5,528 research outputs found
SAX J1808.4-3658, an accreting millisecond pulsar shining in gamma rays?
We report the detection of a possible gamma-ray counterpart of the accreting
millisecond pulsar SAX J1808.4-3658. The analysis of ~6 years of data from the
Large Area Telescope on board the Fermi Gamma-ray Space Telescope (Fermi-LAT)
within a region of 15deg radius around the position of the pulsar reveals a
point gamma-ray source detected at a significance of ~6 sigma (Test Statistic
TS = 32), with position compatible with that of SAX J1808.4-3658 within 95%
Confidence Level. The energy flux in the energy range between 0.6 GeV and 10
GeV amounts to (2.1 +- 0.5) x 10-12 erg cm-2 s-1 and the spectrum is
well-represented by a power-law function with photon index 2.1 +- 0.1. We
searched for significant variation of the flux at the spin frequency of the
pulsar and for orbital modulation, taking into account the trials due to the
uncertainties in the position, the orbital motion of the pulsar and the
intrinsic evolution of the pulsar spin. No significant deviation from a
constant flux at any time scale was found, preventing a firm identification via
time variability. Nonetheless, the association of the LAT source as the
gamma-ray counterpart of SAX J1808.4-3658 would match the emission expected
from the millisecond pulsar, if it switches on as a rotation-powered source
during X-ray quiescence.Comment: 8 pages, 4 figures, accepted by MNRA
A self-consistent approach to the reflection component in 4U 1705-44
High-resolution spectroscopy has recently revealed in many neutron-star Low-Mass X-ray binaries that the shape of the broad iron line observed in the 6.4-6.97 keV range is consistently well fitted by a relativistically smeared line profile. We show here spectral fitting results using a newly developed self-consistent reflection model on XMM-Newton data of the LMXB 4U 1705-44 during a period when the source was in a bright soft state. This reflection model adopts a blackbody prescription for the shape of the impinging radiation field, that we physically associate with the boundary layer emission. \ua9 2010 American Institute of Physics
First record of brown long-eared bat Plecotus auritus (Chiroptera: Vespertilionidae) for Sicily island (Italy)
The knowledge of the bat fauna of Sicily (Southern Italy) is scarce,
fragmentary or sometimes even confusing. A recent review mentioned 20
species for the region, but it is likely that the checklist of bats of
Sicily is still far from being exhaustive. To help fill this gap, in
the past few years specific studies were carried out on the
distribution of bat species in Sicily, especially in the woodlands of
the Nebrodi Mountains. In the municipality of Caronia (Messina
province) has captured a young female brown long-eared bat Plecotus
auritus (Linneaus, 1758). The capture of P. auritus in Sicily
represents the first record of brown long-eared bat Plecotus auritus
for Sicily island, an interesting biogeographical record, as this bat
is mostly known for the north and centre of the Italian peninsul
Ultrasonic Mean Free Path in a Granular Aluminum Film(Physics)
The ultrasonic mean free path has been measured and compared to the electrical mean free path of a thin granular aluminum film. They have been found to differ by an order of magnitude which is believed to indicate that mean free path determined ultrasonically is for the Al metal while the one determined electrically is for the Al-Al_2O_3 matrix structure
ANALYSIS OF SUB-GRID MODELING EFFECTS IN THE SIMULATION OF THE SINGLE-PHASE TURBULENT FLOW IN AN INDUSTRIAL CYCLONE SEPARATOR
In the present work two turbulence modeling approaches, namely Large Eddy Simulation and Detached Eddy Simulation, are employed to predict turbulent, swirling flow within an industrial cyclone separator running at Reynolds number 267,000. The results from three LES models, Smagorinsky, dynamic and Yakhot, and the SST-DES model of Strelets have been compared to experimental results for the average axial and tangential velocities. The Navier-Stokes solver is based on an unstructured, finite volume, cell-centered algorithm such that the details of the geometry can be accurately represented. Based on the comparison with the experimental results, it has been found that the Yakhot model provides the most accurate predictions for the tangential velocities, whereas the dynamic LES and the Smagorinsky models overpredict it and the SST-DES model underpredicts it. However, the conclusions are different regarding the axial velocity. Implications of the turbulence modeling for the particle separation are discussed
The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence
We present an optical (gri) study during quiescence of the accreting
millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran
Telescopio Canarias (GTC) in August 2014. Despite the source being in
quiescence at the time of our observations, it showed a strong optical flaring
activity, more pronounced at higher frequencies (i.e. the g band). Once the
flares were subtracted, we tentatively recovered a sinusoidal modulation at the
system orbital period in all bands, even if a significant phase shift with
respect to an irradiated star, typical of accreting millisecond X-ray pulsars
is detected. We conclude that the observed flaring could be a manifestation of
the presence of an accretion disc in the system. The observed light curve
variability could be explained by the presence of a superhump, which might be
another proof of the formation of an accretion disc. In particular, the disc at
the time of our observations was probably preparing to the new outburst of the
source, that happened just a few months later, in 2015.Comment: 6 pages, 2 figures, 1 table. Accepted for publication in A&
An XMM-Newton and INTEGRAL view on the hard state of EXO 1745-248 during its 2015 outburst
CONTEXT - Transient low-mass X-ray binaries (LMXBs) often show outbursts
lasting typically a few-weeks and characterized by a high X-ray luminosity
( erg/sec), while for most of the time they are
found in X-ray quiescence ( erg/sec). EXO 1745-248
is one of them. AIMS - The broad-band coverage, and the sensitivity of
instrument on board of {\xmm} and {\igr}, offers the opportunity to
characterize the hard X-ray spectrum during {\exo} outburst. METHODS - In this
paper we report on quasi-simultaneous {\xmm} and {\igr} observations of the
X-ray transient {\exo} located in the globular cluster Terzan 5, performed ten
days after the beginning of the outburst (on 2015 March 16th) shown by the
source between March and June 2015. The source was caught in a hard state,
emitting a 0.8-100 keV luminosity of ~{\lumcgs}. RESULTS - The
spectral continuum was dominated by thermal Comptonization of seed photons with
temperature keV, by a cloud with moderate optical depth
and electron temperature keV. A weaker soft
thermal component at temperature --0.7 keV and compatible
with a fraction of the neutron star radius was also detected. A rich emission
line spectrum was observed by the EPIC-pn on-board {\xmm}; features at energies
compatible with K- transitions of ionized sulfur, argon, calcium and
iron were detected, with a broadness compatible with either thermal Compton
broadening or Doppler broadening in the inner parts of an accretion disk
truncated at gravitational radii from the neutron star. Strikingly, at
least one narrow emission line ascribed to neutral or mildly ionized iron is
needed to model the prominent emission complex detected between 5.5 and 7.5
keV. (Abridged)Comment: 14 pages, 6 figure, 2 tables. Accepted for publication on A&A
(21/03/2017
Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model
The aim of this work is to investigate in a physical and quantitative way the
spectral evolution of bright Neutron Star Low-Mass X-ray Binaries (NS LMXBs),
with special regard to the transient hard X-ray tails. We analyzed INTEGRAL
data for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and built
broad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source,
X-ray spectra from different states were fitted with the recently proposed
model compTB. The spectra have been fit with a two-compTB model. In all cases
the first compTB describes the dominant part of the spectrum that we interpret
as thermal Comptonization of soft seed photons (< 1 keV), likely from the
accretion disk, by a 3-5 keV corona. In all cases, this component does not
evolve much in terms of Comptonization efficiency, with the system converging
to thermal equilibrium for increasing accretion rate. The second compTB varies
more dramatically spanning from bulk plus thermal Comptonization of blackbody
seed photons to the blackbody emission alone. These seed photons (R < 12 km,
kT_s > 1 keV), likely from the neutron star and the innermost part of the
system, the Transition Layer, are Comptonized by matter in a converging flow.
The presence and nature of this second compTB component (be it a pure blackbody
or Comptonized) are related to the inner local accretion rate which can
influence the transient behaviour of the hard tail: high values of accretion
rates correspond to an efficient Bulk Comptonization process (bulk parameter
delta > 0) while even higher values of accretion rates suppress the
Comptonization, resulting in simple blackbody emission (delta=0).Comment: 12 pages, 10 figures, accepted for publication in A&
Effective Edge-Fault-Tolerant Single-Source Spanners via Best (or Good) Swap Edges
Computing \emph{all best swap edges} (ABSE) of a spanning tree of a given
-vertex and -edge undirected and weighted graph means to select, for
each edge of , a corresponding non-tree edge , in such a way that the
tree obtained by replacing with enjoys some optimality criterion (which
is naturally defined according to some objective function originally addressed
by ). Solving efficiently an ABSE problem is by now a classic algorithmic
issue, since it conveys a very successful way of coping with a (transient)
\emph{edge failure} in tree-based communication networks: just replace the
failing edge with its respective swap edge, so as that the connectivity is
promptly reestablished by minimizing the rerouting and set-up costs. In this
paper, we solve the ABSE problem for the case in which is a
\emph{single-source shortest-path tree} of , and our two selected swap
criteria aim to minimize either the \emph{maximum} or the \emph{average
stretch} in the swap tree of all the paths emanating from the source. Having
these criteria in mind, the obtained structures can then be reviewed as
\emph{edge-fault-tolerant single-source spanners}. For them, we propose two
efficient algorithms running in and time, respectively, and we show that the guaranteed (either
maximum or average, respectively) stretch factor is equal to 3, and this is
tight. Moreover, for the maximum stretch, we also propose an almost linear time algorithm computing a set of \emph{good} swap edges,
each of which will guarantee a relative approximation factor on the maximum
stretch of (tight) as opposed to that provided by the corresponding BSE.
Surprisingly, no previous results were known for these two very natural swap
problems.Comment: 15 pages, 4 figures, SIROCCO 201
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