114 research outputs found
A New Analysis Method for Reconstructing the Arrival Direction of TeV Gamma-rays Using a Single Imaging Atmospheric Cherenkov Telescope
We present a method of atmospheric Cherenkov imaging which reconstructs the
unique arrival direction of TeV gamma rays using a single telescope. The method
is derived empirically and utilizes several features of gamma-ray induced air
showers which determine, to a precision of 0.12 degrees, the arrival direction
of photons, on an event-by-event basis. Data from the Whipple Observatory's 10
m gamma-ray telescope is utilized to test selection methods based on source
location. The results compare these selection methods with traditional
techniques and three different camera fields of view. The method will be
discussed in the context of a search for a gamma-ray signal from a point source
located anywhere within the field of view and from regions of extended
emission.Comment: 24 pages, 16 figures, accepted for publication in Astroparticle
Physics May 11, 200
Optical Intensity Interferometry with Atmospheric Cherenkov Telescope Arrays
In the 1970s, the Narrabri intensity interferometer was used to measure 32
stellar diameters; some as small as 0.4 milli-arc-seconds (mas). The
interferometer consisted of a pair of 6.5m telescopes with relatively crude
optics, similar to those currently in use as Atmospheric Cherenkov Telescopes
(ACT). We explore the possibility of implementing a modern intensity
interferometer on an ACT array. Developments in fast digital signal processing
technology now make such a system relatively easy to implement, and provide
improved sensitivity. Allowing measurements at short wavelength (<400nm), with
long baselines (> 100m), which are still challenging for Michelson
interferometers, present ACT arrays could be used to probe angular structures
as small as ~0.2mas, and smaller with large array projects already being
discussed. This would provide measurements of stellar diameters, binary
systems, circumstellar environments and, possibly, stellar surface features.
ACT arrays could be used as intensity interferometers during bright moon
periods, providing valuable scientific output for little expense and no impact
on the gamma-ray observing schedule.Comment: Accepted for publicaion in Ap
Towards micro-arcsecond spatial resolution with Air Cherenkov Telescope arrays as optical intensity interferometers
In this poster contribution we highlight the equivalence between an Imaging
Air Cherenkov Telescope (IACT) array and an Intensity Interferometer for a
range of technical requirements. We touch on the differences between a
Michelson and an Intensity Interferometer and give a brief overview of the
current IACT arrays, their upgrades and next generation concepts (CTA, AGIS,
completion 2015). The latter are foreseen to include 30-90 telescopes that will
provide 400-4000 different baselines that range in length between 50m and a
kilometre. Intensity interferometry with such arrays of telescopes attains 50
micro-arcseconds resolution for a limiting V magnitude of ~8.5. This technique
opens the possibility of a wide range of studies, amongst others, probing the
stellar surface activity and the dynamic AU scale circumstellar environment of
stars in various crucial evolutionary stages. Here we discuss possibilities for
using IACT arrays as optical Intensity Interferometers.Comment: Appeared in the proceedings of "The Universe under the Microscope -
Astrophysics at High Angular Resolution", Journal of Physics:Conference
Series (IOP; http://www.iop.org/EJ/toc/1742-6596/131/1
Detection Techniques of Microsecond Gamma-Ray Bursts using Ground-Based Telescopes
Gamma-ray observations above 200 MeV are conventionally made by
satellite-based detectors. The EGRET detector on the Compton Gamma Ray
Observatory (CGRO) has provided good sensitivity for the detection of bursts
lasting for more than 200 ms. Theoretical predictions of high-energy gamma-ray
bursts produced by quantum-mechanical decay of primordial black holes (Hawking
1971) suggest the emission of bursts on shorter time scales. The final stage of
a primordial black hole results in a burst of gamma-rays, peaking around 250
MeV and lasting for a tenth of a microsecond or longer depending on particle
physics. In this work we show that there is an observational window using
ground-based imaging Cherenkov detectors to measure gamma-ray burst emission at
