117 research outputs found
The variable X-ray emission of PSR B0943+10
The old pulsar PSR B0943+10 (P=1.1 s, characteristic age tau=5 Myr) is the
best example of mode-switching radio pulsar. Its radio emission alternates
between a highly organized state with regular drifting subpulses (B mode) and a
chaotic emission pattern (Q mode). We present the results of XMM-Newton
observations showing that the X-ray properties of PSR B0943+10 depend on its
radio state (Hermsen et al. 2013). During the radio fainter state (Q mode) the
X-ray flux is more than a factor two larger than during the B-mode and X-ray
pulsations with about 50% pulsed fraction are detected. The X-ray emission of
PSR B0943+10 in the B-mode is well described by thermal emission with blackbody
temperature kT=0.26 keV coming from a small hot spot with luminosity of 7x10^28
erg/s, in good agreement with the prediction of the partially screened gap
model, which also explains the properties of the radio emission in this mode.
We derived an upper limit of 46% on the X-ray pulsed fraction in the B-mode,
consistent with the geometry and viewing angle of PSR B0943+10 inferred from
the radio data. The higher flux observed during the Q-mode is consistent with
the appearance of an additional component with a power-law spectrum with photon
index 2.2. We interpret it as pulsed non-thermal X-rays produced in the star
magnetosphere. A small change in the beaming pattern or in the efficiency of
acceleration of the particles responsible for the non-thermal emission can
explain the reduced flux of this component during the radio B-mode.Comment: Accepted for publication in MNRA
X-ray emission from hot subdwarfs with compact companions
We review the X-ray observations of hot subdwarf stars. While no X-ray
emission has been detected yet from binaries containing B-type subdwarfs,
interesting results have been obtained in the case of the two luminous O-type
subdwarfs HD 49798 and BD +37 442. Both of them are members of binary systems
in which the X-ray luminosity is powered by accretion onto a compact object: a
rapidly spinning (13.2 s) and massive (1.28 M_sun) white dwarf in the case of
HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD
+37 442. Their study can shed light on the poorly known processes taking place
during common envelope evolutionary phases and on the properties of wind mass
loss from hot subdwarfs.Comment: To be published in the proceedings of the 40th Liege International
Astrophysical Colloquium "Ageing low mass stars: from red giants to white
dwarfs
Search for X-ray emission from subdwarf B stars with compact companion candidates
Stellar evolutionary models predict that most of the early type subdwarf
stars in close binary systems have white dwarf companions. More massive
companions, such as neutron stars or black holes, are also expected in some
cases. The presence of compact stars in these systems can be revealed by the
detection of X-rays powered by accretion of the subdwarf's stellar wind or by
surface thermal emission. Using the Swift satellite, we carried out a
systematic search for X-ray emission from a sample of twelve subdwarf B stars
which, based on optical studies, have been suggested to have degenerate
companions. None of our targets was detected, but the derived upper limits
provide one of the few observational constraints on the stellar winds of early
type subdwarfs. If the presence of neutron star companions is confirmed, our
results constrain the mass loss rates of some of these subdwarf B stars to
values <10^{-13}-10^{-12} Msun/yr.Comment: Accepted for publication in Astronomy and Astrophysic
The effect of X-ray dust-scattering on a bright burst from the magnetar 1E 1547.0-5408
A bright burst, followed by an X-ray tail lasting ~10 ks, was detected during
an XMM-Newton observation of the magnetar 1E 1547.0-5408 carried out on 2009
February 3. The burst, also observed by SWIFT/BAT, had a spectrum well fit by
the sum of two blackbodies with temperatures of ~4 keV and 10 keV and a fluence
in the 0.3-150 keV energy range of ~1e-5 erg/cm2. The X-ray tail had a fluence
of ~4e-8 erg/cm2. Thanks to the knowledge of the distances and relative optical
depths of three dust clouds between us and 1E 1547.0-5408, we show that most of
the X-rays in the tail can be explained by dust scattering of the burst
emission, except for the first ~20-30 s. We point out that other X-ray tails
observed after strong magnetar bursts may contain a non-negligible contribution
due to dust scattering.Comment: 8 pages, 2 tables and 10 figures; accepted to publication in MNRA
Protein structure and function relationships: application of computational approaches to biological and biomedical problems
In this work we have studied several cases by means of different computational approaches for the analysis of the
structure and function relationships.
