Ryde 2009 GRB090902B的光球层辐射

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Title:		Identification and Properties of the Photospheric Emission in GRB090902B
Authors:		Ryde, F.; Axelsson, M.; Zhang, B. B.; McGlynn, S.; Pe'er, A.; Lundman, C.; Larsson, S.; Battelino, M.; Zhang, B.; Bissaldi, E.; Bregeon, J.; Briggs, M. S.; Chiang, J.; de Palma, F.; Guiriec, S.; Larsson, J.; Longo, F.; McBreen, S.; Omodei, N.; Petrosian, V.; Preece, R.; van der Horst, A. J.
Affiliation:		AA(Department of Physics, Royal Institute of Technology (KTH), AlbaNova, SE-106 91 Stockholm, Sweden ; The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, SE-106 91 Stockholm, Sweden ),  )
Publication:		The Astrophysical Journal Letters, Volume 709, Issue 2, pp. L172-L177 (2010). (ApJL Homepage)
Publication Date:		02/2010
Origin:		IOP
ApJ Keywords:		gamma-ray burst: individual: GRB090902B, gamma rays: general, radiation mechanisms: thermal
DOI:		10.1088/2041-8205/709/2/L172
Bibliographic Code:		2010ApJ...709L.172R

Abstract

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 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.

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