232 research outputs found
Correlation between X-ray flux and rotational acceleration in Vela X-1
The results of a search for correlations between X-ray flux and angular acceleration for the accreting binary pulsar Vela X-1 are presented. Results are based on data obtained with the Hakucho satellite during the interval 1982 to 1984. In undertaking this correlation analysis, it was necessary to modify the usual statistical method to deal with conditions imposed by generally unavoidable satellite observing constraints, most notably a mismatch in sampling between the two variables. The results are suggestive of a correlation between flux and the absolute value of the angular acceleration, at a significance level of 96 percent. The implications of the methods and results for future observations and analysis are discussed
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
Subaru optical observations of the old pulsar PSR B0950+08
We report the B band optical observations of an old (17.5 Myr) radiopulsar
PSR B0950+08 obtained with the Suprime-Cam at the Subaru telescope. We detected
a faint object, B=27.07(16). Within our astrometrical accuracy it coincides
with the radio position of the pulsar and with the object detected earlier by
Pavlov et al. (1996) in UV with the HST/FOC/F130LP. The positional coincidence
and spectral properties of the object suggest that it is the optical
counterpart of PSR B0950+08. Its flux in the B band is two times higher than
one would expect from the suggested earlier Rayleigh-Jeans interpretation of
the only available HST observations in the adjacent F130LP band. Based on the B
and F130LP photometry of the suggested counterpart and on the available X-ray
data we argue in favour of nonthermal origin of the broad-band optical spectrum
of PSR B0950+08, as it is observed for the optical emission of the younger,
middle-aged pulsars PSR B0656+14 and Geminga. At the same time, the optical
efficiency of PSR B0950+08, estimated from its spin-down power and the detected
optical flux, is by several orders of magnitude higher than for these pulsars,
and comparable with that for the much younger and more energetic Crab pulsar.
We cannot exclude the presence of a compact, about 1'', faint pulsar nebula
around PSR B0950+08, elongated perpendicular to the vector of its proper
motion, unless it is not a projection of a faint extended object on the pulsar
position.Comment: 8 pages, LaTeX, aa.cls style, 5 PS figures, submitted to A&A. Image
is available in FITS format at
http://www.ioffe.rssi.ru/astro/NSG/obs/0950-subar
Magnetars as cooling neutron stars with internal heating
We study thermal structure and evolution of magnetars as cooling neutron
stars with a phenomenological heat source in a spherical internal layer. We
explore the location of this layer as well as the heating rate that could
explain high observable thermal luminosities of magnetars and would be
consistent with the energy budget of neutron stars. We conclude that the heat
source should be located in an outer magnetar's crust, at densities rho < 5e11
g/cm^3, and should have the heat intensity of the order of 1e20 erg/s/cm^3.
Otherwise the heat energy is mainly emitted by neutrinos and cannot warm up the
surface.Comment: 8 pages, 5 figures, submitted to MNRA
Suzaku Observation of AXP 1E 1841-045 in SNR Kes 73
Anomalous X-ray pulsars (AXPs) are thought to be magnetars, which are neutron
stars with ultra strong magnetic field of -- G. Their energy
spectra below 10 keV are modeled well by two components consisting of a
blackbody (BB) (0.4 keV) and rather steep power-law (POW) function
(photon index 2-4). Kuiper et al.(2004) discovered hard X-ray component
above 10 keV from some AXPs. Here, we present the Suzaku observation of
the AXP 1E 1841-045 at the center of supernova remnant Kes 73. By this
observation, we could analyze the spectrum from 0.4 to 50 keV at the same time.
Then, we could test whether the spectral model above was valid or not in this
wide energy range. We found that there were residual in the spectral fits when
fit by the model of BB + POW. Fits were improved by adding another BB or POW
component. But the meaning of each component became ambiguous in the
phase-resolved spectroscopy. Alternatively we found that NPEX model fit well
for both phase-averaged spectrum and phase-resolved spectra. In this case, the
photon indices were constant during all phase, and spectral variation seemed to
be very clear. This fact suggests somewhat fundamental meaning for the emission
from magnetars.Comment: To appear in the proceedings of the "40 Years of Pulsars: Millisecond
Pulsars, Magnetars and More" conference, held 12-17 August 2007, in Montreal
QC (AIP, in press, eds: C. Bassa, Z. Wang, A. Cumming, V. Kaspi
Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
The relativistic plasma flows onto neutron stars that are accreting material
from stellar companions can be used to probe strong-field gravity as well as
the physical conditions in the supranuclear-density interiors of neutron stars.
