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Searches for High Energy Neutrino Emission in the Galaxy with the Combined Icecube-AMANDA Detector

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Searches for High Energy Neutrino Emission in the Galaxy with the Combined Icecube-AMANDA Detector 1 Searches for High Energy Neutrino Emission in the Galaxy with 2 the Combined IceCube-AMANDA Detector 3 Received ; accepted { 2 { 4 ABSTRACT 5 We present searches for neutrino sources from the Galaxy with energies above ∼ 500 GeV, using the data collected by the South Pole neutrino telescopes Ice- Cube (in incomplete configurations) and AMANDA. Data collected during the season 2007 to 2008 with 22 IceCube strings and during 2008 to 2009 with the 40 string configuration have been used. In the two seasons, a total life time of 276 days and 375 days has been respectively obtained with IceCube. In order to improve the sensitivity of IceCube at low energy, we also used during the first season 143 days and during the second season 306 days of data of the precursor AMANDA which was still in operation. This analysis exploits a large energy range and focuses on neutrino searches from galactic sources at both small and large scales. For instance, the range 180.0 GeV - 20.5 TeV includes 90% of the signal from a E−3 spectrum and the range 2.4 - 750.0 TeV includes 90% of the signal from a E−2 spectrum. The accessible part of the galactic plane in the northern sky, which includes the Local Arm towards the Cygnus region and the Perseus Arm, has been scanned in search of excesses of neutrinos with respect to the background of atmospheric neutrinos. The search was conducted for a candidate list of interesting galactic sources for which we provide flux upper lim- its since no excess was observed. For the Cygnus star forming region we have developed a dedicated search based on the measurement of the spatial correla- tions between events. Finally, we describe a search for neutrino emission from Cygnus X-3 in coincidence with the ejection of relativistic particles. No evidence for a signal is found in any of the searches performed. Upper limits are set for neutrino fluxes from astrophysical sources over the northern sky. The obtained 90% confidence level muon neutrino flux upper limit lies in the range between E3dN=dE ∼ 5:4 − 19:5 × 10−11 TeV2 cm−2 s−1. This upper limit is the most { 3 { stringent one calculated for the neutrino sample collected in IceCube in the 40 strings configuration. 6 Subject headings: acceleration of particles, cosmic rays, neutrinos { 4 { 7 1. Introduction 8 The IceCube neutrino telescope has been successfully completed at the South Pole 9 during December 2010. IceCube is the most sensitive telescope to date to search for high 10 energy neutrino sources. High energy neutrinos from astrophysical sources are intimately 11 related to the efficient acceleration of hadrons and their interaction in the environment of 12 the accelerator. The pp and pγ processes responsible for the production of neutrinos in 13 astrophysical sites will also lead to the generation of gamma-rays of similar energy (Kelner 14 et al. 2006; Kelner & Aharonian 2008). However, it is difficult to infer the contribution 15 of a possible hadronic component from the observed gamma-rays, since gamma-rays can 16 also be produced by relativistic electrons via Inverse Compton scattering, or even subject 17 to absorption if the radiation field at production site is sufficiently intense. Moreover, 18 gamma-rays can be absorbed on their way to us. Measurements of the neutrino flux from 19 astrophysical sources, even if challenging, reveals unique insights into the acceleration 20 mechanisms and into the origin of cosmic-rays. 21 The IceCube neutrino telescope has the potential to discover high energy neutrino sources −11 −12 −2 −1 22 at the flux level of F (E > 1 T eV ) / 10 − 10 erg cm s , for a source spectrum −2 23 following E in the relevant energy range from 1 TeV up to a few PeV (Abbasi et al. 2011). 33 −1 24 This translates in source luminosities of L / 10 ergs assuming a relatively nearby source 25 at a distance of 2 kpc. 26 Among the most promising candidate sources of cosmic-rays in the Galaxy we cite 27 here the remnants of supernovae (both shell-type and pulsar wind nebulae), the jets 28 of microquasars, and the collective winds of massive stars (Hillas 2005; Tavani et al. 29 2009; Corbel& Fermi LAT collaboration 2010; Aharonian et al. 2007; Ohm et al. 2010; 30 Marcowith et al. 2008). Due to the large amount of energy released in a supernova explosion 41 41:8 31 (∼10 -10 erg/s), supernova remnants are prime candidate sources of the galactic cosmic 32 rays. In microquasars, the kinetic energy carried by the jet is at least comparable to the { 5 { 36 33 observed non-thermal luminosities