294 research outputs found
Super-Eddington Emission from Accreting, Highly Magnetised Neutron Stars with a Multipolar Magnetic Field
No full textPulsating ultraluminous X-ray sources (PULXs) are characterized by an extremely large luminosity (>1040 erg s−1). While there is a general consensus that they host an accreting, magnetized neutron star (NS), the problem of how to produce luminosities >100 times the Eddington limit, LE, of a solar mass object is still debated. A promising explanation relies on the reduction of the opacities in the presence of a strong magnetic field, which allows for the local flux to be much larger than the Eddington flux. However, avoiding the onset of the propeller effect may be a serious problem. Here, we reconsider the problem of column accretion on to a highly magnetized NS, extending previously published calculations by relaxing the assumption of a pure dipolar field and allowing for more complex magnetic field topologies. We find that the maximum luminosity is determined primarily by the magnetic field strength near the NS surface. We also investigate other factors determining the accretion column geometry and the emergent luminosity, such as the assumptions on the parameters governing the accretion flow at the disc–magnetosphere boundary. We conclude that a strongly magnetized NS with a dipole component of ∼1013 G, octupole component of ∼1014 G, and spin period ∼1 s can produce a luminosity of ∼1041 erg s−1 while avoiding the propeller regime. We apply our model to two PULXs, NGC 5907 ULX-1, and NGC 7793 P13, and discuss how their luminosity and spin period rate can be explained in terms of different configurations, either with or without multipolar magnetic components
Atmosphere of strongly magnetized neutron stars heated by particle bombardment
No full textThe magnetosphere of strongly magnetized neutron stars, such as magnetars, can sustain large electric currents. The charged particles return to the surface with large Lorentz factors, producing a particle bombardment. We investigate the transport of radiation in the atmosphere of strongly magnetized neutron stars, in the presence of particle bombardment heating. We solve the radiative transfer equations for a gray atmosphere in the Eddington approximation, accounting for the polarization induced by a strong magnetic field. The solutions show the formation of a hot external layer and a low (uniform) temperature atmospheric interior. This suggests that the emergent spectrum may be described by a single blackbody with the likely formation of a optical/infrared excess (below ˜1 eV). We also found that the polarization is strongly dependent on both the luminosity and penetration length of the particle bombardment. Therefore, the thermal emission from active sources, such as transient magnetars, in which the luminosity decreases by orders of magnitude, may show a substantial variation in the polarization pattern during the outburst decline. Our results may be relevant in view of future X-ray polarimetric missions such as IXPE and eXTP
A magnetohydrodynamical model for the formation of episodic jets
No full textEpisodic ejection of plasma blobs has been observed in many black hole systems. While steady, continuous jets are believed to be associated with large-scale open magnetic fields, what causes the episodic ejection of blobs remains unclear. Here by analogy with the coronal mass ejection on the Sun, we propose a magnetohydrodynamical model for episodic ejections from black holes associated with the closed magnetic fields in an accretion flow. Shear and turbulence of the accretion flow deform the field and result in the formation of a flux rope in the disc corona. Energy and helicity are accumulated and stored until a threshold is reached. The system then loses its equilibrium and the flux rope is thrust outward by the magnetic compression force in a catastrophic way. Our calculations show that for parameters appropriate for the black hole in our Galactic centre, the plasmoid can attain relativistic speeds in about 35 min
Letter to Nature: An ultra-relativistic outflow from a neutron star accreting gas from a companion.
