- Email: [email protected]
Search for point-like sources with the ANTARES neutrino telescope
Nuclear Instruments and Methods in Physics Research A 630 (2011) 214–217
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Search for point-like sources with the ANTARES neutrino telescope Francisco Salesa-Greus ´n de Paterna, CSIC-Universitat de Vale ncia, Apdo. de Correos 22085, 46071 Valencia, Spain IFIC - Instituto de Fı´sica Corpuscular, Edificios de Investigacio
For the ANTARES Collaboration a r t i c l e in f o
a b s t r a c t
Available online 12 June 2010
The construction of the ANTARES neutrino telescope in the Mediterranean Sea next to the French coast of Toulon is completed since May 2008. The detector comprises about 900 photomultipliers distributed in 12 lines, at a depth of about 2500 m. Before its completion, the detector was operational with 5 out of the total 12 lines from February to December 2007. The data recorded in that period have been analysed. The search for point-like sources has been carried out using two strategies: a search within a list of several potential cosmic neutrino emitters and an all-sky scanning. The results obtained provide competitive limits for the Southern sky. & 2010 Elsevier B.V. All rights reserved.
Keywords: Neutrino telescope Point sources
1. Introduction
2. Performance of the 2007 detector
In May 2008 the ANTARES collaboration [1] finished the construction and deployment of the largest undersea neutrino telescope. It is located 40 km off the French coast of Toulon at a latitude of 423 50uN. The detector consists of about 900 photomultipliers (PMT) distributed in 12 strings, anchored at about 2500 m depth, each one equipped with 25 floors composed mainly by a triplet of PMTs. ANTARES detects the Cherenkov light induced by the charged particles produced in the interaction of high-energy neutrinos with the matter surrounding the detector. This light propagates in the sea water, and then it is recorded by the three-dimensional array of PMTs. The charge and time information of the hits produced is used to reconstruct the neutrino direction. The events are selected to be up-going in order to reject the atmospheric muon background which, even at 2500 m depth, is six orders of magnitude greater than the atmospheric neutrino flux. One of the physics goals of the ANTARES experiment is the search for point cosmic neutrino sources. The detection of highenergy neutrinos from cosmic objects like active galactic nuclei, gamma rays bursts, supernova remnants, etc. is crucial in order to understand the most energetic processes of the universe as, for instance, the origin of cosmic rays. Due to the small number of events expected from potential sources, different search algorithms have been implemented in order to obtain as much as possible information from the data. In this paper we are going to review the methods used in ANTARES for this search and the results obtained when applied to the 2007 data sample.
The sample used for the analysis presented in this paper comprises the data taken from February 2nd to December 8th 2007. During that period the detector was operating with 5 out of 12 lines. After excluding some periods mainly due to the high optical background caused by the sea bioluminiscence, the final data sample contains 140 days live-time. In the raw data the time and charge of every hit is recorded. This is used in the track reconstruction process based on a maximum-likelihood algorithm which finds the best track [2]. The statistic L ¼ logðLÞ=NDOF þ 0:1ðNcomp 1Þ is the quality track parameter. It takes into account the logarithm of the likelihood per degree of freedom and a factor which counts the number of compatible solutions (i.e. within 11 with respect to the preferred track) found by the reconstruction algorithm. The huge amount of reconstructed tracks can be reduced by selecting the appropriate L value. For the 2007 data sample the chosen cut is L 44:7. Tracks close to the horizon are more likely to be mis-reconstructed so an additional cut rejecting tracks with angles greater than 101 below the horizon was applied. At the end, the resulting sample is composed by 94 events; contribution from misreconstructed down-going events is supposed to be less than 10% according to the simulations. For the analysis of the data two important pieces of information are needed, the angular resolution and the effective area. The angular resolution is defined as the median of the angle between the reconstructed muon track and the actual neutrino direction. According to Monte Carlo (MC) simulation, the attainable angular resolution with 5 lines of ANTARES is better than 0.51 for a En 410 TeV as it is shown in Fig. 1. The good agreement data—MC for the time residuals of the selected events (Fig. 2) gives confidence in the angular resolution estimation.
