Bastieri, D

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29th International Cosmic Ray Conference Pune (2005) 4, 435438Search for Gamma-Ray Bursts with the MAGIC TelescopeD. Bastieri , N. Galante, M. Garczarczyk, M. Gaugand S. Mizobuchion behalf of the MAGIC collaboration(a) Max-Planck-Institut f¤ur Physik, F¤ohringer Ring 6, 80805 M¤unchen, Germany(b) Universita and INFN  Padova, Via Marzolo 8, 35131 Padova, Italy(c) Universita di Siena and INFN  Pisa, Via Buonarroti 2, 56127 Pisa, Italy(d) IFAE, Edici Cn. Facultat Ciencies UAB, 08193 Bellaterra (Barcelona), SpainPresenter: M. Garczarczyk (garcz@mppmu.mpg.de), ger-garczarczyk-M-abs1-og24-posterThe MAGIC Telescope, now taking data with a trigger threshold of 	 , is able to take full advantage ofits fast slewing capability to respond to Gamma-Ray Burst alarms. With the low energy threshold and maximalorientation time to any sky position within a few tens of seconds, MAGIC could be one of the first ground-based experiments to detect prompt emission of Gamma-Ray Bursts (GRB) in the few tens of GeV energydomain. Besides giving an overview of the expected performance, we report about the response to some recentGRB alarms.1. IntroductionGRBs are still, in many aspects, enigmatic cosmic objects. The upper end of their spectra has not been mea-sured so far. In one case (GRB940217) delayed GeV emission has been observed. It is therefore a challangeto search for possible  emission in the GeV domain. Besides fast flaring AGNs, GRBs might be the bestcandidates for testing quantum gravity effects. Such a study would benefit very much from data in the GeVrange. Past ground based experiments were not sensitive since their thresholds were well above the predicted -energy cut-off due to absorption by the extragalactic background light (  -ray horizon).The MAGIC Telescope is a new generation, high performance imaging air Cherenkov telescope (IACT). Itwas commissioned in 2004 and has already observed a few very high energy (VHE) gamma-ray (  ) emittingsources such as the well established sources CRAB, Mkn 421, Mkn501, 1ES1959. It has also seen evidence ofa few new sources [1]. The telescope has been built to explore the  -spectrum starting well below 	 .As many novel elements have been used in the construction, it will still take some time to fully understand thetelescope performance and to reach the design goal of a 	 threshold. The superstructure of the telescopewas built mainly from Carbon Fiber (CF) tubes and some aluminium components in order to reduce weightand inertia in order to eventually allow slewing to any new position in the sky within less than 20 seconds.Contributing to the low weight are novel lightweight all-aluminium mirrors. For the compensation of theresidual deformation of the CF mirror support frame an Active Mirror Control system (AMC) [2] is being used.These features, together with the low energy threshold, give MAGIC the potential to follow up GRBs veryshortly after a fast satellite alert. One of MAGIC’s dedicated goals is the observation of GRBs early after thestart of prompt emission into the early afterglow stadium, where some past observations have shown delayedVHE  emission. Further technical details of MAGIC can be found in [3]MAGIC is part of the GRB Coordinates network (GCN) [4], made up of satellites that can recognise GRBevents with their on-board detectors and can quickly react and send the celestial coordinates of presumed GRBevents. GCN takes care of broadcasting the data to ground instruments like many optical- and radio- telescopesas well as MAGIC.In this work we make quantitative predictions for GRB observation with MAGIC and present results from thefirst tests made with alerts sent by SWIFT and HETE-2.436 D. Bastieri et al2. The Probability to observe GRBsThe main parameter influencing the chances to observe GRBs is the telescope duty-cycle [5]. The duty-cycleof the telescope was calculated as the fraction of the sky area accessible to MAGIC per year. The followingconditions had to be met:1. the Sun must be at zenith angle  ;2. the minimum angular distance from the Moon must be ﬀ ;3. the night sky must be clear and the relative humidity of the air must be below 80 %;4. the wind speed must be below ﬂﬁﬃ .Using the weather data and ephemeredes for the year 2001, we estimate a chance to observe a GRB slightlybelow 10%. Provided that a satellite GRB alarm is received, roughly !!