804 research outputs found
Active Galactic Nuclei with Starbursts: Sources for Ultra High Energy Cosmic Rays
Ultra high energy cosmic ray events presently show a spectrum, which we
interpret here as galactic cosmic rays due to a starburst in the radio galaxy
Cen A pushed up in energy by the shock of a relativistic jet. The knee feature
and the particles with energy immediately higher in galactic cosmic rays then
turn into the bulk of ultra high energy cosmic rays. This entails that all
ultra high energy cosmic rays are heavy nuclei. This picture is viable if the
majority of the observed ultra high energy events come from the radio galaxy
Cen A, and are scattered by intergalactic magnetic fields across most of the
sky.Comment: 4 pages, 1 figure, proceedings of "High-Energy Gamma-rays and
Neutrinos from Extra-Galactic Sources", Heidelber
Evolution of Global Relativistic Jets: Collimations and Expansion with kKHI and the Weibel Instability
One of the key open questions in the study of relativistic jets is their
interaction with the environment. Here, we study the initial evolution of both
electron-proton and electron-positron relativistic jets, focusing on their
lateral interaction with the ambient plasma. We trace the generation and
evolution of the toroidal magnetic field generated by both kinetic
Kelvin-Helmholtz (kKH) and Mushroom instabilities (MI). This magnetic field
collimates the jet. We show that in electron-proton jet, electrons are
perpendicularly accelerated with jet collimation. The magnetic polarity
switches from the clockwise to anti-clockwise in the middle of jet, as the
instabilities weaken. For the electron-positron jet, we find strong mixture of
electron-positron with the ambient plasma, that results in the creation of a
bow shock. Merger of magnetic field current filaments generate density bumps
which initiate a forward shock. The strong mixing between jet and ambient
particles prevents full development of the jet on the studied scale. Our
results therefore provide a direct evidence for both jet collimation and
particle acceleration in the created bow shock. Differences in the magnetic
field structures generated by electron-proton and electron-positron jets may
contribute to observable differences in the polarized properties of emission by
electrons.Comment: 25 pages, 12 figures, ApJ, accepte
Particle Acceleration in Relativistic Electron-positron Jets with Helical Magnetic Fields
The properties of relativistic jets, their interaction with the ambient
environment and particle acceleration due to kinetic instabilities are studied
self-consistently with Particle-in-Cell (PIC) simulations. In this work we
study how a relativistic electron-positron jet containing a helical magnetic
field evolves by focusing on its interaction with the external ambient plasma.
Particularly, 3D PIC simulations are performed using a longer simulation system
than previous studies with an embedded helical magnetic field. An important key
issue in this work is how such a magnetic field affects an electron-positron
jet and how this excites kinetic instabilities such as the Weibel instability
(WI), the kinetic Kelvin-Helmholtz instability (kKHI) and others by further
focusing on how particles accelerate. We do find that kinetic instabilities
along with generated magnetic turbulence are present and consequently
accelerate particles. At the linear stage we observe recollimation-like
features at the center of the simulated jet and later-on as the
electron-positron jet evolves, the magnetic fields generated by the
instabilities become untangled and reorganized into a new topology near the
non-linear phase. We additionally report indications of reconnection near the
end of the non-linear stage, before the magnetic-field becomes untangled, as
electrons get accelerated by multiple magnetic islands in the jet. In the
present study the untangled magnetic field becomes turbulent without any
reformation as it happened in our previous study of an electron-proton jet,
which we will use to additionally compare the present results, obtaining
important insights about the nature of these phenomena applicable to
high-energy astrophysical environments such as Active Galactic Nuclei jets and
Gamma-ray bursts.Comment: 12 pages, 24 figures, submitted to MNRAS Journa
Teleology and Realism in Leibniz's Philosophy of Science
This paper argues for an interpretation of Leibniz’s claim that physics requires both mechanical and teleological principles as a view regarding the interpretation of physical theories. Granting that Leibniz’s fundamental ontology remains non-physical, or mentalistic, it argues that teleological principles nevertheless ground a realist commitment about mechanical descriptions of phenomena. The empirical results of the new sciences, according to Leibniz, have genuine truth conditions: there is a fact of the matter about the regularities observed in experience. Taking this stance, however, requires bringing non-empirical reasons to bear upon mechanical causal claims. This paper first evaluates extant interpretations of Leibniz’s thesis that there are two realms in physics as describing parallel, self-sufficient sets of laws. It then examines Leibniz’s use of teleological principles to interpret scientific results in the context of his interventions in debates in seventeenth-century kinematic theory, and in the teaching of Copernicanism. Leibniz’s use of the principle of continuity and the principle of simplicity, for instance, reveal an underlying commitment to the truth-aptness, or approximate truth-aptness, of the new natural sciences. The paper concludes with a brief remark on the relation between metaphysics, theology, and physics in Leibniz
Supernova explosions of massive stars and cosmic rays
Most cosmic ray particles observed derive from the explosions of massive
stars, which commonly produce stellar black holes in their supernova
explosions. When two such black holes find themselves in a tight binary system
they finally merge in a gigantic emission of gravitational waves, events that
have now been detected. After an introduction (section 1) we introduce the
basic concept (section 2): Cosmic rays from exploding massive stars with winds
always show two cosmic ray components at the same time: (i) the weaker polar
cap component only produced by Diffusive Shock Acceleration with a cut-off at
the knee, and (ii) the stronger component with a down-turn to a steeper
power-law spectrum at the knee, and a final cutoff at the ankle. In section 3
we use the Alpha Magnetic Spectrometer (AMS) data to differentiate these two
cosmic ray spectral components. The ensuing secondary spectra can explain
anti-protons, lower energy positrons, and other secondary particles. Triplet
pair production may explain the higher energy positron AMS data. In section 4
we test this paradigm with a theory of injection based on a combined effect of
first and second ionization potential; this reproduces the ratio of Cosmic Ray
source abundances to source material abundances. In section 5 we interpret the
compact radio source 41.9+58 in the starburst galaxy M82 as a recent binary
black hole merger, with an accompanying gamma ray burst. This can also explain
the Ultra High Energy Cosmic Ray (UHECR) data in the Northern sky. Thus, by
studying the cosmic ray particles, their abundances at knee energies, and their
spectra, we can learn about what drives these stars to produce the observed
cosmic rays.Comment: 151 pages, 6 figures, accepted for publication in Advances in Space
Researc
Inflammatory Mediators in the Mesenteric Lymph Nodes, Site of a Possible Intermediate Phase in the Immune Response to Feline Coronavirus and the Pathogenesis of Feline Infectious Peritonitis?