E-mail address: sagreus@ific.uv.es 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.068
F. Salesa-Greus / Nuclear Instruments and Methods in Physics Research A 630 (2011) 214–217
3
angular resolution [°]
2.5
2
1.5
1
0.5
0 2.5
3
4 4.5 5 log10 (Eν [GeV])
3.5
5.5
6
6.5
7
Fig. 1. Angular resolution for the 2007 configuration. The points represent the medians of the angular error distribution for neutrinos. Below 10 TeV the angular resolution is dominated by the kinematic angle between the neutrino trajectory and the muon track. Above 10 TeV, it depends on the quality of the reconstruction.
1000 900
number of hits
800
215
point out the accumulation of those events over the dominant background. The search methods for point-like sources can be divided into two types, binned and unbinned. The binned methods are based in simple counting of the number of events inside a bin (nb), for instance a grid or a cone, drawn in the sky map of the events of the sample. The number of events inside a bin follows Poisson statistics. Therefore, they have a discrete test statistic which is basically the Poisson probability of detecting nb events, or higher, inside the bin. When the number of events detected in a bin is well above the expectation from the background a discovery can be claimed within a certain confidence level. The binned method used for the analysis of the ANTARES data is a method using a cone shape for the search bin, centred at the source location. The optimization is done on the basis of the minimization of the Model Rejection Factor (MRF) [3]. The unbinned methods take advantage of all the available information such as energy of the events, distribution of the events and any other information found to be relevant to discriminate the signal with respect to the background. This makes them perform better than the binned ones. In the unbinned methods the signal and background events have each one an associated probability. The methods maximize the likelihood between both, and use a continuous test statistic, which is often a sort of likelihood ratio. As in the binned method this test statistic sets the probability of a cluster to be produced by the background. The unbinned method used for the analysis is the Expectation Maximization (EM) algorithm [4]. It is a pattern recognition algorithm that analytically maximizes the likelihood in finite mixture models. These mixture models are different groups of data described by different density components (data and background). The EM uses a pre-clustering algorithm in order to find a set of candidate clusters that exceed a given number of events. The algorithm is applied to the pre-selected clusters.
700 4. Results
600 500 400 300 200 100 0 -20
-10
0
10 20 time residual [ns]
30
40
Fig. 2. Time residual distribution (i.e. the measured time minus the direct propagation time) of the 94 selected events for the point-like source analysis (crosses) compared with Monte Carlo simulation (dashes).
The other main piece of information is the effective area. It is defined as a 100% efficient surface which detects the same number of events than the actual detector. This magnitude allows us to relate the flux of the source with the event rate detected. The effective area increases with the energy of the neutrino, because of the increasing cross-section. For the 5 lines configuration the neutrino effective area becomes greater than 1 m2 for En 4 100 TeV.
3. Methods for point-like sources search Due to the small number of events expected from the sources, it is of the utmost importance to have an efficient method able to
Two point-like source analyses have been performed with the 2007 data sample. First, a list of several potential cosmic neutrino emitters has been chosen in order to search for an accumulation of events from these directions. Afterwards, in the second analysis, an all-sky scanning has been performed. It is important to notice that all the analysis process has been done following a blinding policy where all the selection cuts has been chosen a priori in order not to introduce any bias in the analysis. At the end 94 events were selected for the point-like source analysis for the whole period of 140 live-time days of the 2007 data sample. The unblinded sky maps with these events are plotted in Fig. 3. As the unbinned methods are more powerful, the EM algorithm was used as the method for providing the official results, keeping the binned method (cone shape bin) as a crosscheck of the unbinned results. The results are presented using the P-value quantity which is given by the test statistic and is basically the probability of the background to produce the observed signal. 4.1. List of potential sources A list of 24 candidates sources (galactic and extragalactic) has been selected as the most promising candidates to be detected with the 2007 data sample. In addition, the IceCube most significant excess with 22-line data [5] has also been included giving a final list of 25 sources. The list with the source position in the sky (d, RA), the number of events inside the optimum cone for
216
F. Salesa-Greus / Nuclear Instruments and Methods in Physics Research A 630 (2011) 214–217
60°
0° 45°
90° 135° 180°
225° 270°
360° 315°
ANTARES 5-Lines (140 days) MACRO Source List (2299.5 days) Super-K Source List (1645.9 days) AMANDA-II Source List (1387 days) IC22 Source List (275.7 days) ANTARES 12-Lines (1 yr)
10-7 E2 dΦνμ90% / dE (TeV cm-2 s-1)
30°
0°
-30°
-60° -90°
10-8
10-9
10-10
Fig. 3. Sky map in equatorial coordinates (right ascension vs declination) with the 94 events selected for the point-like source analysis from the 2007 data sample.