accounting for ∼ 10 erg/s or even higher (Gallo et al. 34 2005; Margon 1984). The total energy inject in the interstellar medium produced by the 39 35 winds of OB and Wolf-Rayet (WR) stars can be as high as ∼10 erg/s, like in the case 36 of the Cygnus OB2 association (Lozinskaya et al. 2002). What remains undetermined is 37 the fraction of total energy that goes into cosmic ray acceleration, as well as the amount of 38 cosmic rays interacting close to source. Measurements of the products of the cosmic ray 39 interactions, i.e., both gamma-rays and neutrinos, are expected to shed light to this problem 40 in the upcoming future. Recently, a large family of galactic accelerators have been observed 41 to have the bulk of their gamma-ray emission at energies below 50 TeV. This statement is 42 relative to the limited experimental techniques implied for the survey. Moreover, they show −2 43 a characteristic gamma-ray spectrum softer than dN=dE / E expected from first-order 44 Fermi shock acceleration (Fermi 1949, 1954). In this paper we study the capabilities of −2 45 IceCube to observe sources which depart from the standard dN=dE / E spectrum 46 unbroken up to the PeV scale. This study makes use of the 22- and 40-strings configurations 47 of IceCube as well as the Antarctic Muon And Neutrino Detector Array (AMANDA). With 48 the goal to enhance sensitivity for soft spectra sources, or sources with an energy cutoff 49 below 1 PeV, we have implied AMANDA as low energy extension of IceCube and develop 50 an analysis strategy optimized for a high retention of signal below 10 TeV. The impact 51 of the use of AMANDA as low energy extension of IceCube is shown in Figure 1 and in 52 Figure 1. 53 Moreover, we use our low-energy optimized data samples to search for neutrino 54 emission from the Galaxy at energies above ∼ 500 GeV. The part of the Galaxy accessible 55 to IceCube at the energies considered in this analysis covers the range in galactic longitude ◦ ◦ −2 56 of 210 > l > 40 . For IceCube recent results for an E neutrino spectrum we refer to 57 Abbasi et al. (2011). The search program here reported includes a scan of the part of the 58 galactic Plane accessible to IceCube, a dedicated analysis of the Cygnus region, the search { 6 { Fig. 1.| Effective area for IC22+AMANDA of the galactic source analysis at final level: upper curve for 143 days of lifetime (AMANDA operational) and lower curve for 133 days. { 7 { Fig. 2.| Sensitivity and Discovery Potential of the IC40+AMANDA analysis for E−3 neu- trino spectra. The sensitivity is calculated with the method from Feldman and Cousins (Feldman & Cousins 1998) with the additions proposed in (Conrad et al. 2003) and (Hill 2003) to include a systematic uncertainty of ±17% on the neutrino flux. { 8 { 59 for neutrino emission from a pre-defined list of interesting sources, and a time dependent 60 analysis which searches for neutrino emission from Cygnus X-3 in correlation with radio 61 flares. The paper is organized as follows: in Section 2 we report some technical aspects 62 of IceCube, of AMANDA and on the merging of its data in the IceCube data stream. In 63 Section 3, we report about the analysis methods developed and the respective astrophysical 64 targets considered. Following this, the extraction of the neutrino samples is described in 65 Section 4 and in Section 5 the obtained results are reported. 66 2. The Combined Detector: IceCube and AMANDA 67 2.1. IceCube 68 During the construction phase from 2004 to 2010, the operational configuration of 69 IceCube increased year by year (see Figure 3) to finally cover a volume of approximately 70 one cubic kilometer. IceCube is composed of 86 strings each holding 60 digital optical 71 modules (DOMs). Each DOM is composed by a 10 inch photomultiplier tube (PMT) and 72 the relative read-out electronics all housed in a glass pressure vessel. In the array, 78 out 73 of 86 strings form a hexagonal grid with a typical distance of 125 m between neighboring 74 strings. The vertical distance between DOMs on the same string is 17 m. The remaining 8 75 strings are part of the low energy extension DeepCore (R. Abbasi 2011) and are deployed 76 in the center of the detector with a smaller vertical and horizontal spacing between the 77 DOMs. The DOMs detect Cherenkov radiation emitted by secondary charged particles 78 produced in interactions of high energy neutrinos with nuclei in the ice or the bedrock 79 below the ice. To enhance the detection of light from upwards going particles, the PMTs 80 point downwards. In order to avoid deterioration of the PMT analog signal, the waveforms 81 are digitized directly in the DOMs with a set of Analog Transient Waveform Digitizer 82 (ATWDs) and a Fast Analog to Digital Converters (FADCs) (Abbasi et al.
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