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Öğe A search for untriggered GRB afterglows with ROTSE-III(Iop Publishing Ltd, 2005) Rykoff, ES; Aharonian, F; Akerlof, CW; Alatalo, K; Ashley, MCB; Güver, T; Horns, DWe 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 2003 September to 2005 March. We have an effective coverage of 1.74 deg(2) yr for rapidly fading transients that remain brighter than similar to 17.5 mag 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 four previously unknown cataclysmic variables (CVs) and one 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 similar to 20th 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 by-product of this search, we have an effective similar to 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).Öğe Early-time observations of the GRB 050319 optical transient(Iop Publishing Ltd, 2006) Quimby, RM; Rykoff, ES; Yost, SA; Aharonian, F; Akerlof, CW; Alatalo, K; Ashley, MCBWe present the unfiltered ROTSE-III light curve of the optical transient associated with GRB 050319 beginning 4 s after the cessation of gamma-ray activity. We fit a power-law function to the data using the revised trigger time given by Chincarini and coworkers, and a smoothly broken power-law to the data using the original trigger disseminated through the GCN notices. Including the RAPTOR data from Wozniak and coworkers, the best-fit power-law indices are alpha = -0.854 +/- 0.014 for the single power-law and alpha(1) = -0.364(-0.019)(+0.020), alpha(2) = -0.881(-0.031)(+0.030), with a break at t(b) = 418(-30)(+31) 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 multipeaked burst. As Swift continues to provide prompt GRB locations, it becomes more important to answer the question, when does the afterglow begin?'' in order to correctly interpret the light curves.Öğe Optical light curve and cooling break of GRB 050502A(Iop Publishing Ltd, 2006) Yost, SA; Alatalo, K; Rykoff, ES; Aharonian, F; Akerlof, CW; Ashley, MCB; Blake, CHWe present light curves of the afterglow of GRB 050502A, including very early data at t - t(GRB) < 60 s. The light curve is composed of unfiltered ROTSE-IIIb optical observations from 44 s to 6 hr postburst, R-band MDM observations from 1.6 to 8.4 hr postburst, and PAIRITEL JHKs observations from 0.6 to 2.6 hr postburst. The optical light curve is fit by a broken power law, where t(alpha) steepens from alpha = 1.13 +/- 0.02 to -1.44 +/- 0.02 at similar to 5700s. This steepening is consistent with the evolution expected for the passage of the cooling frequency v(c) through the optical band. Even in our earliest observation at 44 s postburst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistent with either an ISM or a wind fireball model, but slightly favor the ISM interpretation. The expected spectral index in the ISM interpretation is consistent within 1 sigma with the observed spectral index beta = -0.8 +/- 0.1; the wind interpretation would imply a spectral index slightly (similar to 2 sigma) shallower than observed. A small amount of dust extinction at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of beta. In this picture, the early optical decay, with the peak at or below 4.7 x 10(14) Hz at 44 s, requires very small electron and magnetic energy partitions from the fireball.Öğe Prompt optical detection of GRB 050401 with ROTSE-IIIa(Iop Publishing Ltd, 2005) Rykoff, ES; Yost, SA; Krimm, HA; Aharonian, F; Akerlof, CW; Alatalo, K; Ashley, MCBThe ROTSE-IIIa telescope at Siding Spring Observatory, Australia, detected prompt optical emission from Swift GRB 050401. 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.Öğe The anomalous early afterglow of GRB 050801(Iop Publishing Ltd, 2006) Rykoff, ES; Mangano, V; Yost, SA; Sari, R; Aharonian, F; Akerlof, CW; Ashley, MCBThe ROTSE-IIIc telescope at the HESS site, Namibia, obtained the earliest detection of optical emission from a gamma-ray burst (GRB), beginning only 21.8 s from the onset of Swift GRB 050801. The optical light curve does not fade or brighten significantly over the first similar to 250 s, after which there is an achromatic break and the light curve declines in typical power-law fashion. The Swift XRT also obtained early observations starting at 69 s after the burst onset. The X-ray light curve shows the same features as the optical light curve. These correlated variations in the early optical and X-ray emission imply a common origin in space and time. This behavior is difficult to reconcile with the standard models of early afterglow emission.
