320 research outputs found
Superfluid Friction and Late-time Thermal Evolution of Neutron Stars
The recent temperature measurements of the two older isolated neutron stars
PSR 1929+10 and PSR 0950+08 (ages of and yr,
respectively) indicate that these objects are heated. A promising candidate
heat source is friction between the neutron star crust and the superfluid it is
thought to contain. We study the effects of superfluid friction on the
long-term thermal and rotational evolution of a neutron star. Differential
rotation velocities between the superfluid and the crust (averaged over the
inner crust moment of inertia) of rad s for PSR
1929+10 and rad s for PSR 0950+08 would account for their
observed temperatures. These differential velocities could be sustained by
pinning of superfluid vortices to the inner crust lattice with strengths of
1 MeV per nucleus. Pinned vortices can creep outward through thermal
fluctuations or quantum tunneling. For thermally-activated creep, the coupling
between the superfluid and crust is highly sensitive to temperature. If pinning
maintains large differential rotation ( rad s), a feedback
instability could occur in stars younger than yr causing
oscillations of the temperature and spin-down rate over a period of . For stars older than yr, however, vortex creep occurs
through quantum tunneling, and the creep velocity is too insensitive to
temperature for a thermal-rotational instability to occur. These older stars
could be heated through a steady process of superfluid friction.Comment: 26 pages, 1 figure, submitted to Ap
Pulse Frequency Fluctuations of Magnetars
Using \emph{RXTE}, \emph{Chandra}, \emph{XMM-Newton} and \emph{Swift}
observations, we for the first time construct the power spectra and torque
noise strengths of magnetars. For some of the sources, we measure strong red
noise on timescales months to years which might be a consequence of their
outbursts. We compare noise strengths of magnetars with those of radio pulsars
by investigating possible correlations of noise strengths with spin-down rate,
magnetic field and age. Using these correlations, we find that magnetar noise
strengths are obeying similar trends with radio pulsars. On the contrary, we do
not find any correlation between noise strength and X-ray luminosity which was
seen in accretion powered pulsars. Our findings suggest that the noise
behaviour of magnetars resembles that of radio pulsars but they possess higher
noise levels likely due to their stronger magnetic fields.Comment: 18 pages, 1 table, 4 figures, accepted for publication in MNRA
Spatial variability of precipitation regimes over Turkey
Turkish annual precipitation regimes are analysed to provide large-scale perspective and redefine precipitation regions. Monthly total precipitation data are employed for 107 stations (1963–2002). Precipitation regime shape (seasonality) and magnitude (size) are classified using a novel multivariate methodology. Six shape and five magnitude classes are identified, which exhibit clear spatial structure. A composite (shape and magnitude) regime classification reveals dominant controls on spatial variability of precipitation. Intra-annual timing and magnitude of precipitation is highly variable due to seasonal shifts in Polar and Subtropical zones and physiographic factors. Nonetheless, the classification methodology is shown to be a powerful tool that identifies physically-interpretable precipitation regions: (1) coastal regimes for Marmara, coastal Aegean, Mediterranean and Black Sea; (2) transitional regimes in continental Aegean and Southeast Anatolia; and (3) inland regimes across central and Eastern Anatolia. This research has practical implications for understanding water resources, which are under ever growing pressure in Turkey
Quadratic Curvature Gravity with Second Order Trace and Massive Gravity Models in Three Dimensions
The quadratic curvature lagrangians having metric field equations with second
order trace are constructed relative to an orthonormal coframe. In
dimensions, pure quadratic curvature lagrangian having second order trace
constructed contains three free parameters in the most general case. The fourth
order field equations of some of these models, in arbitrary dimensions, are
cast in a particular form using the Schouten tensor. As a consequence, the
field equations for the New massive gravity theory are related to those of the
Topologically massive gravity. In particular, the conditions under which the
latter is "square root" of the former are presented.Comment: 24 pages, to appear in GR
Gravitational waves from inspiralling compact binaries with magnetic dipole moments
We investigate the effects of the magnetic dipole-dipole coupling and the
electromagnetic radiation on the frequency evolution of gravitational waves
from inspiralling binary neutron stars with magnetic dipole moments. This study
is motivated by the discovery of the superstrongly magnetized neutron stars,
i.e., magnetar. We derive the contributions of the magnetic fields to the
