Sinking of a magnetically confined mountain on an accreting neutron star

We perform ideal-magnetohydrodynamic axisymmetric simulations of magnetically confined mountains on an accreting neutron star, with masses less than ~0.12 solar masses. We consider two scenarios, in which the mountain sits atop a hard surface or sinks into a soft, fluid base. We find that the ellipticity of the star, due to a mountain grown on a hard surface, approaches ~2e-4 for accreted masses greater than ~1.2e-3 solar masses, and that sinking reduces the ellipticity by between 25% and 60%. The consequences for gravitational radiation from low-mass x-ray binaries are discussed.Comment: 13 pages, 12 figures, and 3 tables; accepted for publication in MNRA

Gravitational Radiation from Accreting Millisecond Pulsars

It is widely assumed that the observed reduction of the magnetic field of millisecond pulsars can be connected to the accretion phase during which the pulsar is spun up by mass accretion from a companion. A wide variety of reduction mechanisms have been proposed, including the burial of the field by a magnetic mountain, formed when the accreted matter is confined to the poles by the tension of the stellar magnetic field. A magnetic mountain effectively screens the magnetic dipole moment. On the other hand, observational data suggests that accreting neutron stars are sources of gravitational waves, and magnetic mountains are a natural source of a time-dependent quadrupole moment. We show that the emission is sufficiently strong to be detectable by current and next generation long-baseline interferometers. Preliminary results from fully three-dimensional magnetohydrodynamic (MHD) simulations are presented. We find that the initial axisymmetric state relaxes into a nearly axisymmetric configuration via toroidal magnetic modes. A substantial quadrupole moment is still present in the final state, which is stable (in ideal MHD) yet highly distorted.Comment: Proceedings of the 11th Marcel Grossmann Meeting on General Relativity, World Scientific 200

Continuous frequency spectrum of the global hydromagnetic oscillations of a magnetically confined mountain on an accreting neutron star

We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD) frequency spectrum of a polar mountain produced by magnetic burial on an accreting neutron star. Applying the formalism developed by Hellsten & Spies (1979), extended to include gravity, we solve the singular eigenvalue problem subject to line-tying boundary conditions. This spectrum divides into an Alfv\'{e}n part and a cusp part. The eigenfunctions are chirped and anharmonic with an exponential envelope, and the eigenfrequencies cover the whole spectrum above a minimum $\omega_\mathrm{low}$. For equilibria with accreted mass 1.2 \times 10^{-6} \la M_a/M_\odot \la 1.7 \times 10^{-4} and surface magnetic fields 10^{11} \la B_\ast/\mathrm{G} \la 10^{13}, $\omega_\mathrm{low}$ is approximately independent of $B_\ast$, and increases with $M_a$. The results are consistent with the Alfv\'{e}n spectrum excited in numerical simulations with the \textsc{zeus-mp} solver. The spectrum is modified substantially by the Coriolis force in neutron stars spinning faster than $\sim 100$ Hz. The implications for gravitational wave searches for low-mass X-ray binaries are considered briefly.Comment: accepted by MNRA

Improved estimate of the detectability of gravitational radiation from a magnetically confined mountain on an accreting neutron star

We give an improved estimate of the detectability of gravitational waves from magnetically confined mountains on accreting neutron stars. The improved estimate includes the following effects for the first time: three-dimensional hydromagnetic ("fast") relaxation, three-dimensional resistive ("slow") relaxation, realistic accreted masses M_a \la 2 \times 10^{-3} M_\odot, (where the mountain is grown ab initio by injection), and verification of the curvature rescaling transformation employed in previous work. Typically, a mountain does not relax appreciably over the lifetime of a low-mass X-ray binary. The ellipticity reaches $\epsilon \approx 2 \times 10^{-5}$ for $M_a=2\times 10^{-3} M_\odot$. The gravitational wave spectrum for triaxial equilibria contains an additional line, which, although weak, provides valuable information about the mountain shape. We evaluate the detectability of magnetic mountains with Initial and Advanced LIGO. For a standard, coherent matched filter search, we find a signal-to-noise ratio of $d = 28 (M_a/10^{-4} M_\odot) (1+5.5 M_a/10^{-4} M_\odot)^{-1} (D/10 \mathrm{kpc})^{-1} (T_0/14 \mathrm{d})^{1/2}$ for Initial LIGO, where $D$ is the distance and $T_0$ is the observation time. From the nondetection of gravitational waves from low-mass X-ray binaries to date, and the wave strain limits implied by the spin frequency distribution of these objects (due to gravitational wave braking), we conclude that there are other, as yet unmodelled, physical effects that further reduce he quadrupole moment of a magnetic mountain, most notably sinking into the crust.Comment: accepted by MNRA

