333 research outputs found
LOTIS Upper Limits and the Prompt OT from GRB 990123
GRB 990123 established the existence of prompt optical emission from
gamma-ray bursts (GRBs). The Livermore Optical Transient Imaging System (LOTIS)
has been conducting a fully automated search for this kind of simultaneous low
energy emission from GRBs since October 1996. Although LOTIS has obtained
simultaneous, or near simultaneous, coverage of the error boxes obtained with
BATSE, IPN, XTE, and BeppoSAX for several GRBs, image analysis resulted in only
upper limits. The unique gamma-ray properties of GRB 990123, such as very large
fluence (top 0.4%) and hard spectrum, complicate comparisons with more typical
bursts. We scale and compare gamma-ray properties, and in some cases afterglow
properties, from the best LOTIS events to those of GRB 990123 in an attempt to
determine whether the prompt optical emission of this event is representative
of all GRBs. Furthermore, using LOTIS upper limits in conjunction with the
relativistic blast wave model, we weakly constrain the GRB and afterglow
parameters such as density of the circumburster medium and bulk Lorentz factor
of the ejecta.Comment: 5 pages, 2 figures, To appear in Proceedings of the 5th Huntsville
Gamma-Ray Burst Symposiu
LOTIS Search for Early Time Optical Afterglows: GRB 971227
We report on the very early time search for an optical afterglow from GRB
971227 with the Livermore Optical Transient Imaging System (LOTIS). LOTIS began
imaging the `Original' BATSE error box of GRB 971227 approximately 14 s after
the onset of gamma-ray emission. Continuous monitoring of the position
throughout the evening yielded a total of 499 images (10 s integration).
Analysis of these images revealed no steady optical afterglow brighter than
R=12.3 +- 0.2 in any single image. Coaddition of different combinations of the
LOTIS images also failed to uncover transient optical emission. In particular,
assuming a constant early time flux, no optical afterglow brighter than R=14.2
+- 0.2 was present within the first 1200 s and no optical afterglow brighter
than R=15.0 +- 0.2 was present in the first 6.0 h.
Follow up observations by other groups revealed a likely X-ray afterglow and
a possible optical afterglow. Although subsequent deeper observations could not
confirm a fading source, we show that these transients are not inconsistent
with our present knowledge of the characteristics of GRB afterglows. We also
demonstrate that with the upgraded thermoelectrically cooled CCDs, LOTIS is
capable of either detecting very early time optical afterglow or placing
stringent constraints on the relationship between the gamma-ray emission and
the longer wavelength afterglow in relativistic blast wave models.Comment: 17 pages, 3 eps figures, revisions based on reviewers comment
Search for Small Trans-Neptunian Objects by the TAOS Project
The Taiwan-America Occultation Survey (TAOS) aims to determine the number of
small icy bodies in the outer reach of the Solar System by means of stellar
occultation. An array of 4 robotic small (D=0.5 m), wide-field (f/1.9)
telescopes have been installed at Lulin Observatory in Taiwan to simultaneously
monitor some thousand of stars for such rare occultation events. Because a
typical occultation event by a TNO a few km across will last for only a
fraction of a second, fast photometry is necessary. A special CCD readout
scheme has been devised to allow for stellar photometry taken a few times per
second. Effective analysis pipelines have been developed to process stellar
light curves and to correlate any possible flux changes among all telescopes. A
few billion photometric measurements have been collected since the routine
survey began in early 2005. Our preliminary result of a very low detection rate
suggests a deficit of small TNOs down to a few km size, consistent with the
extrapolation of some recent studies of larger (30--100 km) TNOs.Comment: 4 pages, 3 figures, IAU Symposium 23
The Taiwanese-American Occultation Survey: The Multi-Telescope Robotic Observatory
The Taiwanese-American Occultation Survey (TAOS) operates four fully
automatic telescopes to search for occultations of stars by Kuiper Belt
Objects. It is a versatile facility that is also useful for the study of
initial optical GRB afterglows. This paper provides a detailed description of
the TAOS multi-telescope system, control software, and high-speed imaging.Comment: 11 pages, 11 figure
The TAOS Project Stellar Variability I. Detection of Low-Amplitude delta Scuti Stars
We analyzed data accumulated during 2005 and 2006 by the Taiwan-American
Occultation Survey (TAOS) in order to detect short-period variable stars
(periods of <~ 1 hour) such as delta Scuti. TAOS is designed for the detection
of stellar occultation by small-size Kuiper Belt Objects (KBOs) and is
operating four 50cm telescopes at an effective cadence of 5Hz. The four
telescopes simultaneously monitor the same patch of the sky in order to reduce
