Characteristics of Late-phase >100 MeV Gamma-Ray Emission in Solar Eruptive Events

• Published in: The Astrophysical journal 2018-12, Vol.869 (2), p.182
• Main Authors: Share, G. H., Murphy, R. J., White, S. M., Tolbert, A. K., Dennis, B. R., Schwartz, R. A., Smart, D. F., Shea, M. A.
• Format: Article
• Language: English
• Subjects: acceleration of particles; Sun: flares; Sun: particle emission; Sun: X-rays, gamma rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aaebf7

We characterize and catalog 30 solar eruptive events observed by the Fermi Large Area Telescope (LAT) having late-phase >100 MeV γ-ray emission (LPGRE), identified 30 yr ago in what were called long-duration gamma-ray flares. We show that LPGRE is temporally and spectrally distinct from impulsive phase emission in these events. The spectra are consistent with the decay of pions produced by >300 MeV protons and are not consistent with primary electron bremsstrahlung. Impulsive >100 keV X-ray emission was observed in all 27 LPGRE events where observations were made. All but two of the LPGRE events were accompanied by a fast and broad coronal mass ejection (CME). The LPGRE start times range from CME onset to 2 hr later. Their durations range from ∼0.1 to 20 hr and appear to be correlated with durations of >100 MeV solar energetic particle (SEP) proton events. The power-law spectral indices of the >300 MeV protons producing LPGRE range from ∼2.5 to 6.5 and vary during some events. Combined γ-ray line and LAT measurements indicate that LPGRE proton spectra are steeper above 300 MeV than they are below 300 MeV. The number of LPGRE protons >500 MeV is typically about 10× the number in the impulsive phase of the solar eruptive event and ranges in nine events from ∼0.01× to 0.5× the number in the accompanying SEP event, with large systematic uncertainty. What appears to be late-phase electron bremsstrahlung with energies up to ∼10 MeV was observed in one LPGRE event. We discuss how current models of LPGRE may explain these characteristics.