Koul, M K

CERN Document Server

29th International Cosmic Ray Conference Pune (2005) 5, 239242Monte Carlo studies for improving the sensitivity of the TACTICtelescope by using zenith angle dependent Dynamic SupercutsM.K.Koul and A.K.TickooNuclear Research Laboratory, Bhabha Atomic Research Centre, Trombay, Mumbai, 400 085, IndiaPresenter: M.K.Koul (mkkoul@apsara.barc.ernet.in), ind-koul-M-abs1-og27-posterThe sensitivity of atmospheric Cerenkov imaging telescopes is strongly dependent on the rejection of thecosmic ray background events. While the efficiency of using Hillas parameters for performing this segregationhas been confirmed by the detection of several   -ray sources by various independent groups, one has to alsoconsider energy and zenith angle dependence of these parameters so that the energy spectrum of the   -raysource can be reliably determined. The main aim of the present simulation work is to study the comparativeperformance of various   -ray selection methodologies at different zenith angles so that zenith angle dependentDynamic Supercuts procedure can be used for the TACTIC telescope for determining the energy spectrum ofa source. The results of the Monte Carlo simulations, carried out at four different zenith angle values of 15  ,25, 35and 45, indicate that the zenith angle dependent Dynamic Supercuts provide the best combination ofsensitivity and   -ray acceptance as compared to the Supercuts and the Dynamic Supercuts methodologies.1. IntroductionGround-based   -ray astronomy in the energy range above 100 GeV has made dramatic progress in the lastdecade with the rapid development of the atmospheric Cerenkov imaging technique at a number of observa-tories. Since the sensitivity of a Cerenkov imaging telescope is strongly dependent on the rejection of thecosmic-ray background events, it is important to assess the potential of various   -ray selection methodologiesso that the weak   -ray signal can be isolated from the dominant hadron background with a high signal-to-noise ratio. Simulation work pioneered by Hillas [1] to quantitatively predict image features, has led to thedevelopment and successful usage of several   -ray selection methodologies like Supercuts [2] and DynamicSupercuts [3]. While energy dependence of the imaging parameters has been duly considered in the DynamicSupercuts procedure, it has been argued that the zenith angle dependence of these parameters can be ignoredif a candidate   -ray source is observed only upto a zenith angle of  30. Since in the zenith angle range30-45, the shower development takes place at a higher effective altitude, it is obvious that some of the imageparameters ( e.g DISTANCE, LENGTH and WIDTH ) will also have a zenith angle dependence [4]. The mainaim of this paper is thus to study this behaviour by performing detailed Monte Carlo simulation studies forthe TACTIC telescope at 4 different zenith angles between 15- 45so that zenith angle dependent DynamicSupercuts can be used for extracting the weak   -ray signal from the dominant hadron background with anadequate signal-to-noise ratio.2. TACTIC telescope and generation of simulated data-baseThe TACTIC (TeV Atmospheric Cerenkov Telescope with Imaging Camera)   -ray telescope has been set upat Mt.Abu, India ( 24.6N, 72.7E, 1300m asl), for studying emission of TeV   -rays from celestial sources.The telescope uses a F/1 type steerable tessellated light collector of  9.5 m  area made up of 34 x 0.6 mdiameter, front aluminised spherical glass facets. The telescope uses a 349-pixel imaging camera with a uni-form pixel resolution  0.3and a field of view  6x6for recording the images of atmospheric Cerenkovevents produced by an incoming cosmic ray particle or a   -ray photon. The innermost 121 pixels (11  11240 M.K.Koul and A.K.Tickoomatrix, FOV  3.4x3.4) are used for generating the event-trigger based on the 3NCT (Nearest NeighbourNon-Collinear Triplets) topological logic by demanding a signal  7 pe for the 3 pixels which participate inthe