false positives. To detect short-period variables, we used the Fast Fourier
Transform algorithm (FFT) inasmuch as the data points in TAOS light-curves are
evenly spaced. Using FFT, we found 41 short-period variables with amplitudes
smaller than a few hundredths of a magnitude and periods of about an hour,
which suggest that they are low-amplitude delta Scuti stars (LADS). The
light-curves of TAOS delta Scuti stars are accessible online at the Time Series
Center website (http://timemachine.iic.harvard.edu)Comment: Accepted for publication in A
A Close Binary Star Resolved from Occultation by 87 Sylvia
The star BD+29 1748 was resolved to be a close binary from its occultation by
the asteroid 87 Sylvia on 2006 December 18 UT. Four telescopes were used to
observe this event at two sites separated by some 80 km apart. Two flux drops
were observed at one site, whereas only one flux drop was detected at the
other. From the long-term variation of Sylvia, we inferred the probable shape
of the shadow during the occultation, and this in turn constrains the binary
parameters: the two components of BD+29 1748 have a projected separation of
0.097" to 0.110" on the sky with a position angle 104 deg to 107 deg. The
asteroid was clearly resolved with a size scale ranging from 130 to 290 km, as
projected onto the occultation direction. No occultation was detected for
either of the two known moonlets of 87 Sylvia.Comment: 12 pages, 4 figures, 2 tables; submitted to the PAS
Optical Properties of Deep Ice at the South Pole - Absorption
We discuss recent measurements of the wavelength-dependent absorption
coefficients in deep South Pole ice. The method uses transit time distributions
of pulses from a variable-frequency laser sent between emitters and receivers
embedded in the ice. At depths of 800 to 1000 m scattering is dominated by
residual air bubbles, whereas absorption occurs both in ice itself and in
insoluble impurities. The absorption coefficient increases approximately
exponentially with wavelength in the measured interval 410 to 610 nm. At the
shortest wavelength our value is about a factor 20 below previous values
obtained for laboratory ice and lake ice; with increasing wavelength the
discrepancy with previous measurements decreases. At around 415 to 500 nm the
experimental uncertainties are small enough for us to resolve an extrinsic
contribution to absorption in ice: submicron dust particles contribute by an
amount that increases with depth and corresponds well with the expected
increase seen near the Last Glacial Maximum in Vostok and Dome C ice cores. The
laser pulse method allows remote mapping of gross structure in dust
concentration as a function of depth in glacial ice.Comment: 26 pages, LaTex, Accepted for publication in Applied Optics. 9
figures, not included, available on request from [email protected]
First Results From The Taiwanese-American Occultation Survey (TAOS)
Results from the first two years of data from the Taiwanese-American
Occultation Survey (TAOS) are presented. Stars have been monitored
photometrically at 4 Hz or 5 Hz to search for occultations by small (~3 km)
Kuiper Belt Objects (KBOs). No statistically significant events were found,
allowing us to present an upper bound to the size distribution of KBOs with
diameters 0.5 km < D < 28 km.Comment: 5 pages, 5 figure, accepted in Ap
The AMANDA Neutrino Telescope and the Indirect Search for Dark Matter
With an effective telescope area of order 10^4 m^2, a threshold of ~50 GeV
and a pointing accuracy of 2.5 degrees, the AMANDA detector represents the
first of a new generation of high energy neutrino telescopes, reaching a scale
envisaged over 25 years ago. We describe its performance, focussing on the
capability to detect halo dark matter particles via their annihilation into
neutrinos.Comment: Latex2.09, 16 pages, uses epsf.sty to place 15 postscript figures.
Talk presented at the 3rd International Symposium on Sources and Detection of
Dark Matter in the Universe (DM98), Santa Monica, California, Feb. 199
The AMANDA Neutrino Telescope: Principle of Operation and First Results
AMANDA is a high-energy neutrino telescope presently under construction at
the geographical South Pole. In the Antarctic summer 1995/96, an array of 80
optical modules (OMs) arranged on 4 strings (AMANDA-B4) was deployed at depths
between 1.5 and 2 km. In this paper we describe the design and performance of
the AMANDA-B4 prototype, based on data collected between February and November
1996. Monte Carlo simulations of the detector response to down-going
atmospheric muon tracks show that the global behavior of the detector is
understood. We describe the data analysis method and present first results on
atmospheric muon reconstruction and separation of neutrino candidates. The
AMANDA array was upgraded with 216 OMs on 6 new strings in 1996/97
(AMANDA-B10), and 122 additional OMs on 3 strings in 1997/98.Comment: 36 pages, 23 figures, submitted to Astroparticle Physic
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