Alun R. Hughes: Publications and Reports

Bibliography for Alun R. Hughe

Positrons from particle dark-matter annihilation in the Galactic halo: propagation Green's functions

We have made a calculation of the propagation of positrons from dark-matter particle annihilation in the Galactic halo in different models of the dark matter halo distribution using our 3D code, and present fits to our numerical propagation Green's functions. We show that the Green's functions are not very sensitive to the dark matter distribution for the same local dark matter energy density. We compare our predictions with computed cosmic ray positron spectra (``background'') for the ``conventional'' CR nucleon spectrum which matches the local measurements, and a modified spectrum which respects the limits imposed by measurements of diffuse Galactic gamma-rays, antiprotons, and positrons. We conclude that significant detection of a dark matter signal requires favourable conditions and precise measurements unless the dark matter is clumpy which would produce a stronger signal. Although our conclusion qualitatively agrees with that of previous authors, it is based on a more realistic model of particle propagation and thus reduces the scope for future speculations. Reliable background evaluation requires new accurate positron measurements and further developments in modelling production and propagation of cosmic ray species in the Galaxy.Comment: 8 pages, 6 ps-figures, 3 tables, uses revtex. Accepted for publication in Physical Review D. More details can be found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm

The Contribution of Different Supernova Populations to the Galactic Gamma-Ray Background

The contribution of Source Cosmic Rays (SCRs), accelerated and still confined in Supernova Remnants (SNRs), to the diffuse high energy \gr emission above 1 GeV from the Galactic disk is studied. The \grs produced by the SCRs have a much harder spectrum than those generated by the Galactic Cosmic Rays (GCRs). Extending a previous paper, a simple SNR population synthesis is considered and the Inverse Compton emission from the SCR electrons is evaluated in greater detail. Then the combined spectrum of \gr emission from the Galactic Supernova Remnant population is calculated and this emission at low Galactic latitudes is compared with the diffuse \gr emission observed by the EGRET and ground based instruments. The average contribution of SCRs is comparable to the GCR contribution already at GeV energies, due to Supernovae of types II and Ib exploding into the wind bubbles of quite massive progenitor stars, and becomes dominant at \gr energies above 100 GeV. At TeV energies the dominant contribution is from SCRs in SNRs that expand into a uniform interstellar medium. In fact, the sum of hadronic and Inverse Compton \grs would exceed the limits given by the existing experimental data, unless the confinement time $T_{SN}$, i.e. the time until which SNRs confine the main fraction of accelerated SCRs, is as small as $T_{SN}\sim 10^4$ yr and the typical magnetic field strength in SNRs as large as 30 $\mu$G. Both situations are however possible as a result of field amplification through CR backreaction in the acceleration process. It is pointed out that accurate measurements of the low-latitude diffuse Galactic \gr spectrum at TeV-energies can serve as a unique consistency test for CR origin from the Supernova Remnant population as a whole.Comment: To appear in Astrophysical Journa

UHE nuclei propagation and the interpretation of the ankle in the cosmic-ray spectrum

We consider the stochastic propagation of high-energy protons and nuclei in the cosmological microwave and infrared backgrounds, using revised photonuclear cross-sections and following primary and secondary nuclei in the full 2D nuclear chart. We confirm earlier results showing that the high-energy data can be fit with a pure proton extragalactic cosmic ray (EGCR) component if the source spectrum is \propto E^{-2.6}. In this case the ankle in the CR spectrum may be interpreted as a pair-production dip associated with the propagation. We show that when heavier nuclei are included in the source with a composition similar to that of Galactic cosmic-rays (GCRs), the pair-production dip is not present unless the proton fraction is higher than 85%. In the mixed composition case, the ankle recovers the past interpretation as the transition from GCRs to EGCRs and the highest energy data can be explained by a harder source spectrum \propto E^{-2.2} - E^{-2.3}, reminiscent of relativistic shock acceleration predictions, and in good agreement with the GCR data at low-energy and holistic scenarios.Comment: 4 pages, 4 figures, submitted to A&A Letters (minor changes, two figures replaced, two references added

Multi-wavelength constraints on cosmic-ray leptons in the Galaxy

Cosmic rays (CRs) interact with the gas, the radiation field and the magnetic field in the Milky Way, producing diffuse emission from radio to gamma rays. Observations of this diffuse emission and comparison with detailed predictions are powerful tools to unveil the CR properties and to study CR propagation. We present various GALPROP CR propagation scenarios based on current CR measurements. The predicted synchrotron emission is compared to radio surveys, and synchrotron temperature maps from WMAP and Planck, while the predicted interstellar gamma-ray emission is compared to Fermi-LAT observations. We show how multi-wavelength observations of the Galactic diffuse emission can be used to help constrain the CR lepton spectrum and propagation. Finally we discuss how radio and microwave data could be used in understanding the diffuse Galactic gamma-ray emission observed with Fermi-LAT, especially at low energies.Comment: 8 pages, 5 figures; in Proceedings of the 34th International Cosmic Ray Conference (ICRC 2015), The Hague (The Netherlands); Oral contributio

Serine hydroxymethyltransferase: origin of substrate specificity

All forms of serine hydroxymethyltransferase, for which a primary structure is known, have five threonine residues near the active-site lysyl residue (K229) that forms the internal aldimine with pyridoxal phosphate. For Escherichia coli serine hydroxymethyltransferase each of these threonine residues has been changed to an alanine residue. The resulting five mutant enzymes were purified and characterized with respect to kinetic and spectral properties. The mutant enzymes T224A and T227A showed no significant changes in kinetic and spectral properties compared to the wild-type enzyme. The T225A and T230A enzymes exhibited differences in K(m) and k(cat) values but exhibited the same spectral properties as the wild-type enzyme. The four threonine residues at positions 224, 225, 227, and 230 do not play a critical role in the mechanism of the enzyme. The T226A enzyme had nearly normal affinity for substrates and coenzymes but had only 3% of the catalytic activity of the wild-type enzyme. The spectrum of the T226A enzyme in the presence of amino acid substrates showed a large absorption maximum at 343 nm with only a small absorption band at 425 nm, unlike the wild-type enzyme whose enzyme-substrate complexes absorb at 425 nm. Rapid reaction studies showed that when amino acid substrates and substrate analogues were added to the T226A enzyme, the internal aldimine absorbing at 422 nm was rapidly converted to a complex absorbing at 343 nm in a second-order process. This was followed by a very slow first-order formation of a complex absorbing at 425 nm. Variation of the initial rapid second-order process as a function of pH suggested that the anionic form of the amino acid forms the first complex with the enzyme. The results are interpreted as being due to the rapid formation of a gem-diamine complex between amino acids and T226A enzyme with a rate-determining formation of the external aldimine. This suggests that Thr-226 plays an important role in converting the gem-diamine complex to the external aldimine complex. Variation of the kinetic constants with amino acid structure suggests that the T226A enzyme distinguishes between substrates and substrate analogues in the formation of the gem-diamine complex

Effective chiral-spin Hamiltonian for odd-numbered coupled Heisenberg chains

An $L \times \infty$ system of odd number of coupled Heisenberg spin chains is studied using a degenerate perturbation theory, where $L$ is the number of coupled chains. An effective chain Hamiltonian is derived explicitly in terms of two spin half degrees of freedom of a closed chain of $L$ sites, valid in the regime the inter-chain coupling is stronger than the intra-chain coupling. The spin gap has been calculated numerically using the effective Hamiltonian for $L=3,5,7,9$ for a finite chain up to ten sites. It is suggested that the ground state of the effective Hamiltonian is correlated, by examining variational states for the effective chiral-spin chain Hamiltonian.Comment: 9 Pages, Latex, report ICTP-94-28

Prodigious polyphyly in Pleuroceridae (Gastropoda: Cerithioidea)

Phylogenomic studies with hundreds or thousands of loci are rare for most invertebrate groups, including freshwater gastropods. This can prevent understanding of phylogeny, which hinders many areas of research. Pleuroceridae is a family of freshwater snails that is highly imperiled and plays an essential role in the ecology of many freshwater systems of the eastern United States. However, the evolutionary history of the family is not understood, and the systematics of the family has not been revised in a modern framework. Pleurocerids display a variety of egg-deposition behaviors and shell shapes, making the family an ideal system for studying evolution of invertebrate life history and morphology. However, past mitochondrial-based phylogenetic analyses have failed to produce meaningful phylogenetic hypotheses, preventing conclusions about pleurocerid systematics and evolution. Here, we generated a novel anchored hybrid enrichment probe set with phylogenetic utility for Pleuroceridae. We sampled pleurocerids from across their range to test the probe set and generated a backbone phylogeny. Our analyses uncovered striking levels of polyphyly among currently accepted genera. Numerous species were also polyphyletic, indicative of unrecognized diversity. Phylogenetic patterns also revealed considerable convergence of shell morphologies. In contrast, anatomical and life history features appeared to be much less homoplastic. Despite generic paraphyly, high support for most major clades and phylogenetic cohesiveness of non-shell characters indicate utility of the AHE probe set for studying pleurocerid evolution