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Description
Blazars are highly variable gamma-ray sources whose flaring activity provides a direct probe of particle acceleration and radiation processes in relativistic jets. However, the gamma-ray spectral behavior associated with individual flare peaks has not yet been characterized systematically for a large blazar sample. In this work, we investigate whether blazars become spectrally harder or softer during gamma-ray flares by combining long-term Fermi-LAT light curves with source information from the 4LAC catalog.
We identify active gamma-ray states and flare peaks from the Fermi-LAT light curves, and measure the photon-index behavior around each peak. The photon index at the flare peak is compared with the characteristic photon index of the same source, allowing us to evaluate spectral hardening or softening on a source-by-source basis. We also examine daily shifted time windows around the peak to explore whether the spectral change is confined to the flare peak or extends to neighboring time bins.
Applying this framework to a large blazar sample, we identify more than one thousand gamma-ray flare peaks, several hundred of which have sufficient photon-index coverage for statistical analysis. We find that spectral changes during flares are common, with spectral hardening appearing frequently at flare peaks. At the source level, blazars can be separated into several groups according to their flare-related spectral behavior, including hardening-dominated, softening-dominated, mixed, and non-significant cases.
These results suggest that gamma-ray flares in blazars are often accompanied by measurable changes in the particle-energy distribution or emission conditions. The diversity of spectral behavior among sources indicates that flare-related spectral variability is not governed by a single universal mechanism, but may depend on source class, jet environment, and flare properties.