4,485 research outputs found
Evidence that particle acceleration in hotspots of FR II galaxies is not constrained by synchrotron cooling
We study the hotspots of powerful radiogalaxies, where electrons accelerated
at the jet termination shock emit synchrotron radiation. The turnover of the
synchrotron spectrum is typically observed between infrared and optical
frequencies, indicating that the maximum energy of non-thermal electrons
accelerated at the shock is ~TeV for a canonical magnetic field of ~100 micro
Gauss. We show that this maximum energy cannot be constrained by synchrotron
losses as usually assumed, unless the jet density is unreasonably large and
most of the jet upstream energy goes to non-thermal particles. We test this
result by considering a sample of hotspots observed at radio, infrared and
optical wavelengths.Comment: 7 pages, 2 figures. To be appear in the proceedings of the conference
"Cosmic ray origin - beyond the standard models" (San Vito di Cadore, Italy,
September 2016
Particle acceleration and magnetic field amplification in the jets of 4C74.26
We model the multi-wavelength emission in the southern hotspot of the radio
quasar 4C74.26. The synchrotron radio emission is resolved near the shock with
the MERLIN radio-interferometer, and the rapid decay of this emission behind
the shock is interpreted as the decay of the amplified downstream magnetic
field as expected for small scale turbulence. Electrons are accelerated to only
0.3 TeV, consistent with a diffusion coefficient many orders of magnitude
greater than in the Bohm regime. If the same diffusion coefficient applies to
the protons, their maximum energy is only ~100 TeV.Comment: Accepted for publication in ApJ. 6 pages - 2 figures. Minor
correction
Cosmic ray acceleration to ultrahigh energy in radio galaxies
The origin of ultrahigh energy cosmic rays (UHECRs) is an open question. In
this proceeding, we first review the general physical requirements that a
source must meet for acceleration to 10-100 EeV, including the consideration
that the shock is not highly relativistic. We show that shocks in the backflows
of radio galaxies can meet these requirements. We discuss a model in which
giant-lobed radio galaxies such as Centaurus A and Fornax A act as
slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a
more powerful past episode. We also show that Centaurus A, Fornax A and other
radio galaxies may explain the observed anisotropies in data from the Pierre
Auger Observatory, before examining some of the difficulties in associating
UHECR anisotropies with astrophysical sources.Comment: 6 pages, 4 figures. Proceedings of UHECR 2018, 8-12 October 2018,
Paris, Franc
Amplification of perpendicular and parallel magnetic fields by cosmic ray currents
Cosmic ray (CR) currents through magnetised plasma drive strong instabilities
producing amplification of the magnetic field. This amplification helps explain
the CR energy spectrum as well as observations of supernova remnants and radio
galaxy hot spots. Using magnetohydrodynamic (MHD) simulations, we study the
behaviour of the non-resonant hybrid (NRH) instability (also known as the Bell
instability) in the case of CR currents perpendicular and parallel to the
initial magnetic field. We demonstrate that extending simulations of the
perpendicular case to 3D reveals a different character to the turbulence from
that observed in 2D. Despite these differences, in 3D the perpendicular NRH
instability still grows exponentially far into the non-linear regime with a
similar growth rate to both the 2D perpendicular and 3D parallel situations. We
introduce some simple analytical models to elucidate the physical behaviour,
using them to demonstrate that the transition to the non-linear regime is
governed by the growth of thermal pressure inside dense filaments at the edges
of the expanding loops. We discuss our results in the context of supernova
remnants and jets in radio galaxies. Our work shows that the NRH instability
can amplify magnetic fields to many times their initial value in parallel and
perpendicular shocks.Comment: Published in MNRAS. 14 pages, 12 figures, 2 tables. Replacement
corrects some typesetting error
The thermodynamics of prediction
A system responding to a stochastic driving signal can be interpreted as
computing, by means of its dynamics, an implicit model of the environmental
variables. The system's state retains information about past environmental
fluctuations, and a fraction of this information is predictive of future ones.
The remaining nonpredictive information reflects model complexity that does not
improve predictive power, and thus represents the ineffectiveness of the model.
