Spectrum of the Isotropic Diffuse Gamma-ray Background

The isotropic diffuse W-ray background (IGRB) comprises all extragalactic diffuse emission that is not resolved into sources. The initial measurement of the IGRB intensity with the Fermi Large Area Telescope (LAT) was performed in 2010 using the first 10 months of sky-survey data. After improvements were made in event selection and characterization of cosmic-ray backgrounds, a second measurement using 50 months of LAT data allowed for a refinement and a better understanding of the IGRB measurement, this time covering an energy range from 100MeV to 820 GeV. In this second analysis, the IGRB spectrum was measured to about +15%/-30% systematic uncertainty, attributed mostly to the Galactic diffuse foregrounds. It dominates the measurement uncertainties over most of the observed energy range, and poses challenges for the interpretation of potential subtle spectral features in the IGRB spectrum. In the current analysis, therefore, the primary goal is to refine the measurement of the IGRB, employing 8 years of Pass8 LAT data and the individual sources compiled in the 4FGL catalog. A reduction of the systematic uncertainties arising from the diffuse Galactic foreground emission will be achieved through improved modeling of this emission, as well as a careful selection of analysis regions. Other improvements include a wider energy range, starting from 50 MeV, a larger dataset, and more powerful fitting techniques. In these proceedings we give an overview of the methods and foreground models used, and show first results from fitting the foreground model to the pass8 LAT data.