In chapter 2 we describe a method, based on multiple neural networks, that we developed for evaluate the accuracy
of predicted threedimensional
protein structures. This tool has been used in different studies described in this work,
in which the prediction of the 3D structure of the protein under study, has been necessary.
In chapter 3, the interaction among a new class of natural sweeteners (steviol glycosides) and the human sweet taste
receptor, has been analyzed by means of an insilico
docking study, which allowed to identify the preferential
binding site for the steviol glycosides.
In chapter 4 the relationship between the dynamical properties and the function of some psychrophilic enzyme has
been studied. A comparative study (psychrophile vs mesophile) of the thermodynamic properties of two different
enzymes belonging to the elastases and the uracilDNAglycosylases
families has been done. This study, carried out
with molecular dynamic simulations, revealed that the low temperature adaptation is related to the different
flexibility of the psychrophilic compared to the mesophilic enzyme.
In chapter 5, we have studied the structural and functional impact of point mutations on three different proteins
which are involved in serious rare diseases which cause grave metabolic disorders
Discovery of spin-up in the X-ray pulsar companion of the hot subdwarf HD 49798
The hot subdwarf HD 49798 has an X-ray emitting compact companion with a
spin-period of 13.2 s and a dynamically measured mass of 1.28+/-0.05 M_sun,
consistent with either a neutron star or a white dwarf. Using all the available
XMM-Newton and Swift observations of this source, we could perform a
phase-connected timing analysis extending back to the ROSAT data obtained in
1992. We found that the pulsar is spinning up at a rate of
(2.15+/-0.05)x10^{-15} s/s. This result is best interpreted in terms of a
neutron star accreting from the wind of its subdwarf companion, although the
remarkably steady period derivative over more than 20 years is unusual in
wind-accreting neutron stars. The possibility that the compact object is a
massive white dwarf accreting through a disk cannot be excluded, but it
requires a larger distance and/or properties of the stellar wind of HD 49798
different from those derived from the modelling of its optical/UV spectra.Comment: Accepted for publication in MNRA
The Shape of Protein Crowders is a Major Determinant of Protein Diffusion
AbstractAs a model for understanding how molecular crowding influences diffusion and transport of proteins in cellular environments, we combined experimental and theoretical approaches to study the diffusion of proteins in highly concentrated protein solutions. Bovine serum albumin and γ-Globulin were chosen as molecular crowders and as tracers. These two proteins are representatives of the main types of plasma protein and have different shapes and sizes. Solutions consisting of one or both proteins were studied. The self-diffusion coefficients of the fluorescently labeled tracer proteins were measured by means of fluorescence correlation spectroscopy at a total protein concentration of up to 400 g/L. γ-Globulin is found to have a stronger influence as a crowder on the tracer self-diffusion coefficient than Bovine serum albumin. Brownian dynamics simulations show that the excluded volume and the shape of the crowding protein have a significantly stronger influence on translational and rotational diffusion coefficients, as well as transient oligomerization, than hydrodynamic or direct interactions. Anomalous subdiffusion, which is not observed at the experimental fluorescence correlation spectroscopy timescales (>100 μs), appears only at very short timescales (<1 μs) in the simulations due to steric effects of the proteins. We envision that the combined experimental and computational approach employed here can be developed to unravel the different biophysical contributions to protein motion and interaction in cellular environments by systematically varying protein properties such as molecular weight, size, shape, and electrostatic interactions
Diffusion and association processes in biological systems: theory, computation and experiment
Macromolecular diffusion plays a fundamental role in biological processes. Here, we give an overview of recent methodological advances and some of the challenges for understanding how molecular diffusional properties influence biological function that were highlighted at a recent workshop, BDBDB2, the second Biological Diffusion and Brownian Dynamics Brainstorm
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