26 research outputs found
The Hidden Magnetic Field of The Young Neutron Star in Kesteven 79
Recent observations of the central compact object in the Kesteven 79
supernova remnant show that this neutron star (NS) has a weak dipole magnetic
field (a few x 10^{10} G) but an anomalously large (~ 64%) pulse fraction in
its surface X-ray emission. We explore the idea that a substantial sub-surface
magnetic field exists in the NS crust, which produces diffuse hot spots on the
stellar surface due to anisotropic heat conduction, and gives rise to the
observed X-ray pulsation. We develop a general-purpose method, termed
"Temperature Template with Full Transport" (TTFT), that computes the synthetic
pulse profile of surface X-ray emission from NSs with arbitrary magnetic field
and surface temperature distributions, taking into account magnetic atmosphere
opacities, beam pattern, vacuum polarization, and gravitational light bending.
We show that a crustal toroidal magnetic field of order a few x 10^{14} G or
higher, varying smoothly across the crust, can produce sufficiently distinct
surface hot spots to generate the observed pulse fraction in the Kes 79 NS.
This result suggests that substantial sub-surface magnetic fields, much
stronger than the "visible" dipole fields, may be buried in the crusts of some
young NSs, and such hidden magnetic fields can play an important role in their
observational manifestations. The general TTFT tool we have developed can also
be used for studying radiation from other magnetic NSs.Comment: 7 pages, 5 figure
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 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, and the field growth time-scales,
, 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 B150958, J18460258, J11196127 and
J17343333 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
Spin-Orbit Coupling And Tidal Dissipation In Hot Jupiter Systems
Hot Jupiters are giant planets located extremely close to their host stars, with orbital periods less than 5 days. Many aspects of hot Jupiter (HJ) formation remain unclear, but several clues, such as the observed misalignment between their orbital axes and their hosts' spin axes, point to a dynamical origin. In the first portion of this work we explore the stellar spin-orbit dynamics of one such dynamical formation channel, the Lidov-Kozai mechanism. We show that the coupling between the stellar spin and the planet orbit can lead to complex, and sometimes chaotic, behavior of the stellar spin vector. Many features of this behavior arise due to a set of resonances between the stellar spin axis precession timescale and the Lidov-Kozai timescale. Under the assumption that the stellar quadrupole does not induce precession in the planet's orbit, given a system with a set of initial parameters, we show that it is possible to predict whether the system can attain high spin-orbit misalignments. In the second portion of this work, we discuss tidal dissipation in giant planets, another aspect that is crucial to dynamical HJ formation theories. We show that tidal dissipation in the cores of giant planets can be significant, and can help reconcile inconsistencies in the tidal dissipation efficiencies inferred from observations of Jupiter's moons and from high-eccentricity HJ migration theories. Finally, we improve upon existing core tidal dissipation theories by presenting semi-analytical formulae for dissipation in a core surrounded by a polytropic n = 1 envelope
A new code for the Hall-driven magnetic evolution of neutron stars
Over the past decade, the numerical modeling of the magnetic field evolution in astrophysical scenarios has become an increasingly important field. In the crystallized crust of neutron stars the evolution of the magnetic field is governed by the Hall induction equation. In this equation the relative contribution of the two terms (Hall term and Ohmic dissipation) varies depending on the local conditions of temperature and magnetic field strength. This results in the transition from the purely parabolic character of the equations to the hyperbolic regime as the magnetic Reynolds number increases, which presents severe numerical problems. Up to now, most attempts to study this problem were based on spectral methods, but they failed in representing the transition to large magnetic Reynolds numbers. We present a new code based on upwind finite differences techniques that can handle situations with arbitrary low magnetic diffusivity and it is suitable for studying the formation of sharp current sheets during the evolution. The code is thoroughly tested in different limits and used to illustrate the evolution of the crustal magnetic field in a neutron star in some representative cases. Our code, coupled to cooling codes, can be used to perform long-term simulations of the magneto-thermal evolution of neutron stars.This work was partly supported by the Spanish grant AYA 2010-21097-C03-02 and CompStar, a Research Networking Program of the European Science Foundation. D. Viganò is supported by a fellowship from the Prometeo program for research groups of excellence of the Generalitat Valenciana (Prometeo/2009/103)
The imprint of the crustal magnetic field on the thermal spectra and pulse profiles of isolated neutron stars
Isolated neutron stars (NSs) show a bewildering variety of astrophysical manifestations, presumably shaped by the magnetic field strength and topology at birth. Here, using state-of-the-art calculations of the coupled magnetic and thermal evolution of NSs, we compute the thermal spectra and pulse profiles expected for a variety of initial magnetic field configurations. In particular, we contrast models with purely poloidal magnetic fields to models dominated by a strong internal toroidal component. We find that, while the former displays double-peaked profiles and very low pulsed fractions, in the latter, the anisotropy in the surface temperature produced by the toroidal field often results in a single pulse profile, with pulsed fractions that can exceed the 50–60 per cent level even for perfectly isotropic local emission. We further use our theoretical results to generate simulated ‘observed’ spectra, and show that blackbody (BB) fits result in inferred radii that can be significantly smaller than the actual NS radius, even as low as ∼1–2 km for old NSs with strong internal toroidal fields and a high absorption column density along their line of sight. We compute the size of the inferred BB radius for a few representative magnetic field configurations, NS ages and magnitudes of the column density. Our theoretical results are of direct relevance to the interpretation of X-ray observations of isolated NSs, as well as to the constraints on the equation of state of dense matter through radius measurements.This research was supported by NSF grant No. AST 1009396 and NASA grants AR1-12003X, DD1-12053X, GO2-13068X, GO2-13076X (RP), grants AYA 2010-21097-C03-02 (DV, JAP), AYA2009-07391, AYA2012-39303, SGR2009-811, and iLINK 2011-0303 (NR). DV is supported by a fellowship from the Prometeo program for research groups of excellence of the Generalitat Valenciana (Prometeo/2009/103) and NR is supported by a Ramon y Cajal Fellowship
Unifying the observational diversity of isolated neutron stars via magneto-thermal evolution models
Observations of magnetars and some of the high magnetic field pulsars have shown that their thermal luminosity is systematically higher than that of classical radio-pulsars, thus confirming the idea that magnetic fields are involved in their X-ray emission. Here we present the results of 2D simulations of the fully coupled evolution of temperature and magnetic field in neutron stars, including the state-of-the-art kinetic coefficients and, for the first time, the important effect of the Hall term. After gathering and thoroughly re-analysing in a consistent way all the best available data on isolated, thermally emitting neutron stars, we compare our theoretical models to a data sample of 40 sources. We find that our evolutionary models can explain the phenomenological diversity of magnetars, high-B radio-pulsars, and isolated nearby neutron stars by only varying their initial magnetic field, mass and envelope composition. Nearly all sources appear to follow the expectations of the standard theoretical models. Finally, we discuss the expected outburst rates and the evolutionary links between different classes. Our results constitute a major step towards the grand unification of the isolated neutron star zoo.This research was supported by the grants AYA 2010-21097-C03-02 (DV, JAP, JAM); AYA2009-07391, AYA2012-39303, SGR2009-811, and iLINK 2011-0303 (NR); CONICET and PIP-2011-00170 (DNA); NSF grant No. AST 1009396 and NASA grants AR1-12003X, DD1-12053X, GO2-13068X, GO2-13076X (RP). DV is supported by a fellowship from the Prometeo program for research groups of excellence of the Generalitat Valenciana (Prometeo/2009/103) and NR is supported by a Ramon y Cajal Fellowship