A wind model for high energy pulses

A solution to the sigma problem - that of finding a mechanism capable of converting Poynting energy flux to particle-borne energy flux in a pulsar wind - was proposed several years ago by Coroniti and Michel who considered a particular prescription for magnetic reconnection in a striped wind. This prescription was later shown to be ineffective. In this paper, we discuss the basic microphysics of the reconnection process and conclude that a more rapid prescription is permissible. Assuming dissipation to set in at some distance outside the light-cylinder, we compute the resulting radiation signature and find that the synchrotron emission of heated particles appears periodic, in general showing both a pulse and an interpulse. The predicted spacing of these agrees well with observation in the case of the Crab and Vela pulsars. Using parameters appropriate for the Crab pulsar - magnetization parameter at the light cylinder sigma_L = 6 x 10^4, Lorentz factor Gamma=250 - reasonable agreement is found with the observed total pulsed luminosity. This suggest that the high-energy pulses from young pulsars originate not in the co-rotating magnetosphere within the light cylinder (as in all other models) but from the radially directed wind well outside it.Comment: 6 pages, 2 figures. To appear in the Proceedings of the 270. WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan. 21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper. Proceedings are available as MPE-Report 27

A new nearby pulsar wind nebula overlapping the RX J0852.0-4622 supernova remnant

Energetic pulsars can be embedded in a nebula of relativistic leptons which is powered by the dissipation of the rotational energy of the pulsar. The object PSR J0855-4644 is an energetic and fast-spinning pulsar (Edot = 1.1x10^36 erg/s, P=65 ms) discovered near the South-East rim of the supernova remnant (SNR) RX J0852.0-4622 (aka Vela Jr) by the Parkes multibeam survey. The position of the pulsar is in spatial coincidence with an enhancement in X-rays and TeV gamma-rays, which could be due to its putative pulsar wind nebula (PWN). The purpose of this study is to search for diffuse non-thermal X-ray emission around PSR J0855-4644 to test for the presence of a PWN and to estimate the distance to the pulsar. An X-ray observation was carried out with the XMM-Newton satellite to constrain the properties of the pulsar and its nebula. The absorption column density derived in X-rays from the pulsar and from different regions of the rim of the SNR was compared with the absorption derived from the atomic (HI) and molecular (12CO) gas distribution along the corresponding lines of sight to estimate the distance of the pulsar and of the SNR. The observation has revealed the X-ray counterpart of the pulsar together with surrounding extended emission thus confirming the existence of a PWN. The comparison of column densities provided an upper limit to the distance of the pulsar PSR J0855-4644 and the SNR RX J0852.0-4622 (d<900 pc). Although both objects are at compatible distances, we rule out that the pulsar and the SNR are associated. With this revised distance, PSR J0855-4644 is the second most energetic pulsar, after the Vela pulsar, within a radius of 1 kpc and could therefore contribute to the local cosmic-ray e-/e+ spectrum.Comment: 10 pages, 9 Figures. Accepted for publication in A&

Dissociating visuo-spatial and verbal working memory: It’s all in the features

Echoing many of the themes of the seminal work of Atkinson and Shiffrin (1968), this paper uses the Feature Model (Nairne, 1988, 1990; Neath & Nairne, 1995) to account for performance in working memory tasks. The Brooks verbal and visuo-spatial matrix tasks were performed alone, with articulatory suppression, or with a spatial suppression task; the results produced the expected dissociation. We used Approximate Bayesian Computation techniques to fit the Feature Model to the data and showed that the similarity-based interference process implemented in the model accounted for the data patterns well. We then fit the model to data from Guérard and Tremblay (2008); the latter study produced a double dissociation while calling upon more typical order reconstruction tasks. Again, the model performed well. The findings show that a double dissociation can be modelled without appealing to separate systems for verbal and visuo-spatial processing. The latter findings are significant as the Feature Model had not been used to model this type of dissociation before; importantly, this is also the first time the model is quantitatively fit to data. For the demonstration provided here, modularity was unnecessary if two assumptions were made: (1) the main difference between spatial and verbal working memory tasks is the features that are encoded; (2) secondary tasks selectively interfere with primary tasks to the extent that both tasks involve similar features. It is argued that a feature-based view is more parsimonious (see Morey, 2018) and offers flexibility in accounting for multiple benchmark effects in the field

Developmental sensitivity and immunotoxicity of benzo[a]pyrene in the amphibian Xenopus laevis