Typpilannoituksen vaikutus ansarijauhiaisen (Trialeurodes vaporariorum) kuolleisuuteen ja kehitysaikaan tomaatilla (Solanum lycopersicum)

Tutkimuksen tavoitteena oli selvittää, voisiko tomaatin (Solanum lycopersicum) typpilannoitusta vähentämällä hidastaa ansarijauhiaisen (Trialeurodes vaporariorum) populaation kasvunopeutta ja siten parantaa biologisen torjunnan tehokkuutta. Ansarijauhiainen on merkittävä kasvihuonetuholainen, jota torjutaan yleisesti jauhiaiskiilukaisella (Encarsia formosa), mutta torjunnan tehokkuus ei aina ole riittävä, jos ansarijauhiaisen populaation kasvunopeus on suurempi kuin jauhiaiskiilukaisella. Aiemmissa tutkimuksissa ansarijauhiaisen on havaittu hyötyvän tomaatin typpilannoituksesta. Tutkimuksessa jauhiaisia munitettiin vuorokauden ajan klipsihäkeissä tomaateilla, joille annettiin typpipitoisuudeltaan neljää erilaista lannoiteliuosta. Lannoiteliuosten typpipitoisuus muuttui tomaatin kehitysvaiheen perusteella kaupallisten viljelmien käytäntöjen mukaisesti. Jauhiaisten kuolleisuus ja kehitysaika munasta aikuiseksi määritettiin. Kehitysaika laskettiin kummallekin sukupuolelle erikseen. Kuoriutuneista naaraista osa munitettiin kahden viikon ajan gelatiinikapseleista valmistetuissa häkeissä muninnan määrän selvittämiseksi. Typpilannoituksen vaikutusta tomaatin typpipitoisuuteen arvioitiin jauhiaislehtien klorofylli-indeksien perusteella. Tulosten perusteella typpilannoituksella ei ollut merkitsevää vaikutusta jauhiaisten kuolleisuuteen ja kehitysaikaan eikä jauhiaislehtien klorofyllipitoisuuksiin. Yllättävänä havaintona naaraat kuoriutuivat koiraita aikaisemmin kaikilla typpitasoilla: hyönteisillä on yleisempää, että pienikokoisemmat koiraat kuoriutuvat ensin. Kokeen toinen vaihe epäonnistui, eikä muninnan määrää per naaras saatu analysoitua. Näyttäisi siltä, että typpilannoituksen tulisi olla huomattavasti nykykäytäntöä vähäisempi, jotta sillä voisi hillitä jauhiaispopulaation kasvunopeutta. Tämä tulisi kuitenkin varmistaa lisäkokein, ja samalla selvittää vähennetyn typpilannoituksen vaikutukset jauhiaiskiilukaisen torjuntatehoon sekä mahdolliset satovaikutukset. Myös sukupuolten välistä kehitysaikojen eroa olisi tutkittava lisää, jotta saataisiin selville taustalla vaikuttavat syyt tai mekanismit

Relating the Kick Velocities of Young Pulsars with Magnetic Field Growth Timescales Inferred From Braking Indices

