Bretz, T

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29th International Cosmic Ray Conference Pune (2005) 4, 315318Standard Analysis for the MAGIC TelescopeT. Bretz and the MAGIC collaboration(a) Institut f¤ur theoretische Physik und Astrophysik, Universit¤at W¤urzburg, W¤urzburg, Germany(b) Updated collaborator list at: http://magic.mppmu.mpg.de/collaboration/members/index.htmlPresenter: Markus Meyer(tbretz@astro.uni-wuerzburg.de), ger-bretz-T-abs2-og23-posterLarge amounts of data are delivered every observing night by the MAGIC Telescope. New analysis methods areneeded to achieve lower energy thresholds and to discover new sources. To allow for a timely implementationof these new methods or changes in the instrumentation a stable and flexible analysis is fundamental.The event-based analysis package MARS (MAGIC Analysis and Reconstruction Software) offers a suitableframework for such a challenging analysis task. The modular concept provides the possibility to develope andimplement rather easily new methods.As a robust standard analysis MARS is applied to all MAGIC data. Results obtained with this procedure willbe shown.Introduction The MAGIC telescope has been designed in order to be able to detect faint air showers initiatedby cosmic gamma rays with energies below 100 GeV, implying a high trigger rate (currently  Hz). Thetriggered events are digitized by a 300 MHz FADC system. Therefore a large data stream must be recorded andarchived for the analysis. The signal charge and arrival time is calculated from the recorded samples, see [12].Owing to the large fluctuations of low-energy showers, geomagnetic effects, as well as additional backgroundfrom electrons and muons, gamma-/background-separation at low energies is the most challenging task inCherenkov astronomy.In order to facilitate a timely conventional analysis, while at the same time allowing for the testing and inclusionof novel analysis tools, a software project was started some time ago coined STANDARD ANALYSIS. Due to thelarge amounts of data obtained, stability and automation of the analysis chain were given the highest priority[3, 5]. Therefore the implemeted algorithms have to be as robust as possible, e.g. the number of free parametersin cutting distributions must be kept minimal. Other design goals were simple maintainance and flexibility inorder to keep pace with the ongoing development of new methods for background suppression. The event-based analysis package MARS 1 has proven to be a suitable solution for this challenging project. The packageis based on a modular concept and internal coherence [1, 2, 3]. The output of the standard analysis serves as areference for more specialized analyses on individual data sets.Significant progress has been made implementing the complete analysis chain. Running the standard analysisdelivers a set of plots characterizing data quality, image parameter distributions, signal excesses, energy spectra,etc., i.e. sufficient information to allow for a comprehensive first assessment of the scientific content of thedata. In the case of tentatively interesting results from standard analysis, more advanced and specializedinvestigations can be launched, all attachable to the MARS framework.In the following the main steps relevant for standard analysis are discussed and an example output is presented.Quality assurance Before searching for weak signals, quality assurance is essential, because there could befake events in the data, e.g. due to uncontrolled noises. Moreover, malfunctioning pixels could destroy muchof the information contained in the shower image. Firstly, some rather simple precuts are made which removenon-gamma like events, but which do not remove any of the true gamma events. Secondly, a muon analysis[4] is run automatically for all data, providing a continuous control over the optical point-spread-function andthe light calibration of the photomultipliers (PMT). Furthermore, the muon rate can be measured. Due to theMAGIC Analysis and Reconstruction Software316 T. Bretz et alwell-known muon flux, the actual muon rate provides a useful monitor of the quality of the observation night(e.g., the weather conditions). The muon data might also be used to correct the measured gamma ray fluxes forthe effect of changing external conditions which is most important in lightcurve studies of variable sources.Bad pixel treatment Bright stars in the field-of-view can alter the response of some pixels in the PMTcamera. An active gain control levels down the currents of the illuminated pixels, the current fluctuations,however, will increase due to bright stars. Owing to the ALT-AZ mount of the MAGIC telescope, the images ofstars move across the camera during tracking and thus the fluctuations affect different pixels. Pixels affectedby fluctuations larger than five standard deviations from the mean fluctuation and other bad pixels (defectpixels found during calibration, missing central pixel, etc.) are treated by substituting their values with a valueobtained from interpolating the surrounding pixels. Typically less than 	 of all pixels are affected.Image cleaning In addition to the treatment of bad pixels it is necessary to remove the noise in the camerabefore calculating image parameters from statistics of the intensity distribution. In order to avoid an influenceof the noise level (when removing noise relative to the noise level of single pixels) on the shape of the resultingcleaned image, an absolute image cleaning is applied. This algorithm first searches for all pixels with a signalhigher than 8.5 phe  . In addition, all pixels above 4 phe  neighbouring a pixel with 8.5 phe  survive cleaning.Finally, all solitary pixels, which have survived after the first cleaning iteration, are removed.Image Parameters From the intensity distribution of the cleaned image, several image parameters are com-puted as described in [11]: center of gravity (COG  ) standard deviation along the major axis (Length  ) standard deviation along the minor axis (Width  ) distance of COG from the position of the target source in the camera (Dist  ) angle between the major axis of the shower image and the line between COG and the target sourceposition (Alpha,  ) total number of measured photo electrons in the image (Size  ) third moment of the distribution along the major axis (  )In addition to these standard parameters a lot of quality paramters are computed, too, like for example thenumber of islands2 surviving image cleaning.The parameters mentioned above are calculated as follows: Be ﬀ the positions of pixels with signal ﬁ  . ﬂ isthe number of pixels which have survived image cleaning. ﬃ position of source on camera (events with ﬂ !are skipped).Size: #"%$ '&( ﬁ  (1))*,+"%-.