We present the results of a search for untriggered gamma-ray burst (GRB) afterglows with the Robotic Optical Transient Search Experiment-III (ROTSE-III) telescope array. This search covers observations from September 2003 to March 2005. We have an effective coverage of 1.74 deg 2 yr for rapidly fading transients that remain brighter than ∼ 17.5 magnitude for more than 30 minutes. This search is the first large area survey to be able to detect typical untriggered GRB afterglows. Our background rate is very low and purely astrophysical. We have found 4 previously unknown cataclysmic variables (CVs) and 1 new flare star. We have not detected any candidate afterglow events or other unidentified transients. We can place an upper limit on the rate of fading optical transients with quiescent counterparts dimmer than ∼ 20 th magnitude at a rate of less than 1.9 deg −2 yr −1 with 95% confidence. This places limits on the optical characteristics of off-axis (orphan) GRB afterglows. As a byproduct of this search, we have an effective ∼ 52 deg 2 yr of coverage for very slowly decaying transients, such as CVs. This implies an overall rate of outbursts from high galactic latitude CVs of 0.1 deg −2 yr −1 .

We report on follow-up observations of the gamma-ray burst GRB 060927 using the robotic ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hours after the trigger shows a continuum break at λ ≈ 8070Å, produced by neutral hydrogen absorption at z ≈ 5.6. We also detect an absorption line at 8158Å which we interpret as Si II λ 1260 at z = 5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyα profile with a column density with log(N HI /cm −2 ) = 22.50 ± 0.15. We discuss the implications of this work for the use of GRBs as probes of the end of the dark ages and draw three main conclusions: i) GRB afterglows originating from z 6 should be relatively easy to detect from the ground, but rapid near-infrared monitoring is necessary to ensure that they are found; ii) The presence of large H I column densities in some GRBs host galaxies at z > 5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; iii) GRBs appear crucial to locate typical star-forming galaxies at z > 5 and therefore the type of galaxies responsible for the reionization of the universe. Subject headings: gamma rays: bursts (GRB 060927) -cosmology 1 Partly based on observations carried out with the ESO telescopes under programmes 077.D-0661, 077.A-0667, 078.D-0416, and the large programme 177.A-f0591.

We present the unfiltered ROTSE-III light curve of the optical transient associated with GRB 050319 beginning 4 s after the cessation of γ-ray activity. We fit a power-law function to the data using the revised trigger time given by Chincarini et al. (2005), and a smoothly broken power-law to the data using the original trigger disseminated through the GCN notices. Including the RAPTOR data 13 Los Alamos National Laboratory, NIS-2 MS D436, Los Alamos, NM 87545, vestrand@lanl.gov, jwren@nis.lanl.gov from Woźniak et al. (2005), the best fit power-law indices are α = −0.854 ± 0.014 for the single power-law and α 1 = −0.364 +0.020 −0.019 , α 2 = −0.881 +0.030 −0.031 , with a break at t b = 418 +31 −30 s for the smoothly broken fit. We discuss the fit results with emphasis placed on the importance of knowing the true start time of the optical transient for this multi-peaked burst. As Swift continues to provide prompt GRB locations, it becomes more important to answer the question, "when does the afterglow begin" to correctly interpret the light curves.

The robotic ROTSE-III telescope network detected prompt optical emission contemporaneous with the γ-ray emission of Swift events GRB 051109A and GRB 051111. Both datasets have continuous coverage at high signal-to-noise levels from the prompt phase onwards, thus the early observations are readily compared to the Swift XRT and BAT high energy detections. In both cases, the optical afterglow is established, declining steadily during the prompt emission. For GRB 051111, there is evidence of an excess optical component during the prompt emission. The component is consistent with the flux spectrally extrapolated from the γ-rays, using the γ-ray spectral index. A compilation of spectral information from previous prompt detections shows that such a component is unusual. The existence of two prompt optical components -one connected to the high-energy emission, the other to separate afterglow flux, as indicated in GRB 051111 -is not compatible with a simple "external-external" shock model for the GRB and its afterglow.-4telescopes, the GRB field now has several examples of optical lightcurves that begin during, or within seconds after, the γ-ray emission.Broadband prompt emission is one of the least-understood aspects of GRB phenomena. The first prompt optical detection, GRB 990123 (Akerlof et al. 1999), exhibited an optical flare that was interpreted as the signature of a reverse shock passing through the relativistic ejecta (however, for another interpretation, see Liang et al. 1999). Reverse shock emission was expected to be common (Sari & Piran 1999), so it has come as a surprise that nearly all rapidly-detected GRB afterglows show scant evidence of it. There are alternatives to the standard "internal shocks" formulation for prompt emission. Such models include, e.g., external shocks (Meszaros & Rees 1993) or magnetic reconnection (Meszaros et al. 1994;Thompson 1994;Usov 1994) as the mechanism to release energy as γ-rays. The nature of GRB prompt emission is best investigated in conjunction with prompt observations at lower frequencies, with ongoing measurements at the same frequency to connect to the longerlasting, and better understood, afterglow.The Swift XRT's early X-ray observations have revealed a nearly standard morphology seen in most bursts' X-ray afterglow O'Brien et al. 2006). The typical early X-ray afterglow includes two surprises: a stage of relatively slow decay preceding the faster decline, known from pre-Swift observations, hours to days post-burst, and flaring well after the cessation of γ-ray emission. To connect this interesting early behavior to the later afterglow evolution, it is essential to compare such high-energy emission to lower-energy evolution. Such comparisons elucidate which features also occur at low energies, indicating a process affecting the entire early afterglow rather than a separate high-energy component (see GRB 050801, Rykoff et al. 2006).There is a small but growing sample of events for which it is possible to compare the very early optical lightcurve with X-ray emission (o...

The ROTSE-IIIa telescope at Siding Spring Observatory, Australia, detected prompt optical emission from Swift GRB 050401. In this letter, we present observations of the early optical afterglow, first detected by the ROTSE-IIIa telescope 33 s after the start of gamma-ray emission, contemporaneous with the brightest peak of this emission. This GRB was neither exceptionally long nor bright. This is the first prompt optical detection of a GRB of typical duration and luminosity. We find that the early afterglow decay does not deviate significantly from the power-law decay observable at later times, and is uncorrelated with the prompt gamma-ray emission. We compare this detection with the other two GRBs with prompt observations, GRB 990123 and GRB 041219a. All three bursts exhibit quite different behavior at early times.Comment: 4 pages, 3 figures. Accepted for publication in ApJ Letter