12,438 research outputs found
Lx-SFR relation in star forming galaxies
We compare the results of Grimm et al. (2003) and Ranalli et al. (2003) on
the Lx-SFR relation in normal galaxies. Based on the Lx-stellar mass dependence
for LMXBs, we show, that low SFR (SFR<1 Msun/year) galaxies in the Ranalli et
al. sample are contaminated by the X-ray emission from low mass X-ray binaries,
unrelated to the current star formation activity.
The most important conclusion from our comparison is, however, that after the
data are corrected for the ``LMXB contamination'', the two datasets become
consistent with each other, despite of their different content, variability
effects, difference in the adopted source distances, X-ray flux and star
formation rate determination and in the cosmological parameters used in
interpreting the HDF-N data. They also agree well, both in the low and high SFR
regimes, with the predicted Lx-SFR dependence derived from the parameters of
the ``universal'' HMXB luminosity function. This encouraging result emphasizes
the potential of the X-ray luminosity as an independent star formation rate
indicator for normal galaxies.Comment: revised, accepted for publication in MNRAS Letter
Statistical properties of the combined emission of a population of discrete sources: astrophysical implications
We study the statistical properties of the combined emission of a population
of discrete sources (e.g. X-ray emission of a galaxy due to its X-ray binaries
population). Namely, we consider the dependence of their total luminosity
L_tot=SUM(L_k) and of fractional rms_tot of their variability on the number of
sources N or, equivalently, on the normalization of the luminosity function. We
show that due to small number statistics a regime exists, in which L_tot grows
non-linearly with N, in an apparent contradiction with the seemingly obvious
prediction =integral(dN/dL*L*dL) ~ N. In this non-linear regime, the
rms_tot decreases with N significantly more slowly than expected from the rms ~
1/sqrt(N) averaging law. For example, for a power law luminosity function with
a slope of a=3/2, in the non-linear regime, L_tot ~ N^2 and the rms_tot does
not depend at all on the number of sources N. Only in the limit of N>>1 do
these quantities behave as intuitively expected, L_tot ~ N and rms_tot ~
1/sqrt(N). We give exact solutions and derive convenient analytical
approximations for L_tot and rms_tot.
Using the total X-ray luminosity of a galaxy due to its X-ray binary
population as an example, we show that the Lx-SFR and Lx-M* relations predicted
from the respective ``universal'' luminosity functions of high and low mass
X-ray binaries are in a good agreement with observations. Although caused by
small number statistics the non-linear regime in these examples extends as far
as SFR<4-5 Msun/yr and log(M*/Msun)<10.0-10.5, respectively.Comment: MNRAS, accepted for publicatio
Spectrum of a duality-twisted Ising quantum chain
The Ising quantum chain with a peculiar twisted boundary condition is
considered. This boundary condition, first introduced in the framework of the
spin-1/2 XXZ Heisenberg quantum chain, is related to the duality
transformation, which becomes a symmetry of the model at the critical point.
Thus, at the critical point, the Ising quantum chain with the duality-twisted
boundary is translationally invariant, similar as in the case of the usual
periodic or antiperiodic boundary conditions. The complete energy spectrum of
the Ising quantum chain is calculated analytically for finite systems, and the
conformal properties of the scaling limit are investigated. This provides an
explicit example of a conformal twisted boundary condition and a corresponding
generalised twisted partition function.Comment: LaTeX, 7 pages, using IOP style
Cold inelastic collisions between lithium and cesium in a two-species magneto-optical trap
We investigate collisional properties of lithium and cesium which are
simultaneously confined in a combined magneto-optical trap. Trap-loss
collisions between the two species are comprehensively studied. Different
inelastic collision channels are identified, and inter-species rate
coefficients as well as cross sections are determined. It is found that loss
rates are independent of the optical excitation of Li, as a consequence of the
repulsive Li-Cs interaction. Li and Cs loss by inelastic inter-species
collisions can completely be attributed to processes involving optically
excited cesium (fine-structure changing collisions and radiative escape). By
lowering the trap depth for Li, an additional loss channel of Li is observed
which results from ground-state Li-Cs collisions changing the hyperfine state
of cesium.Comment: submitted to Euro. Phys. J. D, special issue on Laser Cooling and
Trappin
Historical Precedents and Early Modern Interpretations: English Histories of America, 1500-1700
In the sixteenth and seventeenth centuries, English scholars and travelers created the first English studies of the environment and inhabitants of the New World. This thesis suggests that historical interpretation formed the intellectual foundations for English colonization in America. Writers applied their knowledge of the past to questions of why to colonize America, how to know the New World, and how to understand the Native Americans. These writers struggled to construct a biblical or classical genealogy to explain how the Native Americans had arrived in the New World and the identity of their ancestors. Debates over various historical narratives for America led to the belief that Native Americans did not possess a knowable or meaningful past. This viewpoint was a consequence of English scholarship, not an accurate assessment based on empirical observation. My thesis concludes that English scholars and travelers strategically invoked history to further their colonial goals and to shape perceptions of foreign peoples as incapable of historical progress
Higgs Limit and b->s gamma Constraints in Minimal Supersymmetry
New limits on the Higgs mass from LEP and new calculations on the radiative
(penguin) decay of the b->s gamma branching ratio restrict the parameter space
of the Constrained Minimal Supersymmetric Standard Model (CMSSM).
We find that for the low tan(beta) scenario only one sign of the Higgs mixing
parameter is allowed, while the high tan(beta scenario is practically excluded,
if one requires all sparticles to be below 1 TeV and imposes radiative
electroweak symmetry breaking as well as gauge and Yukawa coupling unification.
For squarks between 1 and 2 TeV high tan(beta) scenarios are allowed. We
consider especially a new high tan(beta)=64 scenario with triple unification of
all Yukawa couplings of the third generation, which show an infrared fixed
point behaviour.
The upper limit on the mass of the lightest Higgs in the low (high) tan(beta)
scenarios is 97+-6~(120+-2) GeV, where the errors originate predominantly from
the uncertainty in the top mass.Comment: latex + 6 eps figs, 10 pages, IEKP-KA/98-08; References updated in
replacement + 1 figure concerning triple Yukawa unification added for final
publication in Phys. Let
Efficient creation of molecules from a cesium Bose-Einstein condensate
We report a new scheme to create weakly bound Cs molecules from an atomic
Bose-Einstein condensate. The method is based on switching the magnetic field
to a narrow Feshbach resonance and yields a high atom-molecule conversion
efficiency of more than 30%, a factor of three higher than obtained with
conventional magnetic-field ramps. The Cs molecules are created in a single
-wave rotational quantum state. The observed dependence of the conversion
efficiency on the magnetic field and atom density shows scattering processes
beyond two-body coupling to occur in the vicinity of the Feshbach resonance.Comment: 7 pages, 4 figures, submitted to Europhysics Letter
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