#"of all GRB alerts can, in principle,be pursued. From this fraction of tagged GRBs it is possible to estimate how many GRBs might be detected,i.e., that their detection significance will exceed $ . The number of $ can be calculated in first order using theLi-Ma formula: %'&)(*,+.-0/21435-0/7698;:<+=->/210?@-0/A6B8#:(C* ->DﬀE :E- /21 ?F- /A6(1)where*, the so called quality factor, which represents the  -hadron separation efficiency of the analysis is oforder unity at energies close to the threshold.:is the actual acquisition time and-HGare the actual rates. Each-takes into account the particle flux I and the detector sensitivity to the flux itself, expressed as the effective areaof the telescope, J>K6+.LNM7O98. For instance, the expected rate from the GRB is:-D(CPIRQBSUT+=LH8JDK6+=LVM7O98#WXLwhile the cosmic-ray background is:-/76(PIRYXS+=LVM7O98JYZSK6+.LNM7O98#WZL. The effective area of MAGIC closeto Zenith reaches [\]ﬁV^ around 	 and drops to [_BﬁV^ around 	 .The expected VHE GRB flux is derived from the typical power-law spectrum found for GRBs in the BATSEcatalogues, but taking also into account the prediction of the cosmological cut-off due to the absorption ofhigh-energy  ’s by the Metagalactic Radiation Field via the process X`]aUbcSUdfeghij [6]. This estimate alsorequires a simple model for the GRB distribution as a function of the redshift.Of paramount importance for the detection of GRBs is the delay between the GRB onset and the time whenMAGIC can actually start to observe the GRB. This delay sums up from three different times:1. The time needed by the satellite to sum up significant number of  -induced events from a GRB.2. The time elapsing between analysing the events to find the sky coordinates and sending the alert, thebroadcast via GCN and the final acknowledgement by MAGIC. The latter three actions take lk seconds.3. The slewing time needed by MAGIC to point to the GRB position, focussing the mirrors and starting theobservation.From our studies we found that a quick response of a few seconds is very important. For example, a reductionof the response time from 1 minute to 15 seconds increases the number of promptly observable GRBs by afactor five.It should be noted that the zenith angleOenters critically in the estimate of the rate of observable GRBs. Thenumber of observable GRBs, e.g., is not obtained simply by a straightforward multiplication of the duty-cycle+nm[po8by the number of GRBs expected to have a significance%V&q , but is better estimated by:%'rtsBuF(vwyx+zO98;{B+.OB8(2)Search for Gamma-Ray Bursts with the MAGIC Telescope 437wherex+.O98is the duty-cycle in a given bin of zenith angle and{B+zO98is the number of GRBs (extrapolatedfrom BATSE catalogues) actually observable in that given bin of zenith angle. The sum is performed over allzenith angle bins. For this calculation we neglected the influence of the rising threshold as a function of zenithangle.Values of{B+.OB8range between 5 and 24, strongly depending on the delays from the GRB onset to the actualobservation of the GRB by MAGIC. The final result of this analysis is that MAGIC should be able to observeX| ﬂ}~k prompt GRBs per year. Here we define ”prompt” such that MAGIC will start observing while the GRBis still fully active in the X-ray domain.3. Telescope PerformanceCurrently, MAGIC is still in an experimental phase, optimizing the operation for GRB searches. While ini-tially it could be proven that the telescope fulfils the slewing requirements for fast repositioning, we finallyused a conservative step-by-step improvement approach to fully automise the operation, starting with a ratherconservative slow slewing.In April 2005, the Burst Alarm System Gspot (Gamma Sources Pointing Trigger) has been included into thecontrol software of the MAGIC telescope. It receives the alerts from GCN and selects candidates which fulfilthe following criteria:1. the Sun has to be below the astronomical horizon2. the zenith angle of the GRB candidate has to be less than Z3. the