Michael C. B. Ashley

Exceptional astronomical seeing conditions above Dome C in Antarctica.

One of the most important considerations when planning the next generation of ground-based optical astronomical telescopes is to choose a site that has excellent ‘seeing’—the jitter in the apparent position of a star that is caused by light bending as it passes through regions of differing refractive index in the Earths atmosphere. The best mid-latitude sites have a median seeing ranging from 0.5 to 1.0 arcsec (refs 1–5). Sites on the Antarctic plateau have unique atmospheric properties that make them worth investigating as potential observatory locations. Previous testing at the US Amundsen-Scott South Pole Station has, however, demonstrated poor seeing, averaging 1.8 arcsec (refs 6, 7). Here we report observations of the wintertime seeing from Dome C (ref. 8), a high point on the Antarctic plateau at a latitude of 75° S. The results are remarkable: the median seeing is 0.27 arcsec, and below 0.15 arcsec 25 per cent of the time. A telescope placed at Dome C would compete with one that is 2 to 3 times larger at the best mid-latitude observatories, and an interferometer based at this site could work on projects that would otherwise require a space mission.

The ROTSE‐III Robotic Telescope System

The observation of a prompt optical flash from GRB 990123 convincingly demonstrated the value of autonomous robotic telescope systems. Pursuing a program of rapid follow-up observations of gamma- ray bursts, the Robotic Optical Transient Search Experiment (ROTSE) has developed a next-generation instrument, ROTSE-III, that will continue the search for fast optical transients. The entire system was designed as an economical robotic facility to be installed at remote sites throughout the world. There are seven major system components: optics, optical tube assembly, CCD camera, telescope mount, enclosure, environmental sensing and protection, and data acquisition. Each is described in turn in the hope that the techniques developed here will be useful in similar contexts elsewhere.

Looking Into the Fireball: ROTSE-III and Swift Observations of Early Gamma-ray Burst Afterglows

We report on a complete set of early optical afterglows of gamma-ray bursts (GRBs) obtained with the Robotic Optical Transient Search Experiment (ROTSE-III) telescope network from 2005 March through 2007 June. This set is comprised of 12 afterglows with early optical and Swift/X-Ray Telescope observations, with a median ROTSE-III response time of 45 s after the start of γ-ray emission (8 s after the GCN notice time). These afterglows span 4 orders of magnitude in optical luminosity, and the contemporaneous X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray flares, the broadband synchrotron spectra show that the optical and X-ray emission originate in a common region, consistent with predictions of the external forward shock in the fireball model. However, the fireball model is inadequate to predict the temporal decay indices of the early afterglows, even after accounting for possible long-duration continuous energy injection. We find that the optical afterglow is a clean tracer of the forward shock, and we use the peak time of the forward shock to estimate the initial bulk Lorentz factor of the GRB outflow, and find 100 ≲ Γ_0 ≲ 1000, consistent with expectations.

Atmospheric turbulence at the South Pole and its implications for astronomy

To investigate the low-atmosphere turbulence at the South Pole, we have measured, using a SODAR, the temperature fluctuation constant (C 2 ) during winter, as a function of altitude up to 890 m. We found that the turbulence was on average concentrated inside a boundary layer sitting below 270 m. While at the peak of winter the turbulence was stable and clearly bounded, during other seasons there was a more complex turbulence profile which extended to higher altitudes. We found that this behaviour could be explained by the horizontal wind speed conditions whose altitude profile closely matched the turbulence profile. We also observed the presence of a vertical wind velocity change of direction at an altitude range corresponding to the turbulent region. The turbulence gives rise to an average seeing of 1:73 00 , which compares poorly with the best astronomy sites. The location of the turbulence, however, means that the seeing quickly decreases above the boundary layer (dropping to 0:37 00 above 300 m). We also have recorded the largest isoplanatic angle (AO= 3:3 00 ) and the longest coherence time (AO= 2: 9m s)

Looking Into the Fireball: ROTSE-III and Swift Observations of Early GRB Afterglows

