115,633 research outputs found
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
Mohan Thite, Adrian Wilkinson and Pawan Budhwar (Eds), Emerging Indian Multinationals: Strategic Players in a Multipolar World. Oxford, University Press: New Delhi, 2016, 361 pp. (ISBN 978-0-19-946646-7)
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 smaller than the underlying
driving period, where 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
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