Michelle C. Storey

Pulse Profiles, Accretion Column Dips and a Flare in GX 1+4 During a Faint State

The Rossi X-ray Timing Explorer spacecraft observed the X-ray pulsar GX 1+4 for a period of 34 hr on 1996 July 19/20. The source faded from an intensity of ~20 mcrab to a minimum of ≤0.7 mcrab and then partially recovered toward the end of the observation. This extended minimum lasted ~40,000 s. Phase-folded light curves at a barycentric rotation period of 124.36568±0.00020 s show that near the center of the extended minimum the source stopped pulsing in the traditional sense but retained a weak dip feature at the rotation period. Away from the extended minimum, the dips are progressively narrower at higher energies and may be interpreted as obscurations or eclipses of the hot spot by the accretion column. The pulse profile changed from leading-edge bright before the extended minimum to trailing-edge bright after it. Data from the Burst and Transient Source Experiment show that a torque reversal occurred less than 10 days after our observation. Our data indicate that the observed rotation departs from a constant period with a /P value of ~-1.5% per year at a 4.5 σ significance. We infer that we may have serendipitously obtained data, with high sensitivity and temporal resolution about the time of an accretion disk spin reversal. We also observed a rapid flare that had some precursor activity, close to the center of the extended minimum.

Collisions in strong magnetic fields

Exact cross sections for electron-electron collisions and electron-proton collisions in a superstrong magnetic field are derived using the QED formalism developed by Melrose and Parle. The results are compared with those of Langer who used a different QED formalism. The intended application is to collision processes in the accretion columns above neutron stars where magnetic fields of order 109 T are thought to be present. The particular case of electrons initially in their ground states, with one final electron in an excited state is described in detail; this process is thought to be the primary source of photons in X-ray pulsars, through subsequent cyclotron emission.

Nonthermal Radio Emission from the Precataclysmic Binary System V471 Tauri

Recent observations have presented strong evidence for the existence of eclipses in the nonthermal radio emission from the precataclysmic binary system V471 Tau. In this paper we calculate the gyrosynchrotron emission from models assuming a dipole-shaped magnetosphere and a magnetized wind around the K dwarf. The model assumes that electrons are accelerated to mildly relativistic energies in the region where the magnetic fields of the two stars interact and become trapped in the K dwarfs magnetosphere. Comparison with the data indicates that azimuthal drift and energy loss lead to a region of enhanced density of mildly relativistic electrons, extending for over 100° in azimuth and preceding the white dwarf in phase. The data is best fitted by models assuming a power-law drop with azimuthal angle of this enhancement in mildly relativistic electron density.

Quiescent Non-thermal Radio Emission from Stellar Systems

Non-thermal radio emission has been detected from dMe stars, RS CVn binaries and W T Tauri stars. Polarisation and intensity measurements of the quiescent (i.e. non-flaring) emission indicate that the emission is gyrosynchrotron emission from mildly relativistic electrons spiralling in a magnetic field. A three-dimensional dipole magnetic field model for the stellar field is presented and the quiescent gyrosynchrotron emission from such a model is calculated and compared with observations. The model can account for many phenomenological features of quiescent emission. Quantitative comparisons of model results with observations indicate that the electron distribution in the emission region may be a magnetic mirroring distribution.

Radiation transfer of gyrosynchrotron emission in stellar environments

Non-thermal radio emission has been detected from several kinds of active stars. Polarization and intensity measurements of the quiescent (i.e. non-flaring) emission indicate gyrosynchrotron emission. A three-dimensional magnetic field model for the stellar field is presented and the gyrosynchrotron intensity and polarization emerging from such a model is calculated and compared with observations. Model spectra agree well with observations. Model polarization results indicate that the emission region is more complex than the simple model and may indicate the presence of multipole components in the field, small loops of flux near the stellar surface and significant energy loss in the radiating electron distribution over time.

Asymmetric X-ray Pulsar Beam Emission

Many of the X-ray pulsars for which X-ray lightcurves have been presented in the literature exhibit asymmetric emission beams. We postulate the existence of an off-centre magnetic dipole field embedded in the rotating neutron star and show that such a field leads to varying rates of matter transfer between an accretion dise and the neutron star surface, over a rotation period. Assuming that the accretion rate onto the surface is simply related to the luminosity, we show that most of the observed asymmetries can be accounted for by this mechanism.