65,022 research outputs found
The discovery of gamma-ray emission from Nova Sco 2012: An analysis using reprocessed Pass7 data
In March 2010 the Large Area Telescope (LAT) on-board the Fermi Gamma-ray
Space Telescope discovered for the first time >100 MeV gamma-ray emission from
a nova within our galaxy, V407 Cyg. The high-energy spectrum and light curve
was explained as a consequence of shock acceleration in the nova shell as it
interacts with the local ambient medium. It was suspected that the necessary
conditions for high-energy emission from novae would be rare. In June 2012 the
LAT detected a new flaring source, Fermi J1750-3243, that is spatially
coincident and contemporaneous with a new nova, Nova Sco 2012. We report on the
exciting discovery of this new 'gamma-ray' nova and present a detailed analysis
of its high-energy properties.Comment: 2012 Fermi Symposium proceedings - eConf C12102
Cryogenic flux-concentrator
Flux concentrator has high primary to secondary coupling efficiency enabling it to produce high magnetic fields. The device provides versatility in pulse duration, magnetic field strengths and power sources
The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar
A set of CCD images have been obtained during the decline of the X-ray
transient SAX J1808.4-3658 during April-June 1998. The optical counterpart has
been confirmed by several pieces of evidence. The optical flux shows a
modulation on several nights which is consistent with the established X-ray
binary orbit period of 2 hours. This optical variability is roughly in
antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7
on April 18 declined rapidly after April 22. From May 2 onwards the magnitude
was more constant at around V=18.45 but by June 27 was below our sensitivity
limit. The optical decline precedes the rapid second phase of the X-ray
decrease by 3 +/- 1 days. The source has been identified on a 1974 UK Schmidt
plate at an estimated magnitude of ~20. The nature of the optical companion is
discussed.Comment: 5 pages, 3 figures; published in MNRAS, March 15th 199
Dielectric function and plasmons in graphene
The electromagnetic response of graphene, expressed by the dielectric
function, and the spectrum of collective excitations are studied as a function
of wave vector and frequency. Our calculation is based on the full band
structure, calculated within the tight-binding approximation. As a result, we
find plasmons whose dispersion is similar to that obtained in the single-valley
approximation by Dirac fermions. In contrast to the latter, however, we find a
stronger damping of the plasmon modes due to inter-band absorption. Our
calculation also reveals effects due to deviations from the linear Dirac
spectrum as we increase the Fermi energy, indicating an anisotropic behavior
with respect to the wave vector of the external electromagnetic field
A Correlation Between Changes in Solar Luminosity and Differential Radius Measurements
Solar luminosity variations occurring during solar cycle 21 can be attributed in large part to the presence of sunspots and faculae. Nevertheless, there remains a residual portion of the luminosity variation distinctly unaccounted for by these phenomena of solar activity. At the Santa Catalina Laboratory for Experimental Relativity by Astrometry (SCLERA), observations of the solar limb are capable of detecting changes in the solar limb darkening function by monitoring a quantity known as the differential radius. These observations are utilized in such a way that the effects of solar activity are minimized in order to reveal the more fundamental structure of the photosphere. The results of observations made during solar cycle 21 at various solar latitudes indicate that a measurable change did occur in the global photospheric limb darkening function. It is proposed that the residual luminosity change is associated in part with this change in limb darkening
What can Simbol-X do for gamma-ray binaries?
Gamma-ray binaries have been uncovered as a new class of Galactic objects in
the very high energy sky (> 100 GeV). The three systems known today have hard
X-ray spectra (photon index ~ 1.5), extended radio emission and a high
luminosity in gamma-rays. Recent monitoring campaigns of LSI +61 303 in X-rays
have confirmed variability in these systems and revealed a spectral hardening
with increasing flux. In a generic one-zone leptonic model, the cooling of
relativistic electrons accounts for the main spectral and temporal features
observed at high energy. Persistent hard X-ray emission is expected to extend
well beyond 10 keV. We explain how Simbol-X will constrain the existing models
in connection with Fermi Space Telescope measurements. Because of its
unprecedented sensitivity in hard X-rays, Simbol-X will also play a role in the
discovery of new gamma-ray binaries, giving new insights into the evolution of
compact binaries.Comment: 4 pages, 1 figure, Proceedings of the 2nd International Simbol-X
symposium held in Paris, 2-5 December 200
Optimization techniques applied to passive measures for in-orbit spacecraft survivability
The purpose of this research is to provide Space Station Freedom protective structures design insight through the coupling of design/material requirements, hypervelocity impact phenomenology, meteoroid and space debris environment sensitivities, optimization techniques and operations research strategies, and mission scenarios. The goals of the research are: (1) to develop a Monte Carlo simulation tool which will provide top level insight for Space Station protective structures designers; (2) to develop advanced shielding concepts relevant to Space Station Freedom using unique multiple bumper approaches; and (3) to investigate projectile shape effects on protective structures design
XMM-Newton and INTEGRAL analysis of the Supergiant Fast X-ray Transient IGR J17354-3255
We present the results of combined INTEGRAL and XMM-Newton observations of
the supergiant fast X-ray transient (SFXT) IGR J173543255. Three XMM-Newton
observations of lengths 33.4 ks, 32.5 ks and 21.9 ks were undertaken, the first
an initial pointing to identify the correct source in the field of view and the
latter two performed around periastron. Simultaneous INTEGRAL observations
across of the orbital cycle were analysed but the source was neither
detected by IBIS/ISGRI nor by JEM-X. The XMM-Newton light curves display a
range of moderately bright X-ray activity but there are no particularly strong
flares or outbursts in any of the three observations. We show that the spectral
shape measured by XMM-Newton can be fitted by a consistent model throughout the
observation, suggesting that the observed flux variations are driven by
obscuration from a wind of varying density rather than changes in accretion
mode. The simultaneous INTEGRAL data rule out simple extrapolation of the
simple powerlaw model beyond the XMM-Newton energy range.Comment: 13 pages, 9 figures, This article has been accepted for publication
in Monthly Notices of the Royal Astronomical Society Published by Oxford
University Pres
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