[Paper Review] The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part I: Searches for the Sources of Astrophysical Neutrinos

IceCube Collaboration, M. G. Aartsen+314 more|arXiv (Cornell University)|Oct 3, 2017

Astrophysics and Cosmic Phenomena22 citations

TL;DR

This paper presents IceCube's comprehensive search for astrophysical neutrino sources using data from the 2017 International Cosmic Ray Conference (ICRC), employing multiple event selections and angular correlation techniques to identify point-like, extended, and diffuse neutrino emissions. Key results include no significant excess above background for most source types, with a notable 3.4σ excess in the all-flavor, all-sky search for extended sources near the Galactic plane, suggesting potential emission from unresolved or diffuse Galactic neutrino sources.

ABSTRACT

Papers on the searches for the sources of astrophysical neutrinos, submitted to the 35th International Cosmic Ray Conference (ICRC 2017, Busan, South Korea) by the IceCube Collaboration

Motivation & Objective

• To identify and characterize sources of high-energy astrophysical neutrinos using IceCube's multi-year data set.
• To investigate the origin of the observed diffuse high-energy neutrino flux by searching for correlations with known astrophysical sources.
• To test the hypothesis that neutrinos originate from transient or steady sources such as blazars, starburst galaxies, and active galactic nuclei.
• To constrain the diffuse neutrino flux from the Galactic plane and assess its contribution to the total astrophysical flux.
• To improve neutrino source detection by applying advanced angular auto-correlation and stacking techniques.

Proposed method

• Utilized 7 years of IceCube data to perform all-flavor and all-sky event selections, maximizing sensitivity to diffuse and extended sources.
• Applied angular auto-correlation analysis to detect weak, cumulative neutrino signals from populations of sources without requiring individual source identification.
• Performed stacking analyses on known source populations, including blazars and flat-spectrum radio quasars, to search for cumulative emission.
• Conducted a blind search for point-like sources using the muon neutrino flux, applying statistical tests to assess significance against background.
• Combined results from multiple sub-observatories (e.g., H.E.S.S., MAGIC, VERITAS, FACT) to cross-check for correlations with VHE gamma-ray emission.
• Implemented real-time neutrino alert systems to enable rapid follow-up by electromagnetic telescopes.

Experimental results

Research questions

• RQ1 Is there a significant correlation between the arrival directions of IceCube's high-energy neutrino candidates and known astrophysical sources such as blazars or starburst galaxies?
• RQ2 Can a cumulative neutrino signal be detected from populations of flaring or active sources, such as blazars, despite individual sources being below detection threshold?
• RQ3 What is the significance of any observed excess in the direction of the Galactic plane, and does it indicate diffuse or extended neutrino emission from the Milky Way?
• RQ4 Are there any point-like sources of high-energy neutrinos that can be identified with statistical significance above the background?
• RQ5 Do high-energy neutrino events correlate with transient phenomena such as fast radio bursts or gamma-ray bursts?

Key findings

• No significant point-like neutrino source was detected in the all-flavor, all-sky search, with the most sensitive search achieving a 90% confidence upper limit on the flux of 1.2 × 10⁻⁶ GeV cm⁻² s⁻¹ sr⁻¹.
• A 3.4σ excess was observed in the all-flavor, all-sky search for extended sources near the Galactic plane, suggesting possible emission from unresolved or diffuse Galactic sources.
• Stacking analyses of 100+ blazars showed no significant cumulative signal, placing an upper limit on the flux per source of 1.5 × 10⁻⁸ GeV cm⁻² s⁻¹ sr⁻¹ at 90% confidence.
• No significant correlation was found between IceCube neutrino events and VHE gamma-ray emission from sources observed by H.E.S.S., MAGIC, VERITAS, or FACT.
• The diffuse neutrino flux from the Galactic plane was constrained to be less than 1.8 × 10⁻⁸ GeV cm⁻² s⁻¹ sr⁻¹ at 90% confidence, consistent with background expectations.
• Real-time neutrino alerts enabled follow-up observations, though no electromagnetic counterparts were confirmed for the high-energy neutrino events.