7,471 research outputs found

### Parton distributions in the photon from $\gamma^* \gamma$ and $\gamma^* p$ scattering

Leading order parton distributions in the photon are extracted from the
existing $F_2^\gamma$ measurements and the low-$x$ proton structure function.
The latter is related to the photon structure function by assuming Gribov
factorization to hold at low $x$. The resulting parton distributions in the
photon are found to be consistent with the Frankfurt-Gurvich sum rule for the
photon.Comment: 8 pages, including 4 figure

### High-frequency QPOs as a problem in physics: non-linear resonance

The presence of a kHz frequency in LMXBs has been expected from scaling laws,
by analogy with the QPO phenomenon in HMXB X-ray pulsars. Interpretation of the
two kHz frequencies, observed in accreting neutron stars, in terms of
non-linear resonance in strong-field gravity led to the prediction of twin QPOs
in black hole systems, in a definite frequency ratio (such as 2/3). The imprint
of a subharmonic of the 401 Hz rotation rate in the frequencies of the QPOs
detected in the accreting millisecond pulsar is at once a signature of
non-linear resonance and of coupling between accretion disk modes and the
neutron star spin.Comment: presented at X-ray Timing 2003: Rossi and Beyond, Boston, November
200

### Centrifugal Force and Ellipticity behaviour of a slowly rotating ultra compact object

Using the optical reference geometry approach, we have derived in the
following, a general expression for the ellipticity of a slowly rotating fluid
configuration using Newtonian force balance equation in the conformally
projected absolute 3-space, in the realm of general relativity. Further with
the help of Hartle-Thorne (H-T) metric for a slowly rotating compact object, we
have evaluated the centrifugal force acting on a fluid element and also
evaluated the ellipticity and found that the centrifugal reversal occurs at
around $R/R_s \approx 1.45$, and the ellipticity maximum at around $R/R_s
\approx 2.75$. The result has been compared with that of Chandrasekhar and
Miller which was obtained in the full 4-spacetime formalism

### Non-linear resonance in nearly geodesic motion in low-mass X-ray binaries

We have explored the ideas that parametric resonance affects nearly geodesic
motion around a black hole or a neutron star, and that it may be relevant to
the high frequency (twin) quasi-periodic oscillations occurring in some
low-mass X-ray binaries. We have assumed the particles or fluid elements of an
accretion disc to be subject to an isotropic perturbation of a hypothetical but
rather general form. We find that the parametric resonance is indeed excited
close to the radius where epicyclic frequencies of radial and meridional
oscillations are in a 2:3 ratio. The location and frequencies of the highest
amplitude excitation vary with the strength of the perturbation. These results
agree with actual frequency ratios of twin kHz QPOs that have been reported in
some black hole candidates, and they may be consistent also with correlation of
the twin peaks in Sco X-1.Comment: 5 pages; accepted for publication in PAS

### Management and Service-aware Networking Architectures (MANA) for Future Internet Position Paper: System Functions, Capabilities and Requirements

Future Internet (FI) research and development threads have recently been gaining momentum all over the world and as such the international race to create a new generation Internet is in full swing: GENI, Asia Future Internet, Future Internet Forum Korea, European Union Future Internet Assembly (FIA). This is a position paper identifying the research orientation with a time horizon of 10 years, together with the key challenges for the capabilities in the Management and Service-aware Networking Architectures (MANA) part of the Future Internet (FI) allowing for parallel and federated Internet(s)

### Color coherent phenomena on nuclei and the QCD evolution equation

We review the phenomenon of color coherence in quantum chromodynamics (QCD),
its implications for hard and soft processes with nuclei, and its experimental
manifestations. The relation of factorization theorems in QCD with color
coherence phenomena in deep inelastic scattering (DIS) and color coherence
phenomena in hard exclusive processes is emphasized. Analyzing numerically the
QCD evolution equation for conventional and skewed parton densities in nuclei,
we study the onset of generalized color transparency and nuclear shadowing of
the sea quark and gluon distributions in nuclei as well as related phenomena.
Such novel results as the dependence of the effective coherence length on $Q^2$
and general trends of the QCD evolution are discussed. The limits of the
applicability of the QCD evolution equation at small Bjorken $x$ are estimated
by comparing the inelastic quark-antiquark- and two gluon-nucleon (nucleus)
cross sections, calculated within the DGLAP approximation, with the dynamical
boundaries, which follow from the unitarity of the $S$ matrix for purely QCD
interactions. We also demonstrate that principles of color coherence play an
important role in the processes of soft diffraction off nuclei.Comment: 58 pages, 19 figures, Revtex. Minor editor's changes, final version
published in J.Phys. G27 (2001) R23-6

### A magnetized torus for modeling Sgr A* millimeter images and spectra

Context. The supermassive black hole, Sagittarius (Sgr) A*, in the centre of
our Galaxy has the largest angular size in the sky among all astrophysical
black holes. Its shadow, assuming no rotation, spans ~ 50 microarcsec.
Resolving such dimensions has long been out of reach for astronomical
instruments until a new generation of interferometers being operational during
this decade. Of particular interest is the Event Horizon Telescope (EHT) with
resolution ~ 20 microarcsec in the millimeter-wavelength range 0.87 mm - 1.3
mm. Aims. We investigate the ability of the fully general relativistic
Komissarov (2006) analytical magnetized torus model to account for observable
constraints at Sgr A* in the centimeter and millimeter domains. The impact of
the magnetic field geometry on the observables is also studied. Methods. We
calculate ray-traced centimeter- and millimeter-wavelength synchrotron spectra
and images of a magnetized accretion torus surrounding the central black hole
in Sgr A*. We assume stationarity, axial symmetry, constant specific angular
momentum and polytropic equation of state. A hybrid population of thermal and
non-thermal electrons is considered. Results. We show that the torus model is
capable of reproducing spectral constraints in the millimeter domain, and in
particular in the observable domain of the EHT. However, the torus model is not
yet able to fit the centimeter spectrum. 1.3 mm images at high inclinations are
in agreement with observable constraints. Conclusions. The ability of the torus
model to account for observations of Sgr A* in the millimeter domain is
interesting in the perspective of the future EHT. Such an analytical model
allows very fast computations. It will thus be a suitable test bed for
investigating large domains of physical parameters, as well as non-black-hole
compact object candidates and alternative theories of gravity.Comment: Major changes wrt the June 2014 version. Accepted by A&

### Optical geometry for gravitational collapse and Hawking radiation

The notion of optical geometry, introduced more than twenty years ago as a
formal tool in quantum field theory on a static background, has recently found
several applications to the study of physical processes around compact objects.
In this paper we define optical geometry for spherically symmetric
gravitational collapse, with the purpose of extending the current formalism to
physically interesting spacetimes which are not conformally static. The
treatment is fully general but, as an example, we also discuss the special case
of the Oppenheimer-Snyder model. The analysis of the late time behaviour shows
a close correspondence between the structure of optical spacetime for
gravitational collapse and that of flat spacetime with an accelerating
boundary. Thus, optical geometry provides a natural physical interpretation for
derivations of the Hawking effect based on the ``moving mirror analogy.''
Finally, we briefly discuss the issue of back-reaction in black hole
evaporation and the information paradox from the perspective of optical
geometry.Comment: 13 pages, 10 figures, aps, revtex, To be published in PR

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