Yun-Lu Gong | Searcharxiv

arXiv2026

Gamma-Ray Emission from the Crab Pulsar: A 17-Year Fermi-LAT Reanalysis

We present a reanalysis of 17 years of Fermi Large Area Telescope (LAT) observations of the Crab pulsar obtained between 2008 August and 2025 August. Using monthly Jodrell Bank radio ephemerides, we assigned pulse phases to the LAT events and aligned the phase zero across the full data set. From this phase-aligned data set, we derived pulse profiles over 100 MeV to 300 GeV. The pulsed emission remains clearly detectable in the 10 to 20 GeV and 20 to 30 GeV bands, with H-test significances of 32.36 sigma and 11.59 sigma, respectively, but is not significantly detected in the 30 to 300 GeV band. Phase-resolved likelihood analysis was performed over 100 MeV to 30 GeV using 14 phase bins with comparable pulsed statistics. The fixed-window fractional fluxes show that the contribution of Peak 1 (P1) decreases steadily with energy, while those of Peak 2 (P2) and the Bridge increase, with P2 exceeding P1 above 10 GeV. Finally, the same phase-assignment framework also enables an off-pulse analysis from 100 MeV to 1 TeV, confirming the synchrotron and inverse-Compton components that dominate the emission in the selected off-pulse interval.

astro-ph.HE↗

arXiv2026

Fast-Cooling Synchrotron in Decaying Magnetic Fields: Implications for the GRB Spectral Distribution

The prompt-emission spectra of gamma-ray bursts (GRBs) are commonly described by the empirical Band function. The typical low-energy spectral index is $\sim -1$, which poses a challenge to standard synchrotron radiation models. We systematically investigate a fast-cooling synchrotron model with a decaying magnetic field and test, within an observation-consistent pipeline, whether it reproduces the Band-fit parameter distributions in the GBM catalog, in a statistical sense. We solve the electron continuity equation with synchrotron, adiabatic, and synchrotron self-Compton cooling to obtain the time-dependent electron distribution and synthetic spectra; we then forward-fold through the GBM response matrices and recover $(α, β, E_p)$ with Band fits. We find that magnetic-field decay can harden the recovered $α$ relative to the fast-cooling limit in part of parameter space, but the effect is not robust and is sensitive to the location of $E_p$ within the finite band and to spectral curvature; varying key physical scales reshapes the recovered $α$ distribution, indicating that catalog $α$ often represents an effective in-band slope rather than the asymptotic index. SSC cooling provides modest additional hardening and, in our setups, does not stabilize $α$ near the observed peak. Using Monte Carlo samples designed to mimic the observations, the model yields $α$ mostly between $-1.5$ and $-0.8$, but remains centered around $α\approx -1.5$. Overall, while decaying-field fast-cooling synchrotron can partially alleviate overly soft spectra expected from standard fast-cooling synchrotron emission, it still falls short of reproducing the GBM $α$ distribution at the population level, implying that additional physical processes are required.

astro-ph.HE↗