Multi-color Blackbody Emission in GRB 081221
Shu-Jin Hou1,2 & Xue-Feng Wu1
1Purple
2
Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China;
Department 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
3. Data Analysis of GRB 081221
The physics of prompt gamma-ray emission of gamma-ray bursts GRB 081221 trigged Fermi/GBM, Swift/BAT and Konus/Wind
(GRBs), including GRB outflow composition, emission region, and (Pelangeon & Atteia 2008; Tanvir et al. 2008; Wilson-Hodge et
radiation mechanisms, remains as puzzles. According to the al. 2008). We extract the GBM data with the RMFIT package
standard fireball model, the spectrum of GRBs should have thermal and the standard Fermi data analysis tools. The time-integrated
component. However, a large GRB sample detected by spectrum has a broad plateau in the 30-100 keV band, indicating
Compton/BATSE reveals that prompt GRB spectra are non-thermal, that it cannot be represented with the only Band function. We
which can be well fitted with the so-called Band function (Band et find that the mBB function can fit the data quite well, and obtain
al. 1993). The physical radiation mechanism that shapes such a the kTmin = 5.13±0.10 keV, kTmax = 57.24±1.29 keV, m =
spectrum is unclear.
3.65±0.03 (see left panel of Fig. 2). We perform time-resolved
2. Multi-color Blackbody Model
spectral analysis for the GBM Data, using the blackbody plus a
In the standard fireball model of GRBs, thermal emission is
power-law model (e.g., see right panel of Fig. 2). We find that
originated from the photosphere of the expanding GRB outflow
the kT evolves with the flux. The kT changes from 9 to 25 keV,
(e.g., Pe’er et al. 2007). Indeed, time-integrated photons of a GRB
while the power-law photon index of non-thermal component is
come from photospheres of different outflow shells. The
almost unchanged, which is about 1.7.
temperature of these photospheres is not the same. Thus the timeintegrated spectrum of the GRB may be the superimposition of
blackbody with different temperatures, which is also called multicolor 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 Tmin is 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.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.
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 multicolor blackbody is a physical spectral component of GRB
081221, which is superimposed by the pure blackbody with
different temperatures.
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