Analyzing the Data from X-ray Polarimeters with Stokes Parameters

X-ray polarimetry promises to deliver unique information about the geometry of the inner accretion flow of astrophysical black holes and the nature of matter and electromagnetism in and around neutron stars. In this paper, we discuss the possibility to use Stokes parameters - a commonly used tool in radio, infrared, and optical polarimetry - to analyze the data from X-ray polarimeters such as scattering polarimeters and photoelectric effect polarimeters, which measure the linear polarization of the detected X-rays. Based on the azimuthal scattering angle (in the case of a scattering polarimeter) or the azimuthal component of the angle of the electron ejection (in the case of a photoelectric effect polarimeter), the Stokes parameters can be calculated for each event recorded in the detector. Owing to the additive nature of Stokes parameters, the analysis reduces to adding the Stokes parameters of the individual events and subtracting the Stokes parameters characterizing the background (if present). The main strength of this kind of analysis is that the errors on the Stokes parameters can be computed easily and are well behaved - in stark contrast of the errors on the polarization fraction and polarization direction. We demonstrate the power of the Stokes analysis by deriving several useful formulae, e.g. the expected error on the polarization fraction and polarization direction for a detection of $N_S$ signal and $N_{BG}$ background events, the optimal observation times of the signal and background regions in the presence of non-negligible background contamination of the signal, and the minimum detectable polarization (MDP) that can be achieved when following this prescription.Comment: 9 pages, 2 figures, accepted for publication in Astropart. Phy

Labor Force Participation: Timing and Persistence

This paper examines the relative importance of timing and persistence elements in explaining cyclical fluctuations in labor supply. Data from the natural experiment provided by World War I1 and cross-sectional data on American local labor markets, as well as aggregate time-series data are used in the empirical work. We find little evidence that timing effects play an important role in labor market dynamics. The evidence suggests that views emphasizing persistence are more accurate, and that previous employment tends to raise the probability of subsequent employment.

Labor Market Dynamics and Unemployemnt: A Reconsideration

macroeconomics, unemployment

The Dynamics of Youth Unemployment

This paper analyzes the dynamics of youth unemployment. Three broad conclusions emerge. First, the problem of youth joblessness extends beyond the unemployed. We find that over one-half of youth unemployment spells end in labor force withdrawal. Much of youth non-employment is not picked up in the official unemployment statistics, because many young people give up the search for work and leave the labor force. Second, a large part of youth unemployment is accounted for by a relatively small, hard core group of young people who experience long spells of unemployment. While most unemployment spells are short, this is due to the high rates of labor force withdrawal, rather than to job finding. Among male teenagers out of school, for example, we find that over half of unemployment was due to those with more than six months of unemployment in the year. Third, a shortage of attractive jobs is the principle source of long term non-employment. While instability and frequent turnover are major factors in determining the overall pattern of teenage unemployment, we find that the lack of desirable employment opportunities is the crux of the problem for those most seriously affected by youth unemployment.

Demographic Differences in Cyclical Employment Variation

Demographic differences in patterns of employment variation over the business cycle are examined in this paper. Three primary conclusions emerge. First, both participation and unemployment must be considered in any analysis of cyclical changes in the labor market. Second, young people bear a disproportionate share of cyclical employment variation. Third, failure to consider participation has led to undue pessimism about the effect of aggregate demand policy on high unemployment groups. If participation did not surge, reduction in overall unemployment to its 1969 level would reduce the unemployment of almost all demographic groups to very low levels.

Sovereign Wealth Funds: Form and Function in the 21st Century

As representatives of nation-states in global financial markets, sovereign wealth funds (SWFs) share a common form and many functions. Arguably their form and functions owe as much to a shared (global) moment of institutional formation as they owe their form and functions to the hegemony of Anglo-American finance over the late 20th and early 21st centuries. We distinguish between the immediate future for SWFs in the aftermath of the global financial crisis, and two possible long-term scenarios; one of which sees SWFs becoming financial goliaths dominating global markets, while the other sees SWFs morphing into nation-state development institutions that intermediate between financial markets and the long-term commitments of the nation-state sponsors. If the former scenario dominates, global financial integration will accelerate with attendant costs and benefits. If the latter scenario dominates, SWFs are likely to differentiate and evolve, returning, perhaps, to their national traditions and their respective places in a world of contested power and influence. Here, we clarify the assumptions underpinning the conception and formation of sovereign wealth funds over the past twenty years or so in the face of the ‘new’ realities of global finance.Sovereign Wealth Funds, Crisis, Market Performance, Long-term Investment

Intersection Graph of a Module

Let $V$ be a left $R$-module where $R$ is a (not necessarily commutative) ring with unit. The intersection graph \cG(V) of proper $R$-submodules of $V$ is an undirected graph without loops and multiple edges defined as follows: the vertex set is the set of all proper $R$-submodules of $V,$ and there is an edge between two distinct vertices $U$ and $W$ if and only if $U\cap W\neq 0.$ We study these graphs to relate the combinatorial properties of \cG(V) to the algebraic properties of the $R$-module $V.$ We study connectedness, domination, finiteness, coloring, and planarity for \cG (V). For instance, we find the domination number of \cG (V). We also find the chromatic number of \cG(V) in some cases. Furthermore, we study cycles in \cG(V), and complete subgraphs in \cG (V) determining the structure of $V$ for which \cG(V) is planar

Space Trumps Time When Talking About Objects

The nature of the relationship between the concepts of space and time in the human mind is much debated. Some claim that space is primary and that it structures time (cf. Lakoff & Johnson, 1980) while others (cf. Walsh, 2003) maintain no difference in status between them. Using fully immersive virtual reality (VR), we examined the influence of object distance and time of appearance on choice of demonstratives (this and that) to refer to objects. Critically, demonstratives can be used spatially (this/that red triangle) and temporally (this/that month). Experiment 1 showed a pattern of demonstrative usage in VR that is consistent with results found in real‐world studies. Experiments 2, 3, and 4 manipulated both when and where objects appeared, providing scenarios where participants were free to use demonstratives in either a temporal or spatial sense. Although we find evidence for time of presentation affecting object mention, the experiments found that demonstrative choice was affected only by distance. These results support the view that spatial uses of demonstratives are privileged over temporal uses