10-11 Table 1 List of cosmic neutrino source candidates with their corresponding location in the sky. Source name
d (deg)
RA (deg)
nb
P
f90
PSR B1259-63 RCW 86 HESS J1023-575 CIR X-1 HESS J1614-518 GX 339 RX J0852.0-4622 RX J1713.7-3946 PKS 2155-304 Galactic center W28 LS 5039 HESS J1837-069 SS 433 HESS J0632+ 057 ESO 139-G12 PKS 2005-489 Centaurus A PKS 0548-322 H 2356-309 1ES 1101-232 1ES 0347-121 3C 279 RGB J0152+ 017 IC22 hotspot
63.83 62.48 57.76 57.17 51.82 48.79 46.37 39.75 30.22 29.01 23.34 14.83 6.95 4.98 5.81 59.94 48.82 43.02 32.27 30.63 23.49 11.99 5.79 1.79 11.4
195.70 220.68 155.83 230.17 243.58 255.70 133.00 258.25 329.72 266.42 270.43 276.56 279.41 287.96 98.24 264.41 302.37 201.36 87.67 359.78 165.91 57.35 194.05 28.17 153.4
0 0 1 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0
1 1 0.004 1 0.088 1 1 1 1 0.055 1 1 1 1 1 1 1 1 1 1 1 1 0.030 1 1
3.1 3.3 7.6 3.3 5.6 3.8 4.0 4.3 4.2 6.8 4.8 5.0 5.9 7.3 7.4 3.4 3.7 3.9 4.3 4.2 4.6 5.0 9.2 7.0 9.1
The results given are the P-value (P) for both unbinned (official) and binned (crosscheck) methods. The numbers in the last row are the upper limit in number of events for the unbinned method.
the binned search (nb), the P-value (P) obtained with unbinned method, and the corresponding upper limit at 90% C.L. ðf90 Þ are summarized in Table 1. The upper limit value is the normalization constant of the differential muon-neutrino flux assuming a E 2 spectrum (i.e. E2 dfnm =dE r f90 1010 TeV cm2 s1 Þ. It is important to notice that the P-values presented in the table are ‘‘pre-trail’’ values, that is to say, they do not take into account the fact that in the analysis we look at several sources at the same time. In this case the pre-trail values should be corrected multiplying by the number of sources. That gives the ‘‘post-trial’’ values. Applying the EM algorithm to the 2007 data sample, four sources of the list give P-values different to 1, the lowest P-value is obtained in the HESS J1023-575 source with a pre-trial P-value of 0.004. The post-trial value becomes 0.1 which means 1:6s (double sided) significance. For the cone method the same four sources are the ones giving P-values different to 1. The radius of
-80
-60
-40
-20 0 20 declination [°]
40
60
80
Fig. 4. Upper limits obtained with 2007 data sample (5 lines, 140 days live-time), compared with the expectation of 1 year with the complete detector (12 lines), and other experiments.
the cones used range from 31 to 4.51. All the four sources contain one single event inside the cone. Not having any significant excess, the upper limits are computed for the source list. The flux limits obtained are presented in Fig. 4. As can be see in the plot, the limits obtained with the 2007 ANTARES data are comparable with those provided by previous experiments also looking at the Southern sky. This is mainly because of the large effective area of ANTARES for high energies (10–100 TeV). It is important to notice that a significant improvement is expected with the complete detector (solid line). 4.2. All-sky scan For the all-sky scan no a priori assumption is done for the location of a source in the sky. The EM algorithm pre-clustering looks for doubles of events separated less than 51. The most significant excess was found at (d ¼ 63:73 , RA¼243.91), with a post-trial P-value of 0.3 which corresponds to a 1s excess. The binned method, in an independent search, founds four doubles. One of them, the most significant, is also found by the EM confirming therefore the result.
