Chitnis, V R

CERN Document Server

29th International Cosmic Ray Conference Pune (2005) 5, 235238Status of HAGAR, the High Altitude Gamma Ray Observatory atHanleV.R. Chitnis , B.S. Acharya , P.N. Bhat , R. Cowsik, T.P. Prabhu, R. Srinivasan,R. Srivatsanand P.R. Vishwanath(a) Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India(b) Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, IndiaPresenter: V.R. Chitnis (vchitnis@tifr.res.in), ind-chitnis-V-abs1-og27-oralA 7 telescope array is planned to be constructed at Hanle in the Himalayas at an altitude of about 4200m abovemean sea level for detecting celestial gamma-rays. The atmospheric Cerenkov light pool created by the primarygamma-rays is sampled through wave-front sampling technique to infer the direction and energy of the incidentgamma-rays. Monte Carlo simulation calculations were performed to understand the energy threshold andevent trigger rates. One telescope is fabricated and shifted to Hanle after extensive tests at CREST, Hosakote.The current schedule for the fabrication of remaining telescopes, experiment and the energy threshold will bediscussed.1. IntroductionGround based atmospheric Cˇerenkov technique has been used extensively for the study of VHE  -rays fromastronomical objects. Energy thresholds of most of these experiments are typically about few hundred GeVs.There is a strong scientific motivation for lowering energy thresholds of ground based experiments to few tensof GeV and have overlap with satellite based detectors. This will enable the study of spectral cutoffs in AGNspectra and will also allow addressing various issues regarding pulsed emission from pulsars [1]. Thus thereare several attempts to reduce energy thresholds of ground based experiments. Recently, based on stereoscopictechnique, HESS has been able to achieve an energy threshold of about 100 GeV [2, 3]. MAGIC experimentusing very large size light collector is expected to reach energy threshold of about 30 GeV [4, 5]. Similarlyexperiments like CELESTE, STACEE etc used large arrays of mirrors to achieve lower energy threshold.Alternatively it is possible to achieve lower energy threshold using modest size experiment carried out at highaltitudes [6, 7]. One such experiment HAGAR (High Altitude GAmma Ray experiment) is under constructionin Himalayas. This is a collaborative effort between Indian Institute of Astrophysics, Bangalore and TataInstitute of Fundamental Research, Mumbai.2. HAGAR instrument detailsHAGAR telescopes are being setup at Hanle ( 		  N,   E) at an altitude of  ﬀﬂﬁ amsl in thecold desert of Himalayas, which is a fine astronomical site. This array based on wavefront sampling techniquewill consist of 7 telescopes in the form of a hexagon with a spacing of 50 m and one telescope at the centreof the hexagon as shown in figure 1. Each telescope has seven para-axially mounted front coated mirrors ofdiameter 0.9 m with a Photonis phototube of type XP2268B at the focus of each mirror. f/d ratio for thesemirrors is  1. Figure 2 shows the first telescope of HAGAR installed at Hanle.Telescope structure is based on Alt-azimuth design. Each of the axes of the telescope is driven by a steppermotor through a chain of gears with the reduction ratio of about 3000:1 for azimuth drive and 3200:1 forelevation drive. The telescope movement control system comprises of two 17 bit Rotary encoders (Heidenhain,ROC 417), two stepper motors with motor drives (Slo-Syn make) besides the Microcontroller-based Motion236 V.R. Chitnis et al.control interface Units(MCIU). This control system has been developed to achieve the steady-state pointingaccuracy