Decay of positronium in strong magnetic fields

We investigate the decay of bound electron-positron pairs (positronium) in strong magnetic fields (of order 10¹² Gauss, which are assumed for neutron stars) on the basis of a correct treatment of the two-body problem, thus improving previous work by Carr and Sutherland (1978). We find that, even in the presence of a strong magnetic field, the decay of the ground state of positronium must be momentum conserving, whereby the possibility of the one-photon decay is ruled out. We calculate the transition rate for the two-photon annihilation process which turns out to be larger than the field free transition rate by a factor (1/)² B/B _cr (where is the electromagnetic coupling constant, andB _cr=m _e ² c ²/(e)=4.41×10¹³ Gauss).     

Modeling far-infrared observations of young stellar objects

The exotic quantum process of photon splitting has great potential to explain the softness of emission in soft gamma repeaters (SGRs) if they originate in neutron stars with surface fields above the quantum critical fieldB _cr = 4.413×10¹³ Gauss. Splitting becomes prolific at such field strengths: its principal effect is to degrade photon energies, initiating a cascade that softens gamma-ray spectra. Uniform field cascade calculations have demonstrated that emission could be softened to the observed SGR energies for fields exceeding about 10¹⁴ Gauss. Recently, we have determined splitting attenuation lengths and maximum energies for photon escape in neutron star environments including the effects of magnetospheric dipole field geometry. Such escape energies _esc suitably approximate the peak energy of the emergent spectrum, and in this paper we present results for _esc as a function of photon emission angles for polar cap and equatorial emission regions. The escape energy is extremely insensitive to viewing perspective for equatorial emission, arguing in favour of such a site for the origin of SGR activity.     

Photon dispersion in a strong magnetic field with positronium formation: Theory

Positrinium atom is considered in a strong magnetic field in a vector-potential gauged asA _x =–By. The energy spectrum is obtained including its dependence on the centre-of-mass wave vector across the magnetic field. The pole-like contributions into the photon polarization operator coming from the positronium states are calculated and dispersion curves of joint photon-positronium states are obtained as trajectories of poles of the photon Green function in momentum space.When propagating in a strong magnetic field (B0.1B _cr 4×10¹² G) with curved lines of force, a photon is canalized along the magnetic field by adiabatically transforming into a bound electron-positron pair, which is a stronger effect than the analogous photon capture by transforming into an unbound pair at the edge of the continuum discussed previously by the authors. The effect of bound pair formation by-quanta in a strong magnetic field may be important near pulsars,-burst sources, powerful X-ray sources in close binary systems and other astronomical objects, recognized as magnetic neutron stars.     

Gravitational collapse of magnetic neutron stars

Pulsars are presently believed to be rotating neutron stars with frozen-in magnetic fields. Because of the high density of neutron stars, general relativistic effects are important since they effect both the structure and stability of such stars. Besides this, the magnetic field outside the star is also affected. Instead of falling of asr ^(2+l) as in flat space, it is shown that each magnetic multipole varies as a hypergeometric function of radius. A closed form of these hypergeometric functions is given in terms of Legendre functions of the second kind. If the mass of a neutron star exceeds about 2.4m , the star becomes unstable and coliapses. For a quasistatically collapsing body, it is shown that the magnetic field seen by a distant observer vanishes as the radius approaches the gravitational radius.This work was supported in part by the Air Force Office of Scientific Research, Office of Aerospace Research under AFOSR Grant 70-1866.     

Two-photon positron–electron annihilation in a strong magnetic field

We consider the two-photon positron and electron annihilation in flight, it means the annihilating particles exhibit the middly relativistic momenta in a super strong magnetic field. Such particles are present in the corona of pulsars and magnetars. The paper presents how the total emission rate for the two-photon process is affected not only by magnetic field but also by the relativistic momentum of the annihilating particles. We found that the momenta influence significantly the total emission rate and the directions of the emitted photons. Additionally, the total emission for the two-photon process is comparable to that for the one-photon process at the momentum of annihilating particles of about m₀, where m₀ is the electron mass, and the magnetic field close to the critical Schwinger value of 4.41 × 10¹³ G. The latter is reported as a main annihilation channel in a super strong magnetic field. We calculated also the energetic spectra of annihilating photons emitted, which are also affected by the magnetic field and the momenta of the annihilating particles.     

