821 research outputs found
On the possibility of sub-TeV Gamma-ray emission from Cyg X-3
The compact X-ray binary system Cyg X-3 has been recently discovered as a
source of GeV gamma-rays by the AGILE and the {\it Fermi} satellites. It shows
emission features in the GeV gamma-rays similar to other gamma-ray binaries
which were also observed in the TeV gamma-rays (LS 5039 and LSI +61 303). The
question appears whether Cyg X-3 can be also detected in the TeV gamma-rays by
the Cherenkov telescopes.
Here we discuss this problem in detail based on the anisotropic inverse
Compton (IC) e-p pair cascade model successfully applied to TeV gamma-ray
binaries. We calculate the gamma-ray light curves and gamma-ray spectra
expected from the cascade process occurring inside the Cyg X-3 binary system.
It is found that the gamma-ray light curves at GeV energies can be consistent
with the gamma-ray light curve observed by the Fermi for reasonable parameters
of the orbit of the injection source of relativistic electrons. Moreover, we
show that in such a model the sub-TeV gamma-ray emission (above 100 GeV) is
expected to be below sensitivities of the present Cherenkov telescopes assuming
that electrons are accelerated in Cyg X-3 to TeV energies. The next stage
Cherenkov telescopes (MAGIC II, HESS II) should have the energy threshold in
the range 20-30 GeV, in order to have a chance to detect the signal from Cyg
X-3. Otherwise, the positive detection of gamma-rays at energies above a few
tens of GeV requires a telescope with the sensitivity of ~0.1% of Crab Units.
We conclude that detection of sub-TeV gamma-rays from Cyg X-3 by on-ground
telescopes has to probably wait for construction of the Cherenkov Telescope
Array (CTA).Comment: 13 pages, 8 figures, small changes in the text and discussion
extended, accepted to MNRA
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
Improved energy resolution for VHE gamma-ray astronomy with systems of Cherenkov telescopes
We present analysis techniques to improve the energy resolution of
stereoscopic systems of imaging atmospheric Cherenkov telescopes, using the
HEGRA telescope system as an example. The techniques include (i) the
determination of the height of the shower maximum, which is then taken into
account in the energy determination, and (ii) the determination of the location
of the shower core with the additional constraint that the direction of the
gamma rays is known a priori. This constraint can be applied for gamma-ray
point sources, and results in a significant improvement in the localization of
the shower core, which translates into better energy resolution. Combining both
techniques, the HEGRA telescopes reach an energy resolution between 9% and 12%,
over the entire energy range from 1 TeV to almost 100 TeV. Options for further
improvements of the energy resolution are discussed.Comment: 13 Pages, 7 figures, Latex. Astroparticle Physics, in pres
'Expecting': occupant model incorporating anthropometric details of pregnant women
This study reports the research for a design tool related to pregnant
women’s safety during car travel. Anthropometric measurements are taken to
generate an occupant model incorporating pregnancy related changes. These
anthropometric changes mean that a pregnant occupant may be excluded by the
designs, based upon non-pregnant female anthropometry. The paper explains
the generation of a comprehensive parametric computer aided model of a
pregnant occupant, ‘Expecting’. The model can represent different size
pregnant occupants as well as the size differences occurring in standing and
seated postures. This model can be used as a design tool for automotive
designers to help ensure that vehicle designs can accommodate the
anthropometric needs of pregnant occupants
Human cytomegalovirus: taking the strain
In celebrating the 60th anniversary of the first isolation of human cytomegalovirus (HCMV), we reflect on the merits and limitations of the viral strains currently being used to develop urgently needed treatments. HCMV research has been dependent for decades on the high-passage strains AD169 and Towne, heavily exploiting their capacity to replicate efficiently in fibroblasts. However, the genetic integrity of these strains is so severely compromised that great caution needs to be exercised when considering their past and future use. It is now evident that wild-type HCMV strains are not readily propagated in vitro. HCMV mutants are rapidly selected during isolation in fibroblasts, reproducibly affecting gene RL13, the UL128 locus (which includes genes UL128, UL130 and UL131A) and often the UL/b′ region. As a result, the virus becomes less cell associated, altered in tropism and less pathogenic. This problem is not restricted to high-passage strains, as even low-passage strains can harbour biologically significant mutations. Cloning and manipulation of the HCMV genome as a bacterial artificial chromosome (BAC) offers a means of working with stable, genetically defined strains. To this end, the low-passage strain Merlin genome was cloned as a BAC and sequentially repaired to match the viral sequence in the original clinical sample from which Merlin was derived. Restoration of UL128L to wild type was detrimental to growth in fibroblasts, whereas restoration of RL13 impaired growth in all cell types tested. Stable propagation of phenotypically wild-type virus could be achieved only by placing both regions under conditional expression. In addition to the development of these tools, the Merlin transcriptome and proteome have been characterized in unparalleled detail. Although Merlin may be representative of the clinical agent, high-throughput whole-genome deep sequencing studies have highlighted the remarkable high level of interstrain variation present in circulating virus. There is a need to develop systems capable of addressing the significance of this diversity, free from the confounding effects of genetic changes associated with in vitro adaptation. The generation of a set of BAC clones, each containing the genome of a different HCMV strain repaired to match the sequence in the clinical sample, would provide a pathway to address the biological and clinical effects of natural variation in wild-type HCMV
Search for gamma-rays above 400 GeV from Geminga
Observations of Geminga made at the Whipple Observatory using the atmospheric Cherenkov technique during the moonless periods of November 1983 to February 1984 and November 1984 till February 1985 were examined for evidence for the emission of gamma rays with energy in excess of approx 400 GeV. Evidence of either a steady flux or a flux pulsed with a period near 60 seconds were studied. In neither case was any significant effect observed, enabling the establishment 3 of sigma upper limits of 5.5 x 10 to the -11th power photons/sq cm/s and 2.0 x 10 to the -11th power photons/sq cm/s for the steady and pulsed emission respectively. The limit to the pulsed flux is approximately a factor of six below that predicted
Observations of the Crab Nebula at energies 4.10(11)
Since the development of gamma-ray astronomical telescopes, the Crab Nebula has been a prime target for observations. From 100 to 1000 MeV, the pulsar PSR0531 is the dominant source with a light-curve similar to that seen at lower energies; there is also some evidence for longterm amplitude variations but none for emission from the Nebula itself. In the very high energy gamma-ray region there have been reported detections of pulsed emission with longterm time variations from minutes to months. Recently a pulsed flux has been reported that resisted over a long time interval. The detection of a flux from the Nebula at the 3 sigma level at energies of 3x1011eV was reported; there was no evidence of periodic emissions on any time scale during the three years of observations. A new measurement of very high energy gamma rays from the Crab Nebula is reported using the imaging system on the Whipple Observatory 10m reflector
Hercules X-1: Pulsed gamma-rays detected above 150 GeV
The 1.24 second binary pulsar Her X-1, first observed in X-rays in 1971 by UHURU has now been seen as a sporadic gamma ray source from 1 TeV up to at least 500 TeV. In addition, reprocessed optical and infrared pulses are seen from the companion star HZ Herculis. Thus measurements of the Her X-1/HZ Herculis system span 15 decades in energy, rivaling both the Crab pulsar and Cygnus X-3 in this respect for a discrete galactic source
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