347 research outputs found
A note on the cyclic evolution of the pulsar magnetosphere
Positive and negative pulsar breaking indices suggest that some fraction of
the pulsar spindown torque undergoes a cyclic evolution. The observed strong
correlation of `anomalous' breaking indices with pulsar age implies that the
characteristic periodicity timescale is in the range 100 to 10,000 years
depending on the fraction of the spindown torque that undergoes cyclic
evolution, 1 to 100% respectively. We argue that the longest variability
timescale is consistent with a neutron star magnetic cycle similar to the solar
cycle.Comment: Accepted for publication in Astronomy & Astrophysic
The non-integrability of the Zipoy-Voorhees metric
The low frequency gravitational wave detectors like eLISA/NGO will give us
the opportunity to test whether the supermassive compact objects lying at the
centers of galaxies are indeed Kerr black holes. A way to do such a test is to
compare the gravitational wave signals with templates of perturbed black hole
spacetimes, the so-called bumpy black hole spacetimes. The Zipoy-Voorhees (ZV)
spacetime (known also as the spacetime) can be included in the bumpy
black hole family, because it can be considered as a perturbation of the
Schwarzschild spacetime background. Several authors have suggested that the ZV
metric corresponds to an integrable system. Contrary to this integrability
conjecture, in the present article it is shown by numerical examples that in
general ZV belongs to the family of non-integrable systems.Comment: 10 pages, 13 figure
How to observe a non-Kerr spacetime
We present a generic criterion which can be used in gravitational-wave data
analysis to distinguish an extreme-mass-ratio inspiral into a Kerr background
spacetime from one into a non-Kerr background spacetime. The criterion exploits
the fact that when an integrable system, such as the system that describes
geodesic orbits in a Kerr spacetime, is perturbed, the tori in phase space
which initially corresponded to resonances disintegrate so as to form the so
called Birkhoff chains on a surface of section, according to the
Poincar\'{e}-Birkhoff theorem. The KAM curves of these islands in such a chain
share the same ratio of frequencies, even though the frequencies themselves
vary from one KAM curve to another inside an island. On the other hand, the KAM
curves, which do not lie in a Birkhoff chain, do not share this characteristic
property. Such a temporal constancy of the ratio of frequencies during the
evolution of the gravitational-wave signal will signal a non-Kerr spacetime
which could then be further explored.Comment: 4 pages, 2 figure
The pulsar synchrotron: coherent radio emission
We propose a simple physical picture for the generation of coherent radio
emission in the axisymmetric pulsar magnetosphere that is quite different from
the canonical paradigm of radio emission coming from the magnetic polar caps.
In this first paper we consider only the axisymmetric case of an aligned
rotator. Our picture capitalizes on an important element of the MHD
representation of the magnetosphere, namely the separatrix between the
corotating closed-line region (the `dead zone') and the open field lines that
originate in the polar caps. Along the separatrix flows the return current that
corresponds to the main magnetospheric electric current emanating from the
polar caps. Across the separatrix, both the toroidal and poloidal components of
the magnetic field change discontinuously. The poloidal component discontinuity
requires the presence of a significant annular electric current which has up to
now been unaccounted for. We estimate the position and thickness of this
annular current at the tip of the closed line region, and show that it consists
of electrons (positrons) corotating with Lorentz factors on the order of 10^5,
emitting incoherent synchrotron radiation that peaks in the hard X-rays. These
particles stay in the region of highest annular current close to the equator
for a path-length of the order of one meter. We propose that, at wavelengths
comparable to that path-length, the particles emit coherent radiation, with
radiated power proportional to N^2, where N is the population of particles in
the above path-length. We calculate the total radio power in this wavelength
regime and its scaling with pulsar period and stellar magnetic field and show
that it is consistent with estimates of radio luminosity based on observations.Comment: Monthly Notices Letters, in pres
The role of reconnection in the pulsar magnetosphere
The present work is our first attempt to understand the role of reconnection
in the pulsar magnetosphere. Our discussion is based on the observationaly
infered fact that, as the pulsar spins down, the region of closed corotating
dipolar field lines grows with time. This implies that reconnection must take
place in the magnetosphere. We argue that non-dissipative reconnection along
the equatorial current sheet allows for the continuous channeling of pulsar
spindown energy into particle energy, all the way from the light cylinder to
the pulsar wind termination shock, and we propose that this effect may account
for the low \sigma values inferred by observations. We present a simple model
that allows us to relate the magnetic diffusivity in the equatorial current
sheet to an observable pulsar parameter, the braking index n. When n~1, the
global structure of the magnetosphere approaches that of a relativistic split