Cosmic gamma-ray burst spectroscopy

A review is given of the gamma-ray burst energy spectrum measurements on Venera 11 and Venera 12 space probes. The gamma burst continuum approximates in shape thermal brems-strahlung emission of a hot plasma. The radiation temperature varies over a broad range, 50–1000 keV, for different events. Spectra of many bursts contain cyclotron absorption and/or redshifted annihilation lines. Strong variability is typically observed in both continuum and line spectra. These spectral data provide convincing evidence for the gamma-ray bursts being generated by neutron stars with superstrong magnetic fields 10¹²–10¹³ G.     

Electron-positron plasma generation in a magnetar magnetosphere

We consider the electron—positron plasma generation processes in the magnetospheres of magnetars—neutron stars with strong surface magnetic fields, B ? 10¹⁴–10¹⁵ G. We show that the photon splitting in a magnetic field, which is effective at large field strengths, does not lead to the suppression of plasma multiplication, but manifests itself in a high polarization of γ-ray photons. A high magnetic field strength does not give rise to the second generation of particles produced by synchrotron photons. However, the density of the first-generation particles produced by curvature photons in the magnetospheres of magnetars can exceed the density of the same particles in the magnetospheres of ordinary radio pulsars. The plasma generation inefficiency can be attributed only to slow magnetar rotation, which causes the energy range of the produced particles to narrow. We have found a boundary in the \(P - \dot P\) diagram that defines the plasma generation threshold in a magnetar magnetosphere.     

Gamma-ray bursts from nearby neutron stars

We interpret the puzzling-ray bursts as emitted by cooling sparks from the surface of spasmodically accreting, old neutron stars. Their spiky, anisotropic radiation is oriented w.r.t. the galactic disk via interstellar accretion, whose orbital angular momentum tends to counteralign with the galactic spin; in this way, larger source numbers in directions of the galactic disk are compensated by smaller beaming probabilities, resulting in a near-isotropic arrival distribution, as observed by BATSE. The source distances range between 10 pc and 500 pc. Their radiated energies are of order 10³⁵ erg, corresponding to accreted clumps (blades) of typical mass 10¹⁵ g per burst. Magnetic surface field strengths range between 10¹⁰ and 10¹² G, somewhat weaker than those of newborn neutron stars.     

Models of soft gamma-ray repeater emissions

We review emission models of soft gamma-ray repeaters (SGRs) within the context of magnetized neutron star origins. Motivations for moderate field (10^10–12G) versus ultrastrong field ( 5 × 10¹⁴G) neutron stars are considered. Implications for the astrophyiscal models are discussed.     

Vortex lines in neutron star superfluids and decay of pulsar magnetic fields

We adopt that in the interior of neutron stars both the proton and neutron superfluids are in the vortex state. Thus, in the superconducting core the magnetic field is expected to be organized in the form of quantized fluxoids. It is shown that fluxoids are buoyant. This gives rise to a rapid (5×10⁴ yr) expulsion of the magnetic field out of the superconducting core to the subcrustal region, and a subsequent decay within the outer crust. The effect considered may be the physical reason why the characteristic decay-time of pulsar magnetic fields (10⁶ yr) corresponds to the ohmic dissipation time within the neutron star crust. The intersection of two types of vortex lines with each other and its possible consequence for pulsars is briefly discussed.     