trigger-generation. The simulation studies presented here are based on the CORSIKA air-shower simu-lation code [5] and are valid for Mt.Abu observatory altitude of 1300m. The simulated data base for   -rayshowers uses about 25000 showers in the energy range 0.2-20TeV with an impact parameter of 5-250m anda differential spectral index of  -2.6. These showers have been generated at 4 different zenith angles (  =15, 25, 35and 45). Furthermore, a data-base of about 30000 proton initiated showers in the energy range0.4-40 TeV and distributed isotropically within a field of view 6x6, have also been generated. Wavelengthdependence of atmospheric absorption, spectral response of the PMTs, reflection coefficient of mirror facetsand light cones has also been taken into account in the present simulation studies. The data-base, consisting ofnumber of photoelectrons registered by each pixel has been then subjected to noise injection, image cleaningand trigger condition check. The resulting two dimensional ’clean’ Cerenkov image of each triggered eventis then used to determine various image parameters ( viz., SIZE (S), LENGTH(L), WIDTH (W), DISTANCE(D), APLHA (  ) and FRAC2 (F2)).3. Comparative performance of various  -ray selection methodologiesThe optimized image parameter domains for the TACTIC telescope using three different   -ray selectionmethodologies (viz., Supercuts (SC), Dymanic Supercuts (DSC) and zenith angle dependent Dynamic Su-percuts (ZDSC)) are given in Table 1. The optimized image parameter domains of the three   -ray selectionTable 1. Optimised image parameter domains for the three 	 -ray selection methodologies.Parameter Supercuts Dynamic Zenith angle(SC) Supercuts (DSC) dependent Supercuts (ZDSC)SIZE ( S )  50 pe  50 pe  50 peLENGTH ( L ) 0.10  L  0.37  0.10  L  L1 0.10  L  L2 cos  L1 = (0.24 + 0.055 ln(S))L2 = (0.24 + 0.045 ln(S))WIDTH ( W ) 0.07 W  0.170.07 W  W1 0.07 W  W2 cos W1 = (0.08 + 0.035 ln(S))W2 = (0.08 + 0.027 ln(S))DISTANCE ( D ) 0.5 D  1.20.4 D  1.30.4cos  D  1.6cos FRAC2 ( F2 )  0.53 not used not usedALPHA (  )  18 18 18methodologies, valid in the zenith angle range 15-45, have been obtained by demanding that SC, DSC andZDSC methods should yield the best possible sensitivity, 50-60    -ray acceptance and the best combinationof both of these, respectively. The variation of effective collection area as a function of primary energy, for thethree   -ray selection methodologies is shown in Fig.1.The results shown in this figure suggest that the SC method is biased towards lower energies (particularly atlower zenith angles), as might be expected since this method was optimized to give the maximum significance.In order to evaluate the efficiency of a discrimination technique, while it is a common practice to calculate theQuality Factor(Q) (where Q= ﬀ / ﬁ ﬃﬂ , with  and ﬃﬂ being the   -ray and protons acceptance factors, respec-tively after the application of selection cuts), we have instead used a more rigorous approach by calculatingT  "!$# (defined as the minimum observation time required for detecting Crab Nebula at a statistical signifi-cance of N % ). Assuming that the detection sensitivity is limited by statistical fluctuations of the ON-sourceMonte Carlo studies for improving the sensitivity of the TACTIC telescope ... 241 Eγ ( TeV )1 10Effective Collection area  ( m2 ) 101102103104105106Trigger aloneSupercutsDynamic Supercuts (DSC)θ dependent  DSC (a)    Zen.  angle  ( θ ) = 150  Eγ ( TeV )1 10Effective Collection area  ( m2 ) 101102103104105106Trigger aloneSupercutsDynamic  Supercuts  (DSC) θ dependent  DSC  Eγ ( TeV )1 10Effective Collection area  ( m2 ) 101102103104105106Trigger aloneSupercuts  Dynamic supercuts  (DSC)θ  Dependent  DSC  Eγ ( TeV )1 10Effective Collection area  ( m2 ) 101102103104105106Trigger aloneSupercutsDynamic Supercuts (DSC)θ dependent  DSC(b)    Zen.  angle  ( θ ) = 250 (c)    Zen.  angle  ( θ ) = 350 (d)    Zen.  angle  ( θ ) = 450 Figure 1. Effective collection area as a function of the primary 	 -ray energy for the three 	 -ray selection methodologies atdifferent zenith angles : (a) & =15 ' (b) & =25 ' (c) & =35 ' and (d) & =45 ' . The corresponding effective area when no cuts areapplied to the data is also shown here for comparison.