We report on a complete set of early optical afterglows of gamma-ray bursts (GRBs) obtained with the Robotic Optical Transient Search Experiment (ROTSE-III) telescope network from 2005 March through 2007 June. This set is comprised of 12 afterglows with early optical and Swift/X-Ray Telescope observations, with a median ROTSE-III response time of 45 s after the start of γ-ray emission (8 s after the GCN notice time). These afterglows span 4 orders of magnitude in optical luminosity, and the contemporaneous X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray flares, the broadband synchrotron spectra show that the optical and X-ray emission originate in a common region, consistent with predictions of the external forward shock in the fireball model. However, the fireball model is inadequate to predict the temporal decay indices of the early afterglows, even after accounting for possible long-duration continuous energy injection. We find that the optical afterglow is a clean tracer of the forward shock, and we use the peak time of the forward shock to estimate the initial bulk Lorentz factor of the GRB outflow, and find 100 ≲ Γ_0 ≲ 1000, consistent with expectations.

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.

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

The Near-Infrared Sky Emission at the South Pole in Winter

The Antarctic plateau provides superb sites for infrared astronomy, a result of the combination of low temperatures, low levels of precipitable water vapor, high altitude, and atmospheric stability. We have undertaken measurements of the sky background from 1 to 5 μm at the South Pole, using a single channel InSb spectrometer, the Infrared Photometer Spectrometer (IRPS), during the winter (dark) period of 1995. The IRPS records the DC level of the sky flux through a 4° beam and a variety of broadband and narrowband (1%) filters. It can be scanned in elevation from horizon to horizon through the zenith. We find a 20-100 times reduction in the background of thermal emission compared to that from mid-latitude sites such as Siding Spring and Mauna Kea, with typical background levels of 80-200 μJy arcsec-2 at 2.43 μm, 100-300 mJy arcsec-2 at 3.6 μm and ~0.5 Jy arcsec-2 at 4.8 μm. Airglow emission contributes significantly to the sky flux shortward of ~2.4 μm, which is why the Kdark (2.27-2.45 μm) band emission does not drop to the 10-20 μJy arcsec-2 levels originally predicted. The darkest window for IR observations from the South Pole is from 2.35 to 2.45 μm, where the fluxes from the atmosphere may drop to as low as ~50 μJy arcsec-2 at times. Airglow dominates the emission at J (1.25 μm) and H (1.65 μm), but the flux levels of 300-600 μJy arcsec-2 and 800-2000 μJy arcsec-2, respectively, are also one-third to one-half those at temperate sites. We find no evidence for any significant contribution from auroral emission to the Kdark band. During twilight, when the Sun is <10° below the horizon, scattered sunlight contributes to the sky background with a Rayleigh-type spectrum. Scattered moonlight is also evident in the sky emission at the J band when the Moon is up.

Science programs for a 2-m class telescope at Dome C, Antarctica: PILOT, the pathfinder for an international large optical telescope

The cold, dry, and stable air above the summits of the Antarctic plateau provides the best ground-based observing conditions from optical to sub-millimetre wavelengths to be found on the Earth. Pathfinder for an International Large Optical Telescope (PILOT) is a proposed 2 m telescope, to be built at Dome C in Antarctica, able to exploit these conditions for conducting astronomy at optical and infrared wavelengths. While PILOT is intended as a pathfinder towards the construction of future grand-design facilities, it will also be able to undertake a range of fundamental science investigations in its own right. This paper provides the performance specifications for PILOT, including its instrumentation. It then describes the kinds of projects that it could best conduct. These range from planetary science to the search for other solar systems, from star formation within the Galaxy to the star formation history of the Universe, and from gravitational lensing caused by exo-planets to that produced by the cosmic web of dark matter. PILOT would be particularly powerful for wide-field imaging at infrared wavelengths, achieving near diffraction-limited performance with simple tip–tilt wavefront correction. PILOT would also be capable of near diffraction-limited performance in the optical wavebands, as well be able to open new wavebands for regular ground-based observation, in the mid-IR from 17 to 40 μm and in the sub-millimetre at 200 μm.

The Early Optical Afterglow of GRB 030418 and Progenitor Mass Loss

This work has been supported by NASA grants NAG5- 5281 and F006794, NSF grants AST 01-19685 and 01-05221, the Australian Research Council, the University of New South Wales, and the University of Michigan. Work performed at LANL is supported by NASA SR&T through Department of Energy (DOE) contract W-7405-ENG-36 and through internal LDRD funding.