150 research outputs found
The Transition between Nonorthogonal Polarization Modes in PSR B2016+28 at 1404 MHz
Polarization observations of the radio emission from PSR B2016+28 at 1404 MHz
reveal properties that are consistent with two, very different, interpretations
of the pulsar's viewing geometry. The pulsar's average polarization properties
show a rapid change in position angle (PA) near the pulse center, suggesting
that the observer's sightline nearly intersects the star's magnetic pole. But
single pulse, polarization observations of the pulsar show nearly orthogonal
modes of polarization following relatively flat and parallel PA trajectories
across the pulse, suggesting that the sightline is far from the pole.
Additionally, PA histograms reveal a "modal connecting bridge", of unknown
origin, joining the modal PA trajectories over much of the pulse and following
the rapid PA change shown in the average data. The nonorthogonality of
polarization modes is incorporated in a statistical model of radio polarization
to account for the deviations from mode orthogonality that are observed in the
pulsar. The model is used to interpret the rapid PA change and modal connecting
bridge as a longitudinally-resolved transition between modes of nonorthogonal
polarization. Thus, the modal PA trajectories are argued to reflect the
pulsar's true viewing geometry. This interpretation is consistent with the
pulsar's morphological classification, preserves the Radhakrishnan & Cooke
model of pulsar radio emission, and avoids the complication that the modal
connecting bridge might be produced by some other emission mechanism. The
statistical model's ability to simulate the rich variety of polarization
properties observed in the emission lends additional support to the model's
applicability and its underlying assumption that the polarization modes occur
simultaneously.Comment: Accepted for publication in Ap
On the Excess Dispersion in the Polarization Position Angle of Pulsar Radio Emission
The polarization position angles (PA) of pulsar radio emission occupy a
distribution that can be much wider than what is expected from the average
linear polarization and the off-pulse instrumental noise. Contrary to our
limited understanding of the emission mechanism, the excess dispersion in PA
implies that pulsar PAs vary in a random fashion. An eigenvalue analysis of the
measured Stokes parameters is developed to determine the origin of the excess
PA dispersion. The analysis is applied to sensitive, well-calibrated
polarization observations of PSR B1929+10 and PSR B2020+28. The analysis
clarifies the origin of polarization fluctuations in the emission and reveals
that the excess PA dispersion is caused by the isotropic inflation of the data
point cluster formed by the measured Stokes parameters. The inflation of the
cluster is not consistent with random fluctuations in PA, as might be expected
from random changes in the orientation of the magnetic field lines in the
emission region or from stochastic Faraday rotation in either the pulsar
magnetosphere or the interstellar medium. The inflation of the cluster, and
thus the excess PA dispersion, is attributed to randomly polarized radiation in
the received pulsar signal. The analysis also indicates that orthogonal
polarization modes (OPM) occur where the radio emission is heavily modulated.
In fact, OPM may only occur where the modulation index exceeds a critical value
of about 0.3.Comment: Accepted for publication in Ap
Orientation Angles of a Pulsar's Polarization Vector
A statistical model of the polarization of pulsar radio emission is used to
derive the general statistics of a polarization vector's orientation angles.
The theoretical distributions are compared with orientation angle histograms
computed from single-pulse, polarization observations of PSR B2020+28. The
favorable agreement between the theoretical and measured distributions lends
support to the underlying assumptions of the statistical model, and
demonstrates, like recent work on other pulsars, that the handedness of
circular polarization is associated with the radiation's orthogonally polarized
modes. Comprehensive directional statistics of the vector's orientation angles
are also derived, and are shown to follow the Watson bipolar and Fisher
distributions in its limiting forms.Comment: 14 pages, 4 figures, accepted for publication in Ap
Altered hippocampal morphology in unmedicated patients with major depressive illness
Despite converging evidence that major depressive illness is associated with both memory impairment and hippocampal pathology, findings vary widely across studies and it is not known whether these changes are regionally specific. In the present study we acquired brain MRIs (magnetic resonance images) from 31 unmedicated patients with MDD (major depressive disorder; mean age 39.2±11.9 years; 77% female) and 31 demographically comparable controls. Three-dimensional parametric mesh models were created to examine localized alterations of hippocampal morphology. Although global volumes did not differ between groups, statistical mapping results revealed that in MDD patients, more severe depressive symptoms were associated with greater left hippocampal atrophy, particularly in CA1 (cornu ammonis 1) subfields and the subiculum. However, previous treatment with atypical antipsychotics was associated with a trend towards larger left hippocampal volume. Our findings suggest effects of illness severity on hippocampal size, as well as a possible effect of past history of atypical antipsychotic treatment, which may reflect prolonged neuroprotective effects. This possibility awaits confirmation in longitudinal studies
An index to examine the sexual HIV risk of psychiatric service users based on sexual partners
Numerous studies report higher HIV infection rates among psychiatric patients than in the general population.1 Relative to other HIV-affected populations, they have higher rates of HIV-related risk behaviors in fewer sexual occasions, including multiple partners, partners of unknown or positive HIV status, sex in exchange for money, shelter or goods, and low condom use rates.2 We present a new HIV risk index (RI)3 that takes into account differential risk associated with these factors
Epitope Mapping of HIV-Specific CD8+ T cells in a Cohort Dominated by Clade A1 Infection
CD8+ T cell responses are often detected at large magnitudes in HIV-infected subjects, and eliciting these responses is the central aim of many HIV-1 vaccine strategies. Population differences in CD8+ T cell epitope specificity will need to be understood if vaccines are to be effective in multiple geographic regions.In a large Kenyan cohort, we compared responsive CD8+ T cell HIV-1 Env overlapping peptides (OLPs) to Best Defined Epitopes (BDEs), many of which have been defined in clade B infection. While the majority of BDEs (69%) were recognized in this population, nearly half of responsive OLPs (47%) did not contain described epitopes. Recognition frequencies of BDEs were inversely correlated to epitopic sequence differences between clade A1 and BDE (P = 0.019), and positively selected residues were more frequent in "new" OLPs (without BDEs). We assessed the impact of HLA and TAP binding on epitope recognition frequencies, focusing on predicted and actual epitopes in the HLA B7 supertype.Although many previously described CD8 epitopes were recognized, several novel CD8 epitopes were defined in this population, implying that epitope mapping efforts have not been completely exhausted. Expansion of these studies will be critical to understand population differences in CD8 epitope recognition
- …