Extreme TeV Blazars and Lower Limits on Intergalactic Magnetic Fields

The intergalactic magnetic field (IGMF) in cosmic voids can be indirectly probed through its effect on electromagnetic cascades initiated by a source of TeV gamma rays, such as blazars, a subclass of active galactic nuclei. Blazars that are sufficiently luminous at TeV energies, "extreme TeV blazars", can produce detectable levels of secondary radiation from inverse Compton scattering of the electrons in the cascade, provided that the IGMF is not too large. We reveiw recent work in the literature which utilizes this idea to derive constraints on the IGMF for three TeV-detected blazars-1ES 0229+200, 1ES 1218+304, and RGB J0710+591, and we also investigate four other hard-spectrum TeV blazars in the same framework. Through a recently developed detailed 3D particle tracking Monte Carlo simulation code, incorporating all major effects of QED and cosmological expansion, we research effects of major uncertainties such as the spectral properties of the source, uncertainty in the intensity of the UV - far IR extragalactic background light (EBL), under-sampled Very High Energy (VHE; energy > 100 GeV) coverage, past history of gamma-ray emission, source vs. observer geometry, and jet AGN Doppler factor. The implications of these effects on the recently reported lower limits of the IGMF are thoroughly examined to conclude that presently available data are compatible with a zero IGMF hypothesis.Comment: 2012 Fermi Symposium proceedings - eConf C12102

Ancient Lowland Maya neighborhoods: Average Nearest Neighbor analysis and kernel density models, environments, and urban scale

Many humans live in large, complex political centers, composed of multi-scalar communities including neighborhoods and districts. Both today and in the past, neighborhoods form a fundamental part of cities and are defined by their spatial, architectural, and material elements. Neighborhoods existed in ancient centers of various scales, and multiple methods have been employed to identify ancient neighborhoods in archaeological contexts. However, the use of different methods for neighborhood identification within the same spatiotemporal setting results in challenges for comparisons within and between ancient societies. Here, we focus on using a single method—combining Average Nearest Neighbor (ANN) and Kernel Density (KD) analyses of household groups—to identify potential neighborhoods based on clusters of households at 23 ancient centers across the Maya Lowlands. While a one-size-fits all model does not work for neighborhood identification everywhere, the ANN/KD method provides quantifiable data on the clustering of ancient households, which can be linked to environmental zones and urban scale. We found that centers in river valleys exhibited greater household clustering compared to centers in upland and escarpment environments. Settlement patterns on flat plains were more dispersed, with little discrete spatial clustering of households. Furthermore, we categorized the ancient Maya centers into discrete urban scales, finding that larger centers had greater variation in household spacing compared to medium-sized and smaller centers. Many larger political centers possess heterogeneity in household clustering between their civic-ceremonial cores, immediate hinterlands, and far peripheries. Smaller centers exhibit greater household clustering compared to larger ones. This paper quantitatively assesses household clustering among nearly two dozen centers across the Maya Lowlands, linking environment and urban scale to settlement patterns. The findings are applicable to ancient societies and modern cities alike; understanding how humans form multi-scalar social groupings, such as neighborhoods, is fundamental to human experience and social organization

Constraining the Intergalactic Magnetic Field Through its Imprint on Gamma Ray Data from Distant Sources.

Gamma ray photons, with energies >TeV propagating cosmological distances will be attenuated by pair production with diffuse extragalactic background photon fields-both the cosmic microwave background (CMB) radiation and the UV - far-IR extragalactic background light (EBL). The produced electron/positron pairs will subsequently inverse Compton scatter background photons up to GeV - TeV energies, and some of these upscattered photons may also initiate pair production in the formation of an electromagnetic cascade. If an intergalactic magnetic field (IGMF) exists on cosmological length scales of relevance to the cascade, it will deflect the electrons and positrons and will leave an imprint on the resulting spectral, angular, and temporal properties of the cascade radiation. The primary goal of this study was to constrain the properties of the IGMF using data from known sources of TeV gamma-rays.This thesis describes the construction of a high precision, 3-dimensional, particle-tracking Monte Carlo simulation code to model the intergalactic electromagnetic cascade, and uses it to systematically explore the effects of the IGMF on the cascades in multiple observational domains. We then compare the simulations with gamma-ray data from current generation ground-based gamma-ray instruments such as VERITAS, HESS, and MAGIC, sensitive to TeV-scale energies, as well as the Fermi satellite, sensitive to the GeV-scale.This novel technique of constraining the IGMF has rapidly emerged over the last decade as gamma-ray instruments have become more sensitive and as theoretical understanding of the cascade process has progressed. This emerging field has proven to be richly complex and we find that the data from current generation gamma-ray instruments do not allow for an unambiguous upper or lower limit to be placed on the IGMF at present. We do find it likely that the next generation ground based gamma-ray observatory, the Cherenkov Telescope Array (CTA) will be able to detect unambiguous signatures of gamma-ray cascading if the IGMF magnitude is within a certain range, and thus provide a robust constraint on IGMF properties