Collapsed white dwarfs as gamma-ray burst sources

It has been suggested by Usov (1992) that accreting white dwarfs, collapsing to neutron stars may be the sources of the gamma-ray bursts observed at cosmological distances, provided they rotate very fast and have enormously high magnetic fields. In this model the burst's duration is given by the ratio of pulsar kinetic energy and magneticdipole luminosity, so that in order to account for the shortest ( 0.1 s) bursts, the pulsars must rotate very fast (with periodP 0.5 ms) and have magnetic fields of 10¹⁶ - 10¹⁷ G. Though the high pulsar frequency was anticipated (Qadir and Rafique, 1986) and has been shown to be plausible (Abramowicz, 1990), the extremely high magnetic fields seem anomalous as observed neutron stars have fields below 10¹³ G.The problem with Usov's proposal is reduced by incorporating the relativistic corrections for fast rotating magnetic dipoles (Belinskyet al., 1994) or magnetic stars (De Paolis and Qadir, 1994). These corrections substantially enhance the radiation efficiency due to the existence of a magnetic synchrotron effect so that the magnetic field required for the explanation of the shortest gamma-ray bursts is strongly reduced. As such the model becomes much more plausible.     

Some Comments on the Magnetic Braking of CP Stars

The low rotation velocities of magnetic CP stars are discussed. Arguments against the involvement of the magnetic field in the loss of angular momentum are given: (1) the fields are not strong enough in young stars in the stage of evolution prior to the main sequence; (2) there is no significant statistical correlation between the magnetic field strength and the rotation period of CP stars; (3) stars with short periods have the highest fields; (4) a substantial number of stars with very low magnetic fields (B _e < 500 G) have rotation speeds that are typical of other CP stars; (5) simulations of the magnetic fields by Leroy and the author show that the orientation of dipoles inside rotating stars, both slow and fast, is consistent with an arbitrary orientation of the dipoles; and, (6) slow rotators with P>25 days, which form 12% of the total, probably lie at the edge of the velocity distribution for low mass stars. All of these properties conflict with the hypothesis of magnetic braking of CP stars.     

Influence of MHD waves on thermonuclear burning in surface layers of neutron stars: The nature of gamma-bursts

A possibility for gamma-ray bursts to arise due to thermonuclear flashes in the surface layers of accreting neutron stars is discussed. The principal difference of the sources of gamma-ray bursts from bursters is supposed to result from the existence of strong magnetic fields (10¹²–10¹³G) on the neutron star surface. It is shown that the thermonuclear energy released may be rapidly and effectively transported to the outer layers by MHD waves (in particular, by Alfvén waves). A very short growth time and rapid variations of some gamma-ray bursts may be easily explained in this case.     

Equation of state and anisotropy of pressure of magnetars

Magnetars are the neutron stars with the highest magnetic fields up to 10¹⁵–10¹⁶ G. It has been proposed that they are also responsible for a variety of extra-galactic phenomena, ranging from giant flares in nearby galaxies to fast radio bursts. Utilizing a relativistic mean field model and a variable magnetic field configuration, we investigate the effects of strong magnetic fields on the equation of state and anisotropy of pressure of magnetars. It is found that the mass and radius of low-mass magnetars are weakly enhanced under the action of the strong magnetic field, and the anisotropy of pressure can be ignored. Unlike other previous investigations, the magnetic field is unable to violate the mass limit of the neutron stars.     

Symmetry of the neutron and proton superfluidity effects in cooling neutron stars

We investigate the combined effect of neutron and proton superfluidities on the cooling of neutron stars whose cores consist of nucleons and electrons. We consider the singlet state paring of protons and the triplet pairing of neutrons in the cores of neutron stars. The critical superfluid temperatures T _c are assumed to depend on the matter density. We study two types of neutron pairing with different components of the total angular momentum of a Cooper pair along the quantization axis (|m _J |=0 or 2). Our calculations are compared with the observations of thermal emission from isolated neutron stars. We show that the observations can be interpreted by using two classes of superfluidity models: (1) strong proton superfluidity with a maximum critical temperature in the stellar core T _c ^max ?4×10⁹ K and weak neutron superfluidity of any type (T _c ^max ?2×10⁸ K); (2) strong neutron superfluidity (pairing with m _J =0) and weak proton superfluidity. The two types of models reflect an approximate symmetry with respect to an interchange of the critical neutron and proton pairing temperatures.     

