5,332 research outputs found
Gravitational Collapse in One Dimension
We simulate the evolution of one-dimensional gravitating collisionless
systems from non- equilibrium initial conditions, similar to the conditions
that lead to the formation of dark- matter halos in three dimensions. As in the
case of 3D halo formation we find that initially cold, nearly homogeneous
particle distributions collapse to approach a final equilibrium state with a
universal density profile. At small radii, this attractor exhibits a power-law
behavior in density, {\rho}(x) \propto |x|^(-{\gamma}_crit), {\gamma}_crit
\simeq 0.47, slightly but significantly shallower than the value {\gamma} = 1/2
suggested previously. This state develops from the initial conditions through a
process of phase mixing and violent relaxation. This process preserves the
energy ranks of particles. By warming the initial conditions, we illustrate a
cross-over from this power-law final state to a final state containing a
homogeneous core. We further show that inhomogeneous but cold power-law initial
conditions, with initial exponent {\gamma}_i > {\gamma}_crit, do not evolve
toward the attractor but reach a final state that retains their original
power-law behavior in the interior of the profile, indicating a bifurcation in
the final state as a function of the initial exponent. Our results rely on a
high-fidelity event-driven simulation technique.Comment: 14 Pages, 13 Figures. Submitted to MNRA
A general model to optimise CuII labelling efficiency of double-histidine motifs for pulse dipolar EPR applications
JLW is supported by the BBSRC DTP Eastbio. We thank the Leverhulme Trust for support (RPG-2018-397). This work was supported by equipment funding through the Wellcome Trust (099149/Z/12/Z) and BBSRC (BB/R013780/1). We gratefully acknowledge ISSF support to the University of St Andrews from the Wellcome Trust.Electron paramagnetic resonance (EPR) distance measurements are making increasingly important contributions to studies of biomolecules underpinning health and disease by providing highly accurate and precise geometric constraints. Combining double-histidine (dH) motifs with CuII spin labels shows promise for further increasing the precision of distance measurements, and for investigating subtle conformational changes. However, non-covalent coordination-based spin labelling is vulnerable to low binding affinity. Dissociation constants of dH motifs for CuII-nitrilotriacetic acid were previously investigated via relaxation induced dipolar modulation enhancement (RIDME), and demonstrated the feasibility of exploiting the double histidine motif for EPR applications at sub-ÎĽM protein concentrations. Herein, the feasibility of using modulation depth quantitation in CuII-CuII RIDME to simultaneously estimate a pair of non-identical independent KD values in such a tetra-histidine model protein is addressed. Furthermore, we develop a general speciation model to optimise CuII labelling efficiency, in dependence of pairs of identical or disparate KD values and total CuII label concentration. We find the dissociation constant estimates are in excellent agreement with previously determined values, and empirical modulation depths support the proposed model.Publisher PDFPeer reviewe
From AMANDA to IceCube
The first string of the neoteric high energy neutrino telescope IceCube
successfully began operating in January 2005. It is anticipated that upon
completion the new detector will vastly increase the sensitivity and extend the
reach of AMANDA to higher energies. A discussion of the IceCube's discovery
potential for extra-terrestrial neutrinos, together with the prospects of new
physics derived from the ongoing AMANDA research will be the focus of this
paper. Preliminary results of the first antarctic high energy neutrino
telescope AMANDA searching in the muon neutrino channel for localized and
diffuse excess of extra-terrestrial neutrinos will be reviewed using data
collected between 2000 and 2003. Neutrino flux limits obtained with the
all-flavor dedicated UHE and cascade analyses will be described. A first
neutrino spectrum above one TeV in agreement with atmospheric neutrino flux
expectations and no extra-terrestrial contribution will be presented, followed
