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Most Gamma-Ray Bursts (GRBs) detected by the Fermi Gamma-ray Space Telescopeexhibit a delay of up to about 10 seconds between the trigger time of the hardX-ray signal as measured by the Fermi GBM and the onset of the MeV-GeVcounterpart detected by the LAT. This delay may hint at important physics,whether it is due to the intrinsic variability of the inner engine or it isrelated to quantum dispersion effects in the velocity of light propagation fromthe sources to the observer. It is critical to have a proper assessment of howthese time delays affect the overall properties of the light curves. Wecross-correlated the 5 brightest GRBs of the 1st Fermi LAT Catalog by means ofthe continuous correlation function (CCF) and of the Discrete CorrelationFunction (DCF). A maximum in the DCF suggests the presence of a time lagbetween the curves, whose value and uncertainty are estimated through aGaussian fitting of the DCF profile and light curve simulation via a MonteCarlo approach. The cross-correlation of the observed LAT and GBM light curvesyields time lags that are mostly similar to those reported in the literature,but they are formally consistent with zero. The cross-correlation of thesimulated light curves yields smaller errors on the time lags and more than onetime lag for GRBs 090902B and 090926A; for all 5 GRBs, the time lags aresignificantly different from zero and consistent with those reported in theliterature, when only the secondary maxima are considered for those two GRBs.The DCF method evidences the presence of time lags between the LAT and GBMlight curves and underlines their complexity. While this suggests that thedelays should be ascribed to intrinsic physical mechanisms, more sensitivityand larger statistics are needed to assess whether time lags are universallypresent in the early GRB emission and which dynamical time scales they trace.

Time delays between Fermi LAT and GBM light curves of GRBs