Bretz, T

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29th International Cosmic Ray Conference Pune (2005) 4, 311314Comparison of On-Off and Wobble mode observations for MAGICT.Bretz , D.Dorner , B.Riegel , D.Ho¨hne and K.Berger for 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-abs1-og23-oralOne of the main goals of the MAGIC telescope is to reduce the energy threshold accessible to the imagingCherenkov technique. Due to the fact that the background suppression becomes increasingly difficult for lowerenergies it is important to reduce the possible systematic error of the background determination. An observationmode in which the source is not in the center of the camera allows to have a symmetrical sky position, for whichthe background can be determined. This ensures having similar systematics like nearly the same zenith angleand the same weather conditions. A comparison with the classical On-Off observation will be presented.Introduction When observing a gamma ray source candidate with an IACT, standard observations are donesuch that the source is located in the center of the camera. To determine the background behind the source anindependent sky region, under observation conditions as similar as possible, is observed or the background isextrapolated from an off-source region in the same measurement (eg. the region between	and	 in 1, right). In this case having the source in the most privileged position in the camera thereis the possibility of having a fake excess of events being a feature of the background rather than a gammaray source. Instead of observing the source in the center of the camera, the telescope can be pointed to a skyposition slightly off-source (for the MAGIC telescope typically ). The background can then be extractedfrom a so called anti-source position symmetrical w.r.t the camera center. Due to the ALT-AZ mount of thetelescope source and anti-source will rotate around the center of the camera which might smear out systematicsdue to camera defects. The distance between them is chosen such that the background measurement is notinfluenced too much by the source itself. This observation mode is called Wobble Mode because several off-axis positions around the source are observed in an alternating fashion. Typically two position symmetric w.r.tthe source are convenient making source and anti-source more equivalent (eg. zenith angle). It was extensivelyand successfully used by the HEGRA telescopes and is currently used by the H.E.S.S. telescopes.In this paper a comparison of observations made in wobble mode and in the on-off mode is presented.On-Off observation mode In on-off observation mode the source is located in the camera center. Forbackground determination a similar sky region must be observed. Because it cannot be done with the sameinstrument at the same time, it must be done under different conditions (weather, night-sky-background, etc.).Thus scaling of the background measurement is necessary to achieve an agreement of the background levelsof the on- and off-source observations. This scaling factor can only be determined by comparing parts ofthe data not influenced by the signal, eg. events which have their origin not at the source position. Havinggood statistics this scaling factor can be determined accurately enough to get plausible results, however stillintroducing an additional systematic error. Having detected a source, studies on its spectrum are essentialto extract the interesting physics about the source. Therefore the data sample is divided into subsamples ofdifferent energy as determined by an energy estimation algorithm [3]. To be able to calculate the gamma rayflux for each of these energy bins independently it is necessary, that the scaling of the background in all binsproduces reasonable results. Having slightly different conditions for signal and background measurements,e.g. a little higher humidity or the changed star light at another sky region, the scale factor for all energy binsmight not be unique anymore.Furthermore the construction of MAGIC’s camera makes the center of the camera a privileged position. Dueto the gap between two outer sectors of the camera (a sketch is shown in 3) and because only the fine-pixel partof the camera triggers the readout, events are aligned towards the center.312 T.Bretz et alWobble observation mode In wobble observations a position symmetric w.r.t. the camera center for thesource is available. This anti-source position is used to determine the background to be subtracted fromthe signal. This offers the possibility of continuous monitoring of a source without having to interrupt theobservations for taking off data. For reasons of symmetry almost identical conditions for both, signal- andbackground-observation, are available. Consequently in the first order no scaling has to be applied to the back-ground measurement. Due to inhomogeneities (eg. single broken PMTs) in the camera acceptance it might benecessary to apply a small correction. The rotation of the source around the camera center smears out possibleinhomogeneities and further decreases a necessary small correction factor. It also improves the quality of thebackground measurement because it does not correspond to a unique sky position anymore. However, there]2 [deg2ϑ0 0.1 0.2 0.3 0.4 0.5 0.6Counts0102030405060708090]2 [deg2ϑ0 0.1 0.2 0.3 0.4 0.5 0.6Counts020406080100120140160180Figure 1. . Signal- (black dots) and background (gray shaded) as taken from wobble mode Crab observations (loose cuts)and without applying any scaling. Left: low size bin (around ﬀﬂﬁﬂﬃ "! ). Right: medium size bin (around #ﬂﬁﬂﬁﬂﬃ "! ).is some danger that the background measurement is not statistically independent from the signal of a strongsource. Therefore a special anti-source cut has to be applied, i.e. any event analyses can not be assigned to bothpositions. Already simple technique (classical $ method) suppress this bias well. More advanced analysis’,assigning a single source position to each shower image like Whipple’s Disp-method [5], suppress this biasalmost completely. This is done by applying source-position dependent cuts, also as a veto to the anti-sourceposition and vice versa. Figure 1 shows asignal- and background-plot [2] taken from wobble mode Crabobservations without applying any scaling. Figure 2 shows the size distribution (solid line) of Monte Carlogamma events assigned to the source position by surviving such cuts (plus gamma-/background-separationcuts) and the distribution (dashed line) of the same events which were wrongly assigned to the anti-sourceposition only. It can be seen that the there are less than %'& wrongly assigned events in the lowest size binswhich is negligible compared to statistical errors. For lower sizes this effect increases further, but it can besuppressed by more sophisticated cuts or using the Monte Carlo distribution for correction.Discussion An example for a measurement can be seen in 1. For simplicity the binning was not chosenin estimated energy but in size, the total measured intensity of the shower image (which has an almost lin-ear correlation to the energy of the primary particle). The plots show the(-signal (black dots) of a wobblemode measurement of Crab and the measured background (signal of the anti-source). It can be seen thatboth measurements fit quite well and only small scaling corrections might be necessary, introducing onlya small additional systematic error. Furthermore the scaling factor also depends on the way of its deter-mination (eg. unifying the integral between	)*+ ,and-.