One-photon and two-photon annihilation lines in gamma-bursts,I

This paper and subsequent Paper II are an investigation of the annihilation line formation in gamma-ray bursts based on the assumption of positron production in a strong magnetic field (because of one-photon absorption of hard gamma-quanta radiated in the neutron star hot polar spot). We discuss a two-photon annihilation line in this paper. It is shown that if the star magnetic field is greater than 3×10¹² G, the relative flux in the line depends solely on the hardness of the continuum and is, as a rule, less than or about 10–20% of the total flux. This is consistent with the spectral data recorded by ‘Venera-11’ and ‘Venera-12’ space probes. The annihilation region formation above the hot polar spot is discussed, and positron density and annihilation region dimensions are estimated.     

Two-photon annihilation radiation in strong magnetic field: the case of small longitudinal velocities of electrons and positrons

Spectra, angular distributions, and polarization of two-photon annihilation radiation in a magnetic field are studied in detail in the case of small longitudinal velocities of annihilating electrons and positrons which occupy the ground Landau level. Magnetic field essentially affects the annihilation if its magnitudeB is not very low in comparison withB _cr=4.4×10¹³G, which may take place near the surface of a neutron star. The magnetic field broadens the spectra (the width depends on an angle betweenB and a wave vector) and leads to their asymmetry. The angular distribution may be highly anisotropic, being fan-like or pencillike for different photon energies . The total annihilation rate is suppressed by the magnetic field (B ^–3 forBB _cr).The radiation is linearly polarized; the degree and orientation of the polarization depend onB, and . The polarization may reach several tens percent even for comparatively small fieldsB 0.1B _crtypical for neutron stars. This means that the polarization may be detected, e.g., in the annihilation radiation from the gamma-ray bursts.     

The influence of magnetic field and rotation on the onset of convection in the envelopes of neutron stars

The influence of magnetic field and rotation on the occurrence of convective instabilities in the liquid layer of neutron star envelopes has been investigated. The critical wavelength _c , which denotes the boundary between stable and unstable behaviour of convective disturbances, is calculated for a neutron star model as a function of magnetic field and rotation. It is shown that the strength of the magnetic fields of neutron stars strongly suppresses the onset of convection, whereas the limiting effect of rotation acts only if the magnetic field vanishes.     

Gamma-quanta conversion into positronium atoms in a strong magnetic field

The curvature -quanta emitted in the pulsar magnetospheres tangentially to the curved lines of force of the magnetic field are shown to be later canalized along the magnetic field—if it is strong enough,B0.1B _cr =4×10¹²G—by gradually converting into a mutually bound electron-positron pair, i.e. a positronium atom. This happens before the photon reaches the threshold of free-pair creation. The positronium thus arising is stable against the ionizing action of the electric field near the pulsar unless it reaches a critical value about 4×10⁷ CGSE forB10¹³G. This prevents the screening of the electric field up to the distances from the pulsar, where the magnetic field is already below the value of 0.1B _cr and the free pair creation may become essential. This effect provides, at least within the Arons model, a higher theoretical estimate for the total luminosity of pulsars whose field at the surfaceB _s exceeds 0.1B _cr as compared with the conventional one.     

Bimodality of anomalous pulsars?

At present, it is widely believed that anomalous X-ray pulsars (AXPs), soft gamma-ray repeaters (SGRs), rotational radio transients (RRATs), compact central objects (CCOs) in supernova remnants, and X-ray dim isolated neutron stars (XDINSs) belong to different classes of anomalous objects in which the central bodies are isolated neutron stars. Previously, we have shown that AXPs and SGRs can be described in terms of the drift model for parameters of the central neutron star typical of radio pulsars (rotation periods P ～ 0.1–1 s and surface magnetic fields B ～ 10¹¹–10¹³ G). Here, we show that some of the peculiarities of the sources under consideration can be explained by their geometry (in particular, by the angle β between the rotation axis and the magnetic moment). If β ? 10° (an aligned rotator), the drift waves in the outer layers of the neutron star magnetosphere can account for the observed periodicity in the radiation. For large β (a nearly orthogonal rotator), the observed modulation of the radiation and its short bursts can be explained by mass accretion from the ambient medium (e.g., a relic disk).