7,830 research outputs found
The gamma-ray emitting microquasar LSI+61303
LS I +61 303 is one of the most studied X-ray binary systems because of its
two peculiarities: On the one hand being the probable counterpart of the
variable gamma ray source 2CG 135+01 (Gregory and Taylor 1978; Tavani et al.
1998) and on the other hand being a periodic radio source (Taylor and Gregory
1982). The recent discovery of a radio emitting jet extending ca. 200 AU at
both sides of a central core (Massi et al. 2004) in all evidence has shown the
occurrence of accretion/ejection processes in this system. However, the radio
outbursts do not occur at periastron passage, where the accretion is at its
maximum, but several days later. In addition, when the gamma-ray emission of
2CG 135+01 is examined along the orbital phase of LS I +61 303 one sees that
this emission seems to peak at periastron passage (Massi 2004). Here in detail
we analyse the trend of the gamma-ray data versus orbital phase and discuss the
delay between peaks at gamma-rays and in the radio band within the framework of
a two-peak accretion/ejection model proposed by Taylor et al. (1992) and
further developed by Marti' and Paredes (1995).Comment: To be published in the proceedings of the Symposium on High-Energy
Gamma-Ray Astronomy, Heidelberg, July 26-30, 2004 (AIP Proceedings Series
General-Relativistic MHD for the Numerical Construction of Dynamical Spacetimes
We assemble the equations of general relativistic magnetohydrodynamics (MHD)
in 3+1 form. These consist of the complete coupled set of Maxwell equations for
the electromagnetic field, Einstein's equations for the gravitational field,
and the equations of relativistic MHD for a perfectly conducting ideal gas. The
adopted form of the equations is suitable for evolving numerically a
relativistic MHD fluid in a dynamical spacetime characterized by a strong
gravitational field.Comment: 8 pages; scheduled for March 10 issue of Ap
A Medium Resolution Near-Infrared Spectral Atlas of O and Early B Stars
We present intermediate resolution (R ~ 8,000 - 12,000) high signal-to-noise
H- and K-band spectroscopy of a sample of 37 optically visible stars, ranging
in spectral type from O3 to B3 and representing most luminosity classes.
Spectra of this quality can be used to constrain the temperature, luminosity
and general wind properties of OB stars, when used in conjunction with
sophisticated atmospheric model codes. Most important is the need for
moderately high resolutions (R > 5000) and very high signal-to-noise (S/N >
150) spectra for a meaningful profile analysis. When using near-infrared
spectra for a classification system, moderately high signal-to-noise (S/N ~
100) is still required, though the resolution can be relaxed to just a thousand
or two. In the appendix we provide a set of very high quality near-infrared
spectra of Brackett lines in six early-A dwarfs. These can be used to aid in
the modeling and removal of such lines when early-A dwarfs are used for
telluric spectroscopic standards.Comment: 12 pages, 3 tables, 14 figures. AASTex preprint style. To appear in
ApJS, November 2005. All spectra are available by contacting M.M. Hanso
Selective Area Grown Semiconductor-Superconductor Hybrids: A Basis for Topological Networks
We introduce selective area grown hybrid InAs/Al nanowires based on molecular
beam epitaxy, allowing arbitrary semiconductor-superconductor networks
containing loops and branches. Transport reveals a hard induced gap and
unpoisoned 2e-periodic Coulomb blockade, with temperature dependent 1e features
in agreement with theory. Coulomb peak spacing in parallel magnetic field
displays overshoot, indicating an oscillating discrete near-zero subgap state
consistent with device length. Finally, we investigate a loop network, finding
strong spin-orbit coupling and a coherence length of several microns. These
results demonstrate the potential of this platform for scalable topological
networks among other applications.Comment: NBI QDEV 201
5 year Global 3-mm VLBI survey of Gamma-ray active blazars
The Global mm-VLBI Array (GMVA) is a network of 14 3\,mm and 7\,mm capable
telescopes spanning Europe and the United States, with planned extensions to
Asia. The array is capable of sensitive maps with angular resolution often
exceeding 50\,as. Using the GMVA, a large sample of prominent -ray
blazars have been observed approximately 6 monthly from later 2008 until now.
