Oña-Wilhelmi, E

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29th International Cosmic Ray Conference Pune (2005) 4, 247250First pulsar observations with the MAGIC telescopeE. On˜a-Wilhelmi , R. de los Reyes, J.L. Contreras, C. Baixeras, J.A. Barrio,M. Camara, J. Cortina , O.C. de Jager, M.V. Fonseca, M. L  pez, I. Oyaand J.Rico for the MAGIC collaboration.(a) Institut de Fisica d’Altes Energies (IFAE), Universidad Autonoma de Barcelona, 08193, Bellaterra, Spain.(b) Dpto. Fisica Atomic, Nuclear y Molecular, Universidad Complutense de Madrid, 28040, Madrid, Spain(c) Dpto. Fisica, Universidad Autonoma de Barcelona, 08193, Bellaterra, Spain.(d) Space Research Unit, Northwest University, Potchefstroom 2520, South AfricaPresenter: E. On˜a-Wilhelmi (emma@ifae.es), spa-ona-wilhelmi-E-abs1-og22-oralA few regions of the sky containing pulsars have been observed by the MAGIC Telescope [2] during its com-missioning phase, namely PSR B1957+20, and PSR J0218+4232. In this work we report on the analysis ofthese data, looking for  -ray emissions both in continuous and pulsed mode. Constrains to different theoreticalmodels about  -ray emission from pulsars and plerions will be discussed.1. IntroductionDuring the first year of operation of the MAGIC telescope quite some time has been devoted to the Crabpulsar and a dedicated contribution is presented in this conference [16]. Beside Crab, for the current study, asmall subset of pulsar systems was observed and analyzed as part of an ongoing program to search for galacticsources of GeV emission. These sources were chosen based on their similarity to some other detected sourcesand on the prediction of emission given the various models of  -ray production in pulsars. A deeper analysishave been done with two of these sources, PSR B1957+20 and PSR J0218+4232.No evidence has been found of pulsed or unpulsed emission from the pulsars at high energies. These non-detections place limits on photon densities, magnetic field strengths, and emission regions in plerionic systems,since both pulsars are in binary systems. Both, PSR B1957+20 and PSR J0218+4232 are millisecond pulsarsin energetic binary systems, detected in X-rays. The X-ray emission may be due to the interaction of the pulsarwind with the companion. Those energetic systems are believed to be as well  -ray emitters, not only byinteraction between the two stars [15][7] but also, several models of millisecond pulsars [4] propose pulsed -ray emission from the pulsar inner magnetosphere up to  100 GeV. Different modelization of millisecondpulsars ([13] (and reference therein) and [6]) predict a high cutoff energy 	 in their spectra, at a  100 GeV,while for canonical pulsars this cutoff is expected to happen at a few GeV. These models are based in magneticfield strength and pair production considerations: although these pulsars have low surface magnetic fields(  ), their short periods allow them to have large magnetospheric potential drops, but the majority donot produce sufficient pairs to completely screen the accelerating electric field. Thus the spectra are very hardpower-laws with exponential cutoffs up to 100 GeV. On the other hand, ablation and heating of the companionstar are believed to be caused by X- or  -rays generated in a intrabinary shock between the pulsar wind andthe star. This process has been observed by HESS in the similar system PSR B1259-63. The observed flux of -rays, modulated within the orbit, were detected at 5% of the Crab emission.The observations of each of these pulsars were carried out in very different conditions and therefore it isnecessary to analyze them in different ways to extract a steady signal. The different procedures and observationconditions will be explained for each pulsar. In addition statistical tests were applied to the  -like events arrivaltimes to search for pulsed emission. The standard corrections were performed in arrival times and the ephemerisused were obtained from the ATNF database, summarized in Table1.248 E. Ona-Wilhelmi et al.Table 1. PSR J0218+4232  PSR B1957+20 ephemeris (ATNF pulsar database).PSR J0218+4232 PSR B1957+20Epoch (MMJD) 50864.00 48196.00Puls. Frequency (Hz) 430.4610663457 622.122030511Freq. Deriv. s/s -1.4340E-14 -6.5221E-15Puls. Period (s) 0.0023230904678309 0.00160740168480632Rot Period (days) 2.028846084 0.38196660692. Observations and Data Analysis2.1 PSR B1957+20PSR B1957+20 is one of the fastest pulsars known, with period P  1.6 ms and spin down luminosity ﬀﬁﬂﬃ erg  "!