&/(ﬁ102/4302 with 2+"56879 and 9+"$.&/(ﬁ:+");0< with);">= ?A@BDC BFEHGIC7B8EJGKCL?M@BDCONand <+"PQR7ﬃ+"S.TU'V WYXTU.V Z\[ with T +");02]Islands are isolated clusters of pixels connected togetherStandard Analysis for the MAGIC Telescope 317Width and Length: R"_^ `ba8adc e.fhgAikj lMm `onFnpe.fhgAikjmrqFsc t1"u^ `vnwn\c e.f8gxijmylzm`bahape.fhgxikjmrqFsc t(2)Dist and Alpha: {"u|'< |  "~}\?AB8EHGS:Z[ (3)Third moment++"|k5IW|0 sgn SF oW [ 0 sgn S :xWk  [ with 5 +"9{'Ł{c6t(4)Gamma-/Hadron-Separation For the standard analysis of MAGIC data cuts in the distributions of imageparameters were searched, which are most robust w.r.t. possible variation of the external environment (weatherconditions, night-sky-background, etc.) and which can be universally used for comparative analyses and quick-look analysis. A wide range of possible cuts optimized for these purposes were investigated systematically [9].Energy estimation The energy of the primary gamma rays can be estimated from the correlation betweenthe image parameters and Size. A detailed study has been done in [8]Effective observation time The effective observation time rF is defined as the time range, within whichthe recorded number of cosmics events  would be obtained under ideal conditions (without dead-time). w can be determined from the distribution of time differences P between successive cosmics events. Theexponential slope  of this distribution is the event rate for cosmics in the ideal case. If  is the total numberof recorded cosmics events,  w is obtained byﬀFP"In the case of a finite dead time y the distribution (for   1 ) is exponential with the same slope  , fitted ina region of  , which is not affected by the dead time.Effective collection area and correction coefficients The effective collection area  w S. [ has been cal-culated from the number of events Pﬃ produced in a well defined area vﬃ around the telescope and the numberof events surviving the cuts  . The correction coefficients S. w [ accounting for the asymmetric spill-overdue to a steep source spectrum are calculated from the number of events  S. K¡ [ before energy estimation andafter ¢S.vw [ energy estimation.vw/S.["ﬃ0¢S4[£ﬃS.[S4ow["¢S.¤¡[¢S.w[Spectrum calculation The energy spectrum can now be calculated as:¢S.¤¡["W¡S.w¥[0S4w[bw0ﬀFTo make sure that the resulting spectrum is consistent with the originally produced Monte Carlo [10] spectrum,the result is fed into the algorithm and the Monte Carlo events are weighted (assuming that the Monte Carloproduction and integration range is from   H( to  K¦ W , the spectral index 2.6 and the new spectral index  )by weights  . This procedure is followed iteratively until the weighted spectrum and the resulting one agree.In this case also a good agreement between the size distribution of Monte Carlo gammas and Excess events isexpected and can be checked. Therefor the distribution of Monte Carlo gammas is scaled by a factor  :R"9§¨©Mª «x§¬0¨¬/©Aª «­"~® ¯°y±a¯°²Ł¬ﬃ01wﬃThe resulting distribution is equivalent to a Monte Carlo distribution measured in the observation time  Ffrom a source spectrum with spectral index 7  .318 T. Bretz et al]2 [deg2ϑ0 0.1 0.2 0.3 0.4 0.5 0.6Counts01002003004005006007008009001000preliminary, 251 excess events)σ7.7h observation of 1ES1218+603 with MAGIC (3.9Figure 1. ³]plot for 7.7 h of 1ES1218+603 data at zenith angles ´¢µM¶· after cuts as described in [9]. The off-data used isshown gray shaded, it was scaled by the inetgral between 0.2 ¸\¹zº]and 0.5 ¸\¹zº]with a factor 1.13. The Li/Ma-signicanceand the number of excess events was calculated from the data below 0.045 ¸\¹»º](marked by a gray vertical line). Theon-data is shown with full dots.An exemplary results The procedure above is run automatically on all MAGIC data taken as described in[5]. One promising result delivered by this procedure is a marginal detection of 1ES1218+304 [6] as shown infigure 1. Further investigations on the data quality have to be done manually now to ensure that the signal seenis not due to systematic errors or the external environment.Conclusion The concept of the event-based framework MARS featuring the discussed robust StandardAnalysis has been shown to fulfill the experimental requirements. Applied to more than 12 TB correspondingto 600 h of data it has proven to be stable and effective descovering new sources from the data.Acknowledgements We acknowledge the support of the german BMBF (05 CMOMG1/3).References[1] T. Bretz et al., Proceeding of the 28 .¼ ICRC, Tsukuba, Japan (2003)[2] T. Bretz, AIP Conference Proceedings 745, p. 730[3] T. Bretz and D. Dorner, Palaiseau, France, April 27-29, 2005[4] F. Goebel et al., these Proceedings.[5] D. Dorner et al., these Proceedings.[6] M. Meyer et al., these Proceedings.[7] Lessard, R. W., et al., Astopart. Phys., 15, 1-18, 2001[8] K. Berger et al., these Proceedings.[9] B. Riegel, T. Bretz et al., these Proceedings.[10] P. Majumdar et al., these Proceedings.[11] Hillas A. 1985. In Proc. 19 .¼ ICRC (La Jolla), Vol. 3, p. 445[12] H. Bartko et al., astro-ph/0506459

Standard Analysis for the MAGIC Telescope

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