Education in Blackburn, 1870 - 1914

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Divergences in QED on a Graph

We consider a model of quantum electrodynamics (QED) on a graph. The one-loop divergences in the model are investigated by use of the background field method.Comment: 14 pages, no figures, RevTeX4. References and typos adde

Metadata for describing learning scenarios under European Higher Education Area paradigm

In this paper we identify the requirements for creating formal descriptions of learning scenarios designed under the European Higher Education Area paradigm, using competences and learning activities as the basic pieces of the learning process, instead of contents and learning resources, pursuing personalization. Classical arrangements of content based courses are no longer enough to describe all the richness of this new learning process, where user profiles, competences and complex hierarchical itineraries need to be properly combined. We study the intersection with the current IMS Learning Design specification and the additional metadata required for describing such learning scenarios. This new approach involves the use of case based learning and collaborative learning in order to acquire and develop competences, following adaptive learning paths in two structured levels

Some Exact Results on the Potts Model Partition Function in a Magnetic Field

We consider the Potts model in a magnetic field on an arbitrary graph $G$. Using a formula of F. Y. Wu for the partition function $Z$ of this model as a sum over spanning subgraphs of $G$, we prove some properties of $Z$ concerning factorization, monotonicity, and zeros. A generalization of the Tutte polynomial is presented that corresponds to this partition function. In this context we formulate and discuss two weighted graph-coloring problems. We also give a general structural result for $Z$ for cyclic strip graphs.Comment: 5 pages, late

Chemical Raman Enhancement of Organic Adsorbates on Metal Surfaces

Using a combination of first-principles theory and experiments, we provide a quantitative explanation for chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces. With density functional theory calculations of the static Raman tensor, we demonstrate and quantify a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential, a well-defined property of each vibrational mode. A straightforward and general analysis is introduced that allows extraction of chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.Comment: 5 pages, 4 figures and Supplementary material included as ancillary fil

XMM-Newton Observations of Radio Pulsars B0834+06 and B0826-34 and Implications for Pulsar Inner Accelerator

We report the X-ray observations of two radio pulsars with drifting subpulses: B0834 + 06 and B0826 - 34 using \xmm\. PSR B0834 + 06 was detected with a total of 70 counts from the three EPIC instruments over 50 ks exposure time. Its spectrum was best described as that of a blackbody (BB) with temperature $T_s=(2.0^{+2.0}_{-0.9}) \times 10^6$ K and bolometric luminosity of $L_b=(8.6^{+14.2}_{-4.4}) \times 10^{28}$ erg s$^{-1}$. As it is typical in pulsars with BB thermal components in their X-ray spectra, the hot spot surface area is much smaller than that of the canonical polar cap, implying a non-dipolar surface magnetic field much stronger than the dipolar component derived from the pulsar spin-down (in this case about 50 times smaller and stronger, respectively). The second pulsar PSR B0826 - 34 was not detected over 50 ks exposure time, giving an upper limit for the bolometric luminosity $L_b \leq 1.4 \times 10^{29}$ erg s$^{-1}$. We use these data as well as the radio emission data concerned with drifting subpulses to test the Partially Screened Gap (PSG) model of the inner accelerator in pulsars.Comment: Accepted for publication by The Astrophysical Journa

Thermal X-rays from Millisecond Pulsars: Constraining the Fundamental Properties of Neutron Stars

Abridged) We model the X-ray properties of millisecond pulsars (MSPs) by considering hot spot emission from a weakly magnetized rotating neutron star (NS) covered by an optically-thick hydrogen atmosphere. We investigate the limitations of using the thermal X-ray pulse profiles of MSPs to constrain the mass-to-radius ($M/R$) ratio of the underlying NS. The accuracy is strongly dependent on the viewing angle and magnetic inclination. For certain systems, the accuracy is ultimately limited only by photon statistics implying that future X-ray observatories could, in principle, achieve constraints on $M/R$ and hence the NS equation of state to better than $\sim$5%. We demonstrate that valuable information regarding the basic properties of the NS can be extracted even from X-ray data of fairly limited photon statistics through modeling of archival spectroscopic and timing observations of the nearby isolated PSRs J0030+0451 and J2124--3358. The X-ray emission from these pulsars is consistent with the presence of a hydrogen atmosphere and a dipolar magnetic field configuration, in agreement with previous findings for PSR J0437--4715. For both MSPs, the favorable geometry allows us to place interesting limits on the allowed $M/R$ of NSs. Assuming 1.4 M$_{\odot}$, the stellar radius is constrained to be $R > 9.4$ km and $R > 7.8$ km (68% confidence) for PSRs J0030+0451 and J2124--3358, respectively. We explore the prospects of using future observatories such as \textit{Constellation-X} and \textit{XEUS} to conduct blind X-ray timing searches for MSPs not detectable at radio wavelengths due to unfavorable viewing geometry. Using the observational constraints on the pulsar obliquities we are also able to place strong constraints on the magnetic field evolution model proposed by Ruderman.Comment: 9 pages, 7 figures, published in the Astrophysical Journal (Volume 689, Issue 1, pp. 407-415

Properties of dense partially random graphs

We study the properties of random graphs where for each vertex a {\it neighbourhood} has been previously defined. The probability of an edge joining two vertices depends on whether the vertices are neighbours or not, as happens in Small World Graphs (SWGs). But we consider the case where the average degree of each node is of order of the size of the graph (unlike SWGs, which are sparse). This allows us to calculate the mean distance and clustering, that are qualitatively similar (although not in such a dramatic scale range) to the case of SWGs. We also obtain analytically the distribution of eigenvalues of the corresponding adjacency matrices. This distribution is discrete for large eigenvalues and continuous for small eigenvalues. The continuous part of the distribution follows a semicircle law, whose width is proportional to the "disorder" of the graph, whereas the discrete part is simply a rescaling of the spectrum of the substrate. We apply our results to the calculation of the mixing rate and the synchronizability threshold.Comment: 14 pages. To be published in Physical Review

Identification of climatological sub-regions within the Tully mill area

Identifying optimal nitrogen application rates that reduce nitrogen loss without adversely reducing yields would benefit growers and the environment. In order to identify optimal nitrogen application rates throughout the Tully mill area, it is important to identify sub-regions that share similar topographical, soil, farm management, productivity or climatological attributes. While current SIX EASY STEPS nitrogen guidelines enable a hierarchy of district, soil, block and crop nitrogen requirements for sugarcane, it would be beneficial for management zones to also take spatial climate variability information into account. Unfortunately, spatial climate variability within a region, is generally not considered when developing nitrogen management practices. The objective of this paper was to identify sub-regions within the Tully mill area based on climatological attributes as a first step towards better informing nitrogen management decisions. Rainfall, radiation and temperature data were obtained on a 0.05 by 0.05˚ grid (approximately 5 km by 5 km) for sugarcane-growing areas within the Tully Mill region. A K-means clustering algorithm was then used to cluster these grid cells into distinct sub-regions based on seasonal or annual climate data. Two distinct sub-regions were identified based on total annual rainfall and annual average daily radiation data. These sub-regions were identified as a northern and southern sub-region, divided roughly along the Tully River. The northern sub-region was characterised by lower radiation, lower temperatures and higher rainfall than the southern sub-region. Crop simulation models will now be able to use this knowledge to assess if nitrogen management plans should vary between the two sub-regions in Tully

Upper Limits On Periodic, Pulsed Radio Emission from the X-Ray Point Source in Cassiopeia A

The Chandra X-ray Observatory recently discovered an X-ray point source near the center of Cassiopeia A, the youngest known Galactic supernova remnant. We have conducted a sensitive search for radio pulsations from this source with the Very Large Array, taking advantage of the high angular resolution of the array to resolve out the emission from the remnant itself. No convincing signatures of a dispersed, periodic source or of isolated dispersed pulses were found, whether for an isolated or a binary source. We derive upper limits of 30 and 1.3 mJy at 327 and 1435 MHz for the phase-averaged pulsed flux density from this source. The corresponding luminosity limits are lower than those for any pulsar with age less than 10^4 years. The sensitivities of our search to single pulses were 25 and 1.0 Jy at 327 and 1435 MHz. For comparison, the Crab pulsar emits roughly 80 pulses per minute with flux densities greater than 100 Jy at 327 MHz and 8 pulses per minute with flux densities greater than 50 Jy at 1435 MHz. These limits are consistent with the suggestion that the X-ray point source in Cas A adds to the growing number of neutron stars which are not radio pulsars.Comment: accepted by ApJ Letter