by a discussion of a limit for neutralino CDM candidates annihilating in the
center of the Sun.Comment: 15 pages, 8 figures Invited talk contribution at 5th International
Conference on Non-accelerator New Physics (NANP 05), Dubna, Russia, 20-25 Jun
200
The WISE gamma-ray strip parametrization: the nature of the gamma-ray Active Galactic Nuclei of Uncertain type
Despite the large number of discoveries made recently by Fermi, the origins
of the so called unidentified gamma-ray sources remain unknown. The large
number of these sources suggests that among them there could be a population
that significantly contributes to the isotropic gamma-ray background and is
therefore crucial to understand their nature. The first step toward a complete
comprehension of the unidentified gamma-ray source population is to identify
those that can be associated with blazars, the most numerous class of
extragalactic sources in the gamma-ray sky. Recently, we discovered that
blazars can be recognized and separated from other extragalactic sources using
the infrared (IR) WISE satellite colors. The blazar population delineates a
remarkable and distinctive region of the IR color-color space, the WISE blazar
strip. In particular, the subregion delineated by the gamma-ray emitting
blazars is even narrower and we named it as the WISE Gamma-ray Strip (WGS). In
this paper we parametrize the WGS on the basis of a single parameter s that we
then use to determine if gamma-ray Active Galactic Nuclei of the uncertain type
(AGUs) detected by Fermi are consistent with the WGS and so can be considered
blazar candidates. We find that 54 AGUs out of a set 60 analyzed have IR colors
consistent with the WGS; only 6 AGUs are outliers. This result implies that a
very high percentage (i.e., in this sample about 90%) of the AGUs detected by
Fermi are indeed blazar candidates.Comment: 22 pages, 13 figures, Astrophysical Journal in pres
Matching Dynamics with Constraints
We study uncoordinated matching markets with additional local constraints
that capture, e.g., restricted information, visibility, or externalities in
markets. Each agent is a node in a fixed matching network and strives to be
matched to another agent. Each agent has a complete preference list over all
other agents it can be matched with. However, depending on the constraints and
the current state of the game, not all possible partners are available for
matching at all times. For correlated preferences, we propose and study a
general class of hedonic coalition formation games that we call coalition
formation games with constraints. This class includes and extends many recently
studied variants of stable matching, such as locally stable matching, socially
stable matching, or friendship matching. Perhaps surprisingly, we show that all
these variants are encompassed in a class of "consistent" instances that always
allow a polynomial improvement sequence to a stable state. In addition, we show
that for consistent instances there always exists a polynomial sequence to
every reachable state. Our characterization is tight in the sense that we
provide exponential lower bounds when each of the requirements for consistency
is violated. We also analyze matching with uncorrelated preferences, where we
obtain a larger variety of results. While socially stable matching always
allows a polynomial sequence to a stable state, for other classes different
additional assumptions are sufficient to guarantee the same results. For the
problem of reaching a given stable state, we show NP-hardness in almost all
considered classes of matching games.Comment: Conference Version in WINE 201
Infrared Colors of the gamma-ray detected blazars
Blazars constitute the most enigmatic class of extragalactic gamma-ray
sources, and their observational features have been ascribed to a relativistic
jet closely aligned to the line of sight. They are generally divided in two
main classes: the BL Lac objects (BL Lacs) and the Flat Spectrum Radio Quasars
(FSRQs). In the case of BL Lacs the double bumped spectral energy distribution
(SED) is generally described by the Synchrotron Self Compton (SSC) emission,
while for the FSRQs it is interpreted as due to External Compton (EC) emission.