5. Conclusions The ANTARES neutrino telescope is completed and taking data smoothly. The first analysis searching for point-like cosmic sources has been performed with the 2007 data sample where 5 out of the total 12 lines were taking data. A sample of 94 events were selected after the optimization of the cuts on a blinded data. The search methods used were, one binned based on cone shaped bins, and another unbinned based on the EM algorithm. Two analyses were performed. In the first one a list of potential candidate sources were selected. No significant excess was detected for any of them, being the most significant one the HESS J1023-575 with a pre-trial P-value of 0.004 which post-trial becomes a 1:6s (double sided) significance. With this result the corresponding flux limits were obtained. In the all-sky search analysis the most significant excess was found at (d ¼ 63:73 ,
F. Salesa-Greus / Nuclear Instruments and Methods in Physics Research A 630 (2011) 214–217
RA ¼243.91), with a post-trial P-value of 0.3 which corresponds to a 1s excess. References [1] J. Carr, for the ANTARES collaboration, J. Phys. Conf. Ser. 136 (2008) 022047.
217
[2] A. Heijboer, Track reconstruction and point source searches with ANTARES, Ph.D. Thesis, Universiteit van Amsterdam, 2004. [3] G.C. Hill, K. Rawlins, Astroparticle Physics 19 (2003) 393402. [4] J.A. Aguilar, J.J. Hernandez-Rey, Astroparticle Physics 29 (2008) 117. [5] R. Abbasi, et al., IceCube collaboration, Search for Point Sources of High Energy Neutrinos with Final Data from AMANDA-II, eprint arXiv: 0905.2253.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Search for point-like sources with the ANTARES neutrino telescope Francisco Salesa-Greus ´n de Paterna, CSIC-Universitat de Vale ncia, Apdo. de Correos 22085, 46071 Valencia, Spain IFIC - Instituto de Fı´sica Corpuscular, Edificios de Investigacio
For the ANTARES Collaboration a r t i c l e in f o
a b s t r a c t
Available online 12 June 2010
The construction of the ANTARES neutrino telescope in the Mediterranean Sea next to the French coast of Toulon is completed since May 2008. The detector comprises about 900 photomultipliers distributed in 12 lines, at a depth of about 2500 m. Before its completion, the detector was operational with 5 out of the total 12 lines from February to December 2007. The data recorded in that period have been analysed. The search for point-like sources has been carried out using two strategies: a search within a list of several potential cosmic neutrino emitters and an all-sky scanning. The results obtained provide competitive limits for the Southern sky. & 2010 Elsevier B.V. All rights reserved.
Keywords: Neutrino telescope Point sources
1. Introduction
2. Performance of the 2007 detector
In May 2008 the ANTARES collaboration [1] finished the construction and deployment of the largest undersea neutrino telescope. It is located 40 km off the French coast of Toulon at a latitude of 423 50uN. The detector consists of about 900 photomultipliers (PMT) distributed in 12 strings, anchored at about 2500 m depth, each one equipped with 25 floors composed mainly by a triplet of PMTs. ANTARES detects the Cherenkov light induced by the charged particles produced in the interaction of high-energy neutrinos with the matter surrounding the detector. This light propagates in the sea water, and then it is recorded by the three-dimensional array of PMTs. The charge and time information of the hits produced is used to reconstruct the neutrino direction. The events are selected to be up-going in order to reject the atmospheric muon background which, even at 2500 m depth, is six orders of magnitude greater than the atmospheric neutrino flux. One of the physics goals of the ANTARES experiment is the search for point cosmic neutrino sources. The detection of highenergy neutrinos from cosmic objects like active galactic nuclei, gamma rays bursts, supernova remnants, etc. is crucial in order to understand the most energetic processes of the universe as, for instance, the origin of cosmic rays. Due to the small number of events expected from potential sources, different search algorithms have been implemented in order to obtain as much as possible information from the data. In this paper we are going to review the methods used in ANTARES for this search and the results obtained when applied to the 2007 data sample.
