Heather Swan

Detection of GRB 060927 at z = 5.47: Implications for the Use of Gamma-Ray Bursts as Probes of the End of the Dark Ages

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 hr after the trigger shows a continuum break at lambda~8070 A, produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A, which we interpret as Si II lambda1260 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 Lyalpha profile with a column density with log(NH/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: (1) 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; (2) the presence of large H I column densities in some GRB host galaxies at z>5 makes the use of GRBs to probe the reionization epoch via spectroscopy of the red damping wing challenging; and (3) 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. Partly based on observations carried out with the ESO telescopes under programs 077.D-0661, 077.A-0667, 078.D-0416, and the large program 177.A-f0591.

GRB 071003: Broadband Follow-up Observations of a Very Bright Gamma-Ray Burst in a Galactic Halo

The optical afterglow of long-duration GRB 071003 is among the brightest yet to be detected from any GRB, with R ≈ 12 mag in KAIT observations starting 42 s after the GRB trigger, including filtered detections during prompt emission. However, our high-S/N afterglow spectrum displays only extremely weak absorption lines at what we argue is the host redshift of z = 1.60435, in contrast to the three other, much stronger Mg II absorption systems observed at lower redshifts. Together with Keck adaptive optics observations, which fail to reveal a host galaxy coincident with the burst position, our observations suggest a halo progenitor and offer a cautionary tale about the use of Mg II for GRB redshift determination. We present early- through late-time observations spanning the electromagnetic spectrum, constrain the connection between the prompt emission and early variations in the light curve (we observe no correlation), and discuss possible origins for an unusual, marked rebrightening that occurs a few hours after the burst: likely either a late-time refreshed shock or a wide-angle secondary jet. Analysis of the late-time afterglow is most consistent with a wind environment, suggesting a massive star progenitor. Together with GRB 070125, this may indicate that a small but significant portion of star formation in the early universe occurred far outside what we consider a normal galactic disk.

Exploring Broadband GRB Behavior during γ-Ray Emission

S. A. Yost, H. F. Swan, E. S. Rykoff, F. Aharonian, C. W. Akerlof, A. Alday, M. C. B. Ashley, S. Barthelmy, D. Burrows, D. L. Depoy, R. J. Dufour, J. D. Eastman, R. D. Forgey, N. Gehrels, E. Gogus, T. Guver, J. P. Halpern, L. C. Hardin, D. Horns, U. Kizilolu, H. A. Krimm, S. Lepine, E. P. Liang, J. L. Marshall, T. A. McKay, T. Mineo, N. Mirabal, M. Ozel, A. Phillips, J. L. Prieto, R. M. Quimby, P. Romano, G. Rowell, W. Rujopakarn, B. E. Schaefer, J. M. Silverman, R. Siverd, M. Skinner, D. A. Smith, I. A. Smith, S. Tonnesen, E. Troja, W. T. Vestrand, J. C. Wheeler, J. Wren, F. Yuan, and B. Zhang

Prompt Optical Detection of GRB 050401 with ROTSE-IIIa

The 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 γ-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 γ-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.

The Anomalous Early Afterglow of GRB 050801

The ROTSE-IIIc telescope at the H.E.S.S. 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 lightcurve does not fade or brighten significantly over the first � 250 s, after which there is an achromatic break and the lightcurve declines in typical power-law fashion. The Swift/XRT also obtained early observations starting at 69 s after the burst onset. The X-ray lightcurve shows the same features as the optical lightcurve. 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. Subject headings: gamma rays:bursts

A Search for Untriggered GRB Afterglows with ROTSE-III

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 2003 September to 2005 March. We have an effective coverage of 1.74 deg2 yr for rapidly fading transients that remain brighter than ~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 ~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 ~52 deg2 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.

