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 $\gamma$ 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 gif figure

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 $L_X = 7\times 10^{39}$~erg~s$^{-1}-1\times 10^{41}$~erg~s$^{-1}$ 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 $\sim 10^{36-37}$~erg~s$^{-1}$, 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