First Observation of Time Variation in the Solar-Disk Gamma-Ray Flux with Fermi

TL;DR

This study analyzes six years of Fermi-LAT data revealing that the solar-disk gamma-ray flux varies over time and correlates with solar activity, providing insights into cosmic ray interactions and magnetic fields in the solar atmosphere.

Contribution

It presents the first observation of time variation in solar-disk gamma-ray flux and extends the energy range detection up to 100 GeV, revealing new physical insights.

Findings

Detected significant time variation in 1-10 GeV flux anticorrelated with solar activity

Observed gamma rays in 10-30 GeV at >5σ and 30-100 GeV at >2σ

Indicates gamma rays are induced by cosmic rays influenced by solar magnetic fields

Abstract

The solar disk is a bright gamma-ray source. Surprisingly, its flux is about one order of magnitude higher than predicted. As a first step toward understanding the physical origin of this discrepancy, we perform a new analysis in 1-100 GeV using 6 years of public Fermi-LAT data. Compared to the previous analysis by the Fermi Collaboration, who analyzed 1.5 years of data and detected the solar disk in 0.1-10 GeV, we find two new and significant results: 1. In the 1-10 GeV flux (detected at $>5\sigma$), we discover a significant time variation that anticorrelates with solar activity. 2. We detect gamma rays in 10-30 GeV at $>5\sigma$, and in 30-100 GeV at $> 2\sigma$. The time variation strongly indicates that solar-disk gamma rays are induced by cosmic rays and that solar atmospheric magnetic fields play an important role. Our results provide essential clues for understanding the underlying gamma-ray production processes, which may allow new probes of solar atmospheric magnetic fields, cosmic rays in the solar system, and possible new physics. Finally, we show that the Sun is a promising new target for ground-based TeV gamma-ray telescopes such as HAWC and LHAASO.