Search for Gamma-ray Spectral Lines from Dark Matter Annihilation in Dwarf Galaxies with the High-Altitude Water Cherenkov Observatory (1912.05632v1)

Abstract: Local dwarf spheroidal galaxies (dSphs) are nearby dark-matter dominated systems, making them excellent targets for searching for gamma rays from particle dark matter interactions. If dark matter annihilates or decays directly into two gamma rays (or a gamma ray and a neutral particle), a monochromatic spectral line is created. At TeV energies, no other processes are expected to produce spectral lines, making this a very clean indirect dark matter search channel. With the development of event-by-event energy reconstruction, we can now search for spectral lines with the High Altitude Water Cherenkov (HAWC) Observatory. HAWC is a wide field of view survey instrument located in central Mexico that observes gamma rays from <1 TeV to >100 TeV. In this work we present results from a recent search for spectral lines from local, dark matter dominated dwarf galaxies using 1038 days of HAWC data. We also present updated limits on several continuum channels that were reported in a previous publication. Our gamma-ray spectral line limits are the most constraining obtained so far from 20 TeV to 100 TeV.

• The paper investigates gamma-ray spectral lines as indicators of dark matter annihilation in dwarf galaxies using an extensive 1038-day dataset from HAWC.
• It employs a binned maximum likelihood analysis with two-dimensional energy binning to evaluate 11 dwarf spheroidal galaxy fields.
• The study sets the most stringent constraints on dark matter annihilation cross sections between 20 and 100 TeV, guiding future high-energy searches.

Search for Gamma-ray Spectral Lines from Dark Matter Annihilation in Dwarf Galaxies

The paper entitled "Search for Gamma-ray Spectral Lines from Dark Matter Annihilation in Dwarf Galaxies with the High-Altitude Water Cherenkov Observatory" examines the potential detection of gamma-ray spectral lines emanating from dark matter (DM) annihilation within local dwarf spheroidal galaxies (dSphs). These galaxies are preferred targets due to their proximity and high dark matter density, making them excellent candidates for indirect DM searches. Specifically, the analysis focuses on gamma-ray spectral lines, which serve as clear indicators of DM interactions at TeV energies since no other known astrophysical processes are expected to produce such features.

Methodology and Data Analysis

The research utilizes 1038 days of observational data from the High Altitude Water Cherenkov (HAWC) Observatory. HAWC is positioned strategically to observe gamma-ray emissions globally, with sensitivities spanning from below 1 TeV to over 100 TeV. The current research employs an event-by-event energy reconstruction capability from HAWC to scrutinize 11 dSph fields for gamma-ray spectral lines. The analysis implements a binned maximum likelihood approach to determine the presence of monoenergetic emission lines, employing a two-dimensional binning method involving 'fhit' and finite log-energy partitioning.

Findings

The search revealed no statistically significant spectral lines, with the most notable statistical anomaly being a test statistic of 4.47 at 5.7 TeV in the Segue I galaxy, translating to a global significance of 0.45σ. Despite the absence of detectable signals indicative of DM annihilation, the paper successfully constrains the upper limits on the DM annihilation cross section to gamma-ray lines, notably achieving the most stringent constraints from 20 to 100 TeV compared to other observatories. The results were juxtaposed against analogous bounds from other experiments such as H.E.S.S., VERITAS, and MAGIC, highlighting HAWC's capacity to extend such searches to unprecedented TeV scales.

Implications and Future Work

The current research underlines the robustness of HAWC's survey capabilities, providing critical constraints on DM interactions within an energy range and environment where limited systematic noise exists. It advances the indirect detection efforts of DM by laying foundational observational constraints on spectral lines localized within the gamma spectrum. Future work would inherently benefit from expanded datasets, including the integration of new peripheral Cherenkov tanks (outriggers) that are projected to enhance HAWC's sensitivity above 50 TeV by tripling the effective detection area. Consequently, these advancements are anticipated to elevate the search capacity to potential detections up to approximately 1 PeV.

This paper significantly bolsters the indirect detection narrative by narrowing the permissible parameter space for DM properties and strengthening our understanding of dSphs as DM-rich locales, paving the way for future explorations into the fundamental characteristics of dark matter.