PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

All Sky Search for Emission of Gamma Ray above 100 TeVUsing Tibet Air Shower Array

Zhaoyang Feng∗, Yi Zhang∗, C. Liu∗, C. Fan† ∗, H. C. Li‡ ∗, B. Wang∗,H. R. Wu∗,H. B. Hu∗, H. Lu∗, Y. H. Tan∗

(On Behalf of The Tibet ASγ Collaboration)

∗Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences,Beijing 100049, China.

†Department of Physics, Shandong University, Jinan 250100, China.‡Department of Physics, Hebei Normal University, Shijiazhuang 050016, China.

Abstract. An efficient γ/Hadron separation methodis described for the air shower with primary energyabove 100 TeV. The method uses age and size ofthe showers as discrimination variables. For the caseof Tibet air shower array(Tibet AS array), 70%of cosmic rays (CRs) background can be rejectedwhile 84% of γ-rays can be retained. With the datacollected by Tibet AS array from Oct. 2000 to Dec.2008, we searched for both point-like and diffusegamma ray emission in northern sky with energyabove 100 TeV. As a preliminary result, no significantemission is detected.

Keywords: cosmic rays, γ-rays emission, 100 TeV,Tibet air shower array

I. INTRODUCTION

Since the discovery of CRs in 1912, the origin of CRsremains to be a fundamental problem. Many evidencesexist for the electron source through the MeV-TeV γ-rays observation in past twenty years, but compellingevidence is still lacking for hadronic source. In re-cent couple of years, some unidentified TeV sourceshave been observed and listed as candidate sourcesof CRs. However, people tend to believe that decisiveand unambiguous evidence will rely on the positivedetection on the emission of γ-rays above 100 TeV.This is because that synchrotron radiation and Klein-Nishima effect will greatly suppress the 100 TeV γ-raysradiation of electron by its inverse Compton scatteringwith background photon, and the dominant mechanismto generate such high energy gamma ray should be thedecay of secondary π0-mesons from interaction of thevery high energy CRs with the ambient gas [1].

With the wide field of view telescopes, such asEGRET[2], Fermi[3] and MILAGRO[4], the Galacticplane has been shown to be the hottest region for diffusegamma ray emission in MeV-GeV-TeV energy band.And it is nature to expect that the Galactic Plane is thebest place to search for 100 TeV gamma ray emission.

II. EXPERIMENT

Since 1990, Tibet AS array has been success-fully operated at Yangbajing in Tibet, China(90.522◦E,

30.102◦N, 4,300 m a.s.l., 606 g/cm2). The Tibet Iarray was constructed in 1990[5], with 65 scintillationcounters placed on a lattice with 15 m spacing. Thisarray was gradually expanded to the TibetII(1994)[6]and TibetIII(1999)[7]. In the late fall of 1999, Tibet-IIIarray consists of 533 fast-timing scintillation counters(FT counter, measuring up to 15 particles), placed ona 7.5 m(inner) or 15 m( outside) square grid covering36,900 m2, and 36 density scintillation counters (Dcounter) around the FT array, while 52 out of 533 FTcounters also equipped with a wide dynamic range PMT(FT w/D counter, measuring up to 500 particles), placedon a 15 m square grid. After Oct 2000, the number of FTw/D counters increases to 185. From Dec. 2003 to now,the Tibet air shower array consists of 761 fast-timing(FT) counters placed on a 7.5 m square grid covering50,400 m2, while 249 out of 761 FT counters with 15m square grid are FT w/D counters, and 28 D countersaround them.

As discussed below, in order to separate the electro-magnetic showers from the hadronic shower, the ageand size parameters of NKG-function[8] are fitted fromthe lateral distribution measured by the Tibet air showerarray. For a shower with energy above 100 TeV, theparticle density near the core could reach up to severalhundred or even several thousand particles per m2,and this would saturate the FT-PMTs easily. As FTw/D counters have a wide dynamic range, they canprovide correct measurement on the number of particles.Therefore, in this work, we employ the data collectedby 185 FT w/D and 36 D counters covering an area of36,900 m2 (case a) for years between 2000 to 2003,and 249 FT w/D and 28 D counters covering an area of50,400 m2 (case b) for years between 2004-2008. Someof the detectors are discarded in analysis if they don’twork or work improperly.

