179 research outputs found
A propelling neutron star in the enigmatic Be-star ~Cassiopeia
The enigmatic X-ray emission from the bright optical star,
Cassiopeia, is a long-standing problem. Cas is known to be a binary
system consisting of a Be-type star and a low-mass ()
companion of unknown nature orbiting in the Be-disk plane. Here we apply the
quasi-spherical accretion theory onto a compact magnetized star and show that
if the low-mass companion of Cas is a fast spinning neutron star, the
key observational signatures of Cas are remarkably well reproduced.
Direct accretion onto this fast rotating neutron star is impeded by the
propeller mechanism. In this case, around the neutron star magnetosphere a hot
shell is formed that emits thermal X-rays in qualitative and quantitative
agreement with observed properties of the X-ray emission from Cas. We
suggest that Cas and its analogs constitute a new subclass of Be-type
X-ray binaries hosting rapidly rotating neutron stars formed in supernova
explosions with small kicks. The subsequent evolutionary stage of Cas
and its analogs should be the X Per-type binaries comprising low-luminosity
slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission
from Cas, and also establishes evolutionary connections between
various types of rotating magnetized neutron stars in Be-binaries.Comment: 6 pages, accepted for publication in MNRA
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
Orbital phase resolved spectroscopy of 4U1538-52 with MAXI
4U 1538-52, an absorbed high mass X-ray binary with an orbital period of 3.73
days, shows moderate orbital intensity modulations with a low level of counts
during the eclipse. Several models have been proposed to explain the accretion
at different orbital phases by a spherically symmetric stellar wind from the
companion. The aim of this work is to study both the light curve and orbital
phase spectroscopy of this source in the long term. Particularly, the folded
light curve and the changes of the spectral parameters with orbital phase to
analyse the stellar wind of QV Nor, the mass donor of this binary system. We
used all the observations made from the Gas Slit Camera on board MAXI of 4U
1538-52 covering many orbits continuously. We obtained the good interval times
for every orbital phase range which were the input to extract our data. We
estimated the orbital period of the system and then folded the light curves and
we fitted the X-ray spectra with the same model for every orbital phase
spectrum. We also extracted the averaged spectrum of all the MAXI data
available. The MAXI spectra in the 2-20 keV energy range were fitted with an
absorbed Comptonization of cool photons on hot electrons. We found a strong
orbital dependence of the absorption column density but neither the
fluorescence iron emission line nor low energy excess were needed to fit the
MAXI spectra. The variation of the spectral parameters over the binary orbit
were used to examine the mode of accretion onto the neutron star in 4U 1538-52.
We deduce a best value of for QV Nor.Comment: 12 pages, 5 figures, accepted to be published by A&A, corrected typos
(changing bold font to normal one
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
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
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
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