The Fermi Gamma-ray Space Telescope observed the bright and longGRB090902B, lying at a redshift of z = 1.822. Together the Large AreaTelescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover the spectralrange from 8 keV to >300 GeV. Here we show that the prompt burstspectrum is consistent with emission from the jet photosphere combinedwith nonthermal emission described by a single power law with photonindex -1.9. The photosphere gives rise to a strong quasi-blackbodyspectrum which is somewhat broader than a single Planck function and hasa characteristic temperature of ~290 keV. We model the photosphericemission with a multicolor blackbody, and its shape indicates that thephotospheric radius increases at higher latitudes. We derive theaveraged photospheric radius R ph = (1.1 ± 0.3)× 1012 Y 1/4 cm and the bulk Lorentz factorof the flow, which is found to vary by a factor of 2 and has a maximalvalue of Γ = 750 Y 1/4. Here, Y is the ratio betweenthe total fireball energy and the energy emitted in the gamma rays. Wefind that during the first quarter of the prompt phase the photosphericemission dominates, which explains the delayed onset of the observedflux in the LAT compared to the GBM. We interpret the broadband emissionas synchrotron emission at R ~ 4 × 1015 cm. Ouranalysis emphasizes the importance of having high temporal resolutionwhen performing spectral analysis on gamma-ray bursts, since there isstrong spectral evolution.

gamma-ray burst; individual; GRB090902B; gamma rays; general; radiation mechanisms; thermal

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