescope at the center. In the case of four telescopes,only every second corner of the hexagon is occupied. Each telescope consist of a Davies-Cotton reflector of 12m diameter. The focal length is 12 m. The cameras are equipped with circular shaped photomultipliers withcharacteristics similar to the Photonis XP2970 models. Light cones are simulated, which yield a geometricalcollection efficiency for Cherenkov photons hitting the focal plane of 90%. The local trigger system consists ofa simple multiplicity trigger of three PMTs with signals above threshold in a time window of 5 ns. The triggerthresholds are chosen to be at a constant signal-to-noise level. (see Table 1). The array trigger requires at leasttwo telescope with a local trigger in a time window of 10 ns.The current development in high quantum efficiency PMTs (HEPMTs) show, that at least a factor of two in392 G. Maier and J. KnappTable 1.Name number of field of no of PMTs PMT trigger high efficiencytelescopes view per camera radius [mm] threshold [pe] PMTsV4 4 3.5499 15 6.5 noV4(HEPMT) 4 3.5499 15 9.0 yesV4(FINE) 4 3.51498 8.8 3.5 noV4(LFOV) 4 5.51273 15 6.5 noV7 7 3.5499 15 6.5 noimprovement compared to the current available PMTs is feasible [5]. For this analysis, the quantum efficiencyof the Photonis XP2970 is simply multiplied by two. Almost equivalent to the use of HEPMTs, and thereforenot taken into account here, is the extension of the mirror size, which increases the Cherenkov photon collectionarea.2. SimulationsThe Monte Carlo simulations consist of shower simulations with CORSIKA 6.20 [6] followed by detailedmodeling of the telescope arrays [7]. The atmospheric profile is modeled using a U.S. Standard atmosphere.The observation height for the telescope system is 1800 m a.s.l.. Showers of primary gamma-rays are simulatedaccording to a Crab-like spectrum in the energy range from 10 GeV to 10 TeV with a zenith angle of 30. Theenergy spectrum of the Crab Nebula is taken from [8] as 	  ﬀﬂﬁﬃ! #"%$&')( *&+ m ,' s .- TeV /- .The shower cores are distributed randomly on a circular area with a radius of 500 m around the center of thearray. The telescope simulation consists of two parts, the propagation of Cherenkov photons through the opticalsystem and the response of the cameras and electronics. The signal in a PMT is created by summing up singlephoto-electron pulses with appropriate time and amplitude jitters applied. Night sky background, electronicnoise and all efficiencies, including mirror reflectivities, geometrical, quantum, and collection efficiencies, andlosses due to signal transmission have been modeled. The simulation chain has been extensively tested toreproduce the characteristics of the VERITAS-1 telescope [9].3. Event reconstructionThe event reconstruction chain consist of image cleaning, second moment image analysis [10] for the signals ineach camera and the reconstruction of shower direction and impact parameter on the ground, using all availableimages. Images are cleaned by cutting on pixels with low signal-to-noise ratio at values of five and three timesthe pedestal variations for image and border pixels, respectively. In the second moment analysis the axes andwidths of the 2dim Gaussian are determined, that fits the image best. Images of the size of at least five pixelsare chosen for the array reconstruction. The intersection of the elongated longer axes of the different imagesdefines the reconstructed shower direction [11]. Only events with at least three images are used.4. Energy thresholds and trigger ratesFig. 1 shows the differential  -detection rate for a Crab-like source. The figure on the left shows the differentialenergy spectrum for events with at least three telescopes with a local trigger. It shows that four telescopesequipped with high efficiency photomultipliers are, at the trigger level, equivalent to a system with seventelescopes. The energy threshold is conventionally defined as the position of the peak at the energy spectrumof the source convoluted with the effective area with the detector. It is 140 GeV for the system called V4 andMC Studies of possible upgrades of IACTs 393 energy [TeV]10log-1.5 -1 -0.5 0 0.5 1]-1dN/dE [(TeV s)-410-310-210-110110V4V7V4 (HEPMT)triggered events energy [TeV]10log-1.5 -1 -0.5 0 0.5 1]-1dN/dE [(TeV s)-410-310-210-110110V4V7V4 (HEPMT)reconstructed eventsFigure 1. Differential trigger rate for a Crab-like source for events with at least three triggered telescopes (left) and aftercuts on successful reconstruction (right) (see Table 1 for description of the