265 research outputs found
IGR J19294+1816: a new Be-X ray binary revealed through infrared spectroscopy
The aim of this work is to characterize the counterpart to the INTEGRAL High
Mass X-ray Binary candidate IGR J19294+1816 so as to establish its true nature.
We obtained H band spectra of the selected counterpart acquired with the NICS
instrument mounted on the Telescopio Nazionale Galileo (TNG) 3.5-m telescope
which represents the first infrared spectrum ever taken of this source. We
complement the spectral analysis with infrared photometry from UKIDSS, 2MASS,
WISE and NEOWISE databases. We classify the mass donor as a Be star.
Subsequently, we compute its distance by properly taking into account the
contamination produced by the circumstellar envelope. The findings indicate
that IGR J19294+1816 is a transient source with a B1Ve donor at a distance of
kpc, and luminosities of the order of erg s,
displaying the typical behaviour of a Be X-ray binary.Comment: 8 pages, 6 figures, accepted to be published in MNRA
Chandra and Suzaku observations of the Be/X-ray star HD110432
We present an analysis of a pointed 141 ks Chandra high resolution
transmission gratings observation of the Be X-ray emitting star HD110432, a
prominent member of the gamma Cas analogs. The Chandra lightcurve shows a high
variability but its analysis fails to detect any coherent periodicity up to a
frequency of 0.05 Hz. The analysis of the Chandra HETG spectrum shows that, to
correctly describe the spectrum, three model components are needed. Two of
those components are optically thin thermal plasmas of different temperatures
(kT~8-9 and 0.2-0.3 keV respectively). Two different models seem to describe
well the third component. One possibility is a third hot optically thin thermal
plasma at kT=16-21 keV with an Fe abundance Z~0.3Zo, definitely smaller than
for the other two thermal components. Alternatively, the third component can be
described by a powerlaw with a photon index Gamma=1.56. In either case, the
Chandra HETG spectrum establishes that each one of these components must be
modified by distinct absorption columns. The analysis of a non contemporaneous
25 ks Suzaku observation shows the presence of a hard tail extending up to at
least 33 keV. The Suzaku spectrum is described with the sum of two components:
an optically thin thermal plasma at kT ~ 9 keV and a very hot second plasma
with kT ~33 keV or, alternatively, a powerlaw with photon index Gamma=1.58. The
analysis of the Si XIII and S XV He like triplets present in the Chandra
spectrum point to a very dense (n_e ~ 10^13 cm^-3) plasma located either close
to the stellar surface (r<3R_*) of the Be star or, alternatively, very close (r
~1.5R_WD) to the surface of a (hypothetical) WD companion. We argue, however,
that the available data supports the first scenario.Comment: 13 pages, 21 Figures. Accepted for publication in Ap
Magnetostatic bias in multilayer microwires: theory and experiments
The hysteresis curves of multilayer microwires consisting of a soft magnetic
nucleus, intermediate non-magnetic layers, and an external hard magnetic layer
are investigated. The magnetostatic interaction between magnetic layers is
proved to give rise to an antiferromagnetic-like coupling resulting in a
magnetostatic bias in the hysteresis curves of the soft nucleus. This
magnetostatic biasing effect is investigated in terms of the microwire
geometry. The experimental results are interpreted considering an analytical
model taking into account the magnetostatic interaction between the magnetic
layers.Comment: 6 pages, 7 figure
Fabrication and magnetic properties of hard/soft magnetostatically coupled FePt/FeNi multilayer microwires
3 pages, 5 figures.-- PACS: 75.70.Cn; 75.60.Ej;
75.50.Ww; 81.15.Pq; 68.65.Ac; 81.40.EfA family of multilayer microwires with hard/soft biphase magnetic behavior is here introduced. The microwires consist of a Fe63Pt27Si10 hard magnetic nucleus and a Fe20Ni80 soft outer shell separated by an intermediate insulating Pyrex glass microtube. The precursor FePtSi glass-coated microwire is fabricated by quenching and drawing technique, and its L10 hard magnetic phase is grown by postannealing treatment technique. The polycrystalline FeNi soft magnetic outer shell has been deposited by electroplating. The analysis of the low-field hysteresis loops of the FeNi soft phase after premagnetizing until near magnetic saturation provides information about the magnetostatic coupling between phases. The FeNi magnetization curve is shifted toward positive field when the FePt remanent magnetization is positive and vice versa. A systematic analysis of the magnetostatic coupling and the corresponding bias field arising from uncompensated poles of the premagnetized FePt hard phase has been performed. The strength of the bias field is shown to increase with the reduction of thickness of the FeNi layer. These magnetostatically coupled biphase systems are thought to be of large potential interest as sensing elements in sensor devices.The work has been developed under the Project No.
MAT2004-00150 supported by the Spanish Ministry of Education and Science, MEC. J. T. acknowledges a FPU program fellowship from MEC.Peer reviewe
An XMM-Newton view of FeK{\alpha} in HMXBs
We present a comprehensive analysis of the whole sample of available
XMM-Newton observations of High Mass X-ray Binaries (HMXBs) until August, 2013,
focusing on the FeK{\alpha} emission line. This line is a key tool to better
understand the physical properties of the material surrounding the X-ray source
within a few stellar radii (the circumstellar medium). We have collected
observations from 46 HMXBs, detecting FeK{\alpha} in 21 of them. We have used
the standard classification of HMXBs to divide the sample in different groups.
We find that: (1) FeK{\alpha} is centred at a mean value of 6.42 keV.
Considering the instrumental and fits uncertainties, this value is compatible
with ionization states lower than FeXVIII. (2) The flux of the continuum is
well correlated with the flux of the line, as expected. Eclipse observations
show that the Fe fluorescence emission comes from an extended region
surrounding the X-ray source. (3) FeK{\alpha} is narrow (width lower than
0.15keV), reflecting that the reprocessing material does not move at high
speeds. We attempt to explain the broadness of the line in terms of three
possible broadening phenomena: line blending, Compton scattering and Doppler
shifts (with velocities of the reprocessing material V=1000-2000 km/s). (4) The
equivalent hydrogen column (NH) directly correlates with the EW of FeK{\alpha},
displaying clear similarities to numerical simulations. It highlights the
strong link between the absorbing and the fluorescent matter. The obtained
results clearly point to a very important contribution of the donors wind in
the FeK{\alpha} emission and the absorption when the donor is a supergiant
massive star.Comment: Accepted for publication in A&A. 13 pages, 16 figures + Appendice
An extended version of the Ordered Median Tree Location Problem including appendices and detailed computational results
In this paper, we propose the Ordered Median Tree Location Problem (OMT). The
OMT is a single-allocation facility location problem where p facilities must be
placed on a network connected by a non-directed tree. The objective is to
minimize the sum of the ordered weighted averaged allocation costs plus the sum
of the costs of connecting the facilities in the tree. We present different
MILP formulations for the OMT based on properties of the minimum spanning tree
problem and the ordered median optimization. Given that ordered median hub
location problems are rather difficult to solve we have improved the OMT
solution performance by introducing covering variables in a valid reformulation
plus developing two pre-processing phases to reduce the size of this
formulations. In addition, we propose a Benders decomposition algorithm to
approach the OMT. We establish an empirical comparison between these new
formulations and we also provide enhancements that together with a proper
formulation allow to solve medium size instances on general random graphs.Comment: 112 pages, 4 figures, extended version of 'The Ordered Median
Location Problem
Evidence of Compton cooling during an X-ray flare supports a neutron star nature of the compact object in 4U1700-37
Based on new Chandra X-ray telescope data, we present empirical evidence of
plasma Compton cooling during a flare in the non pulsating massive X-ray binary
4U1700-37. This behaviour might be explained by quasispherical accretion onto a
slowly rotating magnetised neutron star. In quiescence, the neutron star in
4U1700-37 is surrounded by a hot radiatively cooling shell. Its presence is
supported by the detection of mHz quasi periodic oscillations likely produced
by its convection cells. The high plasma temperature and the relatively low
X-ray luminosity observed during the quiescence, point to a small emitting area
about 1 km, compatible with a hot spot on a NS surface. The sudden transition
from a radiative to a significantly more efficient Compton cooling regime
triggers an episode of enhanced accretion resulting in a flare. During the
flare, the plasma temperature drops quickly. The predicted luminosity for such
transitions, Lx = 3 x 10^35 erg s-1, is very close to the luminosity of
4U1700-37 during quiescence. The transition may be caused by the accretion of a
clump in the stellar wind of the donor star. Thus, a magnetised NS nature of
the compact object is strongly favoured.Comment: Accepted for publication in MNRA
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