Fermi Gamma-ray Space Telescope

A new version of Pass 8 data

Philippe Bruel
(T. Burnett, S. Digel, G. Johannesson, N. Omodei, M. Wood on behalf of the Fermi-LAT Collaboration)


The current version of the Fermi-LAT data is Pass 8 P302 and is publicly available since 24 June 2015. As for the previous versions, it is possible to select various event selections, depending on the level of residual background that is best suited to the data analysis. When performing the analysis, the residual background is taken into account by adding an isotropic background model to the source model. When Pass 8 data was released, we noticed that the residual background of the SOURCE class was not fully isotropic: it was enhanced by a factor 2 at 1-3 GeV within 20 degrees of the Ecliptic compared to the poles. This anisotropy disappears when using the cleaner ULTRACLEANVETO event class. In this talk we show that this background anisotropy is due to cosmic-ray electrons leaking through the ribbons of the Anti-Coincidence Detector and that this background source can be removed by a set of simple cuts while losing less than 1% of the SOURCE acceptance. This new selection has been used to produce a new version of the LAT data, Pass 8 P305, that will be released soon.