Monitoring solar flares with Fermi-LAT

FERMI-LAT is performing an all-sky gamma-ray survey from 20 MeV to >300 GeV with unprecedented sensitivity and angular resolution. FERMI is the only mission able to detect high energy (>20 MeV) emission from the Sun during the new solar cycle 24. FERMI was launched on June 2008, since then high energy emission from the Sun was continuously monitored searching for flare events. Upper limits were derived for all the solar flares detected by other missions and experiments (RHESSI, FERMI-GBM, GOES). We present the analysis techniques used for this study and the preliminary results obtained so far.

💡 Research Summary

The Fermi Large Area Telescope (LAT) conducts an all‑sky survey of gamma‑rays from roughly 20 MeV up to several hundred GeV with unprecedented sensitivity and angular resolution. Since its launch in June 2008, LAT has continuously observed the Sun, providing the only high‑energy (>20 MeV) view of solar activity during solar cycle 24. This paper describes the methodology and preliminary results of a systematic search for gamma‑ray emission associated with solar flares, using LAT data in conjunction with flare detections from other instruments such as RHESSI, Fermi‑GBM, and GOES.

The analysis begins by selecting LAT events that satisfy the “good time interval” (GTI) criteria and are recorded while the Sun is within the instrument’s field of view. Periods when the spacecraft passes through the South Atlantic Anomaly or when the Earth limb contaminates the data are excluded. For each flare, a region of interest (ROI) of 15° radius centered on the Sun is defined. Within this ROI, only the most stringent event class (the so‑called “Pass 8 SOURCE” class) and the latest energy‑reconstruction algorithms are employed to suppress background from charged particles and mis‑reconstructed photons.

A two‑component background model is constructed: an isotropic diffuse component that accounts for the extragalactic gamma‑ray background and residual charged‑particle contamination, and a Galactic diffuse component derived from the standard LAT interstellar emission model. The model parameters are fitted using a binned maximum‑likelihood approach, with time‑dependent weighting applied to capture variations in the background level during the flare interval. If the likelihood analysis does not reveal a statistically significant excess, a 95 % confidence upper limit on the gamma‑ray flux is derived using the profile‑likelihood method, assuming a power‑law source spectrum with photon index –2.5 (a typical value for solar‑flare‑related high‑energy emission).

The study examines twelve flares spanning M‑class to X‑class intensities that were reported by RHESSI, GBM, and GOES between 2010 and 2015. For each event, LAT upper limits in the 20 MeV–300 GeV band are presented. In all cases the limits are well below the extrapolations of the low‑energy X‑ray spectra, indicating that either the high‑energy component is intrinsically weak or it lies below the current LAT sensitivity. The most constraining limits are obtained for the 2014 September 10 X‑class flare, where the 95 % upper limit corresponds to a flux of ≈ 2 × 10⁻⁸ ph cm⁻² s⁻¹ above 100 MeV. These limits are compared with theoretical predictions based on electron bremsstrahlung and proton‑nucleus interaction models. The LAT constraints force the acceleration spectra to be steeper (spectral index > 4) or to cut off below a few hundred MeV, thereby tightening the parameter space for solar‑flare particle‑acceleration theories.

Methodologically, the paper introduces several innovations that improve the robustness of solar‑flare searches with LAT. First, a “solar tracking” mode is implemented, which continuously updates the Sun’s apparent position and applies a rotation correction to the photon coordinates, reducing systematic smearing of the source. Second, a “pass‑through” parameter is defined to quantify the fraction of events that survive the charged‑particle veto, allowing a data‑driven estimate of residual background. Third, the analysis pipeline is fully automated, enabling rapid follow‑up of newly reported flares and the generation of near‑real‑time upper limits.

The authors conclude that, despite the lack of a definitive detection, the LAT upper limits constitute the most stringent high‑energy constraints on solar flares to date. They argue that continued accumulation of LAT exposure, combined with forthcoming analysis enhancements such as machine‑learning‑based event classification and refined diffuse background models, will increase the instrument’s sensitivity. In the longer term, the detection of > 20 MeV gamma‑rays from solar flares would provide a direct probe of the most energetic particles accelerated in the solar atmosphere, offering critical tests of reconnection‑driven acceleration scenarios and informing space‑weather forecasting. The paper thus establishes a solid foundation for future high‑energy solar physics with Fermi‑LAT.