III. LATERAL DISTRIBUTION RECONSTRUCTIONAND GAMMA/HADRON SEPARATION

The age and size parameters of NKG-function de-scribe longitudinal development of an electromagneticcascade and they are related to the lateral spread of thecascading particles. As an approximation, NKG-function

2 ZHAOYANG FENGet al. SEARCH FOR GAMMA RAY EMISSION ABOVE 100 TEV

also can be used to describe the lateral distribution ofCRs showers. With the same primary energy, γ-showerson average has a younger age than CRs showers, and thelight composition of CRs has younger age than heavyone. A shower is identified as a CRs event if the mea-sured age and size parameters differs significantly fromthat expected when assuming it is an electromagneticone.

Particle number measured by Tibet AS experiment isequivalent to the number of ”MIP” recorded in an areaof 1 m2 after passing through 0.5 cm Lead plate[9]. Itis different from the particle number defined for showersize. As they are positively correlated, particle numbermeasured from each detector can still be used to fitlateral distribution using NKG-function as in equation(1).

µ(r) =Ne

2πr20

Γ(4.5− s)Γ(s)Γ(4.5− 2s)

(r

r0)(s−2)(1 +

r

r0)(s−4.5)

(1)Here Ne is shower size, s is shower age, r0 is the

Moliere radii(130m at Yangbajing), Γ is the gammafunction. µ(r) gives the number of particles seen by a1 m2 counter at a distance r to the core in the planeperpendicular to the shower axis. For the i-th counterat a distance of r to the shower core in the planeperpendicular to the shower axis, the probability forit to see mi particles can be calculated by Poissonequation.

P (mi) =µ(r)mi

mi!e−µ(r) =

µ(r)mi

Γ(mi + 1)e−µ(r) (2)

Then the likelihood function can be construction as

LF =N∏

i=1

P (mi) (3)

The corresponding log-likelihood function is, there-fore:

LLF =N∑

i=1

miln(µ(r))−N∑

i=1

µ(r)−N∑

i=1

ln(Γ(mi + 1))

(4)In equation (3) and (4), N is the total number of

detectors that are normally working and aren’t saturated.Minimizing equation (4) by TMinuit of ROOT pack-

age, size Ne and age s can be obtained.In this work, only events with energy above 50 TeV

were reconstructed. First, the event is reconstructed toget the core position and direction by the traditionalmethod[9]. Then the lateral density distribution is cor-rected to the inclined plane perpendicular to the showeraxis and used to jointly fit the size, age and coreposition of the shower. At last the direction of showeris reconstructed again using new core position in orderto get a better angular resolution.

To optimize the analysis, a full Monte Carlo(MC)simulation has been carried out on the development ofair showers in the atmosphere by Corsika(version 6.204)

with QGSTET01c interaction models[10] and also onthe detector response by Epics(version8.65)[11]. CRs airshower events are generated with zenith from 0o to 60o,and azimuth from 0o to 360o. The HD model of CRscomposition at [9] was adopted. The γ-rays air showerevents are generated with zenith angle lesss than 60o

along the Crab’s orbit with a differential energy spectrumE−2.62. Energy of both CRs and γ-rays is from 50 TeVto 10 PeV. Air Shower events are uniformly thrownwithin 300 m of the array’s center.

The following criteria are applied to select eventsabove 100 TeV.

criteria 1 - Air shower core location: Among the threehottest counters in each event, two should be containedin the inner array with area of 22,500 m2 for case ”a”or 36,900 m2 for case ”b”.

criteria 2 - Shower size:∑

ρFT , which is the sum ofthe number of particles per m2 counted by all the FT-PMTs, should be larger than 400.

criteria 3 - Zenith Angle: the zenith angle of the arrivaldirection should be less than 50o.

In total, 1.46×108 air shower events can pass thecriteria 1-3. As can be seen in Fig. 1a and Fig. 1b, ingeneral, for a given shower size, age of γ-rays tendsto be younger than that of CRs and this characteristicdifference makes the discrimination between γ-inducedshowers and CRs-induced showers possible. And fromFig. 1b and Fig. 1c, we see that MC agrees with datareasonably well. Fig. 1d shows good agreement betweendata and MC for the distribution of shower size, thesmall difference in two lowest bins indicates that sizemay be slightly over estimated in data sample.

