Hydrodynamic Thermonuclear Runaways in Superbursts

We calculate the thermal and dynamical evolution of the surface layers of an accreting neutron star during the rise of a superburst. For the first few hours following unstable 12C ignition, the nuclear energy release is transported by convection. However, as the base temperature rises, the heating time becomes shorter than the eddy turnover time and convection becomes inefficient. This results in a hydrodynamic nuclear runaway, in which the heating time becomes shorter than the local dynamical time. Such hydrodynamic burning can drive shock waves into the surrounding layers and may be the trigger for the normal X-ray burst found to immediately precede the onset of the superburst in both cases where the Rossi X-Ray Timing Explorer was observing.Comment: 4 pages, 3 figures (emulateapj), accepted to ApJ Letter

Experiments with Subversion Over OpenNetInf and CCNx

We describe experiences and insights from adapting the Subversion version control system to use the network service of two information-centric networking (ICN) prototypes: OpenNetInf and CCNx. The evaluation is done using a local collaboration scenario, common in our own project work where a group of people meet and share documents through a Subversion repository. The measurements show a performance benefit already with two clients in some of the studied scenarios, despite being done on un-optimised research prototypes. The conclusion is that ICN clearly is beneficial also for non mass-distribution applications

Subversion Over OpenNetInf and CCNx

We describe experiences and insights from adapting the Subversion version control system to use the network service of two information-centric networking (ICN) prototypes: OpenNetInf and CCNx. The evaluation is done using a local collaboration scenario, common in our own project work where a group of people meet and share documents through a Subversion repository. The measurements show a performance benefit already with two clients in some of the studied scenarios, despite being done on un-optimised research prototypes. The conclusion is that ICN clearly is beneficial also for non mass-distribution applications. It was straightforward to adapt Subversion to fetch updated files from the repository using the ICN network service. The adaptation however neglected access control which will need a different approach in ICN than an authenticated SSL tunnel. Another insight from the experiments is that care needs to be taken when implementing the heavy ICN hash and signature calculations. In the prototypes, these are done serially, but we see an opportunity for parallelisation, making use of current multi-core processors

Mapping Myriad Positionalities in Amish Media Studies

Within the United States popular and academic imaginaries, Amish/Mennonite identities tend to get flattened into a monolithic, anti-modern stereotype simultaneously fearful of information communication technology and yet titillated by their utility. This pan-Amish construction relies on reducing interpersonal and intercultural complexities regarding media into bite-size binaries easily understood by English outsiders, which is critical when distilling information about myriad Amish norms and sects even if it means choosing breadth over depth. As a result, academic literature on the Amish mediascape tends to focus on binaries of traditionalism and modernity from a variety of disciplines, and research has not kept up with the rapidity with which newer information communication technologies are adopted by Amish/Mennonite communities. This research note discusses the budding field and literature of Amish media studies, presents a case study of a public Old Order Amish media-focused event to complicate traditional/modern binaries, and provides suggestions for future scholarship in this area. [Abstract by author

Seeing ethnographically: teaching ethnography as part of CSCW

While ethnography is an established part of CSCW research, teaching and learning ethnography presents unique and distinct challenges. This paper discusses a study of fieldwork and analysis amongst a group of students learning ethnography as part of a CSCW & design course. Studying the students’ practices we explore fieldwork as a learning experience, both learning about fieldsites as well as learning the practices of ethnography. During their fieldwork and analysis the students used a wiki to collaborate, sharing their field and analytic notes. From this we draw lessons for how ethnography can be taught as a collaborative analytic process and discuss extensions to the wiki to better support its use for collaborating around fieldnotes. In closing we reflect upon the role of learning ethnography as a practical hands on – rather than theoretical – pursuit

A Method for Distinguishing Between Transiently Accreting Neutron Stars and Black Holes, in Quiescence

We fit hydrogen atmosphere models to the X-ray data for four neutron stars (three from a previous paper, plus 4U 2129+47) and six black hole candidates (A0620-00, GS 2000+25, GS 1124-68, GS 2023+33, GRO J1655-40, and GRO J0422+32). While the neutron stars are similar in their intrinsic X-ray spectra (similar effective temperatures and emission area radii ~10 km), the spectra of two black hole candidates are significantly different, and the spectra of the remaining four are consistent with a very large parameter space that includes the neutron stars. The spectral differences between the neutron stars and black hole candidates favors the interpretation that the quiescent neutron star emission is predominantly thermal emission from the neutron star surface. Our work suggests that an X-ray spectral comparison in quiescence provides an additional means for distinguishing between neutron stars and black holes. The faint X-ray sources in globular clusters are also a class of objects which can be investigated in this manner.Comment: 33 pages, including 3 ps figures, LaTeX. To appear in Ap

A device for reducing the resistance of transom stern hulls

A novel idea to reduce the resistance of a transom stern hull in displacement and semi-planing modes is investigated. By placing a spoon-shaped device in the recirculating zone behind the transom, the momentum of the forward-moving water will be absorbed, and a pushing force generated on the device. Numerical and experimental techniques are applied on a transom stern hull to optimize the shape and position of the device and to explore in detail the physics behind the gain. For the towed hull at a Froude number of 0.4, the maximum measured resistance reduction is 11%, while the computed maximum reduction is 17%. In self-propulsion with one propeller, the measured power reduction is 15%. The power cannot be computed with the applied propeller model, which is an axial body-force distribution in the propeller disk, but the reduction in thrust using the device is 11%. More significant gains are possible at smaller Froude numbers, while the effect is reduced at higher Froude numbers. Larger gains are achieved by splitting the thrust on two propellers

Thermonuclear Burning on the Accreting X-Ray Pulsar GRO J1744-28

We investigate the thermal stability of nuclear burning on the accreting X-ray pulsar GRO J1744-28. The neutron star's dipolar magnetic field is <3\times 10^{11} G if persistent spin-up implies that the magnetospheric radius is less than the co-rotation radius. After inferring the properties of the neutron star, we study the thermal stability of hydrogen/helium burning and show that thermonuclear instabilities are unlikely causes of the hourly bursts seen at very high accretion rates. We then discuss how the stability of the thermonuclear burning depends on both the global accretion rate and the neutron star's magnetic field strength. We emphasize that the appearance of the instability (i.e., whether it looks like a Type I X-ray burst or a flare lasting a few minutes) will yield crucial information on the neutron star's surface magnetic field and the role of magnetic fields in convection. We suggest that a thermal instability in the accretion disk is the origin of the long (~300 days) outburst and that the recurrence time of these outbursts is >50 years. We also discuss the nature of the binary and point out that a velocity measurement of the stellar companion (most likely a Roche-lobe filling giant with m_K>17) will constrain the neutron star mass.Comment: 19 pages, 3 PostScript figures, uses aaspp4.sty and epsfig.sty, to appear in the Astrophysical Journa