Vela, its X-ray nebula, and the polarization of pulsar radiation

The recent identification of the perpendicular mode of radio polarization as the primary one in the Vela pulsar by Lai et al. (2001) is interpreted in terms of the maser mechanism proposed by Luo & Melrose (1995). We suggest that such a mechanism may also be operative for the parallel mode which opens up the possibility of accounting for all types of polarization observed in pulsars. We propose an alternative interpretation of the arcs in the nebular X-radiation observed by Pavlov et al.(2000) & Helfand et al. (2001) with the Chandra Observatory, and interpreted by the latter as an equatorial wind. We interpret the arcs as traces of the particle beams from the two magnetic poles at the shock front. We also propose that the alignment with the rotation axis of the jet-like feature bisecting the arcs is an effect of projection on the sky plane and that there is no physical jet along the axis of rotation.Comment: 7 pages, 3 figures; version 2; accepted for publication in A&

Multifrequency Observations of Giant Radio Pulses from the Millisecond Pulsar B1937+21

Giant pulses are short, intense outbursts of radio emission with a power-law intensity distribution that have been observed from the Crab Pulsar and PSR B1937+21. We have undertaken a systematic study of giant pulses from PSR B1937+21 using the Arecibo telescope at 430, 1420, and 2380 MHz. At 430 MHz, interstellar scattering broadens giant pulses to durations of $\sim50 \mu$secs, but at higher frequencies the pulses are very short, typically lasting only $\sim1$-$2 \mu$secs. At each frequency, giant pulses are emitted only in narrow (\lsim10 \mus) windows of pulse phase located $\sim 55$-$70 \mu$sec after the main and interpulse peaks. Although some pulse-to-pulse jitter in arrival times is observed, the mean arrival phase appears stable; a timing analysis of the giant pulses yields precision competitive with the best average profile timing studies. We have measured the intensity distribution of the giant pulses, confirming a roughly power-law distribution with approximate index of -1.8, contributing \gsim0.1% to the total flux at each frequency. We also find that the intensity of giant pulses falls off with a slightly steeper power of frequency than the ordinary radio emission.Comment: 21 pages, 10 Postscript figures; LaTeX with aaspp4.sty and epsf.tex; submitted to Ap

Surgical Treatment and Outcome of Extracranial Germ Cell Tumors in Childhood

Germ cell tumors (GCTs) are a heterogeneous group of very rare tumors, benign or malignant, which can occur from newborn to old age. They are thought to arise from primordial germ cells, and are found in a variety of sites. This thesis is composed of 2 major parts. In a first part, the hospital records of 193 infants and children with GCTs, treated between 1960 and 2003 in ErasmusMC-Sophia Children’s Hospital Rotterdam and between 1986 and 2003 in the Academic Hospital of the Free University of Brussels, are studied. The long-term outcome for the various anatomical sites and histologies, is studied. Pitfalls in diagnosis and treatment are reported. A second part of the thesis is devoted to the study of (a) 173 patients with sacrococcygeal teratoma, treated between 1970 and 2003 in the six centers of pediatric surgery in the Netherlands, and of (b) 70 patients with SCT treated between 1960 and 2003 in ErasmusMC-Sophia Children’s Hospital Rotterdam. In these patient populations, risk factors for recurrence are studied, as well as functional results and quality of life. Subject headings: 1. Introduction 2. Influence of Tumor Site and Histology on Long-Term Survival in 193 Children with Extracranial Germ Cell Tumors Treated between 1960 - 2003 3. Ovarian Germ Cell Tumors in Children: A Clinical Study of 66 Patients 4. Testicular Germ Cell Tumors in Children: Management and Outcome in a Series of 20 Patients 5. Strategy for Management of Newborns with Cervical Teratoma 6. Mediastinal Germ Cell Tumors: Clinical Aspects and Outcomes in 7 Children 7. Retroperitoneal Germ Cell Tumors: A Clinical Study of 12 Patients 8. Study of the Factors Associated with Recurrence in Children with Sacrococcygeal Teratoma 9. Factors Associated with Recurrence and Metastatic Disease in Sacrococcygeal Teratoma: Results in the Netherlands (1970-2003) 10. Sacrococcygeal Teratoma: Results of a Retrospective Multicentric Study in Belgium and Luxembourg 11. Long-Term Functional Sequelae of Sacrococcygeal Teratoma: A National Study in the Netherlands 12. General Discussio

Nematic cells with defect-patterned alignment layers

Using Monte Carlo simulations of the Lebwohl--Lasher model we study the director ordering in a nematic cell where the top and bottom surfaces are patterned with a lattice of $\pm 1$ point topological defects of lattice spacing $a$. We find that the nematic order depends crucially on the ratio of the height of the cell $H$ to $a$. When $H/a \gtrsim 0.9$ the system is very well--ordered and the frustration induced by the lattice of defects is relieved by a network of half--integer defect lines which emerge from the point defects and hug the top and bottom surfaces of the cell. When $H/a \lesssim 0.9$ the system is disordered and the half--integer defect lines thread through the cell joining point defects on the top and bottom surfaces. We present a simple physical argument in terms of the length of the defect lines to explain these results. To facilitate eventual comparison with experimental systems we also simulate optical textures and study the switching behavior in the presence of an electric field