The projected shower age distribution shows alsosome discrepancy (see Fig. 1e), with the averaged ageparameter differs by 0.03. This might due to the uncer-tainty related with the fractions of component adoptedin MC simulation. Another possibility maybe that cur-rent MC parameters do not truly simulate the detectorresponse, e.g., the saturation values for each PMTs maydiffer from the real values. Nevertheless, such a smalldifference has no effect to background rejection rateas it can be estimated from data but only has effectto the efficiency of γ-rays signal. Given the fact thateffective cut value on shower age is 0.8 for most of theevents(see below for detail), close to the center valueof the distributions of gamma rays, the effect on thesignal efficiency is again not important (see quantitivediscussion below).

To verify the 0.32o angular resolution of CRs passingcriteria 1-3 which is obtained from MC, traditionalMoon shadow analysis is performed. Fig. 2 shows themoon shadow with the above mentioned data sample(of1.46×108 events). Considering the 0.26o radii of themoon, we use 0.4 degree as the smooth radii. Themaximum deficit point has 13.9 standard deviations andlocates right at the center of the moon, which demon-strates that the reconstructed data is in good quality.

To have an optimal discrimination power between

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

log(Size)4.5 5 5.5 6 6.5 7 7.5

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(f) -raysγMC MC CRsExp. Data

Fig. 1. Experimental data compare with MC CRs and MC γ-rays.(a),(b),(c) is the age VS log(size) plot of MC γ-rays, MC CRs andExperimental data respectively. (d),(e), (f) is size, age and γ/Hadronvariable HF distribution of MC γ-rays, MC CRs and experimental datarespectively. Detailed discussion about this figure can be fount at text.

CRs and γ-rays, we apply a cut on a linear combinationof the age and shower size parameters. As defined inequation (5), this variable HF is highly correlated withshower age parameter. Fig.1f shows the HF distributionsfor MC γ-rays, MC CRs and data respectively, which arevery similar to the distributions of shower age in Fig. 1e.The optimized cut condition is defined in criteria 4 (seebelow) and indicated as pink lines in Fig. 1a, Fig. 1band Fig. 1c.

HF = age + 0.05× log(size) (5)

criteria 4 - γ/Hardon separation: event is identified asa γ-event if HF<1.05, otherwise it is treated as a cosmicray event. As shown in Fig.1e, this cut is effectivelyequivalent to a cut on shower age parameter at a valueof 0.8 when shower size is not a huge one.

Experiment data is therefore divided into two samplesby criteria 4, one is γ-rich sample with 4.41×107

shower events, the other is γ-poor sample with 1.05×108

shower. For γ-rich sample, 70% background events arerejected while 84% gamma events are retained. As beingdiscussed earlier, the averaged shower age parameter forCRs is 0.03 larger in data than in MC and this may leadto an overestimation of gamma efficiency. Suppose theaveraged shower age for gamma in data is also 0.03larger than that in MC, a more accurate efficiency canbe obtained by adding a value of 0.03 to shower agefor MC events before applying all the cuts. With such aconsideration, the γ-rays efficiency is found to be 78%.

According to MC, Mode energy of γ-rays is 105 TeV,there is only 21.5% events with energy less than 100TeV. Mode energy of CRs is 175 TeV. In γ-poor sample,mode energy of γ-rays and CRs is a little higher thanγ-rich sample.

IV. ANALYSIS

Even in the γ-rich sample, CRs events are still over-whelm. We still need to estimate the background number

W Angular Distance(degree) E-1.0 -0.8 -0.6 -0.4 -0.2 -0.0 0.2 0.4 0.6 0.8 1.0

N

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tanc

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gree

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Fig. 2. Moon shadow of events passing criteria 1-3. The center ofthe map is the position of moon.

in each search window carefully. Two independent anal-ysis based on equi-zenith angle method are used in thiswork. One is so-called short distance equi-zenith anglemethod (Method 1). The other is all distance equi-zenithangle method (Method 2)[13].

Method 1 is dedicated to point source search. Withthis method, sky is divided into 0.1o×0.1o grids from0o to 360o in RA and from −9.89o to 70.11o in Dec.The optimized half opening angular resolution of γ-richsample is 0.23o. Circles of radii 0.23o at those gridsare treated as on source window. The angular distancebetween 10 off-windows is 2o, and zenith cut is from4.2o to 50o. We just point out that the data statistic islow in this work, the calculation of significance is usinga Poisson-Gauss method.

Method 2 is sensitivity to diffuse gamma emission.Analysis is as same as [13]. As the data statistic is lowin this work, the Likelihood function[14] is constructedinstead of χ2 function. And for this method, sky isdivided into grids with bin size of 1o in both Zenithdirection from 0o to 50o and azimuth direction from 0o

to 360o , and Sky in Equatorial Coordinates is dividedto bin 2o×2o bins from 0o to 360o in RA and from−9.89o to 70.11o in Dec. The smooth radii in EquatorialCoordinates is 10o.

