686 research outputs found
Current sheets at three-dimensional magnetic nulls:effect of compressibility
The nature of current sheet formation in the vicinity of three-dimensional
(3D) magnetic null points is investigated. The particular focus is upon the
effect of the compressibility of the plasma on the qualitative and quantitative
properties of the current sheet. An initially potential 3D null is subjected to
shearing perturbations, as in a previous paper [Pontin et al., Phys. Plasmas,
in press (2007)]. It is found that as the incompressible limit is approached,
the collapse of the null point is suppressed, and an approximately planar
current sheet aligned to the fan plane is present instead. This is the case
regardless of whether the spine or fan of the null is sheared. Both the peak
current and peak reconnection rate are reduced. The results have a bearing on
previous analytical solutions for steady-state reconnection in incompressible
plasmas, implying that fan current sheet solutions are dynamically accessible,
while spine current sheet solutions are not.Comment: to appear in Physics of Plasmas. This version contains updated
figures and references, additional discussion, and typos are fixed. This is
the second in a series of papers - the first of which (by the same authors)
is located at astro-ph/0701462. A version with higher quality figures can be
found at http://www.maths.dundee.ac.uk/~dpontin
Stability of gold nanowires at large Au-Au separations
The unusual structural stability of gold nanowires at large separations of
gold atoms is explained from first-principles quantum mechanical calculations.
We show that undetected light atoms, in particular hydrogen, stabilize the
experimentally observed structures, which would be unstable in pure gold wires.
The enhanced cohesion is due to the partial charge transfer from gold to the
light atoms. This finding should resolve a long-standing controversy between
theoretical predictions and experimental observations.Comment: 7 pages, 3 figure
Homogeneous heterotic supergravity solutions with linear dilaton
I construct solutions to the heterotic supergravity BPS-equations on products
of Minkowski space with a non-symmetric coset. All of the bosonic fields are
homogeneous and non-vanishing, the dilaton being a linear function on the
non-compact part of spacetime.Comment: 36 pages; v2 conclusion updated and references adde
Searching for differences in Swift's intermediate GRBs
Gamma-ray bursts are usually classified through their high-energy emission
into short-duration and long-duration bursts, which presumably reflect two
different types of progenitors. However, it has been shown on statistical
grounds that a third, intermediate population is needed in this classification
scheme, although an extensive study of the properties of this class has so far
not been done. The large amount of follow-up studies generated during the Swift
era allows us to have a suficient sample to attempt a study of this third
population through the properties of their prompt emission and their
afterglows. Our study is focused on a sample of GRBs observed by Swift during
its first four years of operation. The sample contains those bursts with
measured redshift since this allows us to derive intrinsic properties.
Intermediate bursts are less energetic and have dimmer afterglows than long
GRBs, especially when considering the X-ray light curves, which are on average
one order of magnitude fainter than long bursts. There is a less significant
trend in the redshift distribution that places intermediate bursts closer than
long bursts. Except for this, intermediate bursts show similar properties to
long bursts. In particular, they follow the Epeak vs. Eiso correlation and
have, on average, positive spectral lags with a distribution similar to that of
long bursts. Like long GRBs, they normally have an associated supernova,
although some intermediate bursts have shown no supernova component. This study
shows that intermediate bursts are different from short bursts and, in spite of
sharing many properties with long bursts, there are some differences between
them as well. We suggest that the physical difference between intermediate and
long bursts could be that for the first the ejecta are thin shells while for
the latter they are thick shells.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 17
figures, 5 table
Multi-wavelength observations of afterglow of GRB 080319B and the modeling constraints
We present observations of the afterglow of GRB 080319B at optical, mm and
radio frequencies from a few hours to 67 days after the burst. Present
observations along with other published multi-wavelength data have been used to
study the light-curves and spectral energy distributions of the burst
afterglow. The nature of this brightest cosmic explosion has been explored
based on the observed properties and it's comparison with the afterglow models.