No full textCollimated relativistic outflows—also known as jets—are amongst the most energetic phenomena in the Universe. They are associated with supermassive black holes in distant active galactic nuclei1, accreting stellar-mass black holes and neutron stars in binary systems2 and are believed to be responsible for gamma-ray bursts3. The physics of these jets, however, remains something of a mystery in that their bulk velocities, compositions and energetics remain poorly determined. Here we report the discovery of an ultra-relativistic outflow from a neutron star accreting gas within a binary stellar system. The velocity of the outflow is comparable to the fastest-moving flows observed from active galactic nuclei, and its strength is modulated by the rate of accretion of material onto the neutron star. Shocks are energized further downstream in the flow, which are themselves moving at mildly relativistic bulk velocities and are the sites of the observed synchrotron emission from the jet. We conclude that the generation of highly relativistic outflows does not require properties that are unique to black holes, such as an event horizon
Multi-Messenger Astrophysics of a Millisecond Pulsar Orbiting around a Massive Black Hole
No full textExtreme-mass-ratio and intermediate-mass-ratio binaries with a millisecond pulsar are gravitational-wave sources that emit also electromagnetic radiation. The millisecond pulsars in these binaries have complex orbital and spin dynamics, which are observable because of spin–orbit and spin–spin coupling (through spin–curvature interaction). The strengths of the couplings generally depends on the mass ratio between the pulsar and the black hole. The narrow mass range of neutron stars gives an advantage in parameter extraction as it greatly reduces the search space, in particular, in the determination of the black-hole mass, in gravitational wave experiments and radio pulsar timing observations. Extreme-mass-ratio and intermediate-mass-ratio binaries with a millisecond pulsar will help to resolve the astrophysical problems, concerning the applicability of the M-σ relation for galactic spheroids extending to the very low-mass galaxies and whether or not low-mass dwarf galaxies and globular clusters would harbour a nuclear intermediate-mass black hole. The high-precision that can be achieved in gravitational wave experiments and radio pulsar timing observations will provide an opportunity to directly detect gravitational clock effects that are arisen from spin couplings. Radio monitoring of the orbital and spin evolution of the millisecond pulsar in an extreme-mass-ratio binary can be used as a bootstrap method for correcting the drifts in the phases in the gravitational waves from the extreme-mass-ratio and intermediate-mass-ratio binaries caused by self-force
Polarized Radiation From Inhomogeneous Shocks.
No full textStrongly polarized radiation from AM Herculis binaries is believed to be due to cyclotron emission from hot magnetized plasmas. The flat optically thin spectra and strong IR polarization observed in these binaries cannot be explained by models assuming a homogeneous emission region with a simple geometry. We have therefore studied the cyclotron emission from infinite plasma cylinders with uniform magnetic fields and temperatures, but with a variety of axially symmetric electron density profiles and shown that such inhomogeneous plasmas are able to produce relatively flat spectra which cannot be produced by the homogeneous models. The polarization at low frequencies is shown to be stronger than that due to the homogeneous plasmas and the polarization at high frequencies is weaker. Using a two-core model, in which a high-density core is surrounded by a low-density shell, excellent fits to the optical/IR spectra and polarization of the AM Herculis systems ST LMi, V834 Cen, EF Eri, and BL Hydri are obtained. The emission regions of these systems are typically characterized by a temperature 10 and an average dimensionless plasma parameter 10\sp6. We have also studied the steady-state hydrodynamics of bremsstrahlung-dominated shocks and calculated the cyclotron emission from them. Three types of accretion rate profiles--uniform, axi-symmetric and asymmetric--were considered. The shock-structure is planar for the uniform accretion rate case. The shock due to an axi-symmetric accretion rate is a curved surface. Near the rim of the shock surface, there is an extended low-density region with kT 10keV and 10\sp4; near the symmetry axis, a high-density region exists with 10 and 10\sp6. Bremsstrahlung radiation is emitted from the high-density region and cyclotron emission from both. For asymmetric accretion, the post-shock region is asymmetric and hence produces asymmetric light curves. All these inhomogeneous shocks produce flat optical/IR spectra and strong IR polarization. These models were shown to be consistent with recent photometric and polarimetric observations, implying that the accretion columns in the AM Her systems are highly-structured and cannot be explained by any homogeneous models with a simple geometry
Multi-Messenger Astrophysics of a Millisecond Pulsar Orbiting around a Massive Black Hole
No full textExtreme-mass-ratio and intermediate-mass-ratio binaries with a millisecond pulsar are gravitational-wave sources that emit also electromagnetic radiation. The millisecond pulsars in these binaries have complex orbital and spin dynamics, which are observable because of spin–orbit and spin–spin coupling (through spin–curvature interaction). The strengths of the couplings generally depends on the mass ratio between the pulsar and the black hole. The narrow mass range of neutron stars gives an advantage in parameter extraction as it greatly reduces the search space, in particular, in the determination of the black-hole mass, in gravitational wave experiments and radio pulsar timing observations. Extreme-mass-ratio and intermediate-mass-ratio binaries with a millisecond pulsar will help to resolve the astrophysical problems, concerning the applicability of the M-σ relation for galactic spheroids extending to the very low-mass galaxies and whether or not low-mass dwarf galaxies and globular clusters would harbour a nuclear intermediate-mass black hole. The high-precision that can be achieved in gravitational wave experiments and radio pulsar timing observations will provide an opportunity to directly detect gravitational clock effects that are arisen from spin couplings. Radio monitoring of the orbital and spin evolution of the millisecond pulsar in an extreme-mass-ratio binary can be used as a bootstrap method for correcting the drifts in the phases in the gravitational waves from the extreme-mass-ratio and intermediate-mass-ratio binaries caused by self-force
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