Combining 3\,mm maps from the GMVA with near-in-time 7\,mm maps from the
VLBA-BU-BLAZAR program and 2\,cm maps from the MOJAVE program, we determine the
sub-pc morphology and high frequency spectral structure of -ray
blazars. The magnetic field strength can be estimated at different locations
along the jet under the assumption of equipartition between magnetic field and
relativistic particle energies. Making assumptions on the jet magnetic field
configuration (e.g. poloidal or toroidal), we can estimate the separation of
the mm-wave "core" and the jet base, and estimate the strength of the magnetic
field there. The results of this analysis show that on average, the magnetic
field strength decreases with a power-law , .
This suggests that on average, the mm-wave "core" is \,pc downstream
of the de-projected jet apex and that the magnetic field strength is of the
order \,kG, broadly consistent with the predictions of
magnetic jet launching (e.g. via magnetically arrested disks (MAD)).Comment: 6 pages, 1 figur
Complete-Graph Tensor Network States: A New Fermionic Wave Function Ansatz for Molecules
We present a new class of tensor network states that are specifically
designed to capture the electron correlation of a molecule of arbitrary
structure. In this ansatz, the electronic wave function is represented by a
Complete-Graph Tensor Network (CGTN) ansatz which implements an efficient
reduction of the number of variational parameters by breaking down the
complexity of the high-dimensional coefficient tensor of a
full-configuration-interaction (FCI) wave function. We demonstrate that CGTN
states approximate ground states of molecules accurately by comparison of the
CGTN and FCI expansion coefficients. The CGTN parametrization is not biased
towards any reference configuration in contrast to many standard quantum
chemical methods. This feature allows one to obtain accurate relative energies
between CGTN states which is central to molecular physics and chemistry. We
discuss the implications for quantum chemistry and focus on the spin-state
problem. Our CGTN approach is applied to the energy splitting of states of
different spin for methylene and the strongly correlated ozone molecule at a
transition state structure. The parameters of the tensor network ansatz are
variationally optimized by means of a parallel-tempering Monte Carlo algorithm
Simultaneous Observations of GRS 1758-258 in 1997 by VLA, IRAM, SEST, RXTE and OSSE: Spectroscopy and Timing
We report the results of our multi-wavelength observations of GRS 1758-258
made in August 1997. The energy bands include radio, millimeter, X-ray, and
gamma-ray. The observations enable us to obtain a complete spectrum of the
source over an energy range of 2 - 500 keV. The spectrum shows that GRS
1758-258 was in its hard state. It is well fitted by the Sunyaev-Titarchuk (ST)
Compton scattering model. The spectrum is also fit by a power law with an
exponential cutoff (PLE) plus a soft black-body component. The temperature of
the soft component is about 1.2 keV, and the energy flux is less than 1.5% of
the total X- and gamma-ray flux. The deduced hydrogen column density is in the
range of (0.93 - 2.0) 10^{22} cm^{-2}. No significant iron lines are detected.
The radio emission has a flat energy spectrum. The daily radio, X-ray and
gamma-ray light curves show that GRS 1758-258 was stable during the observation
period, but was highly variable on smaller time scales in X- and gamma-rays.
The power density spectra are typical for the low-state, but we find the photon
flux for the 5 to 10 keV band to be more variable than that in the other two
energy bands (2 - 5 keV and 10 - 40 keV). Harmonically spaced quasi-periodic
oscillations (QPOs) are observed in the power spectra. The phase lags between
the hard photons and the soft photons have a flat distribution over a wide
range of frequencies. A high coherence of about 1.0 (0.01 - 1 Hz) between the
hard photons and the soft photons is also obtained in our observations. We
compare these results with two variation models. Our millimeter observations
did not reveal any conclusive signatures of an interaction between the jet from
GRS 1758-258 and the molecular cloud that lies in the direction of GRS
1758-258.Comment: 32 pages, 13 figures, to appear in ApJ, 2000, V.533, no. 1, Apr. 10.
For better figure resolution, please directly download the paper from
http://spacsun.rice.edu/~lin/publication.htm
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