Our results show that the observed features of the afterglow fits equally good
with the Inter Stellar Matter and the Stellar Wind density profiles of the
circum-burst medium. In case of both density profiles, location of the maximum
synchrotron frequency is below optical and the value of cooling break
frequency is below rays, s after the burst. Also, the
derived value of the Lorentz factor at the time of naked eye brightness is
with the corresponding blast wave size of cm. The
numerical fit to the multi-wavelength afterglow data constraints the values of
physical parameters and the emission mechanism of the burst.Comment: 8 Pages, 3 Figures, Accepted for publication to Astronomy and
Astrophysics on 02/04/200
New perspectives for pharmacological chaperoning treatment in methylmalonic aciduria cblB type
Methylmalonic aciduria cblB type (MMA cblB) is caused by the impairment of ATP:cob(I)alamin adenosyltransferase (ATR), the enzyme responsible for the synthesis of adenosylcobalamin (AdoCbl) from cob(I)alamin. No definitive treatment is available for patients with this condition and novel therapeutic strategies are therefore much needed. Recently, we described a proof-of-concept regarding the use of pharmacological chaperones as a treatment. This work describes the effect of two potential pharmacological chaperones - compound V (N-{[(4-chlorophenyl)carbamothioyl]amino}-2-phenylacetamide) and compound VI (4-(4-(4-fluorophenyl)-5-methyl-1H-pyrazol-3-yl)benzene-1,3-diol) - on six ATR mutants, including the most common, p.Arg186Trp. Comprehensive functional analysis identified destabilizing (p.Arg186Gln, p.Arg190Cys, p.Arg190His, p.Arg191Gln and p.Glu193Lys) and oligomerization (p.Arg186Trp and p.Arg191Gln) mutations. In a cellular model overexpressing the destabilizing/oligomerization mutations, compounds V and VI had a positive effect on the stability and activity of all ATR variants. When provided in combination with hydroxocobalamin a more positive effect was obtained than with the compounds alone, even in mutations previously described as B12 non-responsive. In addition, a normal oligomerization profile was recovered after treatment of the p.Arg186Trp mutant with both compounds. These promising results confirm MMA cblB type as a conformational disorder and hence, pharmacological chaperones as a new therapeutic option alone or in combination with hydroxocobalamin for many patients with MMA cblBThis work was supported by Instituto de Salud Carlos III and (grant
PI13/01239) plus grants from the Fundación Isabel Gemio and Obra
Social de La Caixa to BP; the Research Council of Norway [nr. 185181 to
AM], The KG Jebsen Foundation, and NovoSeeds (Novo Nordisk). AG
was supported by a Ramón y Cajal grant from the Ministerio de Ciencia y
Tecnología. This work was supported also by the European Regional
Development Fund (PI13/01239
GRB 021004: Tomography of a gamma-ray burst progenitor and its host galaxy
We analyse the distribution of matter around the progenitor star of gamma-ray
burst GRB 021004 as well as the properties of its host galaxy with
high-resolution echelle as well as near-infrared spectroscopy. Observations
were taken by the 8.2m Very Large Telescope with the Ultraviolet and Visual
Echelle spectrograph (UVES) and the Infrared Spectrometer And Array Camera
(ISAAC) between 10 and 14 hours after the onset of the event. We report the
first detection of emission lines from a GRB host galaxy in the near-infrared,
detecting H-alpha and the [O III] doublet. These allow an independent
measurement of the systemic redshift (z = 2.3304 +/- 0.0005) which is not
contaminated by absorption as the Ly-alpha line is, and the deduction of
properties of the host galaxy. From the visual echelle spectroscopy, we find
several absorption line groups spanning a range of about 3,000 km/s in velocity
relative to the redshift of the host galaxy. The absorption profiles are very
complex with both velocity-broadened components extending over several 100 km/s
and narrow lines with velocity widths of only 20 km/s. By analogy with QSO
absorption line studies, the relative velocities,widths, and degrees of
ionization of the lines ("line-locking", "ionization--velocity correlation")
show that the progenitor had both an extremely strong radiation field and
several distinct mass loss phases (winds). These results are consistent with
GRB progenitors being massive stars, such as Luminous Blue Variables (LBVs) or
Wolf--Rayet stars, providing a detailed picture of the spatial and velocity
structure of the GRB progenitor star at the time of explosion. The host galaxy
is a prolific star-forming galaxy with a SFR of about 40 solar masses per year.Comment: 11 pages, 5 figures. Accepted for publication in Astronomy and
Astrophysics
Structure and stability of finite gold nanowires
Finite gold nanowires containing less than 1000 atoms are studied using the
molecular dynamics simulation method and embedded atom potential. Nanowires
with the face-centered cubic structure and the (111) oriented cross-section are
prepared at T=0 K. After annealing and quenching the structure and vibrational
properties of nanowires are studied at room temperature. Several of these
nanowires form multi-walled structures of lasting stability. They consist of
concentrical cylindrical sheets and resemble multi-walled carbon nanotubes.
