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arXiv · 1508.06276

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

Kenny C. Y. Ng·John F. Beacom·Annika H. G. Peter·Carsten Rott
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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.

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Kenny C. Y. Ng, John F. Beacom, Annika H. G. Peter, Carsten Rott. 2015-08-25. First Observation of Time Variation in the Solar-Disk Gamma-Ray Flux with Fermi. https://doi.org/10.1103/physrevd.94.023004

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