labels).125 GeV for V4 HEPMT and V7. The respective integral rates of  -rays from the Crab Nebula are 31, 49 and50  ’s/min.Fig. 1 right shows the differential energy spectrum for events available for the reconstruction, i.e. with at leastthree images of more than four pixels each. Four telescopes with HEPMTs have a significantly lower energythreshold (125 GeV) and higher  -ray rate (31  ’s/min) than all other cases (200 GeV, 20  ’s/min for seventelescopes, 225 GeV and 15  ’s/mim for four telescopes). The high quantum efficiency cameras have in generalmuch better defined images with more pixels, which results in the better performance after reconstruction. Theseven telescope array is as sensitive as four telescopes with HEPMTs at low energies, but most images are toosmall to be useful in the reconstruction.The off-axis sensitivity for  -rays with 150 and 500 GeV primary energy is shown in Fig.2 left. Only eventswhich triggered at least three telescopes are included here. Not surprisingly the system equipped with thewide field of view cameras accepts showers out to the largest offset from the pointing direction (   2). Seventelescopes have an acceptance generally about 50% higher than systems with high efficiency cameras or V4.All cameras with 3.5field of view show a strong decrease in acceptance with larger offsets.5. Reconstruction AccuracyThe angular resolution is here defined as the 68% value of the distribution of the differences in space anglebetween actual and reconstructed source position. As can be seen in Fig.2 right, the angular resolution isenergy dependent and is at the level of 0.1or better. Seven telescopes give the best angular resolution for allenergies, systems with high efficiency PMTs give a significant improvement at small energies. This is for thelatter one due to the large images, which results in a better determination of the orientation of the image axes.The performance of the optical system and fluctuations of the lateral extension of the light-emitting region inthe shower limit the precision of the angular reconstruction, better results can only be achieved by averagingover several measurements of the same event. This is the reason why the system with smaller PMTs is mainlybetter at smaller energies than four telescope systems (it has more images with at least five pixels), and theseven telescope system gives a significant improvement over the whole energy range.394 G. Maier and J. Knapp]opointing offset [0 0.5 1 1.5 2 2.5 3 3.5 4trigger efficiency (arbitrary units)00.10.20.30.40.5V4V7V4 (HEPMT)V4 (WFOV) energy [GeV]10log2 2.2 2.4 2.6 2.8 3 3.2 3.4]oangular resolution [0.040.050.060.070.080.090.10.110.12V4V7V4 (FINE)V4 (HEPMT)Figure 2. 3-telescope trigger efciency for off-axis sources (left, open symbols: 150 GeV 0 -ray showers, lled symbols:500 GeV showers). Angular resolution vs. primary energy (right) (see Table 1 for description of the labels).6. ConclusionThis study of possible extension to arrays of imaging Cherenkov telescopes shows that a replacement of thecurrent photomultipliers by high quantum efficiency PMTs give the best overall improvement. In particular thelow-energy performance: the number of images per event and reconstruction accuracy is better or equivalent toseven telescopes. Increasing the number of telescopes has on the other hand additional advantages of flexibilityand the option of observing multiple sources simultaneously. As well, it has to be mentioned, that high quantumefficiency PMTs are still in the R&D phase and presently not available on large scales.AcknowledgmentsGM acknowledges the support as a Feodor Lynen Fellow of the Alexander von Humboldt foundation.References[1] C. Itoh et al., Astronomy & Astrophysics, 402, 443 (2003)[2] F. Aharonian et al., Nature, 432, 75 (2004)[3] C. Baixeras et al., Nucl. Physics B (Proc. Suppl.) 114, 247 (2003)[4] T. Weekes et al., Astroparticle Physics, 17, 221 (2002)[5] D. Paneque et al., NIM A, 504, 109 (2003), E. Lorenz et al., NIM A, 504, 280 (2003)[6] D. Heck et al., Report FZKA 6019, Forschungszentrum Karlsruhe (1998)[7] C.Duke & S.LeBohec, http://www.physics.utah.edu/gammaray/GrISU/[8] M. A. Hillas, The Astrophysical Journal, 503, 744 (1998)[9] G. Maier et al., Monte Carlo Studies of the first VERITAS telescope, 29th ICRC, Pune (2005)[10] M. A. Hillas, Proc. 19th Int. Cosmic Ray Conf., 3, 445 (1985)[11] W. Hofmann, Astroparticle Physics 12, 135 (1999)

Monte Carlo studies of possible upgrades of current arrays of imaging Cherenkov telescopes

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