The sample used for the analysis presented in this paper comprises the data taken from February 2nd to December 8th 2007. During that period the detector was operating with 5 out of 12 lines. After excluding some periods mainly due to the high optical background caused by the sea bioluminiscence, the final data sample contains 140 days live-time. In the raw data the time and charge of every hit is recorded. This is used in the track reconstruction process based on a maximum-likelihood algorithm which finds the best track [2]. The statistic L ¼ logðLÞ=NDOF þ 0:1ðNcomp 1Þ is the quality track parameter. It takes into account the logarithm of the likelihood per degree of freedom and a factor which counts the number of compatible solutions (i.e. within 11 with respect to the preferred track) found by the reconstruction algorithm. The huge amount of reconstructed tracks can be reduced by selecting the appropriate L value. For the 2007 data sample the chosen cut is L 44:7. Tracks close to the horizon are more likely to be mis-reconstructed so an additional cut rejecting tracks with angles greater than 101 below the horizon was applied. At the end, the resulting sample is composed by 94 events; contribution from misreconstructed down-going events is supposed to be less than 10% according to the simulations. For the analysis of the data two important pieces of information are needed, the angular resolution and the effective area. The angular resolution is defined as the median of the angle between the reconstructed muon track and the actual neutrino direction. According to Monte Carlo (MC) simulation, the attainable angular resolution with 5 lines of ANTARES is better than 0.51 for a En 410 TeV as it is shown in Fig. 1. The good agreement data—MC for the time residuals of the selected events (Fig. 2) gives confidence in the angular resolution estimation.
E-mail address: sagreus@ific.uv.es 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.068
F. Salesa-Greus / Nuclear Instruments and Methods in Physics Research A 630 (2011) 214–217
3
angular resolution [°]
2.5
2
1.5
1
0.5
0 2.5
3
4 4.5 5 log10 (Eν [GeV])
3.5
5.5
6
6.5
7
Fig. 1. Angular resolution for the 2007 configuration. The points represent the medians of the angular error distribution for neutrinos. Below 10 TeV the angular resolution is dominated by the kinematic angle between the neutrino trajectory and the muon track. Above 10 TeV, it depends on the quality of the reconstruction.
1000 900
number of hits
800
215
point out the accumulation of those events over the dominant background. The search methods for point-like sources can be divided into two types, binned and unbinned. The binned methods are based in simple counting of the number of events inside a bin (nb), for instance a grid or a cone, drawn in the sky map of the events of the sample. The number of events inside a bin follows Poisson statistics. Therefore, they have a discrete test statistic which is basically the Poisson probability of detecting nb events, or higher, inside the bin. When the number of events detected in a bin is well above the expectation from the background a discovery can be claimed within a certain confidence level. The binned method used for the analysis of the ANTARES data is a method using a cone shape for the search bin, centred at the source location. The optimization is done on the basis of the minimization of the Model Rejection Factor (MRF) [3]. The unbinned methods take advantage of all the available information such as energy of the events, distribution of the events and any other information found to be relevant to discriminate the signal with respect to the background. This makes them perform better than the binned ones. In the unbinned methods the signal and background events have each one an associated probability. The methods maximize the likelihood between both, and use a continuous test statistic, which is often a sort of likelihood ratio. As in the binned method this test statistic sets the probability of a cluster to be produced by the background. The unbinned method used for the analysis is the Expectation Maximization (EM) algorithm [4]. It is a pattern recognition algorithm that analytically maximizes the likelihood in finite mixture models. These mixture models are different groups of data described by different density components (data and background). The EM uses a pre-clustering algorithm in order to find a set of candidate clusters that exceed a given number of events. The algorithm is applied to the pre-selected clusters.
