The Influence of Longitudinal Space Charge Fields on the Modulation Process of Coherent Electron Cooling

The initial modulation in the scheme for Coherent electron Cooling (CeC) rests on the screening of the ion charge by electrons. However, in a CeC system with a bunched electron beam, inevitably, a long-range longitudinal space charge force is introduced. For a relatively dense electron beam, its force can be comparable to, or even greater than the attractive force from the ions. Hence, the influence of the space charge field on the modulation process could be important. If the 3-D Debye lengths are much smaller than the extension of the electron bunch, the modulation induced by the ion happens locally. Then, in that case, we can approximate the long-range longitudinal space charge field as a uniform electric field across the region. As detailed in this paper, we developed an analytical model to study the dynamics of ion shielding in the presence of a uniform electric field. We solved the coupled Vlasov-Poisson equation system for an infinite anisotropic electron plasma, and estimated the influences of the longitudinal space charge field to the modulation process for the experimental proof of the CeC principle at RHIC.Comment: 23 pages, 11 figures, single colum

Transverse Beam Transfer Functions of Colliding Beams in RHIC

We use transverse beam transfer functions to measure tune distributions of colliding beams in RHIC. The tune has a distribution due to the beam-beam interaction, nonlinear magnetic fields - particularly in the interaction region magnets, and non-zero chromaticity in conjunction with momentum spread. The measured tune distributions are compared with calculations

The effect of differential refraction on wave propagation in rotating pulsar magnetospheres

Refraction of wave propagation in a corotating pulsar magnetospheric plasma is considered as a possible interpretation for observed asymmetric pulse profiles with multiple components. The pulsar radio emission produced inside the magnetosphere propagates outward through the rotating magnetosphere, subject to refraction by the intervening plasma that is spatially inhomogeneous. Both effects of a relativistic distribution of the plasma and rotation on wave propagation are considered. It is shown that refraction coupled with rotation can produce asymmetric conal structures of the profile. The differential refraction due to the rotation can cause the conal structures to skew toward the rotation direction and lead to asymmetry in relative intensities between the leading and trailing components. Both of these features are potentially observable.Comment: 9 pages, 7 figures, accepted for publication in MNRA

High-precision geometry of a double-pole pulsar

High time resolution observations of PSR B0906-49 (or PSR J0908-4913) over a wide range of frequencies have enabled us to determine the geometry and beam shape of the pulsar. We have used the position angle traverse to determine highly-constrained solutions to the rotating vector model which show conclusively that PSR B0906-49 is an orthogonal rotator. The accuracy obtained in measuring the geometry is unprecedented. This may allow tests of high-energy emission models, should the pulsar be detected with GLAST. Although the impact parameter, beta, appears to be frequency dependent, we have shown that this is due to the effect of interstellar scattering. As a result, this pulsar provides some of the strongest evidence yet that the position angle swing is indeed related to a geometrical origin, at least for non-recycled pulsars. We show that the beam structures of the main pulse and interpulse in PSR B0906-49 are remarkably similar. The emission comes from a height of ~230 km and is consistent with originating in a patchy cone located about half way to the last open field lines. The rotation axis and direction of motion of the pulsar appear to be aligned.Comment: accepted for publication in MNRAS, 7 pages, 4 figure

