3,966 research outputs found
The long-term evolution of the X-ray pulsar XTE J1814-338: a receding jet contribution to the quiescent optical emission?
We present a study of the quiescent optical counterpart of the Accreting
Millisecond X-ray Pulsar XTE J1814-338, carrying out multiband (BVR) orbital
phase-resolved photometry using the ESO VLT/FORS2. The optical light curves are
consistent with a sinusoidal variability modulated with the orbital period,
showing evidence for a strongly irradiated companion star, in agreement with
previous findings. The observed colours cannot be accounted for by the
companion star alone, suggesting the presence of an accretion disc during
quiescence. The system is fainter in all analysed bands compared to previous
observations. The R band light curve displays a possible phase offset with
respect to the B and V band. Through a combined fit of the multi-band light
curves we derive constraints on the companion star and disc fluxes, on the
system distance and on the companion star mass. The irradiation luminosity
required to account for the observed day-side temperature of the companion star
is consistent with the spin-down luminosity of a millisecond radio pulsar. The
flux decrease and spectral evolution of the quiescent optical emission observed
comparing our data with previous observations, collected over 5 years, cannot
be well explained with the contribution of an irradiated companion star and an
accretion disc alone. The progressive flux decrease as the system gets bluer
could be due to a continuum component evolving towards a lower, bluer spectrum.
While most of the continuum component is likely due to the disc, we do not
expect it to become bluer in quiescence. Hence we hypothesize that an
additional component, such as synchrotron emission from a jet was contributing
significantly in the earlier data obtained during quiescence and then
progressively fading or moving its break frequency toward longer wavelengths.Comment: 7 pages, 8 figures, accepted for publication in Section 7. Stellar
structure and evolution of Astronomy and Astrophysic
Properties of the redback millisecond pulsar binary 3FGL J0212.1+5320
Linares et al. (2016) obtained quasi-simultaneous g', r' and i-band light
curves and an absorption line radial velocity curve of the secondary star in
the redback system 3FGL J0212.1+5320. The light curves showed two maxima and
minima primarily due to the secondary star's ellipsoidal modulation, but with
unequal maxima and minima. We fit these light curves and radial velocities with
our X-ray binary model including either a dark solar-type star spot or a hot
spot due to off-centre heating from an intrabinary shock, to account for the
unequal maxima. Both models give a radial velocity semi-amplitude and
rotational broadening that agree with the observations. The observed secondary
star's effective temperature is best matched with the value obtained using the
hot spot model, which gives a neutron star and secondary star mass of =1.85 and =0.50
, respectively.Comment: 10 pages, 8 figues, accepted by MNRA
Observations of Doppler Boosting in Kepler Lightcurves
Among the initial results from Kepler were two striking lightcurves, for KOI
74 and KOI 81, in which the relative depths of the primary and secondary
eclipses showed that the more compact, less luminous object was hotter than its
stellar host. That result became particularly intriguing because a substellar
mass had been derived for the secondary in KOI 74, which would make the high
temperature challenging to explain; in KOI 81, the mass range for the companion
was also reported to be consistent with a substellar object. We re-analyze the
Kepler data and demonstrate that both companions are likely to be white dwarfs.
We also find that the photometric data for KOI 74 show a modulation in
brightness as the more luminous star orbits, due to Doppler boosting. The
magnitude of the effect is sufficiently large that we can use it to infer a
radial velocity amplitude accurate to 1 km/s. As far as we are aware, this is
the first time a radial-velocity curve has been measured photometrically.
Combining our velocity amplitude with the inclination and primary mass derived
from the eclipses and primary spectral type, we infer a secondary mass of
0.22+/-0.03 Msun. We use our estimates to consider the likely evolutionary
paths and mass-transfer episodes of these binary systems.Comment: 8 pages, 4 figures, ApJ 715, 51 (v4 is updated to match the published
version, including a note added in proof with measured projected rotational
velocities)
Discovery of the Optical Counterparts to Four Energetic Fermi Millisecond Pulsars
In the last few years, over 43 millisecond radio pulsars have been discovered
by targeted searches of unidentified gamma-ray sources found by the Fermi
Gamma-Ray Space Telescope. A large fraction of these millisecond pulsars are in
compact binaries with low-mass companions. These systems often show eclipses of
the pulsar signal and are commonly known as black widows and redbacks because
the pulsar is gradually destroying its companion. In this paper, we report on
the optical discovery of four strongly irradiated millisecond pulsar
companions. All four sources show modulations of their color and luminosity at
the known orbital periods from radio timing. Light curve modelling of our
exploratory data shows that the equilibrium temperature reached on the
companion's dayside with respect to their nightside is consistent with about
10-30% of the available spin-down energy from the pulsar being reprocessed to
increase the companion's dayside temperature. This value compares well with the
range observed in other irradiated pulsar binaries and offers insights about
the energetics of the pulsar wind and the production of gamma-ray emission. In
addition, this provides a simple way of estimating the brightness of irradiated
pulsar companions given the pulsar spin-down luminosity. Our analysis also
suggests that two of the four new irradiated pulsar companions are only
partially filling their Roche lobe. Some of these sources are relatively bright
and represent good targets for spectroscopic follow-up. These measurements
could enable, among other things, mass determination of the neutron stars in
these systems.Comment: 11 pages, 5 tables, 1 figure, 4 online tables. ApJ submitted and
referee
Stress response function of a two-dimensional ordered packing of frictional beads
We study the stress profile of an ordered two-dimensional packing of beads in
response to the application of a vertical overload localized at its top
surface. Disorder is introduced through the Coulombic friction between the
grains which gives some indeterminacy and allows the choice of one constrained
random number per grain in the calculation of the contact forces. The so-called
`multi-agent' technique we use, lets us deal with systems as large as
grains. We show that the average response profile has a double
peaked structure. At large depth , the position of these peaks grows with
, while their widths scales like . and are analogous to
`propagation' and `diffusion' coefficients. Their values depend on that of the
friction coefficient . At small , we get and , with , which means that the peaks get
closer and wider as the disorder gets larger. This behavior is qualitatively
what was predicted in a model where a stochastic relation between the stress
components is assumed.Comment: 7 pages, 7 figures, accepted version to Europhys. Let
Spin precession and inverted Hanle effect in a semiconductor near a finite-roughness ferromagnetic interface
Although the creation of spin polarization in various non-magnetic media via
electrical spin injection from a ferromagnetic tunnel contact has been
demonstrated, much of the basic behavior is heavily debated. It is reported
here for semiconductor/Al2O3/ferromagnet tunnel structures based on Si or GaAs
that local magnetostatic fields arising from interface roughness dramatically
alter and even dominate the accumulation and dynamics of spins in the
semiconductor. Spin precession in the inhomogeneous magnetic fields is shown to
reduce the spin accumulation up to tenfold, and causes it to be inhomogeneous
and non-collinear with the injector magnetization. The inverted Hanle effect
serves as experimental signature. This interaction needs to be taken into
account in the analysis of experimental data, particularly in extracting the
spin lifetime and its variation with different parameters (temperature, doping
concentration). It produces a broadening of the standard Hanle curve and
thereby an apparent reduction of the spin lifetime. For heavily doped n-type Si
at room temperature it is shown that the spin lifetime is larger than
previously determined, and a new lower bound of 0.29 ns is obtained. The
results are expected to be general and occur for spins near a magnetic
interface not only in semiconductors but also in metals, organic and
carbon-based materials including graphene, and in various spintronic device
structures.Comment: Final version, with text restructured and appendices added (25 pages,
9 figures). To appear in Phys. Rev.
The Double Pulsar Eclipses I: Phenomenology and Multi-frequency Analysis
The double pulsar PSR J0737-3039A/B displays short, 30 s eclipses that arise
around conjunction when the radio waves emitted by pulsar A are absorbed as
they propagate through the magnetosphere of its companion pulsar B. These
eclipses offer a unique opportunity to probe directly the magnetospheric
structure and the plasma properties of pulsar B. We have performed a
comprehensive analysis of the eclipse phenomenology using multi-frequency radio
observations obtained with the Green Bank Telescope. We have characterized the
periodic flux modulations previously discovered at 820 MHz by McLaughlin et
al., and investigated the radio frequency dependence of the duration and depth
of the eclipses. Based on their weak radio frequency evolution, we conclude
that the plasma in pulsar B's magnetosphere requires a large multiplicity
factor (~ 10^5). We also found that, as expected, flux modulations are present
at all radio frequencies in which eclipses can be detected. Their complex
behavior is consistent with the confinement of the absorbing plasma in the
dipolar magnetic field of pulsar B as suggested by Lyutikov & Thompson and such
a geometric connection explains that the observed periodicity is harmonically
related to pulsar B's spin frequency. We observe that the eclipses require a
sharp transition region beyond which the plasma density drops off abruptly.
Such a region defines a plasmasphere which would be well inside the
magnetospheric boundary of an undisturbed pulsar. It is also two times smaller
than the expected standoff radius calculated using the balance of the wind
pressure from pulsar A and the nominally estimated magnetic pressure of pulsar
B.Comment: 9 pages, 7 figures, 3 tables, ApJ in pres
Pulsar Wind Nebulae in the SKA era
Neutron stars lose the bulk of their rotational energy in the form of a
pulsar wind: an ultra-relativistic outflow of predominantly electrons and
positrons. This pulsar wind significantly impacts the environment and possible
binary companion of the neutron star, and studying the resultant pulsar wind
nebulae is critical for understanding the formation of neutron stars and
millisecond pulsars, the physics of the neutron star magnetosphere, the
acceleration of leptons up to PeV energies, and how these particles impact the
interstellar medium. With the SKA1 and the SKA2, it could be possible to study
literally hundreds of PWNe in detail, critical for understanding the many open
questions in the topics listed above.Comment: Comments: 10 pages, 3 figures, to be published in: "Advancing
Astrophysics with the Square Kilometre Array", Proceedings of Science,
PoS(AASKA14
Assessing the climate change impacts of biogenic carbon in buildings: a critical review of two main dynamic approaches
Wood is increasingly perceived as a renewable, sustainable building material. The carbon it contains, biogenic carbon, comes from biological processes; it is characterized by a rapid turnover
in the global carbon cycle. Increasing the use of harvested wood products (HWP) from sustainable forest management could provide highly needed mitigation efforts and carbon removals. However,
the combined climate change benefits of sequestering biogenic carbon, storing it in harvested wood products and substituting more emission-intensive materials are hard to quantify. Although different
methodological choices and assumptions can lead to opposite conclusions, there is no consensus on the assessment of biogenic carbon in life cycle assessment (LCA). Since LCA is increasingly relied
upon for decision and policy making, incorrect biogenic carbon assessment could lead to inefficient or counterproductive strategies, as well as missed opportunities. This article presents a critical review
of biogenic carbon impact assessment methods, it compares two main approaches to include time considerations in LCA, and suggests one that seems better suited to assess the impacts of biogenic carbon in buildings
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