accumulated cycles in gravitational waves as , where denotes the strength of the polar magnetic
fields of each neutron star in the binary system. It is found that the effects
of the magnetic fields will be negligible for the detection and the parameter
estimation of gravitational waves, if the upper limit for magnetic fields of
neutron stars are less than G, which is the maximum magnetic
field observed in the soft gamma repeaters and the anomalous X-ray pulsars up
to date. We also discuss the implications of electromagnetic radiation from the
inspiralling binary neutron stars for the precursory X-ray emission prior to
the gamma ray burst observed by the Ginga satellite.Comment: 15 pages, no figures, accepted for publication in Ap
Hopkins Ultraviolet Telescope Observations of Her X-1
We have obtained a far-ultraviolet spectrum of the X-ray binary Hercules
X-1/HZ Herculis using the Hopkins Ultraviolet Telescope aboard the Astro-1
space shuttle mission in 1990 December. This is the first spectrum of Her X-1
that extends down to the Lyman limit at 912 A. We observed emission lines of O
VI, N V, and C IV, and the far UV continuum extending to the Lyman limit. We
examine the conditions of the emitting gas through line strengths, line ratios,
and doublet ratios. The UV flux is lower by about a factor of 2 than expected
at the orbital phase of the observation. We model the UV continuum with a
simple power-law and with a detailed model of an X-ray-illuminated accretion
disk and companion star. The power-law provides a superior fit, as the detailed
model predicts too little flux below 1200 A. We note, however, that there are
uncertainties in the interstellar reddening, in the background airglow
spectrum, and in the long-term phase of the accretion disk. We have searched
the data for UV line and continuum pulsations near the neutron star spin period
but found none at a detectable level.Comment: 19 pages, 5 figures, accepted by the Astrophysical Journa
A Unified Approach to Variational Derivatives of Modified Gravitational Actions
Our main aim in this paper is to promote the coframe variational method as a
unified approach to derive field equations for any given gravitational action
containing the algebraic functions of the scalars constructed from the Riemann
curvature tensor and its contractions. We are able to derive a master equation
which expresses the variational derivatives of the generalized gravitational
actions in terms of the variational derivatives of its constituent curvature
scalars. Using the Lagrange multiplier method relative to an orthonormal
coframe, we investigate the variational procedures for modified gravitational
Lagrangian densities in spacetime dimensions . We study
well-known gravitational actions such as those involving the Gauss-Bonnet and
Ricci-squared, Kretchmann scalar, Weyl-squared terms and their algebraic
generalizations similar to generic theories and the algebraic
generalization of sixth order gravitational Lagrangians. We put forth a new
model involving the gravitational Chern-Simons term and also give three
dimensional New massive gravity equations in a new form in terms of the Cotton
2-form
X-ray Flux Related Timing and Spectral Features of 2S 1417-62
RXTE observations of the X-ray transient pulsar 2S 1417-62 between 1999
November and 2000 August with a total exposure of ksec were
analyzed. Observations include a main outburst followed by a series of mini
outbursts. Changes in pulse morphology and pulse fraction were found to be
related to the changes in X-ray flux. Particularly low X-ray flux regions were
found to have significantly lower pulse fractions with different pulse
morphologies. The 3-60 keV PCA-HEXTE main outburst spectrum was modeled with an
absorbed power law model with high energy cut-off and a Gaussian Iron line
complex feature. Using the same spectral model, individual 3-20 keV PCA spectra
were found to be softer and less absorbed in low X-ray flux regions between
outbursts. Spectral studies showed that hydrogen column density was correlated,
and the power law index was anti-correlated with the 3-20 keV X-ray flux. X-ray
flux related spectral and timing features in 2S 1417-62 except for low X-ray
flux regions were interpreted as a sign of disc accretion with a similar
accretion geometry with a varying mass accretion rate (), whereas
spectral and timing features of the low X-ray flux regions were interpreted as
a sign of possible temporary accretion geometry change prior to the next
periastron where increases again to restore the original accretion
geometry.Comment: 14 pages, 6 figures, accepted for publication in MNRA
Probing stellar winds and accretion physics in high-mass X-ray binaries and ultra-luminous X-ray sources with LOFT
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of high-mass X-ray binaries and
ultra-luminous X-ray sources. For a summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timing. (v2 few typos corrected
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