Feline infectious peritonitis (FIP) is an almost invariably fatal feline coronavirus (FCoV)-induced disease thought to arise from a combination of viral mutations and an overexuberant immune response. Natural initial enteric FCoV infection may remain subclinical, or result in mild enteric signs or the development of FIP; cats may also carry the virus systemically with no adverse effect. This study screened mesenteric lymph nodes (MLNs), the presumed first site of FCoV spread from the intestine regardless of viraemia, for changes in the transcription of a panel of innate immune response mediators in response to systemic FCoV infection and with FIP, aiming to identify key pathways triggered by FCoV. Cats with and without FIP, the latter with and without FCoV infection in the MLN, were compared. Higher expression levels in FIP were found for toll-like receptors (TLRs) 2, 4 and 8. These are part of the first line of defence and suggest a response to both viral structural proteins and viral nucleic acid. Expression of genes encoding inflammatory cytokines and chemokines, including interleukin (IL)-1β, IL-6, IL-15, tumour necrosis factor (TNF)-α, CXCL10, CCL8, interferon (IFN)-α, IFN-β and IFN-γ, was higher in cats with FIP, consistent with inflammatory pathway activation. Expression of genes encoding transcription factors STAT1 and 2, regulating signalling pathways, particularly of the interferons, was also higher. Among cats without FIP, there were few differences between virus-positive and virus-negative MLNs; however, TLR9 and STAT2 expression were higher with infection, suggesting a direct viral effect. The study provides evidence for TLR involvement in the response to FCoV. This could open up new avenues for therapeutic approaches
Microscopic Processes in Global Relativistic Jets Containing Helical Magnetic Fields
In the study of relativistic jets one of the key open questions is their interaction with the environment on the microscopic level. Here, we study the initial evolution of both electron–proton ( e − – p + ) and electron–positron ( e ± ) relativistic jets containing helical magnetic fields, focusing on their interaction with an ambient plasma. We have performed simulations of “global” jets containing helical magnetic fields in order to examine how helical magnetic fields affect kinetic instabilities such as the Weibel instability, the kinetic Kelvin-Helmholtz instability (kKHI) and the Mushroom instability (MI). In our initial simulation study these kinetic instabilities are suppressed and new types of instabilities can grow. In the e − – p + jet simulation a recollimation-like instability occurs and jet electrons are strongly perturbed. In the e ± jet simulation a recollimation-like instability occurs at early times followed by a kinetic instability and the general structure is similar to a simulation without helical magnetic field. Simulations using much larger systems are required in order to thoroughly follow the evolution of global jets containing helical magnetic fields.This work is supported by NSF AST-0908010, AST-0908040, NASA-NNX09AD16G,
NNX12AH06G, NNX13AP-21G, and NNX13AP14G grants. The work of J.N. and O.K. has been supported
by Narodowe Centrum Nauki through research project DEC-2013/10/E/ST9/00662. Y.M. is supported by
the ERC Synergy Grant “BlackHoleCam—Imaging the Event Horizon of Black Holes” (Grant No. 610058).
M.P. acknowledges support through grant PO 1508/1-2 of the Deutsche Forschungsgemeinschaft. Simulations
were performed using Pleiades and Endeavor facilities at NASA Advanced Supercomputing (NAS), and using
Gordon and Comet at The San Diego Supercomputer Center (SDSC), and Stampede at The Texas Advanced
Computing Center, which are supported by the NSF. This research was started during the program “Chirps,
Mergers and Explosions: The Final Moments of Coalescing Compact Binaries” at the Kavli Institute for Theoretical
Physics, which is supported by the National Science Foundation under grant No. PHY05-51164. The first
velocity shear results using an electron positron plasma were obtained during the Summer Aspen workshop
“Astrophysical Mechanisms of Particle Acceleration and Escape from the Accelerators” held at the Aspen Center
for Physics (1–15 September 2013). We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI
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