A nascent neutron star may be exposed to fallback accretion soon after the proto-neutron star stage. This high accretion episode can submerge the magnetic field deep in the crust. The diffusion of the magnetic field back to the surface will take hundreds to millions of years depending on the amount of mass accreted and the consequent depth the field is buried. Neutron stars with large kick velocities will accrete less amount of fallback material leading to shallower submergence of their fields and shorter time-scales for the growth of their fields. We obtain the relation $\tau_{\rm Ohm} \propto v^{-1}$ between the space velocity of the neutron star and Ohmic time-scale for the growth of the magnetic field. We compare this with the relation between the measured transverse velocities, $v_{\perp}$ and the field growth time-scales, $\mu/\dot{\mu}$, inferred from the measured braking indices. We find that the observational data is consistent with the theoretical prediction though the small number of data precludes a strong conclusion. Measurement of the transverse velocities of pulsars B1509$-$58, J1846$-$0258, J1119$-$6127 and J1734$-$3333 would increase the number of the data and strongly contribute to understanding whether pulsar fields grow following fallback accretion.Comment: Accepted to MNRAS Letters. Title and abstract are change

Tracing colliding winds in the UV line orbital variability of gamma-ray binaries

Gamma-ray binaries emit most of their radiated power beyond ~10 MeV. The non-thermal emission is thought to arise from the interaction of the relativistic wind of a rotation-powered pulsar with the stellar wind of its massive (O or Be) companion star. A powerful pulsar creates an extended cavity, filled with relativistic electrons, in the radiatively-driven wind of the massive star. As a result, the observed P Cyg profiles of UV resonant lines from the stellar wind should be different from those of single massive stars. We propose to use UV emission lines to detect and constrain the colliding wind region in gamma-ray binaries. We compute the expected orbital variability of P Cyg profiles depending upon the interaction geometry (set by the ratio of momentum fluxes from the winds) and the line-of-sight to the system. We predict little or no variability for the case of LS 5039 and PSR B1259-63, in agreement with currently available HST observations of LS 5039. However, variability between superior and inferior conjunction is expected in the case of LS I+61 303.Comment: Accepted for publication in MNRA

Pulsar timing irregularities and the imprint of magnetic field evolution

(Abridged) The rotational evolution of isolated neutron stars is dominated by the magnetic field anchored to the solid crust of the star. Assuming that the core field evolves on much longer timescales, the crustal field evolves mainly though Ohmic dissipation and the Hall drift, and it may be subject to relatively rapid changes with remarkable effects on the observed timing properties. We investigate whether changes of the magnetic field structure and strength during the star evolution may have observable consequences in the braking index, which is the most sensitive quantity to reflect small variations of the timing properties that are caused by magnetic field rearrangements. By performing axisymmetric, long-term simulations of the magneto-thermal evolution of neutron stars with state-of-the-art microphysical inputs, we find that the effect of the magnetic field evolution on the braking index can be divided into three qualitatively different stages depending on the age and the internal temperature: a first stage that may be different for standard pulsars (with n~3) or low field neutron stars that accreted fallback matter during the supernova explosion (systematically n<3); in a second stage, the evolution is governed by almost pure Ohmic field decay, and a braking index n>3 is expected; in the third stage, at late times, when the interior temperature has dropped to very low values, Hall oscillatory modes in the neutron star crust result in braking indices of high absolute value and both positive and negative signs. Models with strong (1e14 G) multipolar or toroidal components, even with a weak (~1e12 G) dipolar field are consistent with the observed trend of the timing properties.Comment: 7 pages, 5 figures, accepted for publication in Astronomy & Astrophysics (submitted July 24, 2012

Phase dependent view of Cyclotron lines from model accretion mounds on Neutron Stars

In this paper we make a phase dependent study of the effect of the distortion of local magnetic field due to confinement of accreted matter in X-ray pulsars on the cyclotron spectra emitted from the hotspot . We have numerically solved the Grad-Shafranov equation for axisymmetric static MHD equilibria of matter confined at the polar cap of neutron stars. From our solution we model the cyclotron spectra that will be emitted from the region, using a simple prescription and integrating over the entire mound. Radiative transfer through the accretion column overlying the mound may significantly modify the spectra in comparison to those presented here. However we ignore this in the present paper in order to expose the effects directly attributable to the mound itself. We perform a spin phase dependent analysis of the spectra to study the effect of the viewing geometry.Comment: 13 pages, 19 figure