700 4. Results
600 500 400 300 200 100 0 -20
-10
0
10 20 time residual [ns]
30
40
Fig. 2. Time residual distribution (i.e. the measured time minus the direct propagation time) of the 94 selected events for the point-like source analysis (crosses) compared with Monte Carlo simulation (dashes).
The other main piece of information is the effective area. It is defined as a 100% efficient surface which detects the same number of events than the actual detector. This magnitude allows us to relate the flux of the source with the event rate detected. The effective area increases with the energy of the neutrino, because of the increasing cross-section. For the 5 lines configuration the neutrino effective area becomes greater than 1 m2 for En 4 100 TeV.
3. Methods for point-like sources search Due to the small number of events expected from the sources, it is of the utmost importance to have an efficient method able to
Two point-like source analyses have been performed with the 2007 data sample. First, a list of several potential cosmic neutrino emitters has been chosen in order to search for an accumulation of events from these directions. Afterwards, in the second analysis, an all-sky scanning has been performed. It is important to notice that all the analysis process has been done following a blinding policy where all the selection cuts has been chosen a priori in order not to introduce any bias in the analysis. At the end 94 events were selected for the point-like source analysis for the whole period of 140 live-time days of the 2007 data sample. The unblinded sky maps with these events are plotted in Fig. 3. As the unbinned methods are more powerful, the EM algorithm was used as the method for providing the official results, keeping the binned method (cone shape bin) as a crosscheck of the unbinned results. The results are presented using the P-value quantity which is given by the test statistic and is basically the probability of the background to produce the observed signal. 4.1. List of potential sources A list of 24 candidates sources (galactic and extragalactic) has been selected as the most promising candidates to be detected with the 2007 data sample. In addition, the IceCube most significant excess with 22-line data [5] has also been included giving a final list of 25 sources. The list with the source position in the sky (d, RA), the number of events inside the optimum cone for
216
F. Salesa-Greus / Nuclear Instruments and Methods in Physics Research A 630 (2011) 214–217
60°
0° 45°
90° 135° 180°
225° 270°
360° 315°
ANTARES 5-Lines (140 days) MACRO Source List (2299.5 days) Super-K Source List (1645.9 days) AMANDA-II Source List (1387 days) IC22 Source List (275.7 days) ANTARES 12-Lines (1 yr)
10-7 E2 dΦνμ90% / dE (TeV cm-2 s-1)
30°
0°
-30°
-60° -90°
10-8
10-9
10-10
Fig. 3. Sky map in equatorial coordinates (right ascension vs declination) with the 94 events selected for the point-like source analysis from the 2007 data sample.
10-11 Table 1 List of cosmic neutrino source candidates with their corresponding location in the sky. Source name
d (deg)
RA (deg)
nb
P
f90
PSR B1259-63 RCW 86 HESS J1023-575 CIR X-1 HESS J1614-518 GX 339 RX J0852.0-4622 RX J1713.7-3946 PKS 2155-304 Galactic center W28 LS 5039 HESS J1837-069 SS 433 HESS J0632+ 057 ESO 139-G12 PKS 2005-489 Centaurus A PKS 0548-322 H 2356-309 1ES 1101-232 1ES 0347-121 3C 279 RGB J0152+ 017 IC22 hotspot
63.83 62.48 57.76 57.17 51.82 48.79 46.37 39.75 30.22 29.01 23.34 14.83 6.95 4.98 5.81 59.94 48.82 43.02 32.27 30.63 23.49 11.99 5.79 1.79 11.4
195.70 220.68 155.83 230.17 243.58 255.70 133.00 258.25 329.72 266.42 270.43 276.56 279.41 287.96 98.24 264.41 302.37 201.36 87.67 359.78 165.91 57.35 194.05 28.17 153.4
0 0 1 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0
1 1 0.004 1 0.088 1 1 1 1 0.055 1 1 1 1 1 1 1 1 1 1 1 1 0.030 1 1
3.1 3.3 7.6 3.3 5.6 3.8 4.0 4.3 4.2 6.8 4.8 5.0 5.9 7.3 7.4 3.4 3.7 3.9 4.3 4.2 4.6 5.0 9.2 7.0 9.1
The results given are the P-value (P) for both unbinned (official) and binned (crosscheck) methods. The numbers in the last row are the upper limit in number of events for the unbinned method.
the binned search (nb), the P-value (P) obtained with unbinned method, and the corresponding upper limit at 90% C.L. ðf90 Þ are summarized in Table 1. The upper limit value is the normalization constant of the differential muon-neutrino flux assuming a E 2 spectrum (i.e. E2 dfnm =dE r f90 1010 TeV cm2 s1 Þ. It is important to notice that the P-values presented in the table are ‘‘pre-trail’’ values, that is to say, they do not take into account the fact that in the analysis we look at several sources at the same time. In this case the pre-trail values should be corrected multiplying by the number of sources. That gives the ‘‘post-trial’’ values. Applying the EM algorithm to the 2007 data sample, four sources of the list give P-values different to 1, the lowest P-value is obtained in the HESS J1023-575 source with a pre-trial P-value of 0.004. The post-trial value becomes 0.1 which means 1:6s (double sided) significance. For the cone method the same four sources are the ones giving P-values different to 1. The radius of
-80
-60
-40
-20 0 20 declination [°]
40
60
80
Fig. 4. Upper limits obtained with 2007 data sample (5 lines, 140 days live-time), compared with the expectation of 1 year with the complete detector (12 lines), and other experiments.
the cones used range from 31 to 4.51. All the four sources contain one single event inside the cone. Not having any significant excess, the upper limits are computed for the source list. The flux limits obtained are presented in Fig. 4. As can be see in the plot, the limits obtained with the 2007 ANTARES data are comparable with those provided by previous experiments also looking at the Southern sky. This is mainly because of the large effective area of ANTARES for high energies (10–100 TeV). It is important to notice that a significant improvement is expected with the complete detector (solid line). 4.2. All-sky scan For the all-sky scan no a priori assumption is done for the location of a source in the sky. The EM algorithm pre-clustering looks for doubles of events separated less than 51. The most significant excess was found at (d ¼ 63:73 , RA¼243.91), with a post-trial P-value of 0.3 which corresponds to a 1s excess. The binned method, in an independent search, founds four doubles. One of them, the most significant, is also found by the EM confirming therefore the result.
5. Conclusions The ANTARES neutrino telescope is completed and taking data smoothly. The first analysis searching for point-like cosmic sources has been performed with the 2007 data sample where 5 out of the total 12 lines were taking data. A sample of 94 events were selected after the optimization of the cuts on a blinded data. The search methods used were, one binned based on cone shaped bins, and another unbinned based on the EM algorithm. Two analyses were performed. In the first one a list of potential candidate sources were selected. No significant excess was detected for any of them, being the most significant one the HESS J1023-575 with a pre-trial P-value of 0.004 which post-trial becomes a 1:6s (double sided) significance. With this result the corresponding flux limits were obtained. In the all-sky search analysis the most significant excess was found at (d ¼ 63:73 ,
F. Salesa-Greus / Nuclear Instruments and Methods in Physics Research A 630 (2011) 214–217
RA ¼243.91), with a post-trial P-value of 0.3 which corresponds to a 1s excess. References [1] J. Carr, for the ANTARES collaboration, J. Phys. Conf. Ser. 136 (2008) 022047.
217
[2] A. Heijboer, Track reconstruction and point source searches with ANTARES, Ph.D. Thesis, Universiteit van Amsterdam, 2004. [3] G.C. Hill, K. Rawlins, Astroparticle Physics 19 (2003) 393402. [4] J.A. Aguilar, J.J. Hernandez-Rey, Astroparticle Physics 29 (2008) 117. [5] R. Abbasi, et al., IceCube collaboration, Search for Point Sources of High Energy Neutrinos with Final Data from AMANDA-II, eprint arXiv: 0905.2253.