Periodic variability and binary black hole systems in blazars

We consider the periodic modulation of emission from jets in blazar-type sources. A differential Doppler boosting origin, associated with the helical motion of a radiating component, is analyzed for different periodic driving sources including orbital motion and jet precession in a binary black hole system (BBHS). We emphasize that for non-ballistic helical motion classical travel time effects can lead to strong shortening effects, such that the observed period may be a factor $\gamma_b^2$ smaller than the underlying driving period, where $\gamma_b$ denotes the bulk Lorentz factor of the jet flow. The relevance of the above noted scenarios is discussed for the BL Lac object AO 0235+16.Comment: 6 pages, 1 figure; contribution to: High Energy Gamma-Ray Astronomy: 2nd International Symposium (Heidelberg, July 2004); AIP Conf. Proceedings, eds. F. A. Aharonian, H J. Voelk, and D. Horn

Gamma-Ray Astrophysics in the Time Domain

The last few years have seen gamma-ray astronomy maturing and advancing in the field of time-domain astronomy, utilizing source variability on timescales over many orders of magnitudes, from a decade down to a few minutes and shorter, depending on the source. This review focuses on some of the key science issues and conceptual developments concerning the timing characteristics of active galactic nuclei (AGN) at gamma-ray energies. It highlights the relevance of adequate statistical tools and illustrates that the developments in the gamma-ray domain bear the potential to fundamentally deepen our understanding of the nature of the emitting source and the link between accretion dynamics, black hole physics, and jet ejection.Comment: review paper; accepted for publication in Galaxies, Special Issue Monitoring the Non-Thermal Universe; 17 pages, 11 figure

Gamma-Rays from Non-Blazar AGN

Non-blazar Active Galactic Nuclei (AGN) have emerged as a new gamma-ray emitting source class on the extragalactic sky and started to deepen our understanding of the physical processes and the nature of AGN in general. The detection of Narrow Line Seyfert 1 galaxies in the Fermi-LAT energy regime, for example, offers important information for our understanding of jet formation and radio-loudness. Radio galaxies, on the other hand, have become particularly interesting at high (HE) and very high (VHE) gamma-ray energies. With their jets not directly pointing towards us (i.e. misaligned), they offer a unique tool to probe into the nature of the fundamental (and often hidden) physical processes in AGN. This review highlights and discusses some of the observational and theoretical progress achieved in the gamma-ray regime during recent years, including the evidence for unexpected spectral hardening in Centaurus A and extreme short-term variability as seen in IC 310 and M87.Comment: Invited review paper for the 6th Symposium on High Energy Gamma-Ray Astronomy, to appear in AIP Conference Proc., 11 pages, 6 figure

Improvements and Future Challenges in the Field of Genetically Sensitive Sample Designs

Understanding the sources of individual differences beyond social and economic effects has become a research area of growing interest in psychology, sociology, and economics. A quantitative genetic research design provides the necessary tools for this type of analysis. For a state-of-the-art approach, multigroup data is required. Household panel studies, such as BHPS (Understanding Society) in the UK or the SOEP in Germany, combined with an oversampling of twins, provide a powerful starting point since data from a reasonably large number of non-twin relatives is readily available. In addition to advances in our understanding of genetic and environmental influences on key variables in the social sciences, quantitative genetic analyses of target variables can guide molecular genetic research in the field of employment, earnings, health and satisfaction, as combined twin and sibling or parent data can help overcome serious caveats in molecular genetic research.Genetics, twins, psychology, sociology, economics, heritability, environment, multigroup design, BHPS, SOEP

Personal Bankruptcy Law, Wealth and Entrepreneurship: Theory and Evidence from the Introduction of a "Fresh Start"

A personal bankruptcy law that allows for a "fresh start" after bankruptcy reduces the individual risk involved in entrepreneurial activity. On the other hand, as risk shifts to creditors who recover less of their credit after a debtor's bankruptcy, lenders may charge higher interest rates or ration credit supply, which can hamper entrepreneurship. Both aspects of a more forgiving personal bankruptcy law are less relevant for wealthy potential entrepreneurs who still risk losing their wealth, but tend not to face higher interest rates because they provide collateral. This paper illustrates these effects in a model and tests the hypotheses derived by exploiting the introduction of a "fresh start" policy in Germany in 1999 as a natural experiment, based on representative household panel data. The results indicate that the insurance effect of a more forgiving personal bankruptcy law exceeds the interest effect and on balance encourages less wealthy individuals to enter into entrepreneurship.Personal bankruptcy law, insolvency, entrepreneurship, fresh start

Risky Earnings, Taxation and Entrepreneurial Choice: A Microeconometric Model for Germany

Which role do individual income prospects play in the decision to be an entrepreneur rather than an employee? In a model of occupational choice, higher expected after-tax earnings attract people to self-employment, while more risky net earnings deter risk-averse individuals. In this paper I analyse the expected value and variance of income in self-employment and dependent employment empirically, accounting for selection. Based on this analysis, structural models of self-employment entry and exit under risk are estimated, which include a standard risk aversion parameter. The model predicts that the German income tax reduction of 2000 induced smaller exit rates out of self-employment for men and smaller entry rates for women.Entrepreneurship, Risk, Returns to Self-Employment, Taxation

Gap-type Particle Acceleration in the Magnetospheres of Rotating Supermassive Black Holes

The detection of rapidly variable gamma-ray emission in active galactic nuclei has generated renewed interest in magnetospheric particle acceleration and emission scenarios. In order to explore its potential, we study the possibility of steady gap acceleration around the null surface of a rotating black hole magnetosphere. We employ a simplified (1D) description along with the general relativistic expression of Gauss's law, and we assume that the gap is embedded in the radiation field of a radiatively inefficient accretion flow. The model is used to derive expressions for the radial distribution of the parallel electric field component, the electron and positron charge density, the particle Lorentz factor, and the number density of $\gamma$-ray photons. We integrate the set of equations numerically, imposing suitable boundary conditions. The results show that the existence of a steady gap solution for a relative high value of the global current is in principle possible if charge injection of both species is allowed at the boundaries. We present gap solutions for different choices of the global current and the accretion rate. When put in context, our results suggest that the variable very high energy $\gamma$-ray emission in M87 could be compatible with a magnetospheric origin.Comment: 20 pages, 11 figures; ApJ accepted; minor typos fixed to match published versio

Extreme Associated Functions: Optimally Linking Local Extremes to Large-scale Atmospheric Circulation Structures

We present a new statistical method to optimally link local weather extremes to large-scale atmospheric circulation structures. The method is illustrated using July-August daily mean temperature at 2m height (T2m) time-series over the Netherlands and 500 hPa geopotential height (Z500) time-series over the Euroatlantic region of the ECMWF reanalysis dataset (ERA40). The method identifies patterns in the Z500 time-series that optimally describe, in a precise mathematical sense, the relationship with local warm extremes in the Netherlands. Two patterns are identified; the most important one corresponds to a blocking high pressure system leading to subsidence and calm, dry and sunny conditions over the Netherlands. The second one corresponds to a rare, easterly flow regime bringing warm, dry air into the region. The patterns are robust; they are also identified in shorter subsamples of the total dataset. The method is generally applicable and might prove useful in evaluating the performance of climate models in simulating local weather extremes.Comment: 10 pages, 7 figures, 14 eps figure files; to appear in J. Atmos. Chem. Phy