Vibrations are investigated by diagonalization of the dynamical matrix. It was
found that several percents of vibrational modes are unstable because of
uncompleted restructuring of initial fcc nanowires.Comment: 4 figures in gif forma
Identifying the Location in the Host Galaxy of the Short GRB 111117A with the Chandra Sub-arcsecond Position
We present our successful Chandra program designed to identify, with
sub-arcsecond accuracy, the X-ray afterglow of the short GRB 111117A, which was
discovered by Swift and Fermi. Thanks to our rapid target of opportunity
request, Chandra clearly detected the X-ray afterglow, though no optical
afterglow was found in deep optical observations. The host galaxy was clearly
detected in the optical and near-infrared band, with the best photometric
redshift of z=1.31_{-0.23}^{+0.46} (90% confidence), making it one of the
highest known short GRB redshifts. Furthermore, we see an offset of 1.0 +- 0.2
arcseconds, which corresponds to 8.4 +- 1.7 kpc, between the host and the
afterglow position. We discuss the importance of using Chandra for obtaining
sub-arcsecond X-ray localizations of short GRB afterglows to study GRB
environments.Comment: 17 pages, 11 figures, accepted for publication in Ap
Pre-ALMA observations of GRBs in the mm/submm range
GRBs generate an afterglow emission that can be detected from radio to X-rays
during days, or even weeks after the initial explosion. The peak of this
emission crosses the mm/submm range during the first hours to days, making
their study in this range crucial for constraining the models. Observations
have been limited until now due to the low sensitivity of the observatories in
this range. We present observations of 10 GRB afterglows obtained from APEX and
SMA, as well as the first detection of a GRB with ALMA, and put them into
context with all the observations that have been published until now in the
spectral range that will be covered by ALMA. The catalogue of mm/submm
observations collected here is the largest to date and is composed of 102 GRBs,
of which 88 had afterglow observations, whereas the rest are host galaxy
searches. With our programmes, we contributed with data of 11 GRBs and the
discovery of 2 submm counterparts. In total, the full sample, including data
from the literature, has 22 afterglow detections with redshift ranging from
0.168 to 8.2. GRBs have been detected in mm/submm wavelengths with peak
luminosities spanning 2.5 orders of magnitude, the most luminous reaching
10^33erg s^-1 Hz^-1. We observe a correlation between the X-ray brightness at
0.5 days and the mm/submm peak brightness. Finally we give a rough estimate of
the distribution of peak flux densities of GRB afterglows, based on the current
mm/submm sample. Observations in the mm/submm bands have been shown to be
crucial for our understanding of the physics of GRBs, but have until now been
limited by the sensitivity of the observatories. With the start of the
operations at ALMA, the sensitivity will be increased by more than an order of
magnitude. Our estimates predict that, once completed, ALMA will detect up to
98% of the afterglows if observed during the passage of the peak synchrotron
emission.Comment: 23 pages, 14 figures, 5 tables (one big one!), Accepted for
publication in A&A. Includes the first observation of a GRB afterglow with
ALM
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