SPH simulations of irradiation-driven warped accretion discs and the long periods in X-ray binaries

We present three dimensional smoothed particle hydrodynamics (SPH) calculations of irradiation-driven warping of accretion discs. Initially unwarped planar discs are unstable to the radiation reaction when the disc is illuminated by a central radiation source. The disc warps and tilts and precesses slowly in a retrograde direction; its shape continuously flexes in response to the changing orientation of the Roche potential. We simulate ten systems: eight X-ray binaries, one cataclysmic variable (CV), and a `generic' low mass X-ray binary (LMXB). We adopt system parameters from observations and tune a single parameter: our model X-ray luminosity ($L_{*}$) to reproduce the observed or inferred super-orbital periods. Without exception, across a wide range of parameter space, we find an astonishingly good match between the observed $L_{X}$ and the model $L_{*}$. We conclude irradiation-driven warping is the mechanism underlying the long periods in X-ray binaries. Our Her X-1 simulation simultaneously reproduces the observed $L_{X}$, the "main-" and "short-high" X-ray states and the orbital inclination. Our simulations of SS 433 give a maximum warp angle of $18.6^{\circ}$, a good match to the cone traced by the jets, but this angle is reached only in the outer disc. In all cases, the overall disc tilt is less than \degrees{13} and the maximum disc warp is less than and or equal to \degrees{21}.Comment: 17 pages, 14 figures, shorter abstract (24 lines limit

Engineering Management and Industrial Engineering: Six One Way, a Half Dozen the Other

To some people engineering management (EM) is a specialty within Industrial Engineering (IE), to others EM is broader than IE. Comparing the histories of the two disciplines shows a clear distinction. Both disciplines have their roots in the work of engineering pioneers at the turn of the century. Work in motion and time study, along with other efficiency driven techniques, was the beginning of IE. Both IE and EM can trace their roots to the era of scientific management and the work of the early pioneers. A noted change for IE occurred after World War II when many programs shifted their focus towards operations research. For some this shifted marks the beginning of the distinction between the two disciplines

Encouraging Lifelong Learning for Engineering Management Undergraduates

The current ABET guidelines place an emphasis on life-long learning for our undergraduate students. What is life-long learning? How can we encourage students to consider global issues, current events, or even anything that isn\u27t going to be on the next test ? In this paper we present survey results evaluating habits of undergraduate students entering an engineering management program and seniors related to life-long learning including attending professional society meetings, reading trade publications, reading business related books, and other learning outside of the classroom activities. This paper also presents a two semester effort to increase life-long learning activities among undergraduate engineering management students. Changes were made to an introductory sophomore level EM class. Students were required to participate in lifelong learning activities including reading business books and interviewing managers. These activities were graded as part of the required course. Additionally, the students were asked to identify learning activities they would complete the semester following the course - which would not be reflected in their grades. Recommendations for incorporating life-long learning initiatives in the engineering management undergraduate curriculum are also presented

Is the Compact Source at the Center of Cas A Pulsed?

A 50 ksec observation of the Supernova Remnant Cas A was taken using the Chandra X-Ray Observatory High Resolution Camera (HRC) to search for periodic signals from the compact source located near the center. Using the HRC-S in imaging mode, problems with correctly assigning times to events were overcome, allowing the period search to be extended to higher frequencies than possible with previous observations. In an extensive analysis of the HRC data, several possible candidate signals are found using various algorithms, including advanced techniques developed by Ransom to search for low significance periodic signals. Of the candidate periods, none is at a high enough confidence level to be particularly favored over the rest. When combined with other information, however (e.g., spectra, total energetics, and the historical age of the remnant), a 12 ms candidate period seems to be more physically plausible than the others, and we use it for illustrative purposes in discussing the possible properties of a putative neutron star in the remnant. We emphasize that this is not necessarily the true period, and that a follow-up observation, scheduled for the fall of 2001, is required. A 50 ksec Advanced CCD Imaging Spectrometer (ACIS) observation was taken, and analysis of these data for the central object shows that the spectrum is consistent with several forms, and that the emitted X-ray luminosity in the 0.1 -10 keV band is 10^{33}-10^{35}erg cm^{-2}sec^{-1} depending on the spectral model and the interstellar absorption along the line of sight to the source.Comment: 14 pages, 3 figures Submitted to ApJ 2001 June 2

Comprehensive simulations of superhumps

(Abridged) We use 3D SPH calculations with higher resolution, as well as with more realistic viscosity and sound-speed prescriptions than previous work to examine the eccentric instability which underlies the superhump phenomenon in semi-detached binaries. We illustrate the importance of the two-armed spiral mode in the generation of superhumps. Differential motions in the fluid disc cause converging flows which lead to strong spiral shocks once each superhump cycle. The dissipation associated with these shocks powers the superhump. We compare 2D and 3D results, and conclude that 3D simulations are necessary to faithfully simulate the disc dynamics. We ran our simulations for unprecedented durations, so that an eccentric equilibrium is established except at high mass ratios where the growth rate of the instability is very low. Our improved simulations give a closer match to the observed relationship between superhump period excess and binary mass ratio than previous numerical work. The observed black hole X-ray transient superhumpers appear to have systematically lower disc precession rates than the cataclysmic variables. This could be due to higher disc temperatures and thicknesses. The modulation in total viscous dissipation on the superhump period is overwhelmingly from the region of the disc within the 3:1 resonance radius. As the eccentric instability develops, the viscous torques are enhanced, and the disc consequently adjusts to a new equilibrium state, as suggested in the thermal-tidal instability model. We quantify this enhancement in the viscosity, which is ~10 per cent for q=0.08. We characterise the eccentricity distributions in our accretion discs, and show that the entire body of the disc partakes in the eccentricity.Comment: 18 pages (mn2e LaTeX), 14 figures, 5 tables, Accepted for publication in MNRA

Glideslope perception during aircraft landing

Ideally, when a pilot approaches a runway on their final approach for landing, they must maintain a constant trajectory, or glideslope, of typically 3°-4°. If pilots misperceive their glideslope and alter their flight path accordingly, they are likely to overshoot or undershoot their desired touch down point on the runway. This experiment examined the accuracy of passive glideslope perceptions during simulated fixed-wing aircraft landings. 17 university students were repeatedly exposed to the following four landing scene conditions: (i) a daylight scene of a runway surrounded by buildings and lying on a 100 km deep texture mapped ground plane; (ii) a night scene with only the side runway lights visible; (iii) a night scene with the side, center, near end and far end runway lights visible and a visible horizon line; or (iv) a night scene with a runway outline (instead of discrete lights) and a visible horizon line. Each of these simulations lasted 2 seconds and represented a 130 km/hr landing approach towards a 30 m wide x 1000 m long runway with a glideslope ranging between 1° and 5°. On each experimental trial, participants viewed two simulated aircraft landings (one presented directly after the other): (a) an ideal 3° glideslope landing simulation; and (b) a comparison landing simulation, where the glideslope was either 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5°. Participants simply judged which of the two landing simulations appeared to have the steepest glideslope. As expected, the daylight landing scene simulations were found to produce significantly more accurate glideslope judgments than any of the night landing simulations. However, performance was found to be unacceptably imprecise and biased for all of our landing simulation scenes. Even in daylight conditions, the smallest glideslope difference that could be reliably detected (i.e. resulted in 75% correct levels of performance) exceeded 2º for 11 of our 16 subjects. It is concluded that glideslope differences of up to 2° can not be accurately perceived based on visual information alone, regardless of scene lighting or detail. The additional visual information provided by the ground surface and buildings in the daytime significantly improved performance, however not to a level that would prevent landing incidents