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The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B,lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) andthe Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300GeV is covered. Here we show that the prompt burst spectrum is consistent withemission from the jet photosphere combined with non-thermal emission describedby a single power-law with photon index -1.9. The photosphere gives rise to astrong quasi-blackbody spectrum which is somewhat broader than a single Planckfunction and has a characteristic temperature of ~290keV. We derive thephotospheric radius Rph = (1.1 \pm 0.3) x 10^12 Y^{1/4} cm and the bulk Lorentzfactor of the flow, which is found to vary by a factor of two and has a maximalvalue of Gamma = 750 Y^{1/4}. Here Y is the ratio between the total fireballenergy and the energy emitted in the gamma-rays. We find that during the firstquarter of the prompt phase the photospheric emission dominates, which explainsthe delayed onset of the observed flux in the LAT compared to the GBM. We modelthe photospheric emission with a multi-color blackbody and its shape indicatesthat the photospheric radius increases at higher latitudes. We interpret thebroad band emission as synchrotron emission at R ~ 4x10^15 cm. Our analysisemphasize the importance of having high temporal resolution when performingspectral analysis on GRBs, since there is strong spectral evolution.

Identification and properties of the photospheric emission in GRB090902B