$#&% . The pulsar is in a 9.16-hour binary orbit with a low-mass companion star. Observationsof the pulsar PSR B1957+20 were carried out with MAGIC in October 2004 ( ')(+* - ,'-(+* ) for 6 hours effectiveON time, at low zenith angle ( . 30  ). The observations were done in a standard ON-OFF mode, pointing thetelescope to the pulsar position.The total data set has been analyzed to look for emission in both steady and pulsed modes. Only high qualitydata were considered for the analysis. After calibration and image cleaning, Cherenkov showers were recon-structed using the standard Hillas parameter technique [8]. To reduce the hadronic background, optimum cutson the Hillas parameters were applied to select  -like events. This optimization was done with the RandomForest algorithm [3], which was trained to recognize  -ray events with Crab Nebula ON data from the sameepoch and same zenith angle.For the pulsed emission analysis, the arrival times of the Cherenkov events were registered by a GPS clock. Allarrival times were then transformed to the solar system barycenter using the JPL DE200 planetary ephemeridesand folded modulo the period relevant to the epoch and the source. Some very loose cuts were applied sincethe signal is expected to be dominant at low energies where the Hillas technique fails to discriminate  -raysfrom the hadronic background.Once the necessary corrections are applied, which also take into account the pulsar motion with respect to thecompanion, several periodicity tests were performed: not only /10 to test against a uniform distribution, butalso tests based on Fourier transformations which are able to resolve more sophisticated pulse shapes ( 2	34 andH test) [5].2.2 PSR J0218+4232PSR J0218+4232 was discovered in 1995 [11] as a luminous radio pulsar with pulse period of 2.3 ms. It orbits adegenerate companion with orbital period of about 2 days. The radio profile of the pulsed emission is complex,showing three peaks and a continuous (DC) component.This millisecond pulsar has also been detected in soft and hard X-rays, and belongs to the reduced club ofthose showing X-ray modulation at the rotational period. As an X-ray pulsar it has a high luminosity, hardpower-law shaped X-ray spectrum and a profile composed of narrow pulses. The X-ray emission is consistentwith a structure of non-thermal hard pulses superimposed on a continuum of softer spectrum. The source ispunctual with a size below 1 arc second.First pulsar observations with the MAGIC telescope 249Table 2. PSR J0218+4232  PSR B1957+20 flux upper limits for the steady emission.PULSAR F ( 56$,78	9;:=< ( >?#&%@01ACBD#E0,!$#&% )PSR B1957+20 4.5PSR J0218+4232 1.3In addition of the above properties this pulsar may represent the only detection of a millisecond pulsar in  -rays. It is positionally coincident with the EGRET source 3EG J0222+4253 which has been related with boththe AGN 3C66A and this source. Kuiper et al.[9] reported a 3.5 F detection of a pulsed signal in EGRET datawhose significance increased later on to 4.9 F when it was shown that the peaks in the  profile were alignedwith the X-ray peaks.The analysis of PSR J0218+4232 was a byproduct of dedicated observations on the AGN 3C66A, since forMAGIC the pulsar happens to be in the same field of view of the BL Lac object. Therefore the pulsar appearsat a distance of 0.973 G of the center of the telescope camera, close to the limit of the trigger area (  1 G ). Thisfact implies a reduced effective area and MAGIC sensitivity in a factor  35 H [14].The data analyzed belong to August 2004 ( I-)(+* - I-'-(+* ) and October 2004 ( ,$(+* - IJ(+* ) with a total ON