Buried Magnetic Field Research Articles

Relaxation by thermal conduction of a magnetically confined mountain on an accreting neutron star

A magnetically confined mountain on the surface of an accreting neutron star simultaneously reduces the global magnetic dipole moment through magnetic burial and generates a mass quadrupole moment, which emits gravitational radiation. Previous mountain models have been calculated for idealized isothermal and adiabatic equations of state. Here these models are generalised to include non-zero, finite thermal conduction. Grad-Shafranov equilibria for three representative, polytropic equations of state are evolved over many conduction time-scales with the magnetohydrodynamic solver PLUTO. It is shown that conduction facilitates the flow of matter towards the pole. Consequently the buried magnetic field is partially resurrected starting from an initially polytropic Grad-Shafranov equilibrium. The poleward mass current makes the star more prolate, marginally increasing its detectability as a gravitational wave source, though to an extent which is likely to be subordinate to other mountain physics. Thermal currents also generate filamentary hot spots $(\gtrsim 10^{8} \text{ K})$ in the mountain, especially near the pole where the heat flux is largest, with implications for type I X-ray bursts.

Detecting gravitational waves from mountains on neutron stars in the advanced detector era

Rapidly rotating Neutron Stars (NSs) in Low Mass X-ray Binaries (LMXBs) are thought to be interesting sources of Gravitational Waves (GWs) for current and next generation ground based detectors, such as Advanced LIGO and the Einstein Telescope. The main reason is that many of the NS in these systems appear to be spinning well below their Keplerian breakup frequency, and it has been suggested that torques associated with GW emission may be setting the observed spin period. This assumption has been used extensively in the literature to assess the strength of the likely gravitational wave signal. There is now, however, a significant amount of theoretical and observation work that suggests that this may not be the case, and that GW emission is unlikely to be setting the spin equilibrium period in many systems. In this paper we take a different starting point and predict the GW signal strength for two physical mechanisms that are likely to be at work in LMXBs: crustal mountains due to thermal asymmetries and magnetically confined mountains. We find that thermal crustal mountains in transient LMXBs are unlikely to lead to detectable GW emission, while persistent systems are good candidates for detection by Advanced LIGO and by the Einstein Telescope. Detection prospects are pessimistic for the magnetic mountain case, unless the NS has a buried magnetic field of $B\approx 10^{12}$ G, well above the typically inferred exterior dipole fields of these objects. Nevertheless, if a system were to be detected by a GW observatory, cyclotron resonant scattering features in the X-ray emission could be used to distinguish between the two different scenarios.

Quiescent thermal emission from neutron stars in low-mass X-ray binaries

We monitored the quiescent thermal emission from neutron stars in low-mass X-ray binaries after active periods of intense activity in x-rays (outbursts). The theoretical modeling of the thermal relaxation of the neutron star crust may be used to establish constraints on the crust composition and transport properties, depending on the astrophysical scenarios assumed. We numerically simulated the thermal evolution of the neutron star crust and compared them with inferred surface temperatures for five sources: MXB 1659-29, KS 1731-260, EXO 0748-676, XTE J1701-462 and IGR J17480-2446. We find that the evolution of MXB 1659-29, KS 1731-260 and EXO 0748-676 can be well described within a deep crustal cooling scenario. Conversely, we find that the other two sources can only be explained with models beyond crustal cooling. For the peculiar emission of XTE J1701-462 we propose alternative scenarios such as residual accretion during quiescence, additional heat sources in the outer crust, and/or thermal isolation of the inner crust due to a buried magnetic field. We also explain the very recent reported temperature of IGR J17480-2446 with an additional heat deposition in the outer crust from shallow sources.

Helioseismology in the 1980s and 1990s

AbstractOver more than twenty years, Wojtek Dziembowski and I collaborated on nearly fifty papers, which were concentrated in helioseismology through the 1980s and 1990s, but extended early into the new century. In this review, I discuss the most significant results of this collaboration and some of the underlying sociology that contributed to the intensity and longevity of our collaboration. Our work began with placing limits on the Sun's buried magnetic field and ended with extracting from the solar-cycle dependent oscillation frequency changes the roles (and net result) of competing dynamical drivers of changes in the solar diameter.

Effect of buried magnetic field on bone healing process in rat femurs

Effect of buried magnetic field on bone healing process in rat femurs

Evolution of a buried magnetic field in the central compact object neutron stars

The central compact objects are a newly-emerging class of young neutron stars near the centre of supernova remnants. From X-ray timing and spectral measurements, their magnetic fields are determined to be ~ 10^10-10^11 G, which is significantly lower than that found on most pulsars. Using the latest electrical and thermal conductivity calculations, we solve the induction equation to determine the evolution of a buried crustal or core magnetic field. We apply this model of a buried field to explain the youth and low observed magnetic field of the central compact objects. We obtain constraints on their birth magnetic field and depth of submergence (or accreted mass). Measurement of a change in the observed magnetic field strength would discriminate between the crustal and core fields and could yield uniquely the birth magnetic field and submergence depth. If we consider the central compact objects as a single neutron star viewed at different epochs, then we constrain the magnetic field at birth to be ~ (6-9)x10^11 G. A buried magnetic field can also explain their location in an underpopulated region of the spin period-period derivative plane for pulsars.

An analysis of solar p-mode frequencies extracted from BiSONdata: 1991-1996

We present a comprehensive frequency analysis of Doppler velocity observations of the visible solar disc made by the Birmingham Solar Oscillations Network (BiSON) from 1990-1996, i.e. covering the falling phase of activity cycle 22, up to and including the cycle 22/23 boundary. We have fitted low-degree (low-l) solar p modes in a variety of power spectra of differing lengths generated from these data. The analysis of the extracted frequencies reveals the expected clear solar-cycle dependence; in addition, there is now sufficient accuracy in the data to show that the low-l modal eigenfrequencies are less affected by the solar cycle than their higher l counterparts. The observed low-degree frequency shifts up to approximate to 3900 mu Hz are consistent - at the level of precision of the data - with an inverse mode-mass scaling. At frequencies above this, the blending of modes adjacent in frequency space makes it increasingly difficult to extract reliable frequency estimates. However, our data show indications of a turnover and possibly also an eventual sign change in the solar cycle shifts at frequencies above;approximate to 4000 mu Hz, as seen in higher l data. We have parametrized the observed shifts as a function of the 10.7-cm radio flux, and produced an activity-corrected, average frequency table which incorporates eigenfrequencies from 18 4-month and 9 8-month spectra. We also present the fitted frequencies from a 32-month power spectrum, generated from data collected at or close to solar minimum. In addition, we also searched for frequency asymmetries in the l = 2 mode multiplets. These could result from strong near-surface magnetic activity, or a buried magnetic field. Our fits merely place an upper limit to any mean asymmetry - over the range 10 less than or equal to n less than or equal to 21 - of between approximate to - 80 and approximate to 170 nHz (3 sigma).

The effect of rotation and a buried magnetic field on stellar oscillations

Une methode de perturbation permettant de determiner les modifications de frequence resultantes, causees par la rotation et le champ magnetique interne, sur les oscillations stellaires est presentee. Les effets de differentes vitesses angulaires et configurations du champ magnetique sur les modes acoustiques solaires de grands ordres sont analyses