Recently, we showed that in the [3.4]-[4.6]-[12] micron color- color diagram
the blazar population covers a distinct region (i.e., the WISE blazar Strip,
WBS), clearly separated from the other extragalactic sources that are dominated
by thermal emission. In this paper we investigate the relation between the
infrared and gamma-ray emission for a subset of confirmed blazars from the
literature, associated with Fermi sources, for which WISE archival observations
are available. This sample is a proper subset of the sample of sources used
previously, and the availability of Fermi data is critical to constrain the
models on the emission mechanisms for the blazars. We found that the selected
blazars also lie on the WISE blazar Strip covering a narrower region of the
infrared color-color planes than the overall blazars population. We then found
an evident correlation between the IR and gamma-ray spectral indices expected
in the SSC and EC frameworks. Finally, we determined the ratio between their
gamma-ray and infrared fluxes, a surrogate of the ratio of powers between the
inverse Compton and the synchrotron SED components, and used such parameter to
test different emitting scenarios blazars.Comment: 15 pages, 14 figure, accepted for publication in ApJ, to appear in
2012 March 20 editio
High-Energy Neutrino Astronomy
Kilometer-scale neutrino detectors such as IceCube are discovery instruments
covering nuclear and particle physics, cosmology and astronomy. Examples of
their multidisciplinary missions include the search for the particle nature of
dark matter and for additional small dimensions of space. In the end, their
conceptual design is very much anchored to the observational fact that Nature
accelerates protons and photons to energies in excess of and
eV, respectively. The cosmic ray connection sets the scale of cosmic
neutrino fluxes. In this context, we discuss the first results of the completed
AMANDA detector and the reach of its extension, IceCube. Similar experiments
are under construction in the Mediterranean. Neutrino astronomy is also
expanding in new directions with efforts to detect air showers, acoustic and
radio signals initiated by super-EeV neutrinos.Comment: 9 pages, Latex2e, uses ws-procs975x65standard.sty (included), 4
postscript figures. To appear in Proceedings of Thinking, Observing, and
Mining the Universe, Sorrento, Italy, September 200
Majorana: from atomic and molecular, to nuclear physics
In the centennial of Ettore Majorana's birth (1906-1938?), we re-examine some
aspects of his fundamental scientific production in atomic and molecular
physics, including a not well known short communication. There, Majorana
critically discusses Fermi's solution of the celebrated Thomas-Fermi equation
for electron screening in atoms and positive ions. We argue that some of
Majorana's seminal contributions in molecular physics already prelude to the
idea of exchange interactions (or Heisenberg-Majorana forces) in his later
workson theoretical nuclear physics. In all his papers, he tended to emphasize
the symmetries at the basis of a physical problem, as well as the limitations,
rather than the advantages, of the approximations of the method employed.Comment: to appear in Found. Phy
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Rhythm in the speech of a person with right hemisphere damage: Applying the pairwise variability index
Although several aspects of prosody have been studied in speakers with right hemisphere damage (RHD), rhythm remains largely uninvestigated. This study compares the rhythm of an Australian English speaker with right hemisphere damage (due to a stroke, but with no concomitant dysarthria) to that of a neurologically unimpaired individual. The speakers' rhythm is compared using the pairwise variability index (PVI) which allows for an acoustic characterization of rhythm by comparing the duration of successive vocalic and intervocalic intervals. A sample of speech from a structured interview between a speech and language therapist and each participant was analysed. Previous research has shown that speakers with RHD may have difficulties with intonation production, and therefore it was hypothesized that there may also be rhythmic disturbance. Results show that the neurologically normal control uses a similar rhythm to that reported for British English (there are no previous studies available for Australian English), whilst the speaker with RHD produces speech with a less strongly stress-timed rhythm. This finding was statistically significant for the intervocalic intervals measured (t(8) = 4.7, p < .01), and suggests that some aspects of prosody may be right lateralized for this speaker. The findings are discussed in relation to previous findings of dysprosody in RHD populations, and in relation to syllable-timed speech of people with other neurological conditions
Robust implications on Dark Matter from the first FERMI sky gamma map
We derive robust model-independent bounds on DM annihilations and decays from
the first year of FERMI gamma-ray observations of the whole sky. These bounds
only have a mild dependence on the DM density profile and allow the following
DM interpretations of the PAMELA and FERMI electron/positron excesses: primary
channels mu+ mu-, mu+ mu-mu+mu- or e+ e- e+ e-. An isothermal-like density
profile is needed for annihilating DM. In all such cases, FERMI gamma spectra
must contain a significant DM component, that may be probed in the future.Comment: 16 pages, 8 figures. Final versio
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