obser-vation time of  13 h. The mean observation zenith angle is  15 G . Two different analysis scenarios weretried:1. In the first scenario a lower cut on the number of photoelectrons in the image (size parameter) demandingat least a value of 150, was applied. Images were reconstructed from the source offset position. Thosehigh energy/size images thus selected had enough quality to be treated through a standard set of cuts.This analysis has a nominal threshold of about 115 GeV.2. The second approach was to select only low energy events. It allows to greatly reduce the backgroundfrom cosmic rays events and exploit the soft spectrum that most models predict for the pulsed emissionof pulsars. On the other hand no standard imaging is possible in this regime since images consist of veryfew pixels with small amplitudes.3. ResultsNo excess is observed in the total data set for the millisecond pulsars neither for the binary systems. Theupper limit on the integrated flux above a certain energy (was inferred from the MAGIC sensitivity [10]for a point-like source, as obtained from Monte Carlo calculations. Upper limits at 2 F , 95% confidence level,are listed in Table 2 for the steady emission. No evidence of pulsed emission was detected, either from theorbital modulation (in the case of PSR B1957+20) or from the pulsars at the expected frequency (see Figure. 1and Figure. 2). Upper limits for the pulsed emission using the method explained by O.C de Jager [5] will bepresented at the meeting.4. DiscussionThe data tabulated in Table 2 summarize the upper limits for unpulsed emission from the 2 systems understudy. The energy threshold achieved in this analysis is still high to constrain the emission due to CR (up to50 GeV) in the theoretical spectra [4] for isolated pulsars. However, the flux upper limit for the binary pulsar250 E. Ona-Wilhelmi et al.0123456500 525 550 575 600 625 650 675 700 725 750ν(Hz)Z2 466000680007000072000740000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1φ entriesFigure 1. On the left, KMLN test for a wide range of frequencies around the expected frequency of the millisecond pulsar,marked with a red line. The size of the bin is dened by 1 IFS to make sure that the folding is done for independentfrequencies. On the right, the light curve at the expected frequency is represented. The result is compatible within theerrors to a at distribution (red dots line) [Hz]ν430.4595 430.46 430.4605 430.461 430.4615 430.462 430.4625102 Z051015202530PSR J0218+4232 (E<115 GeV) [Hz]ν430.4595 430.46 430.4605 430.461 430.4615 430.462 430.4625102 Z051015202530PSR J0218+4232 (E>115 GeV)Figure 2. On the left, K LO+P test for a wide range of frequencies around the expected frequency of the millisecond pulsar,marked with a red line, for events below Q 115 GeV . On the right, the same test for events at high energies.PSR B1957+20 is lower than the predicted value of [7]. This discrepancy can perhaps be accounted for bylower efficiency values for protons acceleration and  -ray production.References[1] F.A. Aharonian et al., submitted to Astronomy & Astrophysics (2005).[2] J. Cortina et al., these proceedings (2005).[3] L. Breiman, Machine Learning 45, 5 (2001).[4] T. Bulik, B. Rudak R J. Dyks, MNRAS 317, 97, (2000).[5] O.C. de Jager, ApJ 436, 239 (1994).[6] A. Harding, Proceed. “Towards a Network of Atmospheric Cherenkov Detectors VII”, (2005).[7] A. Harding R O.C. de Jager, “Towards a Network of Atmospheric Cherenkov Detectors V”, (1998).[8] A.M. Hillas, Proceed. ,S$(+* ICRC, Vol.3, 445 (1995).[9] Kuiper, L. et al, A&A, 359, 615, (2000).[10] A. Moralejo, http://wwwmagic.mppmu.mpg.de/physics/results/sensit.jpg (2005).[11] J. Navarro et al., ApJ 455, 55 (1995).[12] Y.I. Neshpor et al. Astron. Letter, 24, 134 (1998).[13] E. On˜a-Wilhelmi et al. Proceed. I-T (+* ICRC, 2457 (2003).[14] J.Rico et al., these proceedings (2005).[15] B.W. Stappers et al., Science, 299 (2003).[16] M. L U pez et al., these proceedings (2005).

First pulsar observations with the MAGIC telescope

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