TY - JOUR
T1 - Identification and properties of the photospheric emission in GRB090902B
AU - Ryde, F.
AU - Axelsson, M.
AU - Zhang, B. B.
AU - McGlynn, S.
AU - Pe'Er, A.
AU - Lundman, C.
AU - Larsson, S.
AU - Battelino, M.
AU - Zhang, B.
AU - Bissaldi, E.
AU - Bregeon, J.
AU - Briggs, M. S.
AU - Chiang, J.
AU - De Palma, F.
AU - Guiriec, S.
AU - Larsson, J.
AU - Longo, F.
AU - McBreen, S.
AU - Omodei, N.
AU - Petrosian, V.
AU - Preece, R.
AU - Van Der Horst, A. J.
PY - 2010
Y1 - 2010
N2 - 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 8keV 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 power law 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 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 ± 0.3) × 1012 Y 1/4 cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of 2 and has a maximal value of Γ = 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 broadband emission as synchrotron emission at R ∼ 4 × 1015 cm. Our analysis emphasizes the importance of having high temporal resolution when performing spectral analysis on gamma-ray bursts, since there is strong spectral evolution.
AB - 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 8keV 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 power law 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 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 ± 0.3) × 1012 Y 1/4 cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of 2 and has a maximal value of Γ = 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 broadband emission as synchrotron emission at R ∼ 4 × 1015 cm. Our analysis emphasizes the importance of having high temporal resolution when performing spectral analysis on gamma-ray bursts, since there is strong spectral evolution.
KW - Gamma rays: general
KW - Gamma-ray burst: individual (GRB090902B)
KW - Radiation mechanisms: thermal
UR - http://www.scopus.com/inward/record.url?scp=77749336341&partnerID=8YFLogxK
U2 - 10.1088/2041-8205/709/2/L172
DO - 10.1088/2041-8205/709/2/L172
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AN - SCOPUS:77749336341
SN - 2041-8205
VL - 709
SP - L172-L177
JO - Astrophysical Journal Letters
JF - Astrophysical Journal Letters
IS - 2 PART 2
ER -