Multi-color Blackbody Emission in GRB 081221

1. Multi-color Blackbody Emission in GRB 081221 Shu-Jin Hou1,2 & Xue-Feng Wu1 1Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China; 2Department of Astronomy, Xiamen University, Xiamen 361005, China Abstract : Through presenting a detailed temporal and spectral analysis on the GBM data of the bright gamma-ray burst GRB 081221, we find that the time-integrated spectrum is well fit with a multi-color blackbody (mBB), yielding the minimum kT = 5.13±0.10 keV, the maximum kT = 57.24±1.29 keV, and the power-law index of the temperature distribution m=3.65±0.03. The time-resolved spectra are adequately fit with a power-law plus a black body. The kT of the main burst rapidly increases from 9 keV to 24.5 keV. The photon index of the power-law component is Γ~1.7. Our results imply that the time-integrated spectrum of GRB 081221 is superimposed to be an mBB by the pure blackbody with different temperatures. 1. Introduction The physics of prompt gamma-ray emission of gamma-ray bursts (GRBs), including GRB outflow composition, emission region, and radiation mechanisms, remains as puzzles. According to the standard fireball model, the spectrum of GRBs should have thermal component. However, a large GRB sample detected by Compton/BATSE reveals that prompt GRB spectra are non-thermal, which can be well fitted with the so-called Band function (Band et al. 1993). The physical radiation mechanism that shapes such a spectrum is unclear. 3. Data Analysis of GRB 081221 GRB 081221 trigged Fermi/GBM, Swift/BAT and Konus/Wind (Pelangeon & Atteia 2008; Tanvir et al. 2008; Wilson-Hodge et al. 2008). We extract the GBM data with the RMFIT package and the standard Fermi data analysis tools. The time-integrated spectrum has a broad plateau in the 30-100 keV band, indicating that it cannot be represented with the only Band function. We find that the mBB function can fit the data quite well, and obtain the kTmin = 5.13±0.10 keV, kTmax = 57.24±1.29 keV, m = 3.65±0.03 (see left panel of Fig. 2). We perform time-resolved spectral analysis for the GBM Data, using the blackbody plus a power-law model (e.g., see right panel of Fig. 2). We find that the kT evolves with the flux. The kT changes from 9 to 25 keV, while the power-law photon index of non-thermal component is almost unchanged, which is about 1.7. 2. Multi-color Blackbody Model In the standard fireball model of GRBs, thermal emission is originated from the photosphere of the expanding GRB outflow (e.g., Pe’er et al. 2007). Indeed, time-integrated photons of a GRB come from photospheres of different outflow shells. The temperature of these photospheres is not the same. Thus the time-integrated spectrum of the GRB may be the superimposition of blackbody with different temperatures, which is also called multi-color blackbody (mBB). For simplicity, we assume that the blackbody temperature distribution is a power-law function of T, which can be formulated as where m is the power-law index of the temperature distribution, and the Tminis the minimum temperature. Then the mBB spectrum can be given by Where Tmax is the maximum temperature. Different spectral shape corresponds to different values ​​of m, as shown in Fig.1 . Fig. 2 Time integrated (left, fitted with an mBB model) and resolved (right, fitted with a power-law plus blackbody model) spectra of GRB 081221 observed by Fermi/GBM. 4. Conclusions: Detailed spectral analysis on the prompt gamma-ray emission of GRB 081221 is presented. Our results indicate that the multi-color blackbody is a physical spectral component of GRB 081221, which is superimposed by the pure blackbody with different temperatures. Fig.1. mBB spectral shapes with different m values: (1) m = -1 corresponds to a pure blackbody; (2) m = -2 corresponds to the thermal component in GRB 090902B; (3) m=-3 corresponds to Comptonized spectrum; (4) m = -4 corresponds to the time-integrated spectrum of GRB 081221. 5. Reference: 1. Band, D., et al., 1993, ApJ, 413, 281 2. Pe’er, A., et al ., 2007, ApJ, 664, L1 3. Pelangeon, A., & Atteia, J.-L., 2008, GCN, 8700, 1 4. Tanvir, N. R., et al ., 2008, GCN, 8698, 1 5. Wilson-Hodge, C. A., 2008, GCN, 8704, 1