5,409 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
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)
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
Radio Pulsars in Binary Systems
This thesis focuses on the study of binary radio pulsars, their evolution and
some specific use of their properties to investigate fundamental physics such
as general relativity and other gravitational theories. The work that we
present here is organized in three main parts. First, we report on the study of
PSR J1744-3922, a binary pulsar presenting a peculiar `flickering' flux
behavior as well as spin and orbital properties that do not correspond to the
expectations of standard evolution scenarios. Second, we conducted an in-depth
analysis of the eclipses in the relativistic double pulsar system PSR
J0737-3039A/B. From our modeling of the eclipses, we precisely determined the
geometry of pulsar B in space and used this information to study the temporal
behavior of the eclipses, which revealed that pulsar B precesses around the
angular momentum of the system in a way that is consistent with the prediction
of general relativity. Third, we searched for the signature of latitudinal
aberration in the pulse profile of pulsar A in the double pulsar system. The
non-detection of this effect allows us to put an upper limit on its amplitude,
which constrains the geometry of pulsar A with respect to our line of sight as
well as its emission geometry. (Abridged)Comment: Ph.D. Thesis, 236 pages, 76 figures, 7 table
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
Generating Gowdy cosmological models
Using the analogy with stationary axisymmetric solutions, we present a method
to generate new analytic cosmological solutions of Einstein's equation
belonging to the class of Gowdy cosmological models. We show that the
solutions can be generated from their data at the initial singularity and
present the formal general solution for arbitrary initial data. We exemplify
the method by constructing the Kantowski-Sachs cosmological model and a
generalization of it that corresponds to an unpolarized Gowdy model.Comment: Latex, 15 pages, no figure
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