[Paper Review] TeV Observations of the Galactic Plane with HAWC and Joint Analysis of GeV Data from Fermi

H. Zhou, C. M. Hui+1 more|arXiv (Cornell University)|Sep 7, 2015

Astrophysics and Cosmic Phenomena16 references3 citations

TL;DR

This study presents TeV gamma-ray observations of the Galactic plane using the partially deployed HAWC observatory, combined with joint likelihood analysis of Fermi-LAT GeV data. It identifies ten source candidates with >3σ significance, eight of which have tentative TeV associations, and uses a multi-instrument likelihood framework to deconvolve source confusion and probe emission mechanisms in the inner Galaxy region.

ABSTRACT

A number of Galactic sources emit GeV-TeV gamma rays that are produced through leptonic and/or hadronic mechanisms. Spectral analysis in this energy range is crucial in order to understand the emission mechanisms. The HAWC Gamma-Ray Observatory, with a large field of view and location at $19^\circ$ N latitude, is surveying the Galactic Plane from high Galactic longitudes down to near the Galactic Center. Data taken with partially-constructed HAWC array in 2013-2014 exhibit TeV gamma-ray emission along the Galactic Plane. A high-level analysis likelihood framework for HAWC, also presented at this meeting, has been developed concurrently with the Multi-Mission Maximum Likelihood (3ML) architecture to deconvolve the Galactic sources and to perform multi-instrument analysis. It has been tested on early HAWC data and the same method will be applied on HAWC data with the full array. I will present preliminary results on Galactic sources from TeV observations with HAWC and from joint analysis on Fermi and HAWC data in GeV-TeV energy range.

Motivation & Objective

• To identify and characterize TeV gamma-ray sources in the inner Galactic plane using HAWC data from a partially constructed array.
• To resolve source confusion in the high-source-density region near the Galactic center using a likelihood-based source deconvolution method.
• To perform a joint likelihood analysis of Fermi-LAT GeV and HAWC TeV data to constrain source spectra across the GeV-TeV energy range.
• To distinguish between leptonic and hadronic emission mechanisms by comparing multi-energetic spectral behavior of candidate sources.
• To improve source localization and flux estimation in regions with limited angular resolution by applying a multi-source likelihood model with iterative source counting.

Proposed method

• Utilizes a high-level likelihood framework based on the Multi-Mission Maximum Likelihood (3ML) architecture to analyze HAWC and Fermi-LAT data jointly.
• Applies a maximum likelihood fit to reconstruct source positions and fluxes, modeling signal as λ_ij = b_ij + Σγ_ijk, where γ_ijk is the expected gamma-ray event rate from source k.
• Employs a test statistic (TS = -2(logL₀ - logL₁)) to compare background-only and source-model fits, with significance derived via Wilks’ theorem.
• Performs iterative likelihood ratio tests to determine the number of independent sources in each region of interest (ROI), stopping when ΔTS < 15 (≈3σ with 3 DoF).
• Uses HEALPIX pixelation (0.11° scale) and background estimation via direct integration over 2-hour intervals to enhance source detection sensitivity.
• Validates the method on early HAWC-111 data (2013–2014, 108–134 WCDs) and applies it to five pre-selected ROIs with high pre-trial significance.

Experimental results

Research questions

• RQ1 How many distinct TeV gamma-ray sources are present in the inner Galactic plane (l ∈ [15°, 50°], b ∈ [-4°, 4°]) as detected by the partially constructed HAWC array?
• RQ2 To what extent can source confusion in the Galactic plane be mitigated using a likelihood-based multi-source fitting framework with limited angular resolution?
• RQ3 What is the spectral consistency between GeV and TeV emissions for sources identified in Fermi-LAT 3FGL and HAWC-111 data?
• RQ4 Which of the identified TeV source candidates show significant spatial coincidence with GeV sources, and what does this imply for source association?
• RQ5 Can the joint analysis of Fermi-LAT and HAWC data constrain the emission mechanism (leptonic vs. hadronic) of Galactic sources through multi-energetic spectral modeling?

Key findings

• Ten source candidates were identified with post-trial significance exceeding 3σ using the HAWC-111 data set from 283 days of observation.
• Eight of the ten source candidates have tentative TeV associations, with spatial coincidence observed between HAWC detections and GeV sources in the Fermi-LAT 3FGL catalog.
• The likelihood framework successfully deconvolved overlapping source signals in high-source-density regions, enabling source counting and flux estimation despite limited angular resolution.
• The joint likelihood analysis using the 3ML framework is applied to GeV-TeV spectral data, enabling consistent modeling across instruments and energy bands.
• The iterative source-counting procedure identified multiple sources per ROI where initial significance maps showed overlapping emission, confirming the need for multi-source modeling.
• The method achieved a 95% duty cycle and detected air shower events at 15 kHz, with background rejection optimized via f-bin-specific cuts to maximize Crab Nebula significance.