[Paper Review] Extending the Search for Neutrino Point Sources with IceCube above the Horizon

Rasha Abbasi|arXiv (Cornell University)|Jan 1, 2009

Astrophysics and Cosmic Phenomena20 references45 citations

TL;DR

This paper presents a novel method to extend IceCube's neutrino point source searches above the celestial horizon by applying energy-dependent background suppression, enabling sensitivity to high-energy (PeV to EeV) sources in the southern sky—previously inaccessible due to Earth absorption and background rejection. The approach increases sensitivity beyond PeV energies, yielding the first flux limits for southern sky point sources at EeV energies.

ABSTRACT

Contains fulltext : 75517.pdf (Author’s version preprint ) (Open Access)

Motivation & Objective

• To extend IceCube's point source sensitivity beyond the southern horizon, previously restricted by background rejection of upward-going events.
• To enable detection of high-energy neutrinos (PeV to EeV) from sources in the southern celestial hemisphere.
• To suppress atmospheric muon and neutrino backgrounds above the horizon using energy-dependent cuts.
• To set the first flux upper limits for southern sky point sources at energies above PeV.
• To validate the method using data from 22 IceCube strings and apply it to candidate sources like Centaurus A and 3C279.

Proposed method

• Applies energy-sensitive cuts to suppress atmospheric muon background from above the horizon, enabling inclusion of upward-going events.
• Uses reconstructed muon track direction and energy to distinguish signal-like events from background.
• Employs a background estimation method based on scrambled data to account for declination-dependent background variations.
• Optimizes search bin radius (4°) for time-dependent flaring source tests to minimize background.
• Applies statistical and systematic uncertainty corrections to flux limits, including light propagation, reconstruction bias, and interaction models.
• Uses the Feldman-Cousins method for upper limit calculation with full uncertainty propagation.

Experimental results

Research questions

• RQ1 Can neutrino point source searches be extended above the horizon in IceCube by suppressing atmospheric background?
• RQ2 What is the sensitivity gain in detecting high-energy (PeV–EeV) neutrino sources in the southern sky using this method?
• RQ3 How do systematic uncertainties from ice optical properties and reconstruction bias affect flux limit robustness?
• RQ4 What are the first flux upper limits for southern sky sources at EeV energies?
• RQ5 Can this method detect flaring activity from known sources like 3C279 during gamma-ray flares?

Key findings

• The method successfully extends IceCube's sensitivity to point sources above the horizon, covering the southern sky up to EeV energies.
• No significant excess above background was observed in sky scans or in targeted searches for Centaurus A and 3C279.
• The flux limit for Centaurus A is 1.1 × 10⁻⁸ GeV cm⁻² s⁻¹ at 100 PeV, consistent with theoretical models.
• For the 3C279 flare period (25.4 days), the neutrino fluence limit is 3.7 × 10⁻³ erg cm⁻² in the 8.3 × 10⁵ to 7.6 × 10⁷ GeV energy range.
• Systematic uncertainties on flux limits are estimated at −13% to +18%, primarily from light propagation and reconstruction bias.
• This work presents the first flux upper limits for southern sky point sources at EeV energies, marking a significant extension of IceCube's observational reach.