Stokes tomography of radio pulsar magnetospheres. II. Millisecond pulsars

The radio polarization characteristics of millisecond pulsars (MSPs) differ significantly from those of non-recycled pulsars. In particular, the position angle (PA) swings of many MSPs deviate from the S-shape predicted by the rotating vector model, even after relativistic aberration is accounted for, indicating that they have non-dipolar magnetic geometries, likely due to a history of accretion. Stokes tomography uses phase portraits of the Stokes parameters as a diagnostic tool to infer a pulsar's magnetic geometry and orientation. This paper applies Stokes tomography to MSPs, generalizing the technique to handle interpulse emission. We present an atlas of look-up tables for the Stokes phase portraits and PA swings of MSPs with current-modified dipole fields, filled core and hollow cone beams, and two empirical linear polarization models. We compare our look-up tables to data from 15 MSPs and find that the Stokes phase portraits for a current-modified dipole approximately match several MSPs whose PA swings are flat or irregular and cannot be reconciled with the standard axisymmetric rotating vector model. PSR J1939+2134 and PSR J0437$-$4715 are modelled in detail. The data from PSR J1939+2134 at 0.61\,GHz can be fitted well with a current-modified dipole at $(\alpha, i) = (22 \pm 2^\circ, 80 \pm 1^\circ)$ and emission altitude 0.4 $r_\text{LC}$. The fit is less accurate for PSR J1939+2134 at 1.414\,GHz, and for PSR J0437$-$4715 at 1.44\,GHz, indicating that these objects may have a more complicated magnetic field geometry, such as a localized surface anomaly or a polar magnetic mountain.Comment: 38 pages, 33 figures, accepted for publication by MNRA

Altitude-dependent polarization in radio pulsars

Because of the corotation, the polarization angle (PA) curve of a pulsar lags the intensity profile by 4r/Rlc rad in pulse phase. I present a simple and short derivation of this delay-radius relation to show that it is not caused by the aberration (understood as the normal beaming effect) but purely by contribution of corotation to the electron acceleration in the observer's frame. Available altitude-dependent formulae for the PA curve are expressed through observables and emission altitude to make them immediately ready to use in radio data modelling. The analytical approximations for the altitude-dependent PA curve are compared with exact numerical results to show how they perform at large emission altitudes. I also discuss several possible explanations for the opposite-than-normal shift of PA curve, exhibited by the pedestal emission of B1929+10 and B0950+08.Comment: 12 pages, 5 figures, accepted by MNRAS after minor change

Faraday Rotation in Pulsar Magnetosphere

The magnetosphere of a pulsar is composed of relativistic plasmas streaming along the magnetic field lines and corotating with the pulsar. We study the intrinsic Faraday rotation in the pulsar magnetosphere by critically examining the wave modes and the variations of polarization properties for the circularly polarized natural modes under various assumptions about the magnetosphere plasma properties. Since it is difficult to describe analytically the Faraday rotation effect in such a plasma, we use numerical integrations to study the wave propagation effects in the corotating magnetosphere. Faraday rotation effect is identified among other propagation effects, such as wave mode coupling and the cyclotron absorption. In a highly symmetrical electron-positron pair plasma, the Faraday rotation effect is found to be negligible. Only for asymmetrical plasmas, such as the electron-ion streaming plasma, can the Faraday rotation effect become significant, and the Faraday rotation angle is found to be approximately proportional to $\lambda^{0.5}$ instead of the usual $\lambda^2$-law. For such electrons-ion plasma of pulsar magnetosphere, the induced rotation measure becomes larger at higher frequencies, and should have opposite signs for the emissions from opposite magnetic poles.Comment: 15 pages, 4 figure, submitted to MNRA

Comparing Geometrical and Delay Radio Emission Heights in Pulsars

We use a set of carefully selected published average multifrequency polarimetric observations for six bright cone dominated pulsars and devise a method to combine the multifrequency polarization position angle (PPA) sweep traverses. We demonstrate that the PPA traverse is in excellent agreement with the rotating vector model over this broad frequency range confirming that radio emission emanates from perfectly dipolar field lines. For pulsars with central core emission in our sample, we find the peak of central core component to lag the steepest gradient of the PPA traverse at several frequencies. Also significant frequency evolution of the core width is observed over this frequency range. The above facts strongly suggest: (a) the peak core emission does not lie on the fiducial plane containing the dipole magnetic axis and the rotation axis, and (b) the core emission does not originate from the polar cap surface.Comment: Accepted for publication in Astronomy and Astrophysic