Burial of the polar magnetic field of an accreting neutron star – I. Self-consistent analytic and numerical equilibria

Payne, D. J. B.; Melatos, A.

doi:10.1111/j.1365-2966.2004.07798.x

Cited by 133 publications

(285 citation statements)

References 32 publications

(116 reference statements)

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“…However, the situation can be different in cases of presently-accreting millisecond pulsars, in which temporarily enhanced accretion events can lead to temporary burial of parts of the stellar magnetic flux, which can diffuse back much faster. Accretion through the funnel streams leads to enhanced accretion towards the magnetic poles (Lovelace, Romanova & Bisnovatyi-Kogan 2005) and to formation of temporary polar 'mountains' at the poles (Payne & Melatos 2004, while the magnetic flux is pushed towards the equator. This field will be redistributed at later times, after a thermonuclear burst.…”

Section: Discussion Of Phase Shifts In Accreting Millisecond Pulsars mentioning

confidence: 99%

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

2012

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Recent measurements of the surface magnetic fields of classical T Tauri stars (CTTSs) and magnetic cataclysmic variables show that their magnetic fields have a complex structure. Investigation of accretion onto such stars requires global three-dimensional (3D) magnetohydrodynamic (MHD) simulations, where the complexity of simulations strongly increases with each higher-order multipole. Previously, we were able to model disc accretion onto stars with magnetic fields described by a superposition of dipole and quadrupole moments. However, in some stars, like CTTS V2129 Oph and BP Tau, the octupolar component is significant and it was necessary to include the next octupolar component. Here, we show results of global 3D MHD simulations of accretion onto stars with superposition of the dipole and octupole fields, where we vary the ratio between components. Simulations show that if octupolar field strongly dominates at the disc-magnetosphere boundary, then matter flows into the ring-like octupolar poles, forming ring-shape spots at the surface of the star above and below equator. The light-curves are complex and may have two peaks per period. In case where the dipole field dominates, matter accretes in two ordered funnel streams towards poles, however the polar spots are meridionally-elongated due to the action of the octupolar component. In the case when the fields are of similar strengths, both, polar and belt-like spots are present. In many cases the light-curves show the evidence of complex fields, excluding the cases of small inclinations angles, where sinusoidal light-curve is observed and 'hides' the information about the field complexity.We also propose new mechanisms of phase shift in stars with complex magnetic fields. We suggest that the phase shifts can be connected with: (1) temporal variation of the star's intrinsic magnetic field and subsequent redistribution of main magnetic poles; (2) variation of the accretion rate, which causes the disc to interact with the magnetic fields associated with different magnetic moments. We use our model to demonstrate these phase shift mechanisms, and we discuss possible applications of these mechanisms to accreting millisecond pulsars and young stars.

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Section: Discussion Of Phase Shifts In Accreting Millisecond Pulsars mentioning

confidence: 99%

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

2012

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“…Kulkarni & Romanova 2008;Romanova et al 2008). In the case of the hypercritical accretion present in the supernova fallback, Rayleigh-Taylor instabilities have been studied by Payne & Melatos (2004; Bernal et al (2010Bernal et al ( , 2013; Mukherjee et al (2013a,b). The simulations of Bernal et al (2013) also show that the height of the unstable magnetic field over the NS surface decreases with increasing accretion rate, for fixed NS magnetic field strength, as expected.…”

Section: Summary and Discussionmentioning

confidence: 99%

Are pulsars born with a hidden magnetic field?

Torres-Forné

Cerdá–Durán

Pons

et al. 2016

Mon. Not. R. Astron. Soc.

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The observation of several neutron stars in the center of supernova remnants and with significantly lower values of the dipolar magnetic field than the average radio-pulsar population has motivated a lively debate about their formation and origin, with controversial interpretations. A possible explanation requires the slow rotation of the proto-neutron star at birth, which is unable to amplify its magnetic field to typical pulsar levels. An alternative possibility, the hidden magnetic field scenario, considers the accretion of the fallback of the supernova debris onto the neutron star as responsible for the submergence (or screening) of the field and its apparently low value. In this paper we study under which conditions the magnetic field of a neutron star can be buried into the crust due to an accreting, conducting fluid. For this purpose, we consider a spherically symmetric calculation in general relativity to estimate the balance between the incoming accretion flow and the magnetosphere. Our study analyses several models with different specific entropy, composition, and neutron star masses. The main conclusion of our work is that typical magnetic fields of a few times 10 12 G can be buried by accreting only 10 −3 − 10 −2 M , a relatively modest amount of mass. In view of this result, the Central Compact Object scenario should not be considered unusual, and we predict that anomalously weak magnetic fields should be common in very young (< few kyr) neutron stars.

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“…Or, whether screening or burial of the magnetic field at the polar caps is possible (Brown & Bildsten 1998;Payne & Melatos 2004;Litwin et al 2001;Payne & Melatos 2007). It needs to be investigated, how much mass could eventually be stored in the magnetically confined mountains, whether matter is continuously leaking out to larger areas of the neutron star surface (due to plasma pressure exceeding the magnetic pressure) and on what timescales an observational effect can be expected.…”

Section: Dependence Of E Cyc On Luminositymentioning

confidence: 99%

Long-term change in the cyclotron line energy in Hercules X-1

Staubert

Klochkov

Wilms³

et al. 2014

A&A

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Aims. We investigate the long-term evolution of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and present evidence of a true long-term decrease in the centroid energy E cyc of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012. Methods. Our results are based on repeated observations of Her X-1 by those X-ray observatories capable of measuring clearly beyond the cyclotron line energy of ∼ 40 keV. Results. The historical evolution of the pulse phase averaged CRSF centroid energy E cyc since its discovery in 1976 is characterized by an initial value around 35 keV, an abrupt jump upwards to beyond ∼ 40 keV between 1990 and 1994, and an apparent decay thereafter. Much of this decay, however, was found to be due to an artifact, namely a correlation between E cyc and the X-ray luminosity L x discovered in 2007. In observations after 2006, however, we now find a statistically significant true decrease in the cyclotron line energy. At the same time, the dependence of E cyc on X-ray luminosity is still valid with an increase of ∼ 5% in energy for a factor of two increase in luminosity. A decrease in E cyc by 4.2 keV over the 16 years from 1996 to 2012 can either be modeled by a linear decay, or by a slow decay until 2006 followed by a more abrupt decrease thereafter. Conclusions. We speculate that the physical reason could be connected to a geometric displacement of the cyclotron resonant scattering region in the polar field or to a true physical change in the magnetic field configuration at the polar cap by the continued accretion.

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Burial of the polar magnetic field of an accreting neutron star – I. Self-consistent analytic and numerical equilibria

Cited by 133 publications

References 32 publications

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

Are pulsars born with a hidden magnetic field?

Long-term change in the cyclotron line energy in Hercules X-1

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