of the servo of ﬃ 10 arc-sec with the maximum slew rate of 30  /minute for each axis. The resultingblind-spot size while tracking the stars near zenith is  1.2  . The telescopes’ movement is maneuvered by thecontrol software developed under Linux. The detailed point-run calibration by sighting large number of starsis being carried out to establish pointing model of the telescope to improve pointing accuracy further.High voltages given to individual PMTs are controlled using C.A.E.N. controller model SY1527 so that PMTgains are constant. Pulses from individual PMTs are brought to the control room through coaxial cables of typeLMR-ultraflex-400. PC based data acquisition and recording system employs CAMAC based instrumentation.DAQ software is written in C under linux environment. Linux device drivers are developed to accomplishinterrupt driven DAQ system.Figure 1. Layout of 7 telescope HAGAR array Figure 2. First HAGAR telescope3. Expected performance of HAGARUsing CORSIKA package [8], Monte Carlo simulations of extensive air showers initiated by  -rays, electrons,protons and  particles incident vertically at the top of the atmosphere were carried out and the nature ofCˇerenkov light distribution at Hanle altitude was studied. The atmospheric attenuation of Cˇerenkov photonsat Hanle altitude is   % as compared to !" % at sea-level. The lateral distribution of Cˇerenkov photondensity for  -ray primaries indicates presence of a ”hump” at a core distance of about 90 m, due to effectivefocusing of Cˇerenkov photons from a range of altitudes [9]. The Cˇerenkov photon density near the shower coreat Hanle is higher by a factor of about 4-5 compared to that at the sea-level for showers of same energy. Thesefeatures effectively reduce the energy threshold of the experiment operated at higher altitudes compared to thesimilar array operated at lower altitudes.Taking into account various design details of HAGAR, energy thresholds and trigger rates are estimated forvarious PMT gains for  -rays and cosmic rays. Results are given in Table 1. Showers of different cosmic rayspecies were generated using appropriate spectral shapes. For generating  -ray showers the spectral shape ofCrab as obtained from Whipple data [10] was used. Figure 3 shows the expected differential  -ray count ratefrom Crab nebula for the PMT gain of 1.3 # 10 $ . The energy threshold, defined as the peak of the differentialtrigger rate distribution, is  	"&%(') for vertically incident showers. At larger zenith angles the thresholdenergy would be proportionately higher. Main advantage of this experiment is the energy threshold which isconsiderably lower than that at sea level and overlaps with the energy range of future satellite based detectors.Status of HAGAR ... 237Figure 3. Expected differential * -ray count rate spec-trum from Crab nebula.Figure 4. Sensitivity of HAGAR array( +-,. ,/1032 456ﬂ7 for 1 Crab)The expected rate of events for a trigger based on 4 out of 7 telescope coincidence logic for PMT gain of1.3 # 10 $ is about 55 Hz from protons, 16 Hz from  particles, 1.3 Hz from electrons and 44/minute from -rays from crab.Figure 4 shows the 5 8 sensitivity of HAGAR. Dotted line corresponds to the case without rejection of cosmicray showers apart from the rejection obtained from 4 out of 7 telescope trigger logic. This sensitivity canbe improved by rejecting off-axis cosmic ray showers and using several shower parameters based on densityfluctuations and timing jitter in tandem [11, 12]. It may be possible to reject about 98% of cosmic ray showersretaining about 35% of gamma ray showers. Sensitivity of HAGAR with