(signal+background) and OFF-source (background) events, the expression for T  !$# is given by( !$#*),+%- ./10325476082:9<;>=@?BA=0DC3EGFHIJEGFH7KLFNMNOQPM.P R/02B4S602:9<;>=UT@V=0WCﬂYX ZEGFHIﬂEGFH7KLFE/[0325476082:9<;>=@?5A=0DC8EGF\H]8EGFH7KLF\H3^(1)where dN  /dE = 2.83  10 _J`a` (E/1TeV) _3  cm _8 s _J` TeV _:` is the differential energy spectrum of the CrabNebula, A  (E) is the effective collection area for   -rays in m  ,P(=4) is the ratio of OFF-region (  =18-90) to ON-region (  =0-18), dJ ﬂ /dE=1.10  10 _8b (E/1TeV) _8 b cm _3 s _J` sr _:` TeV _:` is the differentialenergy spectrum of the protons, A ﬂYX Z (E) is the product of effective collection area and the solid angle forprotons in m  sr. The calculations presented in this paper have been done by using the following values: N % =5,E  cd =20TeV and varying E  !<# (i.e. E  !$# eFfGgih@jIkIgIl=(  ) so that possible inaccuracies resulting from thesharp dependence of the effective collection areas for E mnF fGgh@jIkIg7l=EopH ) can be avoided. Variation of L andT  "!$# as a function of zenith angle for the three   -ray selection methodologies is shown in Fig.2.The reason for having a relatively low   (e.g.  0.27 for SC and  0.60 for DSC at  =25as against themostly quoted values of  0.40 and  0.80 for the two cases, respectively) is that apart from being zenith angledependent, this factor also depends on the pre-filtering procedure ( e.g., like removal of small SIZE events orevents with too large or too small DISTANCE parameter etc.,). Since no such pre-filtering conditions havebeen imposed here, computing L directly on the basis of number of triggered events will naturally make thisparameter relatively small. Referring back to Fig.2b, it is obvious that once a   -ray source has been detected242 M.K.Koul and A.K.Tickoo(a)Zenith  angle (deg)10 20 30 40 50Gamma-ray  retention  factor,  ηη γγ0.10.20.30.40.50.60.70.8Supercuts  (SC)Dynamic Supercuts  (DSC)Zenith angle dependent Dynamic Supercuts  (ZDSC)Zenith  angle (deg)10 20 30 40 50 T min  (hrs)020406080100120140160SCDSC ZDSC(b)                          Quality   FactorZen. ang   (deg)            SC        DSC      ZDSC           15                9.9         5.2          5.6   25              11.1         4.9          6.5   35                9.0         4.4          6.4    45                7.1         3.9          6.3 Figure 2. (a) Variation of 	 -ray acceptance ( qﬃr ) as a function of the zenith angle. (b) Variation of T s[tvu (dened as theminimum observation time required to detect a 5 w*	 -ray signal from the Crab Nubula) as a function of the zenith angle.The corresponding Quality Factor values are also tabulated in the gure for comparison.by using the SC methodology, using the ZDSC methodology as against the DSC method for determining theenergy spectrum saves significantly on the observation time at a marginal additional loss of about 5%   -rayevents.4. ConclusionsThe results of the simulation studies indicate that the zenith angle dependent Dynamic Supercuts (ZDSC)method provides the best combination of   and T  !$# as compared to that of the Supercuts (SC) and DynamicSupercuts (DSC) methods. The effectiveness of this method is however being evaluated by applying it to theactual data collected by the TACTIC telescope.References[1] A. M. Hillas., 18th ICRC, La Jolla (1985) 3, 445.[2] M. Punch et al., 22nd ICRC, Dublin (1991) 1, 464.[3] G.Mohanty et al., Astropart. Phys., 9, 15, (1998).[4] K.Kosack et al., Ap. J, 608, L97 (2004).[5] D.Heck et al., FZKA 6019 Forschungszentrum Karlsruhe, (1998).

Monte Carlo studies for improving the sensitivity of the TACTIC telescope by using zenith angle dependent Dynamic Supercuts

Abstract

Similar works

Full text

Available Versions

CERN Document Server