Neutrino emission via the plasma process in a magnetized plasma

Neutrino emission via the plasma process using the vertex formalism for QED in a strongly magnetized plasma is considered. A new vertex function is introduced to include the axial vector part of the weak interaction. Our results are compared with previous calculations, and the effect of the axial vector coupling on neutrino emission is discussed. The contribution from the axial vector coupling can be of the same order as or greater than the vector vector coupling under certain plasma conditions.Comment: 20 pages, 3 figure

An alternative to the plasma emission model: Particle-In-Cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts

1.5D PIC, relativistic, fully electromagnetic (EM) simulations are used to model EM wave emission generation in the context of solar type III radio bursts. The model studies generation of EM waves by a super-thermal, hot beam of electrons injected into a plasma thread that contains uniform longitudinal magnetic field and a parabolic density gradient. In effect, a single magnetic line connecting Sun to earth is considered, for which several cases are studied. (i) We find that the physical system without a beam is stable and only low amplitude level EM drift waves (noise) are excited. (ii) The beam injection direction is controlled by setting either longitudinal or oblique electron initial drift speed, i.e. by setting the beam pitch angle. In the case of zero pitch angle, the beam excites only electrostatic, standing waves, oscillating at plasma frequency, in the beam injection spatial location, and only low level EM drift wave noise is also generated. (iii) In the case of oblique beam pitch angles, again electrostatic waves with same properties are excited. However, now the beam also generates EM waves with the properties commensurate to type III radio bursts. The latter is evidenced by the wavelet analysis of transverse electric field component, which shows that as the beam moves to the regions of lower density, frequency of the EM waves drops accordingly. (iv) When the density gradient is removed, electron beam with an oblique pitch angle still generates the EM radiation. However, in the latter case no frequency decrease is seen. Within the limitations of the model, the study presents the first attempt to produce simulated dynamical spectrum of type III radio bursts in fully kinetic plasma model. The latter is based on 1.5D non-zero pitch angle (non-gyrotropic) electron beam, that is an alternative to the plasma emission classical mechanism.Comment: Physics of Plasmas, in press, May 2011 issue (final accepted version

Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra

The splitting of photons in the presence of an intense magnetic field has recently found astrophysical applications in polar cap models of gamma-ray pulsars and in magnetar scenarios for soft gamma repeaters. Numerical computation of the polarization-dependent rates of this third order QED process for arbitrary field strengths and energies below pair creation threshold is difficult: thus early analyses focused on analytic developments and simpler asymptotic forms. The recent astrophysical interest spurred the use of the S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In this paper, we present numerical computations of a full proper-time expression for the rate of splitting that was obtained by Stoneham, and is exact up to the pair creation threshold. While the numerical results derived here are in accord with the earlier asymptotic forms due to Adler, our computed rates still differ by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner, reflecting the extreme difficulty of generating accurate S-matrix numerics for fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time rates appear very accurate, and exceed Adler's asymptotic specializations significantly only for photon energies just below pair threshold and for supercritical fields, but always by less than a factor of around 2.6. We also provide a useful analytic series expansion for the scattering amplitude valid at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres

Radio Spectral Evolution of an X-ray Poor Impulsive Solar Flare: Implications for Plasma Heating and Electron Acceleration

We present radio and X-ray observations of an impulsive solar flare that was moderately intense in microwaves, yet showed very meager EUV and X-ray emission. The flare occurred on 2001 Oct 24 and was well-observed at radio wavelengths by the Nobeyama Radioheliograph (NoRH), the Nobeyama Radio Polarimeters (NoRP), and by the Owens Valley Solar Array (OVSA). It was also observed in EUV and X-ray wavelength bands by the TRACE, GOES, and Yohkoh satellites. We find that the impulsive onset of the radio emission is progressively delayed with increasing frequency relative to the onset of hard X-ray emission. In contrast, the time of flux density maximum is progressively delayed with decreasing frequency. The decay phase is independent of radio frequency. The simple source morphology and the excellent spectral coverage at radio wavelengths allowed us to employ a nonlinear chi-squared minimization scheme to fit the time series of radio spectra to a source model that accounts for the observed radio emission in terms of gyrosynchrotron radiation from MeV-energy electrons in a relatively dense thermal plasma. We discuss plasma heating and electron acceleration in view of the parametric trends implied by the model fitting. We suggest that stochastic acceleration likely plays a role in accelerating the radio-emitting electrons.Comment: 22 pages, 10 figure