this hadronic rejection is also shownin the figure as a sold line. This corresponds to the detection of Crab at 5 8 level within a duration of  25minutes.Table 1. Trigger rates and energy thresholds for * -rays and cosmic rays for different trigger conditionsThe corresponding energy thresholds are given in brackets.Trigger 9 4/7 6/7 4/7 6/7 4/7 6/7 4/7 6/7PMT : ; ; < < = = > >Gain?A@ BDCFEGBIH20.8/m - 31.9/m - 25.8 Hz - 8.3 Hz -EGBIBKJ<L -EIEGMNJ<OL - PIQBRJ<LEIEGSIBNJ<LEI@ SDCFEGBUT43.8/m 19.2/m 79.4/m 28.4/m 55.2 Hz 20.2 Hz 15.8 Hz 7.3 HzVIBRJ<OL QBKJ<L?IBKJ<LEIEGBRJ<L PMIBRJ<LSAEGBKJ<LVIBIBNJ<OLEIEGSIBRJ<OLP@ BDCFEGBUT WXEGSAE/m 48.7/mWPUYP /mWQ?mW[ZUBP Hz 53.3 HzWQMHz 16.8 Hz-MIBKJ<L - - -EQBUJ<L -VIBIBKJ<L238 V.R. Chitnis et al.4. Present status and timelineFirst telescope of HAGAR has been extensively tested at CREST, Hosakote near Bangalore and has beenshifted to Hanle in June 2005. Some preliminary tests have been carried out on this telescope at Hanle site.Second telescope is installed at CREST and is ready for tests. In about one and half year from now, HAGAR isexpected to be fully operational. Initial emphasis will be on observations of pulsars including crab and geminga.Pulsed emission has been detected from seven pulsars by EGRET below 10 GeV [13, 14, 15]. However, atenergies above few hundred GeVs only non-pulsed emission from nebula is detected. Based on HEGRA dataupper limit of 1-3% of unpulsed component has been quoted for pulsed component from Crab pulsar [16] in theenergy range of  500 GeV to 10 TeV. EGRET measurement gives indication of steepening of pulsed emissionfor some pulsars. So it will be interesting to study spectrum of pulsed component at the energies accessible toHAGAR. We hope to have simultaneous observations with GLAST for pulsars. Detection of pulsed emissionand its spectral cutoff in the energy range covered by GLAST and HAGAR would enable the differentiationbetween polar cap [17, 18] and outer gap [19, 20] models regarding the emission of  -rays.5. AcknowledgementsMany persons from T.I.F.R. and I.I.A. have contributed towards the design, fabrication and testing of tele-scopes. We thank all of them. We thank Prof. B.V. Sreekantan for encouragement from the very beginning ofthe project.References[1] R.A. Ong, Physics Reports 305, 93 (1998).[2] S. Funk et al., Astropart. Phys. 22, 285 (2004).[3] W. Benbow for the H.E.S.S. collaboration, 2nd Int. Symp. on High Energy Gamma Ray Astronomy,Heidelberg, 2004, APS Conf. Proc. 745, 611 (2005).[4] M. Martinez et al., proc. of 28th ICRC, Tsukuba, OG 2.5, 2815, (2003).[5] R.K. Bock for the MAGIC Collaboration, AIP Conference Proceedings 745, 628 (2005).[6] F. Aharonian et al., Astropart. Phys. 15, 335 (2001).[7] R. Cowsik et al., proc. of 27th ICRC, Hamburg, OG 2.05, 2769 (2001).[8] D. Heck et al., Forschungszentrum Karlsruhe Report, ZKA 6019 (1998).[9] B.S. Acharya et al., proc. of 28th ICRC, Tsukuba, 2999 (2003).[10] A.M. Hillas et al., ApJ 503, 744 (1998).[11] V.R. Chitnis and P.N.Bhat, Astropart. Phys. 15, 29 (2001).[12] V.R. Chitnis and P.N.Bhat, Exp. Astr. 13, 77 (2002).[13] P.L. Nolan et al., A&AS 120, 61 (1996).[14] D.J. Thompson et al., AIP conf. proc. 410, 39 (1997).[15] P.V. Ramanamurthy et al., ApJ 450, 791 (1995).[16] F. Aharonian et al., ApJ 614, 897 (2004).[17] A. Harding, ApJ 245, 267 (1981).[18] J.K. Daugherty and A.K. Harding, ApJ 252, 357 (1982).[19] K.S. Cheng, C. Ho and M.A. Ruderman, ApJ 300, 500 (1986).[20] R.W. Romani, ApJ 470, 469 (1996).

Status of HAGAR, the High Altitude Gamma Ray Observatory at Hanle

Abstract

Similar works

Full text

Available Versions

CERN Document Server