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Register a new userThe inside-out view on neutron star magnetospheres
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We construct hydromagnetic neutron star equilibria which allow for a non-zero electric current distribution in the exterior. The novelty of our models is that the neutron stars interior field is in equilibrium with its magnetosphere, thus bridging the gap between previous work in this area which either solves for the interior assuming a vacuum exterior or solves for the magnetosphere without modelling the star itself. We consider only non-rotating stars in this work, so our solutions are most immediately applicable to slowly-rotating systems such as magnetars. Nonetheless, we demonstrate that magnetospheres qualitatively resembling those expected for both magnetars and pulsars are possible within our framework. The inside-out approach taken in this paper should be more generally applicable to rotating neutron stars, where the interior and exterior regions are again not independent but evolve together.
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Planetary nebulae are ionized clouds of gas formed by the hydrogen-rich envelopes of low- and intermediate-mass stars ejected at late evolutionary stages. The strong UV flux from their central stars causes a highly stratified ionization structure, with species of higher ionization potential closer to the star. Here we report on the exceptional case of HuBi 1, a double-shell planetary nebula whose inner shell presents emission from low-ionization species close to the star and emission from high-ionization species farther away. Spectral analysis demonstrates that the inner shell of HuBi 1 is excited by shocks, whereas its outer shell is recombining. The anomalous excitation of these shells can be traced to its low-temperature [WC10] central star whose optical brightness has declined continuously by 10 magnitudes in a period of 46 years. Evolutionary models reveal that this star is the descendent of a low-mass star ($simeq$1.1 $M_odot$) that has experienced a born-again event whose ejecta shock-excite the inner shell. HuBi 1 represents the missing link in the formation of metal-rich central stars of planetary nebulae from low-mass progenitors, offering unique insight regarding the future evolution of the born-again Sakurais object. Coming from a solar-mass progenitor, HuBi 1 represents a potential end-state for our Sun.
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of magnetospheres of isolated neutron stars. For a summary, we refer to the paper.
Axion dark matter (DM) may convert to radio-frequency electromagnetic radiation in the strong magnetic fields around neutron stars. The radio signature of such a process would be an ultra-narrow spectral peak at a frequency determined by the mass of the axion particle. We analyze data we collected from the Robert C. Byrd Green Bank Telescope in the L-band and the Effelsberg 100-m Telescope in the L-Band and S-band from a number of sources expected to produce bright signals of axion-photon conversion, including the Galactic Center of the Milky Way and the nearby isolated neutron stars RX J0720.4-3125 and RX J0806.4-4123. We find no evidence for axion DM and are able to set some of the strongest constraints to-date on the existence of axion DM in the highly-motivated mass range between ~5-11 $mu$eV.
We demonstrate that observations of glitches in the Vela pulsar can be used to investigate the strength of the crust-core coupling in a neutron star, and suggest that recovery from the glitch is dominated by torque exerted by the re-coupling of superfluid components of the core that were decoupled from the crust during the glitch. Assuming that the recoupling is mediated by mutual friction between the superfluid neutrons and the charged components of the core, we use the observed magnitudes and timescales of the shortest timescale components of the recoveries from two recent glitches in the Vela pulsar to infer the fraction of the core that is coupled to the crust during the glitch, and hence spun up by the glitch event. Within the framework of a two-fluid hydrodynamic model of glitches, we analyze whether crustal neutrons alone are sufficient to drive the glitch activity observed in the Vela pulsar. We use two sets of neutron star equations of state (EOSs), both of which span crust and core consistently and cover a range of the slope of the symmetry energy at saturation density $30 < L <120$ MeV. One set produces maximum masses $approx$2.0$M_{odot}$, the second $approx$2.6$M_{odot}$. We also include the effects of entrainment of crustal neutrons by the superfluid lattice. We find that for medium to stiff EOSs, observations imply $>70%$ of the moment of inertia of the core is coupled to the crust during the glitch, though for softer EOSs $Lapprox 30$MeV as little as $5%$ could be coupled. No EOS is able to reproduce the observed glitch activity with crust neutrons alone, but extending the region where superfluid vortices are strongly pinned into the core by densities as little as 0.016fm$^{-3}$ above the crust-core transition density restores agreement with the observed glitch activity.
Two low mass neutron stars, J0737-3039B and the companion to J1756-2251, show strong evidence of being formed from the collapse of an ONeMg core in an electron capture supernova (ECSN) or in an ultra-stripped iron core collapse supernova (FeCCSN). Using three different systematically generated sets of equations of state we explore the relationship between the moment of inertia of J0737-3039A and the binding energy of the two low mass neutron stars. We find this relationship, a less strict variant of the recently discovered I-Love-Q relations, is nevertheless more robust than a previously explored correlation between the binding energy and the slope of the nuclear symmetry energy L. We find that, if either J0737-3039B or the J1756-2251 companion were formed in an ECSN, no more than 0.06 solar masses could have been lost from the progenitor core, more than four times the mass loss predicted by current supernova modeling. A measurement of the moment of inertia of J0737-3039A to within 10% accuracy from pulsar timing, possible within a decade, can discriminate between formation scenarios such as ECSN or ultra-stripped FeCCSN and, given current constraints on the predicted core mass loss, potentially rule them out. Using the I-Love-Q relations we find that an Advanced LIGO can potentially measure the neutron star tidal polarizability to equivalent accuracy in a neutron star-neutron star merger at a distance of 200 Mpc, thus obtaining similar constraints on the formation scenarios. Such information on the occurrence of ECSNe is important for population synthesis calculations, especially for estimating the rate of binary neutron star mergers and resulting electromagnetic and gravitational wave signals. Further progress needs to be made modeling the core collapse process that leads to low-mass neutron stars, particularly in making robust predictions for the mass loss from the progenitor core.
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