angular distance between the GRB position and the Moon has to be larger than ZIf the above criteria are fulfilled within at most five hours after the GRB onset and the telescope is in observa-tion phase, it automatically moves to the target position and starts the observation. With such a conservativeapproach we can also catch a possible afterglow of a GRB that has occurred during late afternoon.A coarse estimate of the expected observation time of GRBs can be derived from an estimated number of GRBfollow-up observations, which can be expressed in the following formula:%'2nR(C%';7ŁxiA;7A(3)where%)2nis the mean number of observed bursts,%;7Łthe mean number of alerts,xthe duty cycle(including the reduction of sky coverage due to the maximum allowed zenith angle) andA;7Aa reductionfactor due to the non-overlapping sky coverage between the satellites and MAGIC.The claimed GRB alert frequency%2nfrom SWIFT is about 150-200 GRBs/year. With the duty-cycle of 10%and a slightly optimistic overlap factorﬂA;7Aof 0.5, the expected number of GRB responses of MAGIC isin the order of 0.6 to 1.6 per month.During the three months after the implementation of Gspot, improvements of the telescope performance werecarried out. Especially the precision of fast repositioning was carefully tested. During this time a total numberof 24 (SWIFT) + 15 (HETE-2) + 6 (INTEGRAL) = 45 alerts with useful GRB coordinates were received fromGspot. Out of this sample the following GRBs have been observed by MAGIC:438 D. Bastieri et alNr. GRB Satellite Time burst onset '#7Ł'2n#;2ŁGA >rsBuredshift1. GRB050408 HETE 16:22:50 UTC 14 sec 3138 sec ﬀ 1.23572. GRB050421 SWIFT 04:11:52 UTC 58 sec 112 sec k3. GRB050502 SWIFT 02:14:18 UTC 18 sec 990 sec ﬀ 3.7934. GRB050505 SWIFT 23:22:21 UTC 540 sec 793 sec ﬀ 4.275. GRB050509A SWIFT 01:46:29 UTC 16 sec 115 sec p6. GRB050509B SWIFT 04:00:19 UTC 15 sec 368 sec ﬀ 0.2267. GRB050528 SWIFT 04:06:45 UTC 43 sec 77 sec kIt has to be pointed out that for these bursts the fast movement was not yet used.Since May 2005, the acceleration and deceleration speed of the telescope has been increased by a factor 2.6 andthe maximum allowed velocity has been raised by a factor of six. As an example we obtained a characteristicrepositioning in )J>(kp taking 63 seconds and for )J0( 22 seconds. There is still room toincrease the positioning speed by at least a factor two by doubling the acceleration, raising the maximum speedlimit and turning the telescope over the Zenith. These performance improvements are planned for this year.The Active Mirror Control of the MAGIC telescope plays an important role for the GRB observation. Asmentioned in the introduction, the AMC system corrects the small mirror dish deformations depending on thezenith angle. To avoid time losses, a focussing with Look-Up Tables was implemented such that the telescopecan be focussed to the GRB coordinates already during the repositioning. This procedure takes less than 10seconds.4. ConclusionsMAGIC is now operating and can be used to follow the GRB events shortly after their onset. Due to a cho-sen conservative approach to reduce the positioning speed step-by-step it will still take some months for theresponse time to be reduced to less than 20 seconds.The analysis of the eight up to now observed GRBs showed no significant signal. However, taking into accountthe measured redshifts and the threshold energies at the zenith angles at which MAGIC has observed the bursts,no afterglow VHE  rays are expected due to the limited gamma ray horizon [6].5. AcknowledgementWe would like to thank the IAC for excellent working conditions. The support of the German BMBF andMPG, the Italian INFN and the Spanish CICYT is gratefully acknowledged.References[1] Mirzoyan R. for the MAGIC Coll., these proceedings[2] Garczarczyk M. for the MAGIC Coll. in Proc. k.  ICRC, Japan 2003, p. 2935–2938.[3] Cortina J. for the MAGIC Coll., these proceedings[4] See http://gcn.gsfc.nasa.gov/ and references therein.[5] Galante N. for the MAGIC Coll. in Proc. k.  ICRC, Japan 2003, p. 2753–2756.[6] Kneiske T.M et al., A&A 413, 807, 2004

Search for Gamma-Ray Bursts with the MAGIC Telescope

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