energies E greater than 200 MeV. This technique, with a sensitivity for bursts
lasting nanoseconds to several microseconds, is based on the detection of
multi-photon-initiated air showers.Comment: accepted for publication in the Astrophysical Journa
Gamma-Hadron Separation Methods for the VERITAS Array of Four Imaging Atmospheric Cherenkov Telescopes
Ground-based arrays of imaging atmospheric Cherenkov telescopes have emerged
as the most sensitive gamma-ray detectors in the energy range of about 100 GeV
and above. The strengths of these arrays are a very large effective collection
area on the order of 100,000 square meter, combined with excellent single
photon angular and energy resolutions. The sensitivity of such detectors is
limited by statistical fluctuations in the number of Cosmic Ray initiated air
showers that resemble gamma-ray air showers in many ways. In this paper, we
study the performance of simple event reconstruction methods when applied to
simulated data of the Very Energetic Radiation Imaging Telescope Array System
(VERITAS) experiment. We review methods for reconstructing the arrival
direction and the energy of the primary photons, and examine means to improve
on their performance. For a software threshold energy of 300 GeV (100 GeV), the
methods achieve point source angular and energy resolutions of sigma[63%]= 0.1
degree (0.2 degree) and sigma[68%]= 15% (22%), respectively. The main emphasis
of the paper is the discussion of gamma-hadron separation methods for the
VERITAS experiment. We find that the information from several methods can be
combined based on a likelihood ratio approach and the resulting algorithm
achieves a gamma-hadron suppression with a quality factor that is substantially
higher than that achieved with the standard methods used so far.Comment: Astroparticle Physics, in press, 22 pages, 10 figure
SGARFACE: A Novel Detector For Microsecond Gamma Ray Bursts
The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) is operated at
the Whipple Observatory utilizing the Whipple 10m gamma-ray telescope. SGARFACE
is sensitive to gamma-ray bursts of more than 100MeV with durations from 100ns
to 35us and provides a fluence sensitivity as low as 0.8 gamma-rays per m^2
above 200MeV (0.05 gamma-rays per m^2 above 2GeV) and allows to record the
burst time structure.Comment: 29 pages, 14 figures, accepted for publication in Astroparticle
Physic
Stellar Intensity Interferometry with Air Cherenkov Telescope arrays
The present generation of ground-based Very High Energy (VHE) gamma-ray
observatories consist of arrays of up to four large (> 12m diameter) light
collectors quite similar to those used by R. Hanbury Brown to measure stellar
diameters by Intensity Interferometry in the late 60's. VHE gamma-ray
observatories to be constructed over the coming decade will involve several
tens of telescopes of similar or greater sizes. Used as intensity
interferometers, they will provide hundreds of independent baselines. Now is
the right time to re-assess the potential of intensity interferometry so that
it can be taken into consideration in the design of these large facilities.Comment: 11 pages, 9 figures, in procedings of the High Time Resolution
Astrophysics conferenc
New Astrophysical Opportunities Exploiting Spatio-Temporal Optical Correlations
The space-time correlations of streams of photons can provide fundamentally
new channels of information about the Universe. Today's astronomical
observations essentially measure certain amplitude coherence functions produced
by a source. The spatial correlations of wave fields has traditionally been
exploited in Michelson-style amplitude interferometry. However the technology
of the past was largely incapable of fine timing resolution and recording
multiple beams. When time and space correlations are combined it is possible to
achieve spectacular measurements that are impossible by any other means.
Stellar intensity interferometry is ripe for development and is one of the few
unexploited mechanisms to obtain potentially revolutionary new information in
astronomy. As we discuss below, the modern use of stellar intensity
interferometry can yield unprecedented measures of stellar diameters, binary
stars, distance measures including Cepheids, rapidly rotating stars, pulsating
stars, and short-time scale fluctuations that have never been measured before.Comment: Science white paper prepared for the Astro2010 Decadal Revie
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