Spectral Evolution of GRB060904A Observed with Swift and Suzaku- Possibility of Inefficient Electron Acceleration

Abstract

We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku 1 satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the BAT and XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from Γ = 1.51 +0.04 −0.03 to Γ = 5.30 +0.69 −0.59 within a few hundred seconds in the prompt tail. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron acceleration and synchrotron radiation. Then, we applied an alternative spectral fitting using a broken power-law with exponential cutoff (BPEC) model. It is valid to consider the situation that the cutoff energy is equivalent to the synchrotron frequency of the maximum energy electrons in their energy distribution. Since the spectral cutoff appears in the soft X-ray band, we conclude the electron acceleration has been inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. We could successfully trace the temporal history of two characteristic break energies (peak energy and cutoff energy) and they show the time dependence of ∝ t −3 ∼ t −4 while the following afterglow spectra are quite stable. This fact indicates that the emitting material of prompt tail is due to completely different dynamics from the shallow decay component. Therefore we conclude the emission sites of two distinct phenomena obviously differ from each other.

X-Raying Extended Emission and Rapid Decay of Short Gamma-Ray Bursts

“…Therefore, because of the analogies between long and short GRBs, the EXP model may be appropriate to describe the time behaviour of rapid decay in SGRBs. Yonetoku et al (2008) and Moretti et al (2008) reported similar rapid decline in time with strong spectral evolution in long GRBs. Especially Yonetoku et al (2008) interpreted the temporal and spectral behavior as dynamic evolution of a spectral model, i.e., a spectral model of a broken power-law with an exponential cutoff moves through the observational energy window of XRT during the rapid decay phase.…”

“…Yonetoku et al (2008) and Moretti et al (2008) reported similar rapid decline in time with strong spectral evolution in long GRBs. Especially Yonetoku et al (2008) interpreted the temporal and spectral behavior as dynamic evolution of a spectral model, i.e., a spectral model of a broken power-law with an exponential cutoff moves through the observational energy window of XRT during the rapid decay phase. Here the physical interpretation of the break energy and the cutoff energy is the E peak corresponding to the minimum energy of accelerated electrons and the synchrotron cutoff, respectively.…”

The Softening Phenomenon Due to the Curvature Effect: In the Case of an Extremely Short Intrinsic Emission

“…A possible thermal component has also been tried. However, Yonetoku et al (2008) showed that introducing a thermal component is not sufficient to explain all of the spectral softening, and thus additional spectral evolution is required. In investigating the origins for the spectral evolution of GRB 070616, Starling et al (2008) ruled out the possibility that a superposition of two power-laws causes the evolution.…”

“…Many careful analyses revealed that there do exist some bursts with their peak energy E peak decreasing from a higher band to a much lower band. These bursts include: GRB 060124, from 108 keV to 1.3 keV (Butler & Kocevski 2007); GRB 060614, from 8.6 keV to 1.1 keV (Butler & Kocevski 2007;Mangano et al 2007); GRB 060904A, from 163 keV to 2.28 keV (Yonetoku et al 2008); GRB 061121, from 270 keV to 0.95 keV (Butler & Kocevski 2007); GRB 070616, from 135 keV to ∼4 keV (Starling et al 2008). For GRB 070616, the spectral softening evolution was observed even in the prompt emission phase: its duration is T 90 = 402.4s, while the softening starts from 285s and extends to 1200s (Starling et al 2008).…”

Curvature Effect of a Non-Power-Law Spectrum and Spectral Evolution of GRB X-Ray Tails

“…where Γ = β + 1 is the power law photon index, and k is a parameter to define the sharpness of the high energy cutoff in the spectrum, E c (t) is the time-dependent characteristic photon energy, and N 0 (t) is a time-dependent photon flux (in units of photons · keV −1 cm −2 s −1 ) at 1 keV (Arnaud 1996). The choice of this function was encouraged by the fact that the spectral evolution of some GRB tails can be fitted by such an empirical model (Campana et al 2006;ZLZ07;Yonetoku et al 2008). According to Eqs.…”