3,858 research outputs found
Relevance of the observation of UHE gammas to hard X-ray astronomy
A number of consequences of the presence of sources of ultra high energy (UHE) gamma rays, exemplified by Cygnus X-3, are examined. It is shown that there should be a flux of hard X-rays at all Galactic latitudes; a significant flux of extragalactic hard X-rays may also result. Relevance to theories of cosmic ray particle origin and propagation is discussed
Fluctuation relations for heat engines in time-periodic steady states
A fluctuation relation for heat engines (FRHE) has been derived recently. In
the beginning, the system is in contact with the cooler bath. The system is
then coupled to the hotter bath and external parameters are changed cyclically,
eventually bringing the system back to its initial state, once the coupling
with the hot bath is switched off. In this work, we lift the condition of
initial thermal equilibrium and derive a new fluctuation relation for the
central system (heat engine) being in a time-periodic steady state (TPSS).
Carnot's inequality for classical thermodynamics follows as a direct
consequence of this fluctuation theorem even in TPSS. For the special cases of
the absence of hot bath and no extraction of work, we obtain the integral
fluctuation theorem for total entropy and the generalized exchange fluctuation
theorem, respectively. Recently microsized heat engines have been realized
experimentally in the TPSS. We numerically simulate the same model and verify
our proposed theorems.Comment: 9 page
Cosmic Needles versus Cosmic Microwave Background Radiation
It has been suggested by a number of authors that the 2.7K cosmic microwave
background (CMB) radiation might have arisen from the radiation from Population
III objects thermalized by conducting cosmic graphite/iron needle-shaped dust.
Due to lack of an accurate solution to the absorption properties of exceedingly
elongated grains, in existing literature which studies the CMB thermalizing
process they are generally modelled as (1) needle-like spheroids in terms of
the Rayleigh approximation; (2) infinite cylinders; and (3) the antenna theory.
We show here that the Rayleigh approximation is not valid since the Rayleigh
criterion is not satisfied for highly conducting needles. We also show that the
available intergalactic iron dust, if modelled as infinite cylinders, is not
sufficient to supply the required opacity at long wavelengths to obtain the
observed isotropy and Planckian nature of the CMB. If appealing to the antenna
theory, conducting iron needles with exceedingly large elongations (10^4)
appear able to provide sufficient opacity to thermalize the CMB within the iron
density limit. But the applicability of the antenna theory to exceedingly thin
needles of nanometer/micrometer in thickness needs to be justified.Comment: 13 pages, 4 figures; submitted to ApJ
The Heavy Quark Form Factors at Two Loops
We compute the two-loop QCD corrections to the heavy quark form factors in
case of the vector, axial-vector, scalar and pseudo-scalar currents up to
second order in the dimensional parameter . These terms are
required in the renormalization of the higher order corrections to these form
factors.Comment: 131 pages, 3 figure
Heavy quark form factors at two loops in perturbative QCD
We present the results for heavy quark form factors at two-loop order in
perturbative QCD for different currents, namely vector, axial-vector, scalar
and pseudo-scalar currents, up to second order in the dimensional
regularization parameter. We outline the necessary computational details,
ultraviolet renormalization and corresponding universal infrared structure.Comment: 13 pages Latex, Proceedings of XLI International Conference of
Theoretical Physics "Matter to the Deepest", Podlesice, Poland, September
3-8, 2017 and RADCOR 2017, St.~Gilgen Austria, Sept 24-29, 201
Cosmological Constraints on Theories with Large Extra Dimensions
In theories with large extra dimensions, constraints from cosmology lead to
non-trivial lower bounds on the fundamental scale M_F, corresponding to upper
bounds on the radii of the compact extra dimensions. These constraints are
especially relevant to the case of two extra dimensions, since only if M_F is
10 TeV or less do deviations from the standard gravitational force law become
evident at distances accessible to planned sub-mm gravity experiments. By
examining the graviton decay contribution to the cosmic diffuse gamma
radiation, we derive, for the case of two extra dimensions, a conservative
bound M_F > 110 TeV, corresponding to r_2 < 5.1 times 10^-5 mm, well beyond the
reach of these experiments. We also consider the constraint coming from
graviton overclosure of the universe and derive an independent bound M_F > 6.5
h^(-1/2) TeV, or r_2 < .015 h mm.Comment: 10 pages, references adde
Heavy Quark Form Factors at Three Loops in the Planar Limit
We compute the color-planar and complete light quark non-singlet
contributions to the heavy quark form factors in the case of the axialvector,
scalar and pseudoscalar currents at three loops in perturbative QCD. We
evaluate the master integrals applying a new method based on differential
equations for general bases, which is applicable for all first order
factorizing systems. The analytic results are expressed in terms of harmonic
polylogarithms and real-valued cyclotomic harmonic polylogarithms.Comment: 9 pages, 4 figure
On the Similarity between Cluster and Galactic Stellar Initial Mass Functions
The stellar initial mass functions (IMFs) for the Galactic bulge, the Milky
Way, other galaxies, clusters of galaxies, and the integrated stars in the
Universe are composites from countless individual IMFs in star clusters and
associations where stars form. These galaxy-scale IMFs, reviewed in detail
here, are not steeper than the cluster IMFs except in rare cases. This is true
even though low mass clusters generally outnumber high mass clusters and the
average maximum stellar mass in a cluster scales with the cluster mass. The
implication is that the mass distribution function for clusters and
associations is a power law with a slope of -2 or shallower. Steeper slopes,
even by a few tenths, upset the observed equality between large and small scale
IMFs. Such a cluster function is expected from the hierarchical nature of star
formation, which also provides independent evidence for the IMF equality when
it is applied on sub-cluster scales. We explain these results with analytical
expressions and Monte Carlo simulations. Star clusters appear to be the relaxed
inner parts of a widespread hierarchy of star formation and cloud structure.
They are defined by their own dynamics rather than pre-existing cloud
boundaries.Comment: 22 pages, 2 figures, ApJ, 648, in press, September 1, 200
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