Abstract
Aims. We present a detailed analysis of the prompt and afterglow emission of GRB 050410 and GRB 050412 detected by Swift for which no optical counterpart was observed. Methods. We analysed data from the prompt emission detected by the Swift BAT and from the early phase of the afterglow obtained by the Swift narrow field instrument XRT. Results. The $15{-}150$ keV energy distribution of the GRB 050410 prompt emission shows a peak energy at 53$_{-21}^{+40}$ keV. The XRT light curve of this GRB decays as a power law with a slope of $\alpha=$ 1.06 ± 0.04. The spectrum is well reproduced by an absorbed power law with a spectral index $\Gamma_{\rm x}=2.4$ ± 0.4 and a low energy absorption N H = 4$^{+3}_{-2}$ $\times$ 10 21 cm -2 which is higher than the Galactic value. The $15{-}150$ keV prompt emission in GRB 050412 is modelled with a hard ( Γ = 0.7 ± 0.2) power law. The XRT light curve follows a broken power law with the first slope $\alpha_1$ = 0.7 ± 0.4, the break time T break = 254$_{-41}^{+79}$ s and the second slope $\alpha_2$ = 2.8$_{-0.8}^{+0.5}$. The spectrum is fitted by a power law with spectral index $\Gamma_{\rm x}=1.3$ ± 0.2 which is absorbed at low energies by the Galactic column. Conclusions. The GRB 050410 afterglow light curve reveals the expected characteristics of the third component of the canonical Swift light curve. Conversely, a complex phenomenology was detected in the GRB 050412 because of the presence of the very early break. The light curve in this case can be interpreted as being the last peak of the prompt emission. The two bursts present tight upper limits for the optical emission, however, neither of them can be clearly classified as dark. For GRB 050410, the suppression of the optical afterglow could be attributed to a low density interstellar medium surrounding the burst. For GRB 050412, the evaluation of the darkness is more difficult due to the ambiguity in the extrapolation of the X-ray afterglow light curve.
Highlights
Gamma-Ray Bursts (GRBs), the brightest explosions in the universe, produce emission across the whole electromagnetic spectrum from γ-rays to radio wavelengths
The 15−150 keV average energy distribution of the GRB 050410 emission was fitted by a Band model with the peak energy at 53+−4201 keV and a low energy slope of −0.79 ± 0.09 after fixing the high energy slope to −3
The GRB 050412 15−150 keV emission was modelled with a hard (Γ = 0.7 ± 0.2) power law suggesting a peak energy above Burst Alert Telescope (BAT) energy range. – The GRB 050410 X-Ray Telescope (XRT) light curve can be modelled with a single power law with a slope of α = 1.06 ± 0.04
Summary
Gamma-Ray Bursts (GRBs), the brightest explosions in the universe, produce emission across the whole electromagnetic spectrum from γ-rays to radio wavelengths. A possible mechanism proposed for these GRBs is based on a Poynting flux dominated outflow (Zhang & Kobayashi 2005) where the transfer of the energy from the fireball to the medium is delayed, leading to the suppression of the reverse shock, likely responsible for the prompt optical emission, and to an apparent high γ-ray efficiency. The characterisation of optical darkness has been previously based on the upper limit of the optical/NIR afterglow flux (Rol et al 2005; Filliatre et al 2005, 2006) or the optical-to-X-ray spectral index (Jakobsson et al 2004). We present the results on the analysis of the prompt and the afterglow emission of GRB 050410 and GRB 050412 observed by Swift, two bursts for which no optical counterpart was detected.
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