arXiv subjects

Richard Smith

Publications and source records attributed to Richard Smith.

13 recordsLinked to original sources

arXiv2026

First Results from the LSST Shadow Survey: The Restless Luminous Blue Variable AT2017des in the Virgo-Cluster Galaxy, NGC4532

The Legacy Survey of Space and Time (LSST) will start in late-summer 2026, revolutionizing transient astronomy. Here, we present the Dark Energy Camera (DECam) Shadow Survey, which is designed to maximize the science potential of LSST by shadowing LSST observations of local galaxy-cluster fields, producing a nightly cadence of these fields. The Shadow Survey will discover extremely young supernovae (SNe), SN precursors, as well as other explosive transients and exotic phenomena, helping to characterize such transients at unprecedented cadence and depth when combined with LSST. We describe our workflow, pipeline, public data releases, and candidate vetting. As an early result of Shadow, we present the fitful luminous blue variable (LBV) eruptions of AT2017des in the Virgo-Cluster galaxy NGC4532. AT2017des has short-timescale variability (of order 10 days), peaking at around $M_r=-12.5$mag, brighter than normal LBVs, and similar to the more extreme flaring of hot LBVs/SN impostors such as SN2000ch, AT2016blu, and the precursor activity of SN2009ip. Our spectral time-series reveals features typical of these hot LBVs and SN impostors/precursors. Combining our data with long-baseline photometry from additional observatories, we find that the peaks of the outbursts of AT2017des are getting brighter over time, with 2026 peak fluxes being up to 5 times greater than in 2023 and an average brightening of $\sim0.05$ mag yr$^{-1}$. The peaks of AT2017des are more luminous than those of most other LBVs, only being fainter than bright precursors such as SN2009ip, and extreme SN impostors such as AT2016blu. AT2017des may therefore be ``ramping up'' to a terminal explosion.

astro-ph.HE↗

arXiv2026

Constrained Fiducial Inference for Gaussian Models

We propose a new fiducial Markov Chain Monte Carlo (MCMC) method for fitting parametric Gaussian models. We utilize the Cayley transform to decompose the parametric covariance matrix, which in turn allows us to formulate a general data generating algorithm for Gaussian data. Leveraging constrained generalized fiducