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A northern sky survey for both TeV cosmic-ray anisotropy and TeV γ-ray sources using the Tibet air shower array
Nuclear Physics B (Proc. Suppl.) 151 (2006) 121–124 www.elsevierphysics.com
A northern sky survey for both TeV cosmic-ray anisotropy and TeV γ-ray sources using the Tibet air shower array Hongbo Hu a
a
, for the Tibet ASγ Collaboration
Key Laboratory of Particle Astrophysics, IHEP, P.O. Box 918, Beijing 100049, P.R.China
Results on 2-dimensional TeV cosmic ray anisotropy and γ-ray point source search using data taken from TibetHD (Feb.1997-Sep.1999) and Tibet-III (Nov.1999-Oct.2001) arrays are presented. From 0◦ to 60◦ in declination, large scale anisotropy of cosmic-ray intensity at a magnitude of about 0.1% is observed. In the mean while, the well-known steady source Crab Nebula and the high state of the flare type source Mrk421 are detected.
1. Introduction Observation of high energy cosmic-rays (HECR) plays a very important role in our understanding of the origin, acceleration, propagation of cosmic-ray (CR) and other problems of astro-particle physics, among which two kinds of HECR phenomena have been widely investigated by many experiments. One is on the large scale anisotropy of CR intensity, and the other is the sky survey of TeV γ-ray point sources. Because of the low CR flux, those HECR observations have to be made by ground based experiments, such as Imaging Atmospheric Cherenkov Telescopes (IACT) and Extensive Air Shower (EAS) arrays. With good angular resolution and capability of eliminating overwhelming proton background, IACTs are more sensitive in searching for steady emission from TeV point sources. In contrast to IACT experiments, the advantages of a wide field of view and high duty cycle make EAS experiments more competitive in the sky survey of point sources and the measurement of CR’s large scale anisotropy. In the past decade, the Tibet ASγ experiment has successfully observed TeV γ-ray emission from the Crab Nebula [1], Mrk421 [2] and Mrk501 [3]. Recently, the ASγ experiment reported results on TeV CR’s anisotropy due to the Compton-Getting effect [4], and both Milagro [5] and ASγ [6]experiments published their results on northern sky survey of point sources. Here we present our preliminary results on the measurement of CR’s two-dimensional anisotropy and an 0920-5632/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2005.07.023
updated result on all sky survey of γ-ray’s steady sources. 2. Experiment The Tibet air shower experiment has operated at Yangbajing (90.53◦ E, 30.11◦N; 4,300m a.s.l.) in Tibet, China since 1990. It has been upgraded from the Tibet-I, Tibet-II/HD [7] to Tibet-III array [8]. Both Tibet-II/HD and TibetIII arrays have the same mode energy of about 3 TeV for proton-initiated showers and the same angular resolution to better than 0.9◦ . The events are selected by imposing the following five criteria on the reconstructed data: i) each of any four FT detectors should record a signal more than 1.25 particles; ii) the estimated core location should be inside the array; iii) the 2 sum of the number of particles per m detected in each detector ( ρF T ) should be larger than 15; iv) the residual error of direction reconstruction should be less than 1.0 m; v) the zenith angle of the incident direction should be less than 40◦ . After applying those data selection criteria, 1.5 × 109 events are selected from 555.9 live days’ running of the HD array and 5.5×109 events from 456.8 days’ running of the Tibet-III array. 3. Analysis Sitting on an almost horizontal plane, TibetHD and Tibet-III arrays have geometric size much larger than the lateral spread of a typical TeV cosmic-ray shower. As a result, the arrays are
122
H. Hu / Nuclear Physics B (Proc. Suppl.) 151 (2006) 121–124
equally powerful in receiving shower events at the same zenith angle. The so-called equi-zenith angle method was therefore developed to estimate the background number using sideband events. In this work, two techniques based on equi-zenith angle method are employed for the purpose of cross-check. 3.1. Method I (Short distance equi-zenith) To a good approximation, the large scale CR anisotropy can be neglected if the “off-source windows” are chosen to be close enough to the “onsource window”, as the large scale anisotropy only slowly changes in the sky. In this analysis, the sky is divided into 0.1◦ × 0.1◦ cells, from 0◦ to 360◦ in R.A. and from 0◦ to 60◦ in Dec. Circles of radii 0.9◦ centred at those cells are tested as onsource windows. 10 off-source windows are symmetrically aligned on both sides of the on-source window. The number of background events can 10 thus be estimated as Nbkg = i=1 Noff,i /10. In order to correct the non-uniformity of the azimuth distribution, the previous measurement was repeated 35 times (arbitrary number), starting from on-source window, successive dummy on-source windows divide 360◦ of R.A. direction into equipartition. The correction factor due to a non-uniform azimuth distribution is given by: 35
η = j=1 35 j=1
(dummy)
/35
(dummy)
/35
Non,j
Nbkg,j
. Finally, the corrected es-
timation on background number is obtained as: N ∗ bkg = ηNbkg . Consequently, the significance value can be simply calculated as: S=
∗ Non − Nbkg ∗ 1.0637 Nbkg
(1)
3.2. Method II (All distance equi-zenith) In this method, for each 1◦ zenith interval between 0◦ to 40◦ , non-uniform azimuth distributions are corrected by normalized azimuth distributions, to keep the total event number unchanged but make the azimuth distribution flat. All events in the equi-zenith belt except those inside the on-source window are taken as off-source events. In this case the large scale anisotropy effect can no longer be ignored. The celestial space
from 0◦ to 360◦ in R.A. and from 0◦ to 60◦ in Dec. are binned into cells with bin size 0.1◦ in both directions. In the observer’s coordinate, zenith angle θ is divided from 0◦ to 40◦ by step size 0.08◦ , and azimuth angle φ is binned by a zenith dependent bin width (0.08◦ / sinθ). For each local sidereal time (LST) interval bin (24s) m, a cell in (θ, φ) space (n, l) is mapped to a celestial cell (αi , δj ), i.e. R.A. bin i and Dec. bin j. Denoting I(i, j) the cosmic-ray intensity in cell (i, j), the equi-zenith condition leads to the following χ2 function, as in eqn.[2]. From eqn.[2] , I(i, j), as a function of R.A. and Dec. and its error ΔI(i, j), can be solved numerically by the iteration method, and CR’s large scale anisotropy can be measured in two dimensions. In order to search for γ-ray point sources, we should remove CR’s large scale anisotropy. As in Fig.1, the measured CR intensity in a belt of 10◦ width along Dec. is projected to R.A. direction, in 8◦ bin size. The centre of the belts moves from 0◦ to 60◦ , in 1◦ step size. Smoothing those curves and having them subtracted in the corresponding 1◦ width belt gives an anisotropy effect corrected CR intensity Icorr (i, j). With this large scale anisotropy subtracted cosmic-ray intensity, the excess number of events in cell (i, j) can be calculated as Ns (i, j) = N (i, j) − N (i, j) / Icorr (i, j) and its uncertainties Ns (i, j) = ΔIcorr (i, j) · N (i, j) / Icorr (i, j). Taking into account the angular resolution, events in a cone of 0.9◦ are summed. The significance for an on-source window centered at (i, j) can be calculated as: Ns (i , j ) i ,j ∈cone S (i, j) = (3) 2 ΔNs (i , j ) i ,j ∈cone
4. Results Fig.1 shows the intensity map containing the contribution from the large scale anisotropy with a demonstration in its subtraction when searching for γ ray point sources. This anisotropy distribution is consistent with the prediction made
123
H. Hu / Nuclear Physics B (Proc. Suppl.) 151 (2006) 121–124
χ2 =
m, n, l
2 Nobs (m, n, l) I(i, j) − l = l Nobs (m, n, l ) / I(i , j ) l = l 1 ; 2 Nobs (m, n, l) I 2 (i, j) + ( l = l 1)2 l = l Nobs (m, n, l ) / I (i , j )
(2)
Declination(deg)
Map of Intensity 60
995
50
0.9
5 00
1.0
40
0.9
30
20
990 0.9
0 02
0.9
1.0
10
15
1.00
985
010 1.0 5 01 1.0
0
98
5
02
0
1.0 0
50
100
150
200
250
300
350
Relative Intensity
RA(deg) 1.003 1.002 1.001 1 0.999 0.998 0.997
0
50
100
150
200
250
300
RA(deg) 350
Figure 1. Upper panel: contour plot of the relative intensity of cosmic-rays in the surveyed northern sky. Lower panel: a demonstration on the subtraction of cosmic-ray’s large scale anisotropy. The anisotropy distribution for the narrow belt centred at 9◦ in Dec. is estimated by projecting the intensity measurement from a wider belt (10◦ width is chosen). This distribution is further smoothed and later subtracted from the narrow belt. by Hall et al. [9–11]. The significance distributions from all directions are shown in Fig.2a and Fig.2b for the two analysis methods respectively. As for method II, the large scale anisotropy has to be subtracted. By changing the bin size and the smoothing parameters, the significance measurement is found to have 0.2σ systematic uncertainty. The perfect agreement on the negative side with a normal distribution (Fig.2a,2b) indicates that the systematic effects are well under control for both analyses. As for the positive side, a wider shoulder exists with significance values greater than 4σ. The dominant contributions are due to the well known stable γ-ray source Crab and transient source Mrk421. After removing their contributions, in a way that those cells in a distance shorter than 2◦ to the two sources are excluded, the dash-dotted line in Fig.2a,2b shows the significance distribution from the rest of the cells which agrees much better with a normal dis-
tribution but a discrepancy still clearly remains. This can be understood as local event number excess. Table 1 lists 21 prominent directions with significance values greater than 4σ. 5. Conclusion A wide northern sky survey for TeV γ-ray source and CR’s large scale anisotropy in Dec. band between 0◦ to 60◦ is performed using data of Tibet HD and III air shower arrays obtained from 1997 to 2001. The CR’s large scale anisotropy with a magnitude about 0.1% is observed in 2dimensions, and the well known Crab and Mrk421 are observed at 4.8σ and 5.2σ respectively. This indicates that the Tibet shower array is a sensitive apparatus for TeV γ-ray astronomy and has a potential to find flare type γ-ray sources, such as BL-Lac type AGNs. In addition, 19 other prominent directions with significances greater than 4σ are found. However, more statistics are necessary to either confirm or to rule out those candidates.
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H. Hu / Nuclear Physics B (Proc. Suppl.) 151 (2006) 121–124
χ / ndf 2
Number of events
Constant
5
10
Sigma
104
543.9 / 99
1.01 ± 0.00
(a)
Constant
105
414.3 / 98 8.562e+04 ± 72 -0.001202 ± 0.000685
Mean Sigma
104
1.006 ± 0.000
(b)
103
103 χ2 / ndf
102
Constant
102
543.9 / 99 8.527e+04 ± 71
Mean -0.0002687 ± 0.0006878
102
10
Sigma
10
1 10-1
-4
2
χ / ndf
-2
4
4.5
0
5
5.5
2
4 6 Significance
-0.001202 ± 0.000685 1.006 ± 0.000
1 10-1
10-1 -6
Mean Sigma
10
1 3.5
414.3 / 98 8.562e+04 ± 72
Constant
102
1.01 ± 0.00
10
1
10-1 -6
χ2 / ndf
Significances Distribution (Method II)
8.527e+04 ± 71 -0.0002687 ± 0.0006878
Mean
Number of events
Significances Distribution (Method I)
-4
3.5
-2
4
4.5
0
5
5.5
2
4 6 Significance
Figure 2. The significance distribution from (a) Method I (b) Method II. The solid lines are from all cells; the dash-dotted lines are from cells after removing those which are at a distance to Crab or Mrk421 closer than 2◦ ; the dashed lines are the best Gaussian fit to the solid line histograms.
No. R.A.(◦ ) DEC.(◦ ) Sig.(σ) 1 38.9 13.9 4.2 2 39.2 31.9 4.1 3 52.5 20.7 4.1 4 66.8 12.3 4.3 70.2 11.9 4.7 5 6 70.4 18.0 4.1 7 78.9 18.9 4.0 83.3 21.8 4.8 8 9 88.8 30.2 5.2 10 165.5 38.4 5.2 11 210.6 7.5 4.0 12 221.7 32.7 4.2 13 253.2 58.8 4.2 14 278.3 38.4 4.4 286.6 5.5 4.6 15 16 301.7 8.6 4.1 304.4 36.7 4.0 17 18 309.5 49.1 4.3 19 309.9 39.6 4.5 20 318.0 40.6 4.4 21 333.0 34.8 4.0 Table 1 Prominent directions with significance values greater than 4σ. Crab and Mrk421 are detected with 4.8σ and 5.2σ respectively in a distance of 0.5◦ from their nominal position. Asterisks indicate the existence of at least one counterpart at a distance closer than 1◦ to this candidate.
Acknowledgments This work is supported in part by Grants-inAid for Scientific Research on Priority Area, for Scientific Research and also for International Science Research from the Ministry of Education, Science, Sports and Culture in Japan, and for International Science Research from the Committee of the Natural Science Foundation and the Chinese Academy of Sciences in China. REFERENCES 1. Amenomori, M., et al., 1999, ApJ.Lett, 525, L93. 2. Amenomori, M., et al., 2003, ApJ, 598, 242. 3. Amenomori, M., et al., 2000, ApJ, 532, 302. 4. Amenomori, M. et al., 2004, Phys. Rev. Lett., 93, 061101. 5. Atkins, R., 2004, astro-ph/0403097V1. 6. S. W. Cui., et al., 2003, Proc. 28th ICRC, 2315-2318. 7. Amenomori, M. et al., 2001, AIP, CP558, High Energy Gamma-Ray Astronomy, P557. 8. Amenomori,M., et al., 2001, Proc. 27th ICRC, 2, 573-576. 9. Hall, D.L. et al., 1997, Proc. 25th ICRC 2, 137. 10. Hall, D.L. et al., 1999, JGR 104, 6737. 11. Duldig, M.L., 2001, PASA, Vol 18, No.1.
A northern sky survey for both TeV cosmic-ray anisotropy and TeV γ-ray sources using the Tibet air shower array Hongbo Hu a
a
, for the Tibet ASγ Collaboration
Key Laboratory of Particle Astrophysics, IHEP, P.O. Box 918, Beijing 100049, P.R.China
Results on 2-dimensional TeV cosmic ray anisotropy and γ-ray point source search using data taken from TibetHD (Feb.1997-Sep.1999) and Tibet-III (Nov.1999-Oct.2001) arrays are presented. From 0◦ to 60◦ in declination, large scale anisotropy of cosmic-ray intensity at a magnitude of about 0.1% is observed. In the mean while, the well-known steady source Crab Nebula and the high state of the flare type source Mrk421 are detected.
1. Introduction Observation of high energy cosmic-rays (HECR) plays a very important role in our understanding of the origin, acceleration, propagation of cosmic-ray (CR) and other problems of astro-particle physics, among which two kinds of HECR phenomena have been widely investigated by many experiments. One is on the large scale anisotropy of CR intensity, and the other is the sky survey of TeV γ-ray point sources. Because of the low CR flux, those HECR observations have to be made by ground based experiments, such as Imaging Atmospheric Cherenkov Telescopes (IACT) and Extensive Air Shower (EAS) arrays. With good angular resolution and capability of eliminating overwhelming proton background, IACTs are more sensitive in searching for steady emission from TeV point sources. In contrast to IACT experiments, the advantages of a wide field of view and high duty cycle make EAS experiments more competitive in the sky survey of point sources and the measurement of CR’s large scale anisotropy. In the past decade, the Tibet ASγ experiment has successfully observed TeV γ-ray emission from the Crab Nebula [1], Mrk421 [2] and Mrk501 [3]. Recently, the ASγ experiment reported results on TeV CR’s anisotropy due to the Compton-Getting effect [4], and both Milagro [5] and ASγ [6]experiments published their results on northern sky survey of point sources. Here we present our preliminary results on the measurement of CR’s two-dimensional anisotropy and an 0920-5632/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2005.07.023
updated result on all sky survey of γ-ray’s steady sources. 2. Experiment The Tibet air shower experiment has operated at Yangbajing (90.53◦ E, 30.11◦N; 4,300m a.s.l.) in Tibet, China since 1990. It has been upgraded from the Tibet-I, Tibet-II/HD [7] to Tibet-III array [8]. Both Tibet-II/HD and TibetIII arrays have the same mode energy of about 3 TeV for proton-initiated showers and the same angular resolution to better than 0.9◦ . The events are selected by imposing the following five criteria on the reconstructed data: i) each of any four FT detectors should record a signal more than 1.25 particles; ii) the estimated core location should be inside the array; iii) the 2 sum of the number of particles per m detected in each detector ( ρF T ) should be larger than 15; iv) the residual error of direction reconstruction should be less than 1.0 m; v) the zenith angle of the incident direction should be less than 40◦ . After applying those data selection criteria, 1.5 × 109 events are selected from 555.9 live days’ running of the HD array and 5.5×109 events from 456.8 days’ running of the Tibet-III array. 3. Analysis Sitting on an almost horizontal plane, TibetHD and Tibet-III arrays have geometric size much larger than the lateral spread of a typical TeV cosmic-ray shower. As a result, the arrays are
122
H. Hu / Nuclear Physics B (Proc. Suppl.) 151 (2006) 121–124
equally powerful in receiving shower events at the same zenith angle. The so-called equi-zenith angle method was therefore developed to estimate the background number using sideband events. In this work, two techniques based on equi-zenith angle method are employed for the purpose of cross-check. 3.1. Method I (Short distance equi-zenith) To a good approximation, the large scale CR anisotropy can be neglected if the “off-source windows” are chosen to be close enough to the “onsource window”, as the large scale anisotropy only slowly changes in the sky. In this analysis, the sky is divided into 0.1◦ × 0.1◦ cells, from 0◦ to 360◦ in R.A. and from 0◦ to 60◦ in Dec. Circles of radii 0.9◦ centred at those cells are tested as onsource windows. 10 off-source windows are symmetrically aligned on both sides of the on-source window. The number of background events can 10 thus be estimated as Nbkg = i=1 Noff,i /10. In order to correct the non-uniformity of the azimuth distribution, the previous measurement was repeated 35 times (arbitrary number), starting from on-source window, successive dummy on-source windows divide 360◦ of R.A. direction into equipartition. The correction factor due to a non-uniform azimuth distribution is given by: 35
η = j=1 35 j=1
(dummy)
/35
(dummy)
/35
Non,j
Nbkg,j
. Finally, the corrected es-
timation on background number is obtained as: N ∗ bkg = ηNbkg . Consequently, the significance value can be simply calculated as: S=
∗ Non − Nbkg ∗ 1.0637 Nbkg
(1)
3.2. Method II (All distance equi-zenith) In this method, for each 1◦ zenith interval between 0◦ to 40◦ , non-uniform azimuth distributions are corrected by normalized azimuth distributions, to keep the total event number unchanged but make the azimuth distribution flat. All events in the equi-zenith belt except those inside the on-source window are taken as off-source events. In this case the large scale anisotropy effect can no longer be ignored. The celestial space
from 0◦ to 360◦ in R.A. and from 0◦ to 60◦ in Dec. are binned into cells with bin size 0.1◦ in both directions. In the observer’s coordinate, zenith angle θ is divided from 0◦ to 40◦ by step size 0.08◦ , and azimuth angle φ is binned by a zenith dependent bin width (0.08◦ / sinθ). For each local sidereal time (LST) interval bin (24s) m, a cell in (θ, φ) space (n, l) is mapped to a celestial cell (αi , δj ), i.e. R.A. bin i and Dec. bin j. Denoting I(i, j) the cosmic-ray intensity in cell (i, j), the equi-zenith condition leads to the following χ2 function, as in eqn.[2]. From eqn.[2] , I(i, j), as a function of R.A. and Dec. and its error ΔI(i, j), can be solved numerically by the iteration method, and CR’s large scale anisotropy can be measured in two dimensions. In order to search for γ-ray point sources, we should remove CR’s large scale anisotropy. As in Fig.1, the measured CR intensity in a belt of 10◦ width along Dec. is projected to R.A. direction, in 8◦ bin size. The centre of the belts moves from 0◦ to 60◦ , in 1◦ step size. Smoothing those curves and having them subtracted in the corresponding 1◦ width belt gives an anisotropy effect corrected CR intensity Icorr (i, j). With this large scale anisotropy subtracted cosmic-ray intensity, the excess number of events in cell (i, j) can be calculated as Ns (i, j) = N (i, j) − N (i, j) / Icorr (i, j) and its uncertainties Ns (i, j) = ΔIcorr (i, j) · N (i, j) / Icorr (i, j). Taking into account the angular resolution, events in a cone of 0.9◦ are summed. The significance for an on-source window centered at (i, j) can be calculated as: Ns (i , j ) i ,j ∈cone S (i, j) = (3) 2 ΔNs (i , j ) i ,j ∈cone
4. Results Fig.1 shows the intensity map containing the contribution from the large scale anisotropy with a demonstration in its subtraction when searching for γ ray point sources. This anisotropy distribution is consistent with the prediction made
123
H. Hu / Nuclear Physics B (Proc. Suppl.) 151 (2006) 121–124
χ2 =
m, n, l
2 Nobs (m, n, l) I(i, j) − l = l Nobs (m, n, l ) / I(i , j ) l = l 1 ; 2 Nobs (m, n, l) I 2 (i, j) + ( l = l 1)2 l = l Nobs (m, n, l ) / I (i , j )
(2)
Declination(deg)
Map of Intensity 60
995
50
0.9
5 00
1.0
40
0.9
30
20
990 0.9
0 02
0.9
1.0
10
15
1.00
985
010 1.0 5 01 1.0
0
98
5
02
0
1.0 0
50
100
150
200
250
300
350
Relative Intensity
RA(deg) 1.003 1.002 1.001 1 0.999 0.998 0.997
0
50
100
150
200
250
300
RA(deg) 350
Figure 1. Upper panel: contour plot of the relative intensity of cosmic-rays in the surveyed northern sky. Lower panel: a demonstration on the subtraction of cosmic-ray’s large scale anisotropy. The anisotropy distribution for the narrow belt centred at 9◦ in Dec. is estimated by projecting the intensity measurement from a wider belt (10◦ width is chosen). This distribution is further smoothed and later subtracted from the narrow belt. by Hall et al. [9–11]. The significance distributions from all directions are shown in Fig.2a and Fig.2b for the two analysis methods respectively. As for method II, the large scale anisotropy has to be subtracted. By changing the bin size and the smoothing parameters, the significance measurement is found to have 0.2σ systematic uncertainty. The perfect agreement on the negative side with a normal distribution (Fig.2a,2b) indicates that the systematic effects are well under control for both analyses. As for the positive side, a wider shoulder exists with significance values greater than 4σ. The dominant contributions are due to the well known stable γ-ray source Crab and transient source Mrk421. After removing their contributions, in a way that those cells in a distance shorter than 2◦ to the two sources are excluded, the dash-dotted line in Fig.2a,2b shows the significance distribution from the rest of the cells which agrees much better with a normal dis-
tribution but a discrepancy still clearly remains. This can be understood as local event number excess. Table 1 lists 21 prominent directions with significance values greater than 4σ. 5. Conclusion A wide northern sky survey for TeV γ-ray source and CR’s large scale anisotropy in Dec. band between 0◦ to 60◦ is performed using data of Tibet HD and III air shower arrays obtained from 1997 to 2001. The CR’s large scale anisotropy with a magnitude about 0.1% is observed in 2dimensions, and the well known Crab and Mrk421 are observed at 4.8σ and 5.2σ respectively. This indicates that the Tibet shower array is a sensitive apparatus for TeV γ-ray astronomy and has a potential to find flare type γ-ray sources, such as BL-Lac type AGNs. In addition, 19 other prominent directions with significances greater than 4σ are found. However, more statistics are necessary to either confirm or to rule out those candidates.
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H. Hu / Nuclear Physics B (Proc. Suppl.) 151 (2006) 121–124
χ / ndf 2
Number of events
Constant
5
10
Sigma
104
543.9 / 99
1.01 ± 0.00
(a)
Constant
105
414.3 / 98 8.562e+04 ± 72 -0.001202 ± 0.000685
Mean Sigma
104
1.006 ± 0.000
(b)
103
103 χ2 / ndf
102
Constant
102
543.9 / 99 8.527e+04 ± 71
Mean -0.0002687 ± 0.0006878
102
10
Sigma
10
1 10-1
-4
2
χ / ndf
-2
4
4.5
0
5
5.5
2
4 6 Significance
-0.001202 ± 0.000685 1.006 ± 0.000
1 10-1
10-1 -6
Mean Sigma
10
1 3.5
414.3 / 98 8.562e+04 ± 72
Constant
102
1.01 ± 0.00
10
1
10-1 -6
χ2 / ndf
Significances Distribution (Method II)
8.527e+04 ± 71 -0.0002687 ± 0.0006878
Mean
Number of events
Significances Distribution (Method I)
-4
3.5
-2
4
4.5
0
5
5.5
2
4 6 Significance
Figure 2. The significance distribution from (a) Method I (b) Method II. The solid lines are from all cells; the dash-dotted lines are from cells after removing those which are at a distance to Crab or Mrk421 closer than 2◦ ; the dashed lines are the best Gaussian fit to the solid line histograms.
No. R.A.(◦ ) DEC.(◦ ) Sig.(σ) 1 38.9 13.9 4.2 2 39.2 31.9 4.1 3 52.5 20.7 4.1 4 66.8 12.3 4.3 70.2 11.9 4.7 5 6 70.4 18.0 4.1 7 78.9 18.9 4.0 83.3 21.8 4.8 8 9 88.8 30.2 5.2 10 165.5 38.4 5.2 11 210.6 7.5 4.0 12 221.7 32.7 4.2 13 253.2 58.8 4.2 14 278.3 38.4 4.4 286.6 5.5 4.6 15 16 301.7 8.6 4.1 304.4 36.7 4.0 17 18 309.5 49.1 4.3 19 309.9 39.6 4.5 20 318.0 40.6 4.4 21 333.0 34.8 4.0 Table 1 Prominent directions with significance values greater than 4σ. Crab and Mrk421 are detected with 4.8σ and 5.2σ respectively in a distance of 0.5◦ from their nominal position. Asterisks indicate the existence of at least one counterpart at a distance closer than 1◦ to this candidate.
Acknowledgments This work is supported in part by Grants-inAid for Scientific Research on Priority Area, for Scientific Research and also for International Science Research from the Ministry of Education, Science, Sports and Culture in Japan, and for International Science Research from the Committee of the Natural Science Foundation and the Chinese Academy of Sciences in China. REFERENCES 1. Amenomori, M., et al., 1999, ApJ.Lett, 525, L93. 2. Amenomori, M., et al., 2003, ApJ, 598, 242. 3. Amenomori, M., et al., 2000, ApJ, 532, 302. 4. Amenomori, M. et al., 2004, Phys. Rev. Lett., 93, 061101. 5. Atkins, R., 2004, astro-ph/0403097V1. 6. S. W. Cui., et al., 2003, Proc. 28th ICRC, 2315-2318. 7. Amenomori, M. et al., 2001, AIP, CP558, High Energy Gamma-Ray Astronomy, P557. 8. Amenomori,M., et al., 2001, Proc. 27th ICRC, 2, 573-576. 9. Hall, D.L. et al., 1997, Proc. 25th ICRC 2, 137. 10. Hall, D.L. et al., 1999, JGR 104, 6737. 11. Duldig, M.L., 2001, PASA, Vol 18, No.1.