Search

We report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 ± 0.3. We next present a carefully selected sample of 19 Swift GRBs, intended to estimate in an unbiased way the GRB redshift distribution, including the mean redshift (zmean) as well as constraints on the fraction of high-redshift bursts. We find that zmean = 2.7 and that at least 5% of the GRBs originate at z > 5. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. The high mean redshift of this GRB sample and the wide redshift range clearly demonstrates the suitability of GRBs as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A γ-ray burst (GRB) has been found with a redshift comparable to the most distant quasars and galaxies: GRB 050904 at z = 6.29 ± 0.01, making it the most distant X-ray source known. The X-ray lightcurve is not a power-law like many afterglows, but is dominated by large amplitude variability from a few minutes to at least half a day. The spectra soften during this time from a power-law with photon index Γ = 1.2 to 1.9. The spectra are well-described by an absorbed power-law with possible evidence of very large intrinsic absorption. There is no evidence for discrete features. This is in spite of the spectrum’s very high signal-to-noise ratio, since GRB 050904 was extraordinarily bright in X-rays. In the first days after the burst, it was by far the brightest known X-ray source at z > 4. In the first minutes after the burst, the X-ray flux was > 10-9 erg cm-2 s-1 in the 0.2–10 keV band, corresponding to an apparent luminosity between 10 5 and 106 times greater than the brightest X-ray quasars at similar distances. More photons were acquired in the first minutes with Swift-XRT than XMM-Newton and Chandra have obtained in ∼ 300 ks of pointed observations of z > 5 AGN. The huge X-ray fluence detected from GRB 050904 is a clear demonstration of concept for efficient X-ray studies of the high-z IGM with new large area, high-resolution X-ray detectors, and shows that GRBs in their early phases are the only backlighting bright enough for X-ray absorption studies of the intervening matter at high redshift. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

BOOTES-IR is the natural evolution of BOOTES (Burst Observer and Optical Transient Exploring System) towards the near-infrared (NIR). Using the experience gathered during the last 7 years, we are developing a powerful tool for the detection and study of Gamma-Ray Burst (GRB) afterglows. The system includes a custom developed robotic observatory software (RTS2) that manages all the instruments and devices of the observatory to make them work together in an optimal way. Additionally we have created a package of real time reduction and analysis tools (JIBARO) that allow us to automatically identify GRB afterglow candidates. The telescope, a 0.6m Ritchey-Chrètien, now in commissioning phase, is capable of pointing with a typical delay of 5 seconds between arrival of the alert and start of the first exposure. The NIR camera, with 1024 × 1024 pixels and a field of view of 12′ × 12′ will see first light in Spring 2006 and is expected to be in normal operation during the Summer. An optical camera working in parallel will complement observations. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]