Time Delays Between LLE and GBM light curves of GRBs

Time delays between X-ray and MeV-GeV light curves of gamma-ray bursts (GRBs) observed by the Fermi Gamma-ray Space Telescope offer crucial insights into their emission regions and mechanisms. This study presents a comprehensive cross-correlation analysis of GRB light curves from the Gamma-ray Burst Monitor (GBM) and Low-Energy events (LLE) from the Large Area Telescope (LAT) on board Fermi. Using the discrete correlation function (DCF) method, we cross-correlated GBM data in the 10-100 keV range with those in the 150 keV - 1 MeV and 30-100 MeV ranges. Our analysis included all GRBs available in the Fermi LLE Online Catalogue, comprising both Long and Short GRBs, with 10-1000 keV fluences ranging from \(1 \times 10^{-7}\) to \(1 \times 10^{-4}\) erg/cm\(^2\). Time lags were derived from the DCF maxima using Gaussian fitting, with uncertainties estimated via Monte Carlo simulations. We found that most GRBs exhibit significant time delays between GBM and LLE data, with both positive and negative lags on the order of seconds, indicating that the emission above 30 MeV can either lag behind or precede the keV emission. In contrast, the onset of emission below 100 keV is systematically delayed by fractions of a second compared to the start of emission at a few hundred keV. These delays could indicate variations in the spectral properties of GRBs between the keV and MeV-GeV emissions, possibly due to different emission regions or mechanisms. The presence of a possible high-energy afterglow for some sources can also be hypothesized. Our findings suggest that analyzing temporal properties, in addition to spectral characteristics, provides new methods to differentiate GRB emission components, potentially addressing systematic and low-statistics issues. Furthermore, this approach enhances the understanding of the complex emission processes in GRBs, offering fresh insights into their underlying physics.