Global Neutrino Heating in Hyperaccretion Flows

The neutrino-dominated accretion flow (NDAF) with accretion rates \dot{M} = 0.01 - 10 M_{\sun} s^{-1} is a plausible candidate for the central engine of gamma-ray bursts (GRBs). This hyperaccretion disk is optically thin to neutrinos in the radial direction, therefore the neutrinos produced at one radius can travel for a long distance in the disk. Those neutrinos can thus be absorbed with certain probability by the disk matter at the other radius and heat the disk there. The effect of this "global neutrino heating" has been ignored in previous works and is the focus of this paper. We find that around the "ignition" radius r_{ign}, the global neutrino heating rate could be comparable to or even larger than the local viscous heating rate thus must be an important process. Two possible consequences are in order if the "global neutrino heating" is taken into account: i) the temperature of the disk is slightly raised and the "ignition" radius r_{ign} slightly shifts to a larger radius, both lead to the increasing of the total neutrino flux; ii) what is more interesting is that, the temperature of the ADAF just beyond r_{ign} may be raised above the virial temperature thus the accretion will be suppressed. In this case, the activity of the black hole is expected to oscillate between an active and inactive phases. The timescale of the active phases is estimated to be \sim 1 second. If the timescale of the inactive phase is comparable to or less than this value, this intermittent activity may explain the slow variability component of the GRBs. Self-consistent global calculations of NDAFs with the "global neutrino heating" included are required in the future to more precisely evaluate this effect.Comment: 9 pages, 5 figures; more discussions and references added; accepted for publication in MNRA

Tabletop robot to aid in arm rehabilitation of stroke patients

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2005.Includes bibliographical references.The aim of this project was to design and build a tabletop robot that could move the arm of a patient with limited motor function around on a table in any given trajectory to aid the patient in regaining function. The design that resulted from bench level experiments was that of an arm brace mounted to a moving base. The base needed to be omni directional to accommodate all possible trajectories of motion, the arm brace needed to be able to move vertically as well as be flexible enough to accommodate yaw, pitch and roll of the forearm. After choosing a three-wheeled design using TransWheelsTM, the base of the robot was built and programmed by Adam Kraft. The arm brace, which I designed and built, had a rack and pinion setup with a variable voltage regulator to control its vertical motion and a foam and linear spring combination to allow for yaw, pitch and roll while still providing for support. Testing of the prototype proved extremely valuable in refining the requirements of the robot as well as the design. Issues that were discovered during testing of the robot included drift of the robot, the inability to orient the robot the same each time, the slipping of the pinion on the rack if too much downward force is applied to the arm brace and the stiffness of the arm brace during yaw, pitch and roll.(cont.) Several suggestions were made for possible solutions to the issues, all which seem very feasible to implement. As it is the robot can only move the patient's arm, the patient cannot move the robot since the motors are not back-drivable. This is an impediment in allowing the patient to initiate movement, which is a critical part of therapy. Solutions were proposed that are worth further examination to determine their feasibility. In addition, with a few changes, the robot act as a guide to move a patient's passive arm along a preprogrammed trajectory to aid the patient in performing tasks such as reaching. Even if the robot is unable to match its initial goal, it has great potential to become a valuable asset to stroke patients with limited arm motor function.by Yuan Shu.S.B