Timothy Arlen - Academia.edu (original) (raw)

Papers by Timothy Arlen

The intergalactic magnetic field (IGMF) in cosmic voids can be indirectly probed through its effe... more 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 active galactic nuclei (AGN). AGN that are sufficiently luminous at TeV energies, "extreme TeV blazars" can produce detectable levels of secondary radiation from inverse Compton (IC) scattering of the electrons in the cascade, provided that the IGMF is not too large. We review 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 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 UV-far IR extragalactic background light (EBL), undersampled 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.

Author(s): Arlen, Timothy | Advisor(s): Vassiliev, Vladimir V | Abstract: Gamma ray photons, with... more Author(s): Arlen, Timothy | Advisor(s): Vassiliev, Vladimir V | Abstract: Gamma ray photons, with energies gTeV 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 descri...

Bulletin of the American Physical Society, 2005

COLLABORATION-Dodecane is deposited at sub-monolayer coverages onto a Au(111) surface, and the 2-... more COLLABORATION-Dodecane is deposited at sub-monolayer coverages onto a Au(111) surface, and the 2-dimensional gas crystallizes into islands, which can again sublimate to a 2-dimensional gas at higher substrate temperatures. We observe island formation and subsequent sublimation, using low energy helium reflectivity. When the dodecane molecules are deposited onto the gold surface, the specular intensity decreases as a result of loss of surface order, but recovers when the formation of dodecane islands leaves large empty surfaces of gold. The islanding process was observed in real time using specularity data for substrate temperatures of 40-400 K. Two sets of specularity data were obtained: 1) Specular recovery curves following the deposition of dodecane on the Au(111) surface recorded the islanding process as a function of time, and 2) Equilibrium specular intensity was recorded as a function of substrate temperature (heating/cooling curves). A computer model of the dynamics of the sub-monolayer growth process using Monte Carlo simulations was developed, showing excellent agreement with experimental data of dodecane on the gold substrate. The simulations reveal insights regarding the intermolecular potential and corrugation potential of the molecules on the gold surface.

The intergalactic magnetic field (IGMF) in cosmic voids can be indirectly probed through its effe... more 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.

The Astrophysical Journal, 2010

Experimental Astronomy, 2011

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtaine... more Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV-10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

Very High Energy (VHE) gamma rays (E>100 GeV) emitted from the blazar class of AGN interact wi... more Very High Energy (VHE) gamma rays (E>100 GeV) emitted from the blazar class of AGN interact with the diffuse far-IR to UV extragalactic background light (EBL) in intergalactic space, producing an electron-positron pair. These leptons, deflected by the intergalactic magnetic field (IGMF) in turn inverse Compton scatter CMB photons, rapidly radiating away their energy and producing a cascade of GeV-TeV photons which can then be observed by ground-based imaging atmospheric Cherenkov telescopes (IACTs) such as VERITAS and space based instruments like the FERMI Gamma Ray Space Telescope. By modeling the change in blazar spectra due to absorption on the EBL and subsequent bending of electron/positron pairs by the magnetic field, it may be possible to provide limits on parameters of the currently poorly understood IGMF, as well as constraints on the EBL spectral energy density in the far-IR to UV region. Results of computer simulations of cascading in intergalactic space which utilizes the fully relativistic description of both Pair Production and inverse Compton scattering in the expanding universe will be presented.

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. C... more The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve fullsky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project.

Bulletin of the American Physical Society, 2005

The intergalactic magnetic field (IGMF) in cosmic voids can be indirectly probed through its effe... more 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.

The Astrophysical Journal, 2010

Experimental Astronomy, 2011