Volume 565, May 2014
|Number of page(s)||9|
|Section||Stellar structure and evolution|
|Published online||12 May 2014|
Time delays between Fermi-LAT and GBM light curves of gamma-ray bursts
SISSA-ISAS, via Bonomea 265,
2 Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
3 Department of Physics and Optical Engineering, ORT Braude, PO Box 78, Carmiel, Israel
4 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, via Gobetti 101, 40129 Bologna, Italy
5 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56122 Pisa, Italy
6 INFN, Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
7 ASI Science Data Center, Frascati, Italy
8 Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44122 Ferrara, Italy
Received: 9 September 2013
Accepted: 5 March 2014
Aims. Most gamma-ray bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope exhibit a delay of up to about 10 seconds between the trigger time of the hard X-ray signal as measured by the Fermi Gamma-ray Burst Monitor (GBM) and the onset of the MeV−GeV counterpart detected by the Fermi Large Area Telescope (LAT). This delay may hint at important physics, whether it is due to the intrinsic variability of the inner engine or related to quantum dispersion effects in the velocity of light propagation from the sources to the observer. Therefore, it is critical to have a proper assessment of how these time delays affect the overall properties of the light curves.
Methods. We cross-correlated the 5 brightest GRBs of the 1st Fermi-LAT Catalog by means of the continuous correlation function (CCF) and of the discrete correlation function (DCF). The former is suppressed because of the low number counts in the LAT light curves. A maximum in the DCF suggests there is a time lag between the curves, whose value and uncertainty are estimated through a Gaussian fitting of the DCF profile and light curve simulation via a Monte Carlo approach.
Results. The cross-correlation of the observed LAT and GBM light curves yields time lags that are mostly similar to those reported in the literature, but they are formally consistent with zero. The cross-correlation of the simulated light curves yields smaller errors on the time lags and more than one time lag for GRBs 090902B and 090926A. For all 5 GRBs, the time lags are significantly different from zero and consistent with those reported in the literature, when only the secondary maxima are considered for those two GRBs.
Conclusions. The DCF method proves the presence of (possibly multiple) time lags between the LAT and GBM light curves in a given GRB and underlines the complexity of their time behavior. While this suggests that the delays should be ascribed to intrinsic physical mechanisms, more sensitivity and more statistics are needed to assess whether time lags are universally present in the early GRB emission and which dynamical time scales they trace.
Key words: cosmology: observations / gamma rays: general / gamma-ray burst: general
© ESO, 2014
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