Plasma inhomogeneities orbiting a few kilometres above the stars are observable
as X-ray brightness fluctuations on the millisecond dynamical timescale of the
flows. Two frequencies in the kilohertz range dominate these fluctuations: the
twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the
origins of these oscillations (based on orbital motions) all predict that they
should be related to the stellar spin frequency, but tests have been difficult
because the spins were not unambiguously known. Here we report the detection of
kHz QPOs from a pulsar whose spin frequency is known. Our measurements
establish a clear link between kHz QPOs and stellar spin, but one not predicted
by any current model. A new approach to understanding kHz QPOs is now required.
We suggest that a resonance between the spin and general relativistic orbital
and epicyclic frequencies could provide the observed relation between QPOs and
spin.Comment: Published in the 2003 July 3 issue of Natur
Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered
that disk-accreting neutron stars with weak magnetic fields produce three
distinct types of high-frequency X-ray oscillations. These oscillations are
powered by release of the binding energy of matter falling into the strong
gravitational field of the star or by the sudden nuclear burning of matter that
has accumulated in the outermost layers of the star. The frequencies of the
oscillations reflect the orbital frequencies of gas deep in the gravitational
field of the star and/or the spin frequency of the star. These oscillations can
therefore be used to explore fundamental physics, such as strong-field gravity
and the properties of matter under extreme conditions, and important
astrophysical questions, such as the formation and evolution of millisecond
pulsars. Observations using RossiXTE have shown that some two dozen neutron
stars in low-mass X-ray binary systems have the spin rates and magnetic fields
required to become millisecond radio-emitting pulsars when accretion ceases,
but that few have spin rates above about 600 Hz. The properties of these stars
show that the paucity of spin rates greater than 600 Hz is due in part to the
magnetic braking component of the accretion torque and to the limited amount of
angular momentum that can be accreted in such systems. Further study will show
whether braking by gravitational radiation is also a factor. Analysis of the
kilohertz oscillations has provided the first evidence for the existence of the
innermost stable circular orbit around dense relativistic stars that is
predicted by strong-field general relativity. It has also greatly narrowed the
possible descriptions of ultradense matter.Comment: 22 pages, 7 figures, updated list of sources and references, to
appear in "Short-period Binary Stars: Observation, Analyses, and Results",
eds. E.F. Milone, D.A. Leahy, and D. Hobill (Dordrecht: Springer,
http://www.springerlink.com
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
Gravitational waves from rapidly rotating neutron stars
Rapidly rotating neutron stars in Low Mass X-ray Binaries have been proposed
as an interesting source of gravitational waves. In this chapter we present
estimates of the gravitational wave emission for various scenarios, given the
(electromagnetically) observed characteristics of these systems. First of all
we focus on the r-mode instability and show that a 'minimal' neutron star model
(which does not incorporate exotica in the core, dynamically important magnetic
fields or superfluid degrees of freedom), is not consistent with observations.
We then present estimates of both thermally induced and magnetically sustained
mountains in the crust. In general magnetic mountains are likely to be
detectable only if the buried magnetic field of the star is of the order of
G. In the thermal mountain case we find that gravitational
wave emission from persistent systems may be detected by ground based
interferometers. Finally we re-asses the idea that gravitational wave emission
may be balancing the accretion torque in these systems, and show that in most
cases the disc/magnetosphere interaction can account for the observed spin
periods.Comment: To appear in 'Gravitational Waves Astrophysics: 3rd Session of the
Sant Cugat Forum on Astrophysics, 2014', Editor: Carlos F. Sopuert
Dextral and sinistral Amphidromus inversus (Gastropoda: Pulmonata: Camaenidae) produce dextral sperm
Coiling direction in pulmonate gastropods is determined by a single gene via a maternal effect, which causes cytoskeletal dynamics in the early embryo of dextral gastropods to be the mirror image of the same in sinistral ones. We note that pulmonate gastropod spermatids also go through a helical twisting during their maturation. Moreover, we suspect that the coiling direction of the helical elements of the spermatozoa may affect their behaviour in the female reproductive tract, giving rise to the possibility that sperm chirality plays a role in the maintenance of whole-body chiral dimorphism in the tropical arboreal gastropod Amphidromus inversus (MĂŒller, 1774). For these reasons, we investigated whether there is a relationship between a gastropodâs body chirality and the chirality of the spermatozoa it produces. We found that spermatozoa in A. inversus are always dextrally coiled, regardless of the coiling direction of the animal itself. However, a partial review of the literature on sperm morphology in the Pulmonata revealed that chiral dimorphism does exist in certain species, apparently without any relationship with the coiling direction of the body. Though our study shows that body and sperm chirality follows independent developmental pathways, it gives rise to several questions that may be relevant to the understanding of the chirality of spermatid ultrastructure and spermatozoan motility and sexual selection
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