V. RESULT AND DISCUSSION

Fig. 3 shows the one dimensional significance distri-bution for γ-rich sample with method 1. The distributioncan be described well by a normal distribution. Nosignificant point source is found in our field of view.

Fig. 4 shows the two dimensional significancemap. The top panel is with method 2 for the γ-richsample. No significance emission is found. It is worthnoting that stretching along the galactic plane, in regionof 120<l<200, |b|<5 and in region of Cygnus, eventexcesses are seen but not in high significance. They canbe attributed to the fluctuation or anisotropy of CRs.

2D significance map for the γ-poor sample is madefor a check, as shown in the middle panel of Fig 4.This time no excess along the galactic plane can beseen. Instead, an excess parallel to the Galactic plane isseen. More study is needed to further understand thoseexcesses.

4 ZHAOYANG FENGet al. SEARCH FOR GAMMA RAY EMISSION ABOVE 100 TEV

Mean 0.0006± 0.0026

Sigma 0.0004± 1.0634

Significance-6 -4 -2 0 2 4 6

Num

ber o

f Grid

s

1

10

210

310

410

510Mean 0.0006± 0.0026

Sigma 0.0004± 1.0634

Fig. 3. One dimensional Significance distribution of Method 1 usingγ-rich sample from 0o to 360o in RA and from −9.89o ro 70.11o

in Dec. The blue solid line indicates a Gaussian fit to the data.

0 50 100 150 200 250 300 350

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ree)

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Fig. 4. 2D significance map for energy above 100TeV by usingMethod 2 analysis with Tibet air shower array data taken between2000 and 2008. Top: For the γ-rich sample; Middle: For the γ-poorsample; Bottom: For the ”γ-clean sample” made by subtracting theγ-poor intensity map from the γ-rich map. The black solid point isthe Galactic Plane (b=0). The black solid line indicates |b|=5. b isthe galactic latitude. The smooth radii is 10o.

Dec

.(de

gree

)

010203040506070

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gree

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Rel

. int

ensi

ty

0.999

1.000

1.001/ndf 17.6/162χ

1.2e-04±Amp. 6.0e-04 0.8± 6.8 φ

Fig. 5. Local solar time CRs intensity map of events passing criteria1-3 using Method 2 analysis from −9.89o ro 70.11o; Top:Intensitymap; Middle: Significance map; Bottom: Relative intensity projectingin the local solar time. The fitting function is in the form of Amp×cos[2π(T-φ)/24], where the local solar Time T and φ are in units ofhour and Amp is the amplitude.

To remove as much as possible the large scaleanisotropy of CRs, the intensity of the γ-poor sampleis subtracted from that of γ-rich sample. The result isshown in the bottom panel of Fig. 4. The excesses alongthe galactic plane remain though the significance is notimproved.

Finally, as a check of the method 2 analysis, the CRsintensity map in the local solar time frame is producedwith events passing the criteria 1-3. With 10 degreesmooth angle, Fig. 5 shows the intensity map(Top),significance map(Middle) and relative intensity in localsolar time direction. The observed anisotropy agreeswith the expected CG effect due to the terrestrial orbitalmotion around the Sun.

VI. SUMMARY

A γ/Hadron separation method is developed for theair shower with primary energy above 100 TeV. Themethod can keep 84% of γ-rays when rejecting 70%of CRs background. Based on this method, Tibet airshower array data taken from Oct. 2000 to Dec. 2008are used to search for the γ-rays emission above 100TeV in northern sky. No γ-rays emision above 100 TeVis detected. Tibet-MUON experiment [15] would be themost potential detector to get positive result at 100 TeVregion.

VII. ACKNOWLEDGMENTS

The collaborative experiment of the Tibet Air ShowerArrays has been performed under the auspices of theMinistry of Science and Technology of China and theMinistry of Foreign Affairs of Japan. This work wassupported in part by a Grant-in-Aid for Scientific Re-search on Priority Areas from the Ministry of Education,Culture, Sports, Science and Technology, by Grants-in-Aid for Science Research from the Japan Societyfor the Promotion of Science in Japan, and by theGrants from the National Natural Science Foundationof China and the Chinese Academy of Sciences. C.Fan is supported by the Natural Science Foundation ofShandong Province, China( No.Q2006A02).

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Tibet AS+MD Project