monopole where the pulsar spindown energy is carried by the electromagnetic
field. However, for values of n>1.5, almost all field lines close inside the
pulsar wind termination shock, and thus most of the electromagnetic pulsar
spindown energy flux is effectively transformed into particle energy in the
equatorial current sheet
The magnetic field topology in the reconnecting pulsar magnetosphere
We show that toroidal magnetic field annihilation in the equatorial current
sheet of the pulsar magnetosphere is related to how fast poloidal magnetic
field lines close as we move away from the light cylinder. This determines the
reconnection radial electric field which directly accelerates particles in the
neutral layer inside the equatorial current sheet. The efficiency of poloidal
magnetic field closure near the light cylinder may be measurable through the
pulsar braking index. We argue that, the lower the efficiency of pair
formation, the higher the braking index. We also argue that synchrotron
radiation reaction in the neutral layer does not inhibit the accelerated
particles from reaching the maximum energy of about 10^16 eV available in the
open pulsar magnetosphere.Comment: Accepted for publication in Astronomy and Astrophysic
The Cosmic Battery and the Inner Edge of the Accretion Disk
The Poynting-Robertson Cosmic Battery proposes that the innermost part of the
accretion disk around a black hole is threaded by a large scale dipolar
magnetic field generated in situ, and that the return part of the field
diffuses outward through the accretion disk. This is different from the
scenario that the field originates at large distances and is carried inward by
the accretion flow. In view of the importance of large scale magnetic fields in
regulating the processes of accretion and outflows, we study the stability of
the inner edge of a magnetized disk in general relativity when the distribution
of the magnetic field is the one predicted by the Poynting-Robertson Cosmic
Battery. We found that as the field grows, the inner edge of the disk gradually
moves outward. In a fast spinning black hole with a>0.8M the inner edge moves
back in towards the black hole horizon as the field grows beyond some threshold
value. In all cases, the inner part of the disk undergoes a dramatic structural
change as the field approaches equipartition.Comment: 7 pages, 3 figures, accepted for publication in Monthly Notices of
the RA
Populating Stellar Orbits Inside a Rotating, Gaseous Bar
In an effort to better understand the formation and evolution of barred
galaxies, we have examined the properties of equatorial orbits in the effective
potential of one model of a rapidly rotating, steady-state gas-dynamical bar
that has been constructed via a self-consistent hydrodynamical simulation.
Using a ``Restriction Hypothesis'' to determine initial conditions, we find
that a significant fraction of orbits in this potential are quasi-ergodic and
that regular orbits have a ``bowtie'' shape in contrast to the more typical x1
orbits. This bowtie orbit should give a boxy-peanut shape to such systems.Comment: Accepted for publication in The Astrophysical Journal; 29 pages, 29
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The Great Pretenders Among the ULX Class
The recent discoveries of pulsed X-ray emission from three ultraluminous
X-ray (ULX) sources have finally enabled us to recognize a subclass within the
ULX class: the great pretenders, neutron stars (NSs) that appear to emit X-ray
radiation at isotropic luminosities ~erg~s~erg~s only because their emissions are strongly beamed toward
our direction and our sight lines are offset by only a few degrees from their
magnetic-dipole axes. The three known pretenders appear to be stronger emitters
than the presumed black holes of the ULX class, such as Holmberg II \& IX X-1,
IC10 X-1, and NGC300 X-1. For these three NSs, we have adopted a single
reasonable assumption, that their brightest observed outbursts unfold at the
Eddington rate, and we have calculated both their propeller states and their
surface magnetic-field magnitudes. We find that the results are not at all
different from those recently obtained for the Magellanic Be/X-ray pulsars: the
three NSs reveal modest magnetic fields of about 0.3-0.4~TG and beamed
propeller-line X-ray luminosities of ~erg~s,
substantially below the Eddington limit.Comment: To appear in Research in Astronomy and Astrophysic
Force-free magnetosphere of an aligned rotator with differential rotation of open magnetic field lines
Here we briefly report on results of self-consistent numerical modeling of a
differentially rotating force-free magnetosphere of an aligned rotator. We show
that differential rotation of the open field line zone is significant for
adjusting of the global structure of the magnetosphere to the current density
flowing through the polar cap cascades. We argue that for most pulsars
stationary cascades in the polar cap can not support stationary force-free
configurations of the magnetosphere.Comment: 5 pages, 4 figures. Presented at the conference "Isolated Neutron
Stars: from the Interior to the Surface", London, April 24-28, 2006; to
appear in Astrophysics and Space Science. Significantly revised version, a
mistake found by ourselfs in the numerical code was corrected, all presented
results are obtained with the correct version of the cod
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