The immune system is increasingly recognized as a vulnerable and sensitive target of contaminant exposure. Disruption of this critical system can lead to significant morbidity and/or mortality of organisms in contaminated environments. Amphibians can be simultaneously exposed to both pathogens and contaminants within aquatic habitats, making contaminant-induced immunotoxicity particularly relevant for this taxonomic group. Polycyclic aromatic hydrocarbons (PAHs) are a major group of environmental contaminants that widely occur as mixtures in aquatic environments, including amphibian habitats. Benzo[a]pyrene (B[a]P) is recognized as the most potent toxicant of PAH mixtures and is the most extensively studied PAH with demonstrated adverse effects on the immune system of many vertebrate species. Understanding how aquatic contaminants, such as B[a]P, can impact the immune system of amphibians is imperative since contaminant-induced immunotoxicity is hypothesized to exacerbate disease-driven amphibian population decline. Thus, the overall goal of this thesis was to gain a better understanding of developmental and contaminant-induced changes in the immune system of amphibians, and ultimately how exposure to B[a]P modulates the response to immune stimulation. Specific research questions were addressed through four studies conducted within the broader goal of this thesis. The objective of the first study was to characterize the expression profiles of various pro-inflammatory cytokines in the model amphibian species, Xenopus laevis, throughout larval developmental stages and determine the impacts of thyroidal modulation on their expression. Results suggest that expression of two cytokines, tumor necrosis factor- (TNF) and interleukin-1β (IL-1β) varied over development, increasing with developmental stage, while interferon- (IFN-) remained relatively unchanged. Exogenous manipulation of thyroidal status was successful, as demonstrated by an altered rate of development and thyroid gland histology; however, thyroidal modulation had negligible effects on basal cytokine expression. The dynamic nature of the tadpole immune system suggested that the immunotoxic impacts of environmental contaminants may depend on the stage of development at the time of exposure. Therefore, the second study aimed to identify the sensitive developmental phases for sub-lethal effects of B[a]P exposure in terms of the development (stage, weight and length) and changes in the expression of immune- and detoxification-related genes. Exposure to B[a]P at all life-stages tested (embryo-larval, pre-metamorphic, and pro-metamorphic tadpoles) resulted in CYP1A7 mRNA induction (43, 53, and 47-fold, respectively) as well as stage-specific modulation of pro-inflammatory cytokine expression. Exposure of pre-metamorphic tadpoles to B[a]P demonstrated the greatest effect on weight, length and stage of development. Taken together, these initial studies indicate that the unique development of the tadpole immune system influences their susceptibility to contaminant-induced immune modulation despite having biotransformation capacity across tadpole life-stages. In the subsequent two studies, the ability of B[a]P to alter immune variables and impair a coordinated immune response was evaluated using an immune challenge model both in vivo and in vitro. Juvenile X. laevis were exposed to B[a]P for seven days and then immune stimulated with an injection of lipopolysaccharide (LPS). The highest concentration of B[a]P (350 g/L) impaired the inflammatory response to LPS as indicated by an inability to induce granulocyte:lymphocyte (G:L) ratio or IL-1β mRNA expression, demonstrating that B[a]P can impair the inflammatory immune response to a simulated pathogen. Next, to assess the mechanisms underlying B[a]P-induced effects in amphibians, X. laevis kidney A6 epithelial cell line were used to evaluate the cellular response to B[a]P exposure and identify if B[a]P could directly alter the ability of these cells to respond to an immune stimulation. A time-course of B[a]P exposure showed the progression from up-regulation of biotransformation-related genes, to reactive oxygen species production, and ultimately to cytotoxic effects; pre-exposure to B[a]P did not appear to impair the ability of the A6 cells to mount an inflammatory response to LPS. Taken together, the immune challenge models showed that B[a]P exposure can alter immune variables and impact the ability of juvenile X. laevis to appropriately respond to immune stimulation and that B[a]P-induced biostransformation pathways and production of reactive oxygen species may play a role in this response. Overall, the research presented in this thesis contributes to our understanding of developmental and contaminant-induced changes in the immune system of amphibians. This research provides a strong foundation for further study into the mechanisms underlying chemical-induced immunotoxicity in amphibians. The tools and approaches developed as part of this research could be used in screening chemicals for immunotoxic potential in amphibians and be expanded to evaluate immune responses of ecologically relevant species that are under real risk of compromised immune function and increased disease in contaminated environments