Population Synthesis of Accreting Neutron Stars Emitting Gravitational Waves

The fastest-spinning neutron stars in low-mass X-ray binaries, despite having undergone millions of years of accretion, have been observed to spin well below the Keplerian break-up frequency. We simulate the spin evolution of synthetic populations of accreting neutron stars in order to assess whether gravitational waves can explain this behaviour and provide the distribution of spins that is observed. We model both persistent and transient accretion and consider two gravitational-wave-production mechanisms that could be present in these systems: thermal mountains and unstable $r$-modes. We consider the case of no gravitational-wave emission and observe that this does not match well with observation. We find evidence for gravitational waves being able to provide the observed spin distribution; the most promising mechanisms being a permanent quadrupole, thermal mountains and unstable $r$-modes. However, based on the resultant distributions alone it is difficult to distinguish between the competing mechanisms.Comment: 12 pages, 12 figures. Accepted for publication in MNRA

Star formation bursts in isolated spiral galaxies

We study the response of the gaseous component of a galactic disc to the time dependent potential generated by N-body simulations of a spiral galaxy. The results show significant variation of the spiral structure of the gas which might be expected to result in significant fluctuations in the Star Formation Rate (SFR). Pronounced local variations of the SFR are anticipated in all cases. Bursty histories for the global SFR, however, require that the mean surface density is much less (around an order of magnitude less) than the putative threshold for star formation. We thus suggest that bursty star formation histories, normally attributed to mergers and/or tidal interactions, may be a normal pattern for gas poor isolated spiral galaxies.Comment: 7 pages, 7 figures To be published in Monthly Notices Roy. Astr. So

Incentivizing Exploration with Heterogeneous Value of Money

Recently, Frazier et al. proposed a natural model for crowdsourced exploration of different a priori unknown options: a principal is interested in the long-term welfare of a population of agents who arrive one by one in a multi-armed bandit setting. However, each agent is myopic, so in order to incentivize him to explore options with better long-term prospects, the principal must offer the agent money. Frazier et al. showed that a simple class of policies called time-expanded are optimal in the worst case, and characterized their budget-reward tradeoff. The previous work assumed that all agents are equally and uniformly susceptible to financial incentives. In reality, agents may have different utility for money. We therefore extend the model of Frazier et al. to allow agents that have heterogeneous and non-linear utilities for money. The principal is informed of the agent's tradeoff via a signal that could be more or less informative. Our main result is to show that a convex program can be used to derive a signal-dependent time-expanded policy which achieves the best possible Lagrangian reward in the worst case. The worst-case guarantee is matched by so-called "Diamonds in the Rough" instances; the proof that the guarantees match is based on showing that two different convex programs have the same optimal solution for these specific instances. These results also extend to the budgeted case as in Frazier et al. We also show that the optimal policy is monotone with respect to information, i.e., the approximation ratio of the optimal policy improves as the signals become more informative.Comment: WINE 201

Impact of Students\u27 Perception of Safety on Academic Success

A student\u27s perception of safety in a school will impact his or her academic achievement. If a student does not feel safe in a school, he or she will be distracted, and will have a diminished concentration span, lack of focus and increased anxiety, all of which will negatively impact his or her achievement and academic potential. It is important for educational leaders, therefore, to create cultures that are safe, welcoming, and conducive to a student\u27s academic growth. To do this, educational leaders, at all levels of the school district--from district administrators, to building administrators, to teachers and other staff in a school--must be purposeful and intentional in their actions and plans. This culture is a prerequisite to the learning process. If the environment is not conducive to academic achievement and growth, then academic achievement and growth will not occur. This paper will explore the background of this issue, will define the scope of the problem, will examine the importance of cultures in a school environment, will explore theoretical constructs that support the need for creating safe cultures, and will discuss the role of various educational leaders in the process