The dark side of ROTSE-III prompt GRB observations

Wepresent severalcasesofopticalobservationsduring � -raybursts(GRBs) whichresulted in promptlimitsbutno detection of optical emission. These limits constrain the prompt optical flux densities and the optical brightness relative to the � -ray emission. The derived constraints fall within the range of properties observed in GRBs with prompt optical detections, although at the faint end of optical/� -rayflux ratios. The currently accessible prompt optical limits do not require a different set of intrinsic or environmental GRB properties, relative to the events with prompt optical detections. Subject headingg gamma rays: bursts Online material: color figure

Optical light curve and cooling break of GRB 050502A

We present light curves of the afterglow of GRB 050502A, including very early data at ttGRB <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 lightcurveisfitbyabrokenpowerlaw,wheretsteepensfrom � ¼� 1:13 � 0:02to � 1:44 � 0:02at � 5700s.This steepening is consistent with the evolution expected for the passage of the cooling frequencyc 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 consistentwitheitheranISMorawindfireballmodel,butslightlyfavortheISMinterpretation.Theexpectedspectral index in the ISM interpretation is consistent within 1 � with the observed spectral index � ¼� 0:8 � 0:1; the wind interpretation would imply a spectral index slightly (� 2 � ) shallower than observed. A small amount of dust extinc- tion at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of � .I n this picture, the early optical decay, with the peak at or below 4:7 ;10 14 Hz at 44 s, requires very small electron and magnetic energy partitions from the fireball. Subject headingg gamma rays: bursts

An Estimation of the Gamma-Ray Burst Afterglow Apparent Optical Brightness Distribution Function

By using recent publicly available observational data obtained in conjunction with the NASA Swift gamma-ray burst (GRB) mission and a novel data analysis technique, we have been able to make some rough estimates of the GRB afterglow apparent optical brightness distribution function. The results suggest that 71% of all burst afterglows have optical magnitudes with mR < 22.1 at 1000 s after the burst onset, the dimmest detected object in the data sample. There is a strong indication that the apparent optical magnitude distribution function peaks at mR ≈ 19.5. Such estimates may prove useful in guiding future plans to improve GRB counterpart observation programs. The employed numerical techniques might find application in a variety of other data analysis problems in which the intrinsic distributions must be inferred from a heterogeneous sample.

Photometry and spectroscopy of GRB 060526: a detailed study of the afterglow and host galaxy of a z = 3.2 gamma-ray burst

Aims. With this paper we want to investigate the highly variable afterglow light curve and environment of gamma-ray burst (GRB) 060526 at z = 3: 221. Methods. We present one of the largest photometric datasets ever obtained for a GRB afterglow, consisting of multi-color photometric data from the ultraviolet to the near infrared. The data set contains 4 12 data points in total to which we add additional data from the literature. Furthermore, we present low-resolution high signal-to-noise spectra of the afterglow. The afterglow light curve is modeled with both an analytical model using broken power law fits and with a broad-band numerical model wh ich includes energy injections. The absorption lines detec ted in the spectra are used to derive column densities using a multi-ion single-component curve-of-growth analysis from which we derive the metallicity of the host of GRB 060526. Results. The temporal behaviour of the afterglow follows a double broken power law with breaks at t = 0: 090�0: 005 and t = 2: 401�0: 061 days. It shows deviations from the smooth set of power laws that can be modeled by additional energy injections from the central engine, although some significant microvariability remains. The broadband spect ral-energy distribution of the afterglow shows no significa nt extinction along the line of sight. The metallicity derived from S II and Fe II of [S/H] = ‐0.57� 0.25 and [Fe/H] = ‐1.09� 0.24 is relatively high for a galaxy at that redshift but comparable to the metallicity of other GRB hosts at similar redshifts. At the position of the afterglow, no host is detect ed to F775W(AB)= 28.5 mag with the HST, implying an absolute magnitude of the host M(1500 A)>‐18.3 mag which is fainter than most long-duration hosts, although the GRB may be associated with a faint galaxy at a distance of 11 kpc.