We expose the fundamental equivalence between this model inefficiency and
thermodynamic inefficiency, measured by dissipation. Our results hold
arbitrarily far from thermodynamic equilibrium and are applicable to a wide
range of systems, including biomolecular machines. They highlight a profound
connection between the effective use of information and efficient thermodynamic
operation: any system constructed to keep memory about its environment and to
operate with maximal energetic efficiency has to be predictive.Comment: 5 pages, 1 figur
Polymorphism in cyclohexanol
The crystal structures and phase behaviour of phase II and the metastable phases III0 and III of cyclohexanol, C6H11OH,
have been determined using high-resolution neutron powder,
synchrotron X-ray powder and single-crystal X-ray diffraction techniques. Cyclohexanol-II is formed by a transition from the plastic phase I cubic structure at 265 K and crystallizes in a tetragonal structure, space group Pïżœ4421c (Z0 = 1), in which the molecules are arranged in a hydrogen-bonded tetrameric ring motif. The structures of phases III0 and III are monoclinic, space groups P21/c (Z0 = 3) and Pc (Z0 = 2), respectively, and are characterized by the formation of hydrogen-bonded molecular chains with a threefold-helical and wave-like nature, respectively. Phase III crystallizes at 195 K from a sample of phase I that is supercooled to ca 100 K. Alternatively, phase III may be grown via phase III0, the latter transforming from supercooled phase I at ca 200 K. Phase III0 is particularly unstable and is metastable with respect to both I and II. Its growth is realised only under very restricted conditions, thus making its characterization especially
challenging. The cyclohexanol molecules adopt a chair
conformation in all three phases with the hydroxyl groups in
an equatorial orientation. No evidence was found indicating
hydroxyl groups adopting an axial orientation, contrary to the majority of spectroscopic literature on solid-state cyclohexanol; however, the H atom of the equatorial OH groups is found to adopt both in-plane and out-of-plane orientations
Successful use of axonal transport for drug delivery by synthetic molecular vehicles
We report the use of axonal transport to achieve intraneural drug delivery. We constructed a novel tripartite complex of an axonal transport facilitator conjugated to a linker molecule bearing up to a hundred reversibly attached drug molecules. The complex efficiently enters nerve terminals after intramuscular or intradermal administration and travels within axonal processes to neuron cell bodies. The tripartite agent provided 100-fold amplification of saturable neural uptake events, delivering multiple drug molecules per complex. _In vivo_, analgesic drug delivery to systemic and to non-targeted neural tissues was greatly reduced compared to existing routes of administration, thus exemplifying the possibility of specific nerve root targeting and effectively increasing the potency of the candidate drug gabapentin 300-fold relative to oral administration
Modelling spatial and inter-annual variations of nitrous oxide emissions from UK cropland and grasslands using DailyDayCent
This work contributes to the Defra funded projects AC0116: âImproving the nitrous oxide inventoryâ, and AC0114: âData Synthesis, Management and Modellingâ. Funding for this work was provided by the UK Department for Environment, Food and Rural Affairs (Defra) AC0116 and AC0114, the Department of Agriculture, Environment and Rural Affairs for Northern Ireland, the Scottish Government and the Welsh Government. Rothamsted Research receives strategic funding from the Biotechnology and Biological Sciences Research Council. This study also contributes to the projects: N-Circle (BB/N013484/1), U-GRASS (NE/M016900/1) and GREENHOUSE (NE/K002589/1).Peer reviewedPublisher PD
Empirically Constrained Color-Temperature Relations. II. uvby
(Abriged) A new grid of theoretical color indices for the Stromgren uvby
photometric system has been derived from MARCS model atmospheres and SSG
synthetic spectra for cool dwarf and giant stars. At warmer temperatures this
grid has been supplemented with the synthetic uvby colors from recent Kurucz
atmospheric models without overshooting. Our transformations appear to
reproduce the observed colors of extremely metal-poor turnoff and giant stars
(i.e., [Fe/H]<-2). Due to a number of assumptions made in the synthetic color
calculations, however, our color-temperature relations for cool stars fail to
provide a suitable match to the uvby photometry of both cluster and field stars
having [Fe/H]>-2. To overcome this problem, the theoretical indices at
intermediate and high metallicities have been corrected using a set of color
calibrations based on field stars having accurate IRFM temperature estimates
and spectroscopic [Fe/H] values. Encouragingly, isochrones that employ the
transformations derived in this study are able to reproduce the observed CMDs
(involving u-v, v-b, and b-y colors) for a number of open and globular clusters
(including M92, M67, the Hyades, and 47Tuc) rather well. Moreover, our
interpretations of such data are very similar, if not identical, with those
given by VandenBerg & Clem (2003, AJ, 126, 778) from a consideration of BV(RI)c
observations for the same clusters. In the present investigation, we have also
analyzed the observed Stromgren photometry for the classic Population II
subdwarfs, compared our "final" (b-y)-Teff relationship with those derived
empirically in a number of recent studies, and examined in some detail the
dependence of the m1 index on [Fe/H].Comment: 70 pages, 26 figures. Accepted for publication in AJ (Feb 2004).
Postscript version with high resolution figures and complete Table 3
available at http://astrowww.phys.uvic.ca/~jclem/uvb
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