Proton Cyclotron Features in Thermal Spectra of Ultra-magnetized Neutron Stars

A great deal of interest has been recently raised in connection with the possibility that soft $\gamma$-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) contain {\em magnetars}, young neutron stars endowed with magnetic fields $\gtrsim 10^{14}$ G. In this paper we calculate thermal spectra from ultra-magnetized neutron stars for values of the luminosity and magnetic field believed to be relevant to SGRs and AXPs. Emergent spectra are found to be very close to a blackbody at the star effective temperature and exhibit a distinctive absorption feature at the proton cyclotron energy $E_{c,p}\simeq 0.63 (B/10^{14} {\rm G})$ keV. The proton cyclotron features (PCFs) are conspicuous (equivalent width of up to many hundreds eV) and relatively broad ($\Delta E/E \sim 0.05-0.2$). The detection of the PCFs is well within the capabilities of present X-ray spectrometers, like the HETGS and METGS on board Chandra. Their observation might provide decisive evidence in favor of the existence of magnetars.Comment: 7 pages, 4 figures, minor changes included, typos corrected. Accepted for publication in Ap

Observations of radio pulses from CU Virginis

The magnetic chemically peculiar star CU Virginis is a unique astrophysical laboratory for stellar magnetospheres and coherent emission processes. It is the only known main sequence star to emit a radio pulse every rotation period. Here we report on new observations of the CU Virginis pulse profile in the 13 and 20\,cm radio bands. The profile is known to be characterised by two peaks of 100$\%$ circularly polarised emission that are thought to arise in an electron-cyclotron maser mechanism. We find that the trailing peak is stable at both 13 and 20\,cm, whereas the leading peak is intermittent at 13\,cm. Our measured pulse arrival times confirm the discrepancy previously reported between the putative stellar rotation rates measured with optical data and with radio observations. We suggest that this period discrepancy might be caused by an unknown companion or by instabilities in the emission region. Regular long-term pulse timing and simultaneous multi-wavelength observations are essential to clarify the behaviour of this emerging class of transient radio source.Comment: Accepted by MNRAS Letters; 5 pages, 2 figures, 3 table

Spin-Dependent Cyclotron Decay Rates in Strong Magnetic Fields

Cyclotron decay and absorption rates have been well studied in the literature, focusing primarily on spectral, angular and polarization dependence. Astrophysical applications usually do not require retention of information on the electron spin state, and these are normally averaged in obtaining the requisite rates. In magnetic fields, higher order quantum processes such as Compton scattering become resonant at the cyclotron frequency and its harmonics, with the resonances being formally divergent. Such divergences are usually eliminated by accounting for the finite lifetimes of excited Landau states. This practice requires the use of spin-dependent cyclotron rates in order to obtain accurate determinations of process rates very near cyclotronic resonances, the phase space domain most relevant for certain applications to pulsar models. This paper develops previous results in the literature to obtain compact analytic expressions for cyclotron decay rates/widths in terms of a series of Legendre functions of the second kind; these expressions can be expediently used in astrophysical models. The rates are derived using two popular eigenstate formalisms, namely that due to Sokolov and Ternov, and that due to Johnson and Lippmann. These constitute two sets of eigenfunctions of the Dirac equation that diagonalize different operators, and accordingly yield different spin-dependent cyclotron rates. This paper illustrates the attractive Lorentz transformation characteristics of the Sokolov and Ternov formulation, which is another reason why it is preferable when electron spin information must be explicitly retained.Comment: 11 pages, 2 embedded figures, apjgalley format, To appear in The Astrophysical Journal, Vol 630, September 1, 2005 issu

Magnetic Photon Splitting: the S-Matrix Formulation in the Landau Representation

Calculations of reaction rates for the third-order QED process of photon splitting in strong magnetic fields traditionally have employed either the effective Lagrangian method or variants of Schwinger's proper-time technique. Recently, Mentzel, Berg and Wunner (1994) presented an alternative derivation via an S-matrix formulation in the Landau representation. Advantages of such a formulation include the ability to compute rates near pair resonances above pair threshold. This paper presents new developments of the Landau representation formalism as applied to photon splitting, providing significant advances beyond the work of Mentzel et al. by summing over the spin quantum numbers of the electron propagators, and analytically integrating over the component of momentum of the intermediate states that is parallel to field. The ensuing tractable expressions for the scattering amplitudes are satisfyingly compact, and of an appearance familiar to S-matrix theory applications. Such developments can facilitate numerical computations of splitting considerably both below and above pair threshold. Specializations to two regimes of interest are obtained, namely the limit of highly supercritical fields and the domain where photon energies are far inferior to that for the threshold of single-photon pair creation. In particular, for the first time the low-frequency amplitudes are simply expressed in terms of the Gamma function, its integral and its derivatives. In addition, the equivalence of the asymptotic forms in these two domains to extant results from effective Lagrangian/proper-time formulations is demonstrated.Comment: 19 pages, 3 figures, REVTeX; accepted for publication in Phys. Rev.

Circular Polarization Induced by Scintillation in a Magnetized Medium

A new theory is presented for the development of circular polarization as radio waves propagate through the turbulent, birefringent interstellar medium. The fourth order moments of the wavefield are calculated and it is shown that unpolarized incident radiation develops a nonzero variance in circular polarization. A magnetized turbulent medium causes the Stokes parameters to scintillate in a non-identical manner. A specific model for this effect is developed for the case of density fluctuations in a uniform magnetic field.Comment: 16 pages, 1 figure, Phys. Rev. E, accepte