3,570 research outputs found
The reflection spectrum of the low-mass X-ray binary 4U 1636-53
We present 3-79 keV NuSTAR observations of the neutron star low-mass X-ray
binary 4U 1636-53 in the soft, transitional and hard state. The spectra display
a broad emission line at 5-10 keV. We applied several models to fit this line:
A GAUSSIAN line, a relativistically broadened emission line model, KYRLINE, and
two models including relativistically smeared and ionized reflection off the
accretion disc with different coronal heights, RELXILL and RELXILLLP. All
models fit the spectra well, however, the KYRLINE and RELXILL models yield an
inclination of the accretion disc of with respect to the line
of sight, which is at odds with the fact that this source shows no dips or
eclipses. The RELXILLLP model, on the other hand, gives a reasonable
inclination of . We discuss our results for these models in this
source and the possible primary source of the hard X-rays.Comment: 9 pages, 8 figure
The virial theorem and exact properties of density functionals for periodic systems
In the framework of density functional theory, scaling and the virial theorem
are essential tools for deriving exact properties of density functionals.
Preexisting mathematical difficulties in deriving the virial theorem via
scaling for periodic systems are resolved via a particular scaling technique.
This methodology is employed to derive universal properties of the
exchange-correlation energy functional for periodic systems.Comment: Accepted in PRB(R) 201
A re-analysis of the NuSTAR and XMM-Newton broad-band spectrum of Ser~X-1
Context: Ser X-1 is a well studied LMXB which clearly shows a broad iron
line. Recently, Miller et al. (2103) have presented broad-band, high quality
NuSTAR data of SerX-1.Using relativistically smeared self-consistent reflection
models, they find a value of R_in close to 1.0 R_ISCO (corresponding to 6 R_g),
and a low inclination angle, less than 10 deg. Aims: The aim of this paper is
to probe to what extent the choice of reflection and continuum models (and
uncertainties therein) can affect the conclusions about the disk parameters
inferred from the reflection component. To this aim we re-analyze all the
available public NuSTAR and XMM-Newton. Ser X-1 is a well studied source, its
spectrum has been observed by several instruments, and is therefore one of the
best sources for this study. Methods: We use slightly different continuum and
reflection models with respect to those adopted in literature for this source.
In particular we fit the iron line and other reflection features with
self-consistent reflection models as reflionx (with a power-law illuminating
continuum modified with a high energy cutoff to mimic the shape of the incident
Comptonization spectrum) and rfxconv. With these models we fit NuSTAR and
XMM-Newton spectra yielding consistent spectral results. Results: Our results
are in line with those already found by Miller et al. (2013) but less extreme.
In particular, we find the inner disk radius at about 13 R_g and an inclination
angle with respect to the line of sight of about 27 deg. We conclude that,
while the choice of the reflection model has little impact on the disk
parameters, as soon as a self-consistent model is used, the choice of the
continuum model can be important in the precise determination of the disk
parameters from the reflection component. Hence broad-band X-ray spectra are
highly preferable to constrain the continuum and disk parameters.Comment: 13 pages including 8 figures. Accepted for publication in A&
Energiaa ryhmästä – Plusfactor-hyvinvointivalmennus auttoi lisäämään työelämän voimavaroja
Artikkeli kuvaa Plusfactor-hyvinvointivalmennuksen tuloksia ja kokemuksia osallistujien näkökulmasta. Osallistujat ovat asiantuntija- ja yrittäjänaisia ja he ovat osallistuneet prosessiin 7 kuukauden ajan vuonna 2016
Updating the orbital ephemeris of the dipping source XB 1254-690 and the distance to the source
XB 1254-690 is a dipping low mass X-ray binary system hosting a neutron star
and showing type I X-ray bursts. We aim at obtaining more accurate orbital
ephemeris and at constraining the orbital period derivative of the system for
the first time. In addition, we want to better constrain the distance to the
source in order to locate the system in a well defined evolutive scenario. We
apply for the first time an orbital timing technique to XB 1254-690, using the
arrival times of the dips present in the light curves that have been collected
during 26 years of X-ray pointed observations performed from different space
missions. We estimate the dip arrival times using a statistical method that
weights the count-rate inside the dip with respect to the level of the
persistent emission outside the dip. We fit the obtained delays as a function
of the orbital cycles both with a linear and a quadratic function. We infer the
orbital ephemeris of XB 1254-690 improving the accuracy of the orbital period
with respect to previous estimates. We infer a mass of M
M for the donor star, in agreement with the estimations already
present in literature, assuming that the star is in thermal equilibrium while
it transfers part of its mass via the inner Lagrangian point, and assuming a
neutron star mass of 1.4 M. Using these assumptions, we also
constrain the distance to the source, finding a value of 7.6 kpc.
Finally, we discuss the evolution of the system suggesting that it is
compatible with a conservative mass transfer driven by magnetic braking.Comment: 13 pages, 5 figures, accepted for publication in Research in
Astronomy and Astrophysics (RAA
Evidence of a non-conservative mass transfer for XTE J0929-314
Context. In 1998 the first accreting millisecond pulsar, SAX J1808.4-3658,
was discovered and to date 18 systems showing coherent, high frequency (> 100
Hz) pulsations in low mass X-ray binaries are known. Since their discovery,
this class of sources has shown interesting and sometimes puzzling behaviours.
In particular, apart from a few exceptions, they are all transient with very
long X-ray quiescent periods implying a quite low averaged mass accretion rate
onto the neutron star. Among these sources, XTE J0929-314 has been detected in
outburst just once in about 15 years of continuous monitoring of the X-ray sky.
Aims. We aim to demonstrate that a conservative mass transfer in this system
will result in an X-ray luminosity that is higher than the observed, long-term
averaged X-ray luminosity. Methods. Under the hypothesis of a conservative mass
transfer driven by gravitational radiation, as expected for this system given
the short orbital period of about 43.6 min and the low mass of the companion
implied by the mass function derived from timing techniques, we calculate the
expected mass transfer rate in this system and predict the long-term averaged
X-ray luminosity. This is compared with the averaged, over 15 years, X-ray flux
observed from the system, and a lower limit of the distance to the source is
inferred. Results. This distance is shown to be > 7.4 kpc in the direction of
the Galactic anticentre, implying a large height, > 1.8 kpc, of the source with
respect to the Galactic plane, placing the source in an empty region of the
Galaxy. We suggest that the inferred value of the distance is unlikely.
(abridged)Comment: 6 pages, 2 figures, accepted for publication in Astronomy &
Astrophysics (A&A
Velocity and magnetic fields within 1000 AU from a massive YSO
We want to study the velocity and magnetic field morphology in the vicinity
(<1000 AU) of a massive young stellar object (YSO), at very high spatial
resolution (10-100 AU). We performed milli-arcsecond polarimetric observations
of the strong CH3OH maser emission observed in the vicinity of an O-type YSO,
in G023.01-00.41. We have combined this information with the velocity field of
the CH3OH masing gas previously measured at the same angular resolution. We
analyse the velocity and magnetic fields in the reference system defined by the
direction of the molecular outflow and the equatorial plane of the hot
molecular core at its base, as recently observed on sub-arcsecond scales. We
provide a first detailed picture of the gas dynamics and magnetic field
configuration within a radius of 2000 AU from a massive YSO. We have been able
to reproduce the magnetic field lines for the outer regions (>600 AU) of the
molecular envelope, where the magnetic field orientation shows a smooth change
with the maser cloudlets position (0.2 degree/AU). Overall, the velocity field
vectors well accommodate with the local, magnetic field direction, but still
show an average misalignment of 30 degrees. We interpret this finding as the
contribution of a turbulent velocity field of about 3.5 km/s, responsible for
braking up the alignment between the velocity and magnetic field vectors. We do
resolve different gas flows which develop both along the outflow axis and
across the disk plane, with an average speed of 7 km/s. In the direction of the
outflow axis, we establish a collimation of the gas flow, at a distance of
about 1000 AU from the disk plane. In the disk region, gas appears to stream
outward along the disk plane for radii greater than 500-600 AU, and inward for
shorter radii.Comment: 7 pages, 4 figures, 1 table, accepted by Astronomy & Astrophysic
Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808
We report on the discovery of coherent pulsations at a period of 2.9 ms from
the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We
observed X-ray pulsations at a frequency of Hz in three different
observations of the source performed with XMM-Newton and NuSTAR during the
source outburst. This newly discovered accreting millisecond pulsar is part of
an ultra-compact binary system characterised by an orbital period of
minutes and a projected semi-major axis of lt-ms. Based on the mass
function we estimate a minimum companion mass of 0.024 M, which
assumes a neutron star mass of 1.4 M and a maximum inclination angle
of (derived from the lack of eclipses and dips in the light-curve
of the source). We find that the companion star's Roche-Lobe could either be
filled by a hot ( K) pure helium white dwarf with a 0.028
M mass (implying ) or an old (>5 Gyr) brown dwarf
with metallicity abundances between solar/sub-solar and mass ranging in the
interval 0.0650.085 M (16 < < 21). During the outburst the
broad-band energy spectra are well described by a superposition of a weak
black-body component (kT 0.5 keV) and a hard cutoff power-law with photon
index 1.7 and cut-off at a temperature kT 130 keV. Up to
the latest Swift-XRT observation performed on 2016 July 19 the source has been
observed in outburst for almost 150 days, which makes MAXI J0911-655 the second
accreting millisecond X-ray pulsar with outburst duration longer than 100 days.Comment: 7 pages, 5 figures, accepted for publication in A&
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