*/0), introducing another smallsystematic error. Extrapolating an accurate background from the large-region into the low-region, inthe low-energy case as shown in the plot, seems problematic. Different functions may fit the backgroundwell at largeor $ , while giving quite different predictions for the background in the signal region. well.Having identical conditions for signal and background measurement in wobble mode means also an impor-tant reduction in the work necessary to ensure the compatibility of both measurements, eg. making sure thatComparison of On-Off and Wobble mode observations for MAGIC 313Size [phe]210 310 410Counts110210310Distribution of Size (number of photoelectrons)Figure 2. Size distribution obtained from Monte Carlo [4]gamma ray events surviving the cuts assigned to the sourceposition (solid) and the anti-source position (dashed).the image parameter distributions fit well which isinfluenced by environmental conditions like weather,zenith angle, etc. Because the compatibility of signaland background measurement is given by constructionthe results are more robust.Comparison To avoid systematic errors in on-offobservation mode due to wrong scaling, as explainedearlier, a background measurement must be done underconditions as similar as possible to the ones taking on-data. Therefore any on-measurement must be followedby a prompt off-measurement. To get enough statis-tics it is essential to take roughly the same amount ofoff- and on-data. On the other hand wobble mode ob-servations with the MAGIC telescope result in a lossof sensitivity of less than& . This is due to moreshowers not contained in the trigger area (mainly theinner part of the camera) anymore or leaking out of thecamera due their size. Figure 3 shows a comparison ofthe distribution of the center-of-gravity of Monte Carlogammas in the camera after cuts f or on-off and wobbleobservation mode. For this study the position informa-tion of the shower in the camera has not been used, it might further improve the sensitivity in wobble mode.To compare both observation modes, data of the Crab Nebula has been taken at similar zenith angles in bothFigure 3. Distribution of center-of-gravity of the intensity distribution of Monte Carlo [4] gamma showers. Left: Thesource centered in the camera. Right: The source 0.4 1 off center. The loss into the coarse pixel region of the camera inwobble-mode is small, while there are also showers on the left side which now are fully contained in the camera or the nepixel region.314 T.Bretz et alTable 1. Top: Information about the observations and results. The off-data duration only differs from on-data in the orderof seconds. Bottom: Information derived from the measurement shown.Significance BackgroundObservation Mode Data Period Zd range Observation Time (Li/Ma) Scale FactorOn/Off Sep’04 - Jan’05 5.6-23.7(On) 14837s 4.12h 30.5 0.94Wobble Sep’04 - Jan’05 5.6-17.213642s 3.79h 22.3 1.00Excess Background Excess Background SignificanceObservation Mode Threshold Events Rate per On-TimeOn/Off 205-236GeV 2233 1727 9.0evt/min 7.0evt/min 15.2 243 5Wobble 205-236GeV 1621 1864 7.1evt/min 8.2evt/min 11.5 2 3 5modes. The analysis is explained in more details in [2] and [1]. The data analyzed has gone though a strictselection (w.r.t. weather conditions and apparatus performance). Robust and conservative cuts as [2] have beenused for both datasets. They were trained dividing the samples each in two subsamples. Gamma-/background-separation has been optimized for maximum significance for both subsamples independently. This, however,raises the analysis threshold, but gives the most reliable output for the a study like this. After being convincedthat both subsamples gave similar cut coefficients the full sample was analyzed with an artificial compromisebetween the two sets of cut coefficients. The energy threshold has been determined as the maximum bin in adN/dE distribution after cuts of MC gammas produced with a spectral index of -2.6. Table 1 (top) contains theinformation about the data analyzed for this study and the results. Table 1 (bottom) contains derived valueswhich are more useful for a comparison of both modes.Conclusions When comparing the on-off and wobble observation modes in the framework of the MAGICtelescope, three aspects deserve particular attention: 1) loss of sensitivity for off-axis observations in the smallcamera, 2) efficiency of gamma-/background-separation at low energies and 3) the possibility of continuousmonitoring of the source on wobble mode. It has been shown that the decrease in sensitivity for wobbleobservations employing a 0.4offset from the camera center is compensated by the possibility of observingthe source continuously, assuming that in on-off mode half of the time is devoted to the off observations. Thiscan be compensated by defining more than one statistical independent off-region in the same distance to thecamera center. The necessity of having non-overlapping on and off regions on the camera makes it difficult touse the wobble mode described here for extended sources ( 67%). The analysis, specially at low energies,benefits from the precision gained in the background determination in wobble observations. If the backgroundis determined from off observations, scaling and uncontrollable statistical and systematic effects (e.g. due tovarying night sky conditions) render the reproducibility of the measurements more difficult, implying generallylarger errors of the background estimate in the signal region.Acknowledgments We acknowledge the support of the german BMBF (05 CMOMG1/3).References[1] T. Bretz, Standard Analysis for the MAGIC telescope. These Proceedings.[2] B. Riegel, T. Bretz, A systematic study of the interdependence of image parameter. These Proceedings.[3] K. Berger, these Proceedings.[4] P. Majumdar et al., these Proceedings.[5] Lessard, R. W., et al., Astopart. Phys., 15, 1-18, 2001

Comparison of On-Off and Wobble mode observations for MAGIC

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