ANTARES-IceCube combined search for neutrino point- sources in the Southern Hemisphere

Abstract. A search for cosmic neutrino point-like sources covering the Southern hemisphere is presented, using 9 years and 7 years of data from the ANTARES and IceCube neutrino telescopes, respectively. The advantageous field of view of ANTARES as well as the high statistics provided by IceCube are exploited to open a window in the Southern sky where the sensitivity to point-sources improves by a factor ∼ 2 compared to individual analyses. An unbinned maximum likelihood method is used to search for a localised excess of muon events over the expected background. Before applying the method to the unblinded dataset, the sensitivity and discovery potential of the search are computed through dedicated pseudo-experiments.


Introduction
Neutrino telescopes can play a crucial role in investigating the physical properties of astrophysical high-energy emitters or in the possible discovery of new astrophysical sources. The high-energy field is currently led by the IceCube [1] and ANTARES [2] experiments. The different characteristics of the two telescopes, in particular the larger instrumented volume of IceCube and the better visibility of the Southern sky for neutrino energies below 100 TeV of ANTARES, allow for a gain in sensitivity by combining the datasets from both experiments in a joint search for point-sources in the Southern Hemisphere. The data samples employed in this search are described in Section 2. The analysis method is presented in Section 3, while in Section 4 the expected performance of the search is shown. The conclusions are given in Section 5. Due to the different layouts and locations of the telescopes, each sample has a different efficiency for detecting events from potential sources. The larger size of IceCube guarantees a larger effective area for neutrino energies above ∼ 100 TeV. However, in order to minimise the down-going background of atmospheric muons in the sample of through-going tracklike events when looking at the Southern sky, the IceCube point-source analysis introduced a declination-dependent energy cut which strongly suppresses low energy events in the final data sample. ANTARES, which can use the Earth as a filter against atmospheric muons in the Southern sky, maintains a larger effective area in this energy and declination range. The relative contribution of each sample j, C j (δ, dΦ/dE), defined as the ratio of the expected number of signal events for the given sample to that for all samples, depends on the source spectrum, dΦ/dE, and declination, δ. Figure 1 shows the relative fraction of signal events for an unbroken E −γ spectrum, with γ = 2 (left) and γ = 2.5 (right) as a function of the source declination. While for an E −2 spectrum all samples significantly contribute to most of the Southern sky, for a softer spectrum, the contribution of high energy neutrinos is lower, and therefore the relative contribution of the ANTARES sample increases.
Relative fraction of signal events Relative fraction of signal events A maximum likelihood estimation is performed to identify clusters of cosmic neutrinos from point-like sources over the background of randomly distributed atmospheric neutrinos. The likelihood exploits the energy and directional information of each event to distinguish signallike clusters from clusters produced by random agglomerations of background events. The used likelihood is defined as where j indicates one of the seven data samples (ANTARES tracks, ANTARES showers, IC40, IC59, IC79, IC86 or 2012-2015), i indicates an event belonging to the j-th sample, S j i is the value of the signal probability density function (PDF) for the i-th event in the j-th sample, B j i indicates the value of the background PDF, N j is the total number of events in the j-th sample, and n j s is the number of signal events fitted in the j-th sample. The number of fitted signal events n j s in each sample is related to the total fitted number of signal events n s by the relative contribution of each sample (introduced in Section 2), C j (δ, dΦ/dE), through the relation n j s = n s · C j (δ, dΦ/dE). The signal and background PDFs are given by the product of a directional and an energy term. The definition of these PDFs is slightly different for the ANTARES and the IceCube samples and can be found in [3] and [4], respectively.
In the likelihood maximisation, the values of the number of signal events n s and the signal spectral index γ are left free to vary. Values between 1 and 4 are allowed for the spectral index. In order to estimate the signal-likeness of a cluster and to determine the significance of any observation, a test statistic is computed as wheren s andγ are the values that maximise the likelihood. The behavior of Q for different signal strengths is determined from pseudo-experiments (PEs). In each PE, a skymap is generated with a number of background events equal to the number of selected events in data. The declination distribution is drawn from the measured one, while the right ascension distribution is assumed to be uniform. In addition, signal events are injected according to the investigated spectrum at a given source position.

Expected performances
In order to estimate the potential of the combined search to discover a neutrino source, the 5σ discovery flux and the 90% C.L. sensitivity are calculated for an E −γ neutrino spectrum, with γ equal to 2.0 and 2.5. The results are shown in Fig. 2 in comparison to the curves from the individual IceCube and ANTARES analyses.
It is found that the sensitivity and the discovery flux improve by a factor ∼ 2 in different regions of the Southern sky, depending on the energy spectrum of the source. For an E −2 spectrum, the most relevant improvement is achieved in a region of the sky that is centred approximately at the declination of the Galactic Centre (δ = −29 • ). δ sin

Conclusions
A combined point-source analysis of data from the ANTARES and IceCube detectors in the Southern sky has been presented. Preliminary results show that combining the data from both experiments allows to achieve the best point-source sensitivity in critical overlap regions in the Southern sky, where in particular neutrino emission from Galactic sources may be found. This analysis is ongoing, but unblinding of the data is expected in (early) 2019.