Preventing extinction and outbreaks in chaotic populations

Interactions in ecological communities are inherently nonlinear and can lead to complex population dynamics including irregular fluctuations induced by chaos. Chaotic population dynamics can exhibit violent oscillations with extremely small or large population abundances that might cause extinction and recurrent outbreaks, respectively. We present a simple method that can guide management efforts to prevent crashes, peaks, or any other undesirable state. At the same time, the irregularity of the dynamics can be preserved when chaos is desirable for the population. The control scheme is easy to implement because it relies on time series information only. The method is illustrated by two examples: control of crashes in the Ricker map and control of outbreaks in a stage-structured model of the flour beetle Tribolium. It turns out to be effective even with few available data and in the presence of noise, as is typical for ecological settings.Comment: 10 pages, 6 figure

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-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

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

From B Modes to Quantum Gravity and Unification of Forces

It is commonly anticipated that gravity is subject to the standard principles of quantum mechanics. Yet some (including Einstein) have questioned that presumption, whose empirical basis is weak. Indeed, recently Freeman Dyson has emphasized that no conventional experiment is capable of detecting individual gravitons. However, as we describe, if inflation occurred, the Universe, by acting as an ideal graviton amplifier, affords such access. It produces a classical signal, in the form of macroscopic gravitational waves, in response to spontaneous (not induced) emission of gravitons. Thus recent BICEP2 observations of polarization in the cosmic microwave background will, if confirmed, provide empirical evidence for the quantization of gravity. Their details also support quantitative ideas concerning the unification of strong, electromagnetic, and weak forces, and of all these with gravity.Comment: 4 pages, no figures. v2: minor typos corrected, reference added. v3: very minor typo corrected. Winning entry in Gravity Research Foundation essay competitio