15,004 research outputs found
Role of the particle's stepping cycle in an asymmetric exclusion process: A model of mRNA translation
Messenger RNA translation is often studied by means of statistical-mechanical
models based on the Asymmetric Simple Exclusion Process (ASEP), which considers
hopping particles (the ribosomes) on a lattice (the polynucleotide chain). In
this work we extend this class of models and consider the two fundamental steps
of the ribosome's biochemical cycle following a coarse-grained perspective. In
order to achieve a better understanding of the underlying biological processes
and compare the theoretical predictions with experimental results, we provide a
description lying between the minimal ASEP-like models and the more detailed
models, which are analytically hard to treat. We use a mean-field approach to
study the dynamics of particles associated with an internal stepping cycle. In
this framework it is possible to characterize analytically different phases of
the system (high density, low density or maximal current phase). Crucially, we
show that the transitions between these different phases occur at different
parameter values than the equivalent transitions in a standard ASEP, indicating
the importance of including the two fundamental steps of the ribosome's
biochemical cycle into the model.Comment: 9 pages, 9 figure
Control of spin relaxation in semiconductor double quantum dots
We propose a scheme to manipulate the spin relaxation in vertically coupled
semiconductor double quantum dots. Up to {\em twelve} orders of magnitude
variation of the spin relaxation time can be achieved by a small gate voltage
applied vertically on the double dot. Different effects such as the dot size,
barrier height, inter-dot distance, and magnetic field on the spin relaxation
are investigated in detail. The condition to achieve a large variation is
discussed.Comment: 5 pages, 4 figures, to be published in PR
The accretion environment of Supergiant Fast X-ray Transients probed with XMM-Newton
Supergiant fast X-ray transients (SFXTs) are characterized by a remarkable
variability in the X-ray domain, widely ascribed to the accretion from a clumpy
stellar wind. In this paper we performed a systematic and homogeneous analysis
of sufficiently bright X-ray flares from the SFXTs observed with XMM-Newton to
probe spectral variations on timescales as short as a few hundred of seconds.
Our ultimate goal is to investigate if SFXT flares and outbursts are triggered
by the presence of clumps and eventually reveal whether strongly or mildly
dense clumps are required. For all sources, we employ a technique developed by
our group, making use of an adaptive rebinned hardness ratio to optimally
select the time intervals for the spectral extraction. A total of twelve
observations performed in the direction of five SFXTs are reported. We show
that both strongly and mildly dense clumps can trigger these events. In the
former case, the local absorption column density may increase by a factor of
>>3, while in the latter case, the increase is only by a factor of 2-3 (or
lower). Overall, there seems to be no obvious correlation between the dynamic
ranges in the X-ray fluxes and absorption column densities in SFXTs, with an
indication that lower densities are recorded at the highest fluxes. This can be
explained by the presence of accretion inhibition mechanism(s). We propose a
classification of the flares/outbursts from these sources to drive future
observational investigations. We suggest that the difference between the
classes of flares/outbursts is related to the fact that the mechanism(s)
inhibiting accretion can be overcome more easily in some sources compared to
others. We also investigate the possibility that different stellar wind
structures, rather than clumps, could provide the means to temporarily overcome
the inhibition of accretion in SFXTs.Comment: Accepted for publication on A&
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
Chemical abundances in the nucleus of the Sagittarius dwarf spheroidal galaxy
We present Iron, Magnesium, Calcium, and Titanium abundances for 235 stars in
the central region of the Sagittarius dwarf spheroidal galaxy (within 9.0
arcmin ~70 pc from the center) from medium-resolution Keck/DEIMOS spectra. All
the considered stars belong to the massive globular cluster M54 or to the
central nucleus of the galaxy (Sgr,N). In particular we provide abundances for
109 stars with [Fe/H] > -1.0, more than doubling the available sample of
spectroscopic metallicity and alpha-elements abundance estimates for Sgr dSph
stars in this metallicity regime. Also, we find the first confirmed member of
the Sagittarius dwarf spheroidal with [Fe/H]< -2.0 based on analysis of iron
lines. We find for the first time a metallicity gradient in the Sgr,N
population, whose peak iron abundance goes from [Fe/H]=-0.38 for R < 2.5 arcmin
to [Fe/H]=-0.57 for 5.0 < R < 9.0 arcmin. On the other hand the trends of
[Mg/Fe], [Ca/Fe], and [Ti/Fe] with [Fe/H] are the same over the entire region
explored by our study. We reproduce the observed chemical patterns of the
Sagittarius dwarf spheroidal as a whole with a chemical evolution model
implying a high mass progenitor ( M_(DM)=6 X 10^{10} Msun ) and a significant
event of mass-stripping occurred a few Gyr ago, presumably starting at the
first peri-Galactic passage after infall.Comment: Accepted for publication to A&A, 12 pages, 14 figures, 1 tabl
Structure and in vitro digestibility of grass pea (Lathyrus sativus L.) flour following transglutaminase treatment
The impact of transglutaminase (TG) modification on microstructure and in vitro protein and starch digestibility of grass pea flour was investigated. Results demonstrated that grass pea flour proteins act as effective substrate of TG. Microstructural results showed that the addition of TG produced a more compact structure likely due to TG-catalyzed heteropolymers. Nutritional properties such as slowly digestible starch and expected glycemic index values followed the order: grass pea flour incubated in the absence of TG>grass pea flour incubated in the presence of TG>raw flour. The TG-catalyzed heteropolymers were easily digested as demonstrated by in vitro oral and gastric digestion carried out under physiological conditions. Therefore, TG-modified grass pea flour can be considered as a new source of starch and proteins suitable for feeding a large spectrum of population
XMM-Newton and Swift observations of XTE J1743-363
XTEJ1743-363 is a poorly known hard X-ray transient, that displays short and
intense flares similar to those observed from Supergiant Fast X-ray Transients.
The probable optical counterpart shows spectral properties similar to those of
an M8 III giant, thus suggesting that XTEJ1743-363 belongs to the class of the
Symbiotic X-ray Binaries. In this paper we report on the first dedicated
monitoring campaign of the source in the soft X-ray range with XMM-Newton and
Swift/XRT. T hese observations confirmed the association of XTEJ1743-363 with
the previously suggested M8 III giant and the classification of the source as a
member of the Symbiotic X-ray binaries. In the soft X-ray domain, XTEJ1743-363
displays a high absorption (~6x10^22 cm^-2 ) and variability on time scales of
hundreds to few thousand seconds, typical of wind accreting systems. A
relatively faint flare (peak X-ray flux 3x10^-11 erg/cm^2/s) lasting ~4 ks is
recorded during the XMM-Newton observation and interpreted in terms of the wind
accretion scenario.Comment: Accepted for publication on A&
Approximate analytical description of the nonaffine response of amorphous solids
An approximation scheme for model disordered solids is proposed that leads to
the fully analytical evaluation of the elastic constants under explicit account
of the inhomogeneity (nonaffinity) of the atomic displacements. The theory is
in quantitative agreement with simulations for central-force systems and
predicts the vanishing of the shear modulus at the isostatic point with the
linear law {\mu} ~ (z - 2d), where z is the coordination number. The vanishing
of rigidity at the isostatic point is shown to be a consequence of the
canceling out of positive affine and negative nonaffine terms
First-order transitions for some generalized XY models
In this note we demonstrate the occurrence of first-order transitions in
temperature for some recently introduced generalized XY models, and also point
out the connection between them and annealed site-diluted (lattice-gas)
continuous-spin models
- …