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 ) and the Gamma-ray Burst Monitor ( GBM ) cover the spectral range from 8 keV to > 300 GeV .
Here we show that the prompt burst spectrum is consistent with emission from the jet photosphere combined with nonthermal emission described by a single powerlaw with photon index -1.9 .
The photosphere gives rise to a strong quasi-blackbody spectrum which is somewhat broader than a single Planck function and has a characteristic temperature of \sim 290 keV .
We model the photospheric emission with a multicolor blackbody and its shape indicates that the photospheric radius increases at higher latitudes .
We derive the averaged photospheric radius R _ { ph } = ( 1.1 \pm 0.3 ) \times 10 ^ { 12 } Y ^ { 1 / 4 } \mathrm { cm } and the bulk Lorentz factor of the flow , which is found to vary by a factor of two and has a maximal value of \Gamma = 750 Y ^ { 1 / 4 } .
Here Y is the ratio between the total fireball energy and the energy emitted in the gamma-rays .
We find that during the first quarter of the prompt phase the photospheric emission dominates , which explains the delayed onset of the observed flux in the LAT compared to the GBM .
We interpret the broad band emission as synchrotron emission at R \sim 4 \times 10 ^ { 15 } cm .
Our analysis emphasize the importance of having high temporal resolution when performing spectral analysis on GRBs , since there is strong spectral evolution .