SEARCHES for STEADY SOURCES of COSMIC NEUTRINOS in ANTARES

J. P. Gómez-González, on behalf of the ANTARES CollaborationIFIC (CSIC-Universitat de València) [ e-mail: jpablo@ific.uv.es ]

Two searches were conducted: 1) An all-sky search anywhere in the visible sky [-90°, +48°]. 2) A candidate list search where the likelihood of the events is evaluated at the direction of an a priori selected list of sources (both galactic and extra-galactic) which are known to be TeV gamma-ray emitters.

Cosmic accelerator reach the detector, not deflected

absorbed by matter and EBL

p deflected by magnetic fields, GZK effect

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Data collected between years 2007 and 2010 were used. The total livetime of the analysis is 813 days, including a period of 183 days when only five detection lines were installed.

The reconstruction method used [3] is an iterative procedure consisting of different steps of increasing sophistication. The final solution is based on the maximization of the likelihood describing the hit time residuals. The quality of the track fit (Λ) can be used to reject muons mis-reconstructed as upgoing. Further event selection requires the uncertainty on the track direction (β) to be smaller than one degree.

The search for steady sources of cosmic neutrinos is the search for a localized excess of events above the background. In this analysis it was used an unbinned method based on the maximization of the likelihood function describing the events under two hypothesis; a) that only backgroung (bg) events are present, and b) that, additionally, there is a cluster of signal (sg) events in data. The number of hits used in the reconstruction was introduced in the likelihood to better discriminate between signal and background.

A likelihood ratio defines the test statistic (Q) necessary to calculate the significance of the analysis. This is done by comparing the observed Q value with the distributions obtained simulating a large number of bg only pseudo-experiments where the events are scrambled in right ascension.

The performance of a neutrino telescope is described by its angular resolution and effective area. The angular resolution governs the capability for signal to background rejection, while the effective area allows to translate event rates into neutrino fluxes. Both parameters are estimated from simulations. In this analysis atmospheric muons and atmospheric neutrinos were simulated using MUPAGE [4] and GENHEN [5] packages.

23rd European Cosmic Ray Symposium, Moscow (Russia) July, 3-7, 2012

References:[1] M. Ageron et. al., Nucl. Inst. and Meth. In Phys. Res., A 656 (2011) 11-38.[2] S. Adrián-Martínez et. al., Ap. J., A 743 (2011) L14.[3] A. Heijboer, PhD thesis, Universitet van Amsterdam, 2004.[4] G. Carminati, M. Bazzotti, A. Margiotta, and M. Spurio, Comp. Phys. Comm., 179 (2008) 915-923.[5] D. Bailey, PhD thesis, University of Oxford, 2002.[6] G. J. Feldman and R.D. Cousins, Phys. Rev. D57 (1998) 3873.[7] A. Kappes, J. Hinton, C. Stegmann and F. A. Aharonian, Astrophys. J. 656 (2007) 870.

The basic sensor element of the ANTARES detector is the Optical Module (OM); a pressure resistant glass sphere housing one 10” PMT and the accompanying electronics. There are 885 OMs grouping in triplets along 25 storeys in 12 lines.

Neutrinos can travel large distance without being deflected or absorbed, pointing back to their sources of emission

Neutrino telescopes aim to detect high energy neutrinos coming from the decay of secondary particles produced in the interaction of cosmic rays near their sites of acceleration. Detection of a source of cosmic neutrinos will be strong evidence for hadronic acceleration and will shed light on the problem of origin of cosmic rays. ANTARES [1] is the largest deep sea neutrino telescope in operation. The detector consists of a matrix of 885 light sensors distributed on 12 flexible lines which are anchored at a depth of 2475 m in the bottom of the Mediterranean Sea. Neutrinos with energies above the GeV and interacting in the surroundings of the ANTARES site can be detected by observing the Cherenkov light induced by the relativistic muons originated in charged current reactions.

No statistically significant excess of events has been found in any of the searches performed. The most significant cluster, which was found at coordinates (α,δ) = (-46.5°,-65.0°) in the all-sky search, has a post-trial p-value of 2.6%. In the candidate list, HESS J1023-575 presented the largest deviation from background, with 41% post-trial significance. Feldman-Cousins [6] upper limits at 90% CL for an E-2

ν spectrum were obtained, therefore, for the 51 sources in the candidate list.

1] INTRODUCTION

2] DATA SELECTION, TRACK RECONSTRUCTION and DETECTOR PERFORMANCE

3] SEARCH METHOD DESCRIPTION

4] SEARCHES AND RESULTS

At energies above 10 TeV the incident neutrino and the resulting muon are highly collinear. This feature make it possible to do astronomy using neutrinos.

5] CONCLUSIONS

A search for time-integrated sources of cosmic neutrinos has been performed using 813 of data livetime gathered during 4 years of the ANTARES detector operation. Any significant excess of events was found in data. The most signal-like cluster (2.2 σ, double sided convention) of events was found at coordinates (α,δ) = (-46.5º,-65.0º) in the full-sky search. In the candidate list search, HESS J1023-575 had the lowest p-value with 41% probability to be produced by the background. Upper limits on the E-2

ν flux were obtained for 51 sources in the Southern sky. These are 2.7 times more restrictive than those obtained in [2]. Limits and MRFs have been also calculated assuming neutrino flux emission models from [7] for the gamma-ray sources RX J1713.7-3946 and Vela X.

The final sample of neutrino candidate events was obtained for Λ≥ -5.2 for an optimal discovery potential (Sec 3). This consists of only 3058 events from a total of ~4x108 triggered events. Simulations agree well with data, for a neutrino (muon) flux uncertainty of 30% (50%). Systematic effects are considered analogously to [2].

5 events were found within 1 degree of the most-significance cluster location

Upper limits on the E-2ν flux obtained for sources

in the candidate list; results from other neutrino experiments are included for comparison

P-value skymap of pre-trial significances

Neutrino flux models, 90% CL upper limits, and MRF for RX J1713.7-3946 (left) and Vela X (right)Test statistic for the only bg case and

when 3, 6 or 9 sg events are addedDiscovery power using (or not) the

number of hits in the likelihood search

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Neutrino effective area as a

function of the energy and for three declination bands

Distributions of the quality of the

reconstruction parameter (left), the reconstructed zenith

angle (right), and error estimate (up)

Abstract: ANTARES is the largest Neutrino Telescope operating in the Northern Hemisphere. Placed at the bottom of the Mediterranean Sea, about 40 km off the coast of Toulon (France), it is composed by 885 photomultiplier-tubes (PMTs) which detect the Cherenkov light emitted in the interaction of high energy neutrinos close or inside the detector. One of the main goals of the experiment is the identification of a source of cosmic neutrinos, which are likely to be produced in the interaction of high energy hadrons in several astrophysical scenarios. The discovery of a source of neutrinos will be, thus, a clear indication for hadronic acceleration mechanisms and shed light on the problem of the origin of cosmic rays (CRs). This works present such a search using 813 days of data collected in ANTARES between years 2007 and 2010. Not having found any significance excess of events upper limits to the E-2

ν flux are given for sources in the Southern sky. Additionally, results using specific emission models for two well know gamma-ray TeV sources are discussed.

Assuming flux models discussed in [7] upper limits and MRFs have been obtained for the supernova remnant RX J1713.7-3946 and the pulsar wind nebula Vela X. In both cases the limits are the most restrictive ones obtained to date for the considered emission models.