The circumburst environment of a FRED GRB: Study of the prompt emission and X-ray/optical afterglow of GRB 051111

Aims.We report a multi-wavelength analysis of the prompt emission and early afterglow of GRB 051111 and discuss its properties in the context of current fireball models. Methods.The detection of GRB 051111 by the Burst Alert Telescope on-board Swift triggered early BVRi' observations with the 2-m robotic Faulkes Telescope North in Hawaii, as well as X-ray observations with the Swift X-Ray Telescope. Results.The prompt $\gamma$-ray emission shows a classical FRED profile. The optical afterglow light curves are fitted with a broken power law, with ${\alpha}_1=0.35$ to ${\alpha}_2=1.35$ and a break time around 12 min after the GRB. Although contemporaneous X-ray observations were not taken, a power law connection between the $\gamma$-ray tail of the FRED temporal profile and the late XRT flux decay is feasible. Alternatively, if the X-ray afterglow tracks the optical decay, this would represent one of the first GRBs for which the canonical steep-shallow-normal decay typical of early X-ray afterglows has been monitored optically. We present a detailed analysis of the intrinsic extinction, elemental abundances and spectral energy distribution. From the absorption measured in the low X-ray band we find possible evidence for an overabundance of some $\alpha$ elements such as oxygen, [O/Zn] = 0.7 $\pm$ 0.3, or, alternatively, for a significant presence of molecular gas. The IR-to-X-ray Spectral Energy Distribution measured at 80 min after the burst is consistent with the cooling break lying between the optical and X-ray bands. Extensive modelling of the intrinsic extinction suggests dust with big grains or grey extinction profiles. The early optical break is due either to an energy injection episode or, less probably, to a stratified wind environment for the circumburst medium

GRB 050410 and GRB 050412: Are they really dark gamma-ray bursts?

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 $\pm$ 0.04. The spectrum is well reproduced by an absorbed power law with a spectral index $\Gamma_{\rm x}=2.4$ $\pm$ 0.4 and a low energy absorption $N_{\rm H}$ = 4 +3-2 $\times$ 1021 cm-2 which is higher than the Galactic value. The 15-150 keV prompt emission in GRB 050412 is modelled with a hard ($\Gamma$ = 0.7 $\pm$ 0.2) power law. The XRT light curve follows a broken power law with the first slope $\alpha_1$ = 0.7 $\pm$ 0.4, the break time $T_{\rm break}$ = 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 $\Gamma_{\rm x}=1.3$ $\pm$ 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

Swift observations of GRB 070110: An extraordinary X-ray afterglow powered by the central engine

We present a detailed analysis of Swift multiwavelength observations of GRB 070110 and its remarkable afterglow. The early X-ray light curve, interpreted as the tail of the prompt emission, displays a spectral evolution already seen in other gamma-ray bursts. The optical afterglow shows a shallow decay up to ~2 days after the burst, which is not consistent with standard afterglow models. The most intriguing feature is a very steep decay in the X-ray flux at ~2 × 10^4 s after the burst, ending an apparent plateau. The abrupt drop of the X-ray light curve rules out an external shock as the origin of the plateau in this burst and implies long-lasting activity of the central engine. The temporal and spectral properties of the plateau phase point toward a continuous central engine emission rather than the episodic emission of X-ray flares. We suggest that the observed X-ray plateau is powered by a spinning-down central engine, possibly a millisecond pulsar, which dissipates energy at an internal radius before depositing energy into the external shock

Swift observations of GRB 070110: An extraordinary X-ray afterglow powered by the central engine

We present a detailed analysis of Swift multiwavelength observations of GRB 070110 and its remarkable afterglow. The early X-ray light curve, interpreted as the tail of the prompt emission, displays a spectral evolution already seen in other gamma-ray bursts. The optical afterglow shows a shallow decay up to ~2 days after the burst, which is not consistent with standard afterglow models. The most intriguing feature is a very steep decay in the X-ray flux at ~2 × 10^4 s after the burst, ending an apparent plateau. The abrupt drop of the X-ray light curve rules out an external shock as the origin of the plateau in this burst and implies long-lasting activity of the central engine. The temporal and spectral properties of the plateau phase point toward a continuous central engine emission rather than the episodic emission of X-ray flares. We suggest that the observed X-ray plateau is powered by a spinning-down central engine, possibly a millisecond pulsar, which dissipates energy at an internal radius before depositing energy into the external shock

GRB 070724B: The first gamma ray burst localized by SuperAGILE and its Swift X-ray afterglow

GRB 070724B is the first gamma ray burst localized by SuperAGILE, the hard X-ray monitor aboard the AGILE satellite. The coordinates of the event were published ~19 h after the trigger. The Swift X-Ray Telescope pointed at the SuperAGILE location and detected the X-ray afterglow inside the SuperAGILE error circle. The AGILE gamma-ray Tracker and Minicalorimeter did not detect any significant gamma ray emission associated with GRB 070724B in the MeV and GeV range, neither prompt nor delayed. Searches for the optical afterglow were performed by the Swift UVOT and the Palomar automated 60-inch telescopes, resulting in no significant detection. Similarly, the Very Large Array did not detect any radio afterglow. This is the first GRB event associated with an X-ray afterglow with a firm upper limit in the 100 MeV-30 GeV energy range