Afterglow Light Curves and Broken Power Laws: A Statistical Study

In gamma-ray burst research it is quite common to fit the afterglow light curves with a broken power law to interpret the data. We apply this method to a computer simulated population of afterglows and find systematic differences between the known model parameters of the population and the ones derived from the power law fits. In general, the slope of the electron energy distribution is overestimated from the pre-break light curve slope while being underestimated from the post-break slope. We also find that the jet opening angle derived from the fits is overestimated in narrow jets and underestimated in wider ones. Results from fitting afterglow light curves with broken power laws must therefore be interpreted with caution since the uncertainties in the derived parameters might be larger than estimated from the fit. This may have implications for Hubble diagrams constructed using gamma-ray burst data.Comment: 4 pages, 5 figures, accepted for publication in ApJ Letter

New Calculation of Secondary Antiprotons in Cosmic Rays

A dramatic increase in the accuracy and statistics of space-borne cosmic ray (CR) measurements has yielded several breakthroughs over the last several years. The most puzzling is the rise in the positron fraction above 10 GeV over the predictions of the propagation models assuming pure secondary production. Antiprotons are produced in CR interactions with interstellar gas and are,therefore, called secondary. These are the same interactions that produce charged and neutral mesons which decay to secondary electrons and positrons andγ-rays. However, in contrast to CR electrons and positrons that can be produced copiously in pulsars, there is no known astrophysical source of primary ̄p. Therefore, ̄pdata and their correct interpretation hold the key to the resolution of many astrophysical puzzles. We calculated ̄pproduction inp p-,pA-, and AA-interactions using EPOS-LHC and QGSJET-II-04, two of the most advanced Monte Carlo generators tuned to accelerator data including those from the LHC. The new cross sections are being incorporated into the GALPROP code to allow an accurate calculation of the spectrum of secondary ̄pin CRs