252 research outputs found
Trans-sonic propeller stage
We follow the approach used by Davies and Pringle (1981) and discuss the
trans-sonic substage of the propeller regime. This substage is intermediate
between the supersonic and subsonic propeller substages. In the trans-sonic
regime an envelope around a magnetosphere of a neutron star passes through a
kind of a reorganization process. The envelope in this regime consists of two
parts. In the bottom one turbulent motions are subsonic. Then at some distance
the turbulent velocity becomes equal to the sound velocity.
During this substage the boundary propagates outwards till it
reaches the outer boundary, and so the subsonic regime starts.
We found that the trans-sonic substage is unstable, so the transition between
supersonic and subsonic substages proceeds on the dynamical time scale. For
realistic parameters this time is in the range from weeks to years.Comment: 8 pages with figures, submitted to Astron. Astroph. Transaction
A new model of a tidally disrupted star
A new semi-analytical model of a star evolving in a tidal field is proposed.
The model is a generalization of the so-called 'affine' stellar model. In our
model the star is composed of elliptical shells with different parameters and
different orientations, depending on time and on the radial Lagrangian
coordinate of the shell. The evolution equations of this model are derived from
the virial relations under certain assumptions, and the integrals of motion are
identified. It is shown that the evolution equations can be deduced from a
variational principle. The evolution equations are solved numerically and
compared quantitatively with the results of 3D numerical computations of the
tidal interaction of a star with a supermassive black hole. The comparison
shows very good agreement between the main ``integral'' characteristics
describing the tidal interaction event in our model and in the 3D computations.
Our model is effectively a one-dimensional Lagrangian model from the point of
view of numerical computations, and therefore it can be evolved numerically
times faster than the 3D approach allows. This makes our model
well suited for intensive calculations covering the whole parameter space of
the problem.Comment: This version is accepted for publication in ApJ. Stylistic and
grammatical changes, new Appendix adde
TRIS II: search for CMB spectral distortions at 0.60, 0.82 and 2.5 GHz
With the TRIS experiment we have performed absolute measurements of the sky
brightness in a sky circle at at the frequencies
0.60, 0.82 and 2.5 GHz. In this paper we discuss the techniques used to
separate the different contributions to the sky emission and give an evaluation
of the absolute temperature of the Cosmic Microwave Background. For the
black-body temperature of the CMB we get: at GHz; at GHz; at
GHz. The first error bar is statistic (1) while the second
one is systematic. These results represent a significant improvement with
respect to the previous measurements. We have also set new limits to the
free-free distortions, ,
and slightly improved the Bose-Einstein upper limit, , both at 95% confidence level.Comment: accepted for publication in The Astrophysical Journa
Nonlinear variations in axisymmetric accretion
We subject the stationary solutions of inviscid and axially symmetric
rotational accretion to a time-dependent radial perturbation, which includes
nonlinearity to any arbitrary order. Regardless of the order of nonlinearity,
the equation of the perturbation bears a form that is similar to the metric
equation of an analogue acoustic black hole. We bring out the time dependence
of the perturbation in the form of a Li\'enard system, by requiring the
perturbation to be a standing wave under the second order of nonlinearity. We
perform a dynamical systems analysis of the Li\'enard system to reveal a saddle
point in real time, whose implication is that instabilities will develop in the
accreting system when the perturbation is extended into the nonlinear regime.
We also model the perturbation as a high-frequency travelling wave, and carry
out a Wentzel-Kramers-Brillouin analysis, treating nonlinearity iteratively as
a very feeble effect. Under this approach both the amplitude and the energy
flux of the perturbation exhibit growth, with the acoustic horizon segregating
the regions of stability and instability.Comment: 15 pages, ReVTeX. Substantially revised with respect to the previous
version. One figure and a new section on travelling waves (Sec. VI) have been
added. The bibliography has been revised. arXiv admin note: substantial text
overlap with arXiv:1207.107
Flux Expulsion - Field Evolution in Neutron Stars
Models for the evolution of magnetic fields of neutron stars are constructed,
assuming the field is embedded in the proton superconducting core of the star.
The rate of expulsion of the magnetic flux out of the core, or equivalently the
velocity of outward motion of flux-carrying proton-vortices is determined from
a solution of the Magnus equation of motion for these vortices. A force due to
the pinning interaction between the proton-vortices and the neutron-superfluid
vortices is also taken into account in addition to the other more conventional
forces acting on the proton-vortices. Alternative models for the field
evolution are considered based on the different possibilities discussed for the
effective values of the various forces. The coupled spin and magnetic evolution
of single pulsars as well as those processed in low-mass binary systems are
computed, for each of the models. The predicted lifetimes of active pulsars,
field strengths of the very old neutron stars, and distribution of the magnetic
fields versus orbital periods in low-mass binary pulsars are used to test the
adopted field decay models. Contrary to the earlier claims, the buoyancy is
argued to be the dominant driving cause of the flux expulsion, for the single
as well as the binary neutron stars. However, the pinning is also found to play
a crucial role which is necessary to account for the observed low field binary
and millisecond pulsars.Comment: 23 pages, + 7 figures, accepted for publication in Ap
Theory of high-energy emission from the pulsar/Be-star system PSR 125963 I: radiation mechanisms and interaction geometry
We study the physical processes of the PSR B1259-63 system containing a 47 ms
pulsar orbiting around a Be star in a highly eccentric orbit. Motivated by the
results of a multiwavelength campaign during the January 1994 periastron
passage of PSR B1259-63, we discuss several issues regarding the mechanism of
high-energy emission. Unpulsed power law emission from the this system was
detected near periastron in the energy range 1-200 keV. We find that the
observed high energy emission from the PSR B1259-63 system is not compatible
with accretion or propeller-powered emission. Shock-powered high-energy
emission produced by the pulsar/outflow interaction is consistent with all high
energy observations. By studying the evolution of the pulsar cavity we
constrain the magnitude and geometry of the mass outflow outflow of the Be
star. The pulsar/outflow interaction is most likely mediated by a collisionless
shock at the internal boundary of the pulsar cavity. The system shows all the
characteristics of a {\it binary plerion} being {\it diffuse} and {\it compact}
near apastron and periastron, respectively. The PSR B1259-63 cavity is subject
to different radiative regimes depending on whether synchrotron or inverse
Compton (IC) cooling dominates the radiation of electron/positron pairs
advected away from the inner boundary of the pulsar cavity. The highly
non-thermal nature of the observed X-ray/gamma-ray emission near periastron
establishes the existence of an efficient particle acceleration mechanism
within a timescale shown to be less than s. A synchrotron/IC
model of emission of e\pm-pairs accelerated at the inner shock front of the
pulsar cavity and adiabatically expanding in the MHD flow provides an excellent
explanation of the observed time variableX-ray flux and spectrum from the PSRComment: 68 pages, accepted for publication in the Astrophys. J. on Aug. 26,
199
Magnetically Accreting Isolated Old Neutron Stars
Previous work on the emission from isolated old neutron stars (IONS)
accreting the inter-stellar medium (ISM) focussed on gravitational capture -
Bondi accretion. We propose a new class of sources which accrete via magnetic
interaction with the ISM. While for the Bondi mechanism, the accretion rate
decreases with increasing NS velocity, in magnetic accretors (MAGACs="magics")
the accretion rate increases with increasing NS velocity. MAGACs will be
produced among high velocity (~> 100 km s-1) high magnetic field (B> 1e14 G)
radio pulsars - the ``magnetars'' - after they have evolved first through
magnetic dipole spin-down, followed by a ``propeller'' phase (when the object
sheds angular momentum on a timescale ~< 1e10 yr). The properties of MAGACS may
be summarized thus: dipole magnetic fields of B~>1e14 G; minimum velocities
relative to the ISM of >25-100 km s-1, depending on B, well below the median in
the observed radio-pulsar population; spin-periods of >days to years; accretion
luminosities of 1e28- 1e31 ergs s-1 ; and effective temperatures kT=0.3 - 2.5
keV if they accrete onto the magnetic polar cap. We find no examples of MAGACs
among previously observed source classes (anomalous X-ray pulsars,
soft-gamma-ray repeaters or known IONS). However, MAGACs may be more prevelant
in flux-limited X-ray catalogs than their gravitationally accreting
counterparts.Comment: ApJ, accepte
On the Nature of Part Time Radio Pulsars
The recent discovery of rotating radio transients and the quasi-periodicity
of pulsar activity in the radio pulsar PSR B193124 has challenged the
conventional theory of radio pulsar emission. Here we suggest that these
phenomena could be due to the interaction between the neutron star
magnetosphere and the surrounding debris disk. The pattern of pulsar emission
depends on whether the disk can penetrate the light cylinder and efficiently
quench the processes of particle production and acceleration inside the
magnetospheric gap. A precessing disk may naturally account for the
switch-on/off behavior in PSR B193124.Comment: 9 pages, accepted to ApJ
The role of the outer boundary condition in accretion disk models: theory and application
The influence of the outer boundary condition (OBC) on the dynamics and
radiation of optically thin accretion flow is investigated. Bremsstrahlung and
synchrotron radiations amplified by Comptonization are taken into account and
two-temperature plasma assumption is adopted. The three OBCs we adopted are the
temperatures of the electrons and ions and the specific angular momentum of the
accretion flow at a certain outer boundary. We find that when the general
parameters such as the mass accretion rate and the viscous parameter are fixed,
the peak flux at various bands such as radio, IR and X-ray, can differ by as
large as several orders of magnitude under different OBCs in our example. Our
results indicate that OBC is both dynamically and radiatively important
therefore should be regarded as a new ``parameter'' in accretion disk models.
We apply the above results to the compact radio source Sgr A* and find that the
discrepancy between the mass accretion rate favored by ADAF models in the
literature and that favored by the three dimensional hydrodynamical simulation
can be naturally resolved by seriously considering the outer boundary condition
of the accretion flow.Comment: 23 pages, 9 figures,accepted by the Astrophysical Journa
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