Le Coultre, P

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29th International Cosmic Ray Conference Pune (2005) 4, 399402Observation of a Flare Signal from a Fixed Position in the NorthernHemisphere through Muons with L3+CP. Le Coultre for the L3+C collaborationInstitute of Particle Physics, ETH CH-8093 Z¤urich, SwitzerlandPresenter: P. Le Coultre (Pierre.Le.Coultre@CERN.ch), swi-lecoultre-P-abs1-og23-oralThe observation with the L3+C spectrometer of a three days lasting enhancement of cosmic ray muons origi-nating from a fixed position in the northern sky is discussed. The ”flare” was detected in a by sized skycell with a chance probability of  	. The signal exhibits a clear time evolution, persists up to muonenergies above 50 GeV, suggests a neutral primary and was recorded within opening angles of down toless than	. It happened between 51773.489 and 51776.333 MJD (17-20 Aug. 2000). The origin is locatedat a galactic longitude of      ﬁﬀﬂand a latitude of ﬁﬃ  ﬀﬂ.1. IntroductionTo-day some 20 point sources are known emitting TeV gamma-ray signals. The measured high-energy gammaspectra are unfortunately incomplete in the sense that there is 1.) a lack of data in the energy range between! 10 and 300 GeV, and 2.) a cut-off at energies above typically 5 to 10 TeV for distant sources, due to theabsorption of the gamma rays by interactions with the infrared and background radiation, or due to absorptionin the neighbourhood of the acceleration region. Another characteristic of these sources is that the signal canbe of steady, and " or periodic, and " or sporadic nature. These facts indicate the problems to be met while tryingto determine the mechanism(s) at work producing and accelerating particles to high energies.To test models one has first to try to find new sources in order to increase the statistics of particular data for thedifferent kind of sources, and secondly to build more sensitive detectors, capable to measure the spectra alsoin the energy range between 10 and 300 GeV. Such efforts have been undertaken in the last few years (TAC-TIC, CANGAROO, HESS, MAGIC, VERITAS) and a lot of new results are expected soon. While imagingCherenkov detectors can detect only one source at a time with relatively good angular and energy resolution,sky survey air shower experiments have the opportunity to continuously search large sky regions for new sig-nals, although with slightly less good angular and energy resolution, as well as with higher energy thresholds.In this paper the search for point sources is presented based on the detection of secondary muons above 20GeV with the L3+C set-up [1]. High energy muons originating from the decay of photoproduced pions orK-mesons have practically the same direction as the incoming gamma ray and point also back to the source(effective pointing: #$	[2]). Of course the detection probability is reduced, due to the much smaller cross-section for photo-production compared to the electron-pair production cross-section. Nevertheless, if the fluxand the energy of the signal is large enough, a source could be found. The original idea of the L3+C collab-oration was not to detect sources emitting steady signals, since the energy flow of all known sources is belowthe detection threshold of the L3+C detector, but to record possible intense, very high energy flare-signals, oreven gamma ray burst signals.Deep underground detectors, like e.g. MACRO and NUSEX [3] have also been used for this purpose. Butprobably due to the high muon energy threshold, MACRO has not recorded any signal in even a rather longlasting sky survey. The SOUDAN collaboration [4], on the contrary, has observed a flare signal from Cyg X-3in 1985 and a second signal in 1991. This type of signals could never be observed again by another experiment.Few GeV muons have been recorded with air shower scintillator arrays to separate gamma initiated showersfrom nuclei initiated showers. Only values for upper gamma ray fluxes have been published. The MILAGROair shower experiment [5] offers a continuous sky survey capability for relatively low primary gamma energies.400 Pierre Le CoultreThe L3+C spectrometer is located at a shallow depth of 30 m, with a muon energy-threshold around 15 GeV,and therefore sensitive to primary gamma energies below 200 GeV. Other experimental advantages are a con-tinuous sky survey (zenith angletoﬁ), selection of the muon momentum threshold off-line, continuousmonitoring of the background, following of candidate sources across the sky, good angular resolution ( #   for %'&)(	*GeV), excellent pointing precision ( #+	 ), a geometrical acceptance of 200 ,.-0/21 .The L3+C spectrometer [1] was installed at the Large Electron-Positron collider, LEP, CERN, Geneva. Threelayers of cylindrically arranged drift chambers are mounted inside a 0.5 T solenoidal magnet of 1000 ,vol-ume. To record the arrival time of cosmic muons 202 , - of scintillators are installed on the top of the ironyoke. The detector was located 450 m above sea level, at a latitude of 46.25N and 6.02E, and 30 m belowground. It provided a precision measurement of the atmospheric muon momentum spectrum between 20 and3000 GeV [6].2. Search procedure for short bursts or flaresThe whole data taking period (July to November 1999 and May to November 2000, effective live-time =252.4 days) has been divided into twelve sets of different time window lengths, lasting 3minutes, with mranging from 1 to 12. The sky has been subdivided into 6 sets of different sized rectangular cells ( 46587ﬂ9;: ): -<9>=	ﬁ 7	ﬁﬁ7  7 ﬁ72? 7? and<4@=<9ﬁ"ACBD9 , but rounded to integer number of divisions).The number of background events for a given cell at a given time and for a particular time window is deter-mined in the following way: Background regions BG( 4E5F7ﬂ9;: ) of hollow rectangular shape around the cell ofinterest were defined with half widths GIHJ=LKEM<9 to G'NO=<9P	in 9 , G'HM<4Q"<9ﬀto G'NM<4Q"<9ﬀin 4 . The ratio of the event numbers measured in the particular cell and the corresponding background regionhas been precisely determined beforehand during the whole data taking period and stored in a table. Theseratios are then used to determine the actual background event number in the particular cell of interest for theparticular time after having determined the number of background events in the surrounding background regionat the same time. This procedure takes care of the time and geometrical dependent detection efficiency of thedetector. The significance of a signal is determined with the widely used Li-Ma prescription [7].The trial numbers have been calculated for each particular time bin- and cell- size, removing statistically un-significant contributing combinations. In addition a MC calculation has been performed to get the importanceof the cell size on the significance of a possible signal. The result showed that for different assumed gammaspectra the cell size has not a very strong effect on the significance of a possible signal. Therefore only three”optimized” cell sizes have been kept for the effective search, corresponding to the particular time binning:  SR? ﬀFfor T'RU Vmin time binning, 	ﬁWR  ﬀ2for ﬁXYRU Zmin time binning, and 	ﬁ SR  ﬀ2for ﬁ[\R T-min time binning.In total 12M3 skymaps have been retained, corresponding to the 12 choices of time windows and the 3 sizesof cells. Accordingly the chance probability was precisely determined for each cell.3. ResultsOne and only one candidate has been found from the sky survey described above. It appeared in the sky surveywith the T-minutes time window and a cell size of   ﬀ-. The number of events inside the ”signal” cell wasKﬁ and the number of background events ? . The excess of events above the background is therefore	?LKand the significance is equal to?^], according to the Li-Ma prescription. But the chance probability toobserve a background fluctuation as an excess is  QF	*_`for this particular analysis. The exact position of theexcess has been determined fitting the event distribution by minimizing the negative log-likelihood function:Right ascension 4a=b	Kﬁ ?)$	Kﬀﬂand declination 9c=dRJ	ﬁ$	*eW)  ﬀﬂ, or in galactic coordinates:Observation of a Flare Signal....L3+C 401longitude =    fg  ﬁﬀ2, latitude = ﬁ ﬁﬃfh  ﬀF.The fitted angular resolution amounts toLK Eg+	?(see Figure 1). The ”excess” occurred between the Modified Julian Dates = 51773.489 and 51776.333 MJD,corresponding to the calendar date and UT time of 11:44 17 " 08 " 2000 – 08:00 20 " 08 " 2000. A careful studyabout the detector performances has not revealed any malfunctioning at the flare time.Four remarkable facts enhance the confidence that the observed ”excess” may not simply be a background fluc-tuation: 1.) The signal shows a clear time evolution as can be seen in Figure 4. The signal is clearly observedin time windows ranging from	*Zup to	*TiVminutes. 2.) An integral energy spectrum of the flare could beextracted thanks to the presence of the signal for several applied energy thresholds (Figure 2. For comparisonthe result of a CORSIKA simulation [8] for integral j -spectra with different power indices of the differentialprimary gamma spectrum are displayed, as well as the simulated muon background spectrum (solid line). Alldata points and curves are normalized to 1.0 at a momentum threshold of 20 GeV). 3.) The muon charge ratio,as a function of the opening angle containing the excess, suggests a neutral primary (Figure 3). 4.) The signalwas observed for 3 different cell sizes: k? ﬀ- ,   ﬀ- and 	 ﬀ- (not shown here).)σ Significance (1234567West-East [degree]-3 -2 -1 0 1 2 3South-North [degree]-3-2-10123Center Position from Fitting39.3% C.L. (1 s.d.)90.0% C.L.95.0% C.L.South-North [degree]Muon Momentum Threshold [GeV/c]Number of Events (normalized)0.1120 40 60 80 100−2.50−3.00−3.50−4.00−4.50−5.00−5.50ExcessBackgroundMC BackgroundMC GammaSpectralSlopeIntegralRadius of the Cone [degree]0.4 0.6 0.8 1 1.2 1.4Charge Ratio0.20.40.60.811.21.41.61.8BackgroundGamma ExpectedDataCharge RatioFigure 1. Source map and Figure 2. Integral number of excess Figure 3. Muon charge-ratiosignificance. events vs. energy threshold. vs. size of opening angle.4. ConclusionsAn excess of events with a chance probability to be a background fluctuation of  l 	*`is a relatively weaksignature (but note the conservative approach of this analysis). Nevertheless the characteristics of the signal,the duration, the evolution in time, the energy spectrum, the muon charge ratio and the angular distribution, arequite suggestive for a Blazar like flare. It was not possible to attribute the flare to a known source: Within aradius of one finds a gamma source (3EGJ 1133+0033), as well as several QSOs and unidentified objects. Abig difficulty occurs when estimating the necessary gamma flux to produce such a large muon signal in L3+C.According to the observed muon spectrum and Monte Carlo calculations (CORSIKA) the gamma spectrummust be very steep and the low energy part of the flux extremely large. E.g. for a spectral index ofR4.5 thegamma differential fluxes would be of the order of ?Om	*-, nm	*ﬁoand enm	*pTCrab flux units at energies1, 10 and 30 TeV respectively. No such signal has ever been observed. Unfortunately at the time of the flare nosatellite was pointing in this direction, nor any ground based detector active - at least to our knowledge. TheMAGIC collaboration has performed a 5 hours search for a remnant signal in February this year, but found nosignal.402 Pierre Le CoultreMJD UTC51400 51500 51600 51700 51800]σSignificance [-505 min82Date UTC06/08/00 13/08/00 20/08/00 27/08/00-505MJD UTC51400 51500 51600 51700 51800]σSignificance [-505 min92Date UTC06/08/00 13/08/00 20/08/00 27/08/00-505MJD UTC51400 51500 51600 51700 51800]σSignificance [-505 min102Date UTC06/08/00 13/08/00 20/08/00 27/08/00-505MJD UTC51400 51500 51600 51700 51800]σSignificance [-505 min112Date UTC06/08/00 13/08/00 20/08/00 27/08/00-505MJD UTC51400 51500 51600 51700 51800]σSignificance [-505 min122Date UTC06/08/00 13/08/00 20/08/00 27/08/00-505MJD UTC51400 51500 51600 51700 51800]σSignificance [-505 min132Date UTC06/08/00 13/08/00 20/08/00 27/08/00-505MJD UTC51400 51500 51600 51700 51800]σSignificance [-505 min142Date UTC06/08/00 13/08/00 20/08/00 27/08/00-505Figure 4. The significances of the signal found in the ”flare” cell are are shown for different time windows,ranging from Zmin to T8Vmin, between MJD 51650 and 51850. On the right hand side a zoomed image isshown around the signal time (vertical, dashed line).References[1] L3+C collab.: O. Adriani et al., Nucl. Inst. Meth. A 488 (2002) 209.[2] L3 collab.: P. Achard et al. et al.,Astropart.Phys.23(2005) 411.[3] MACRO collab.: M. Ambrosio et al., Astropart.Phys. 18 (2002) 615;NUSEX collab.: G. Battistoni et al., Phys.Lett. B115 (1985) 465. .[4] SOUDAN collab.: M.L. Marshak et al., Phys.Rev.Lett. 54 (1985) 2079, and 55 (1985) 1965, and M.A. Thomson etal., Phys.Lett. B269 (1991) 220.[5] MILAGRO collab.: R. Atkins et al., Astrophys. J.Lett. 533 (2000) L119, and 525 (1999) L25.[6] L3 collab.: P.Achard et al., Phys. Lett. B 598 (2004) 15.[7] T.P. Li and Y.Q. Ma, Astrophys. J. 272 (1983) 317.[8] D. Heck et al., Technical report, FZKA 6019, Forschungszentrum Karlsruhe (1998).

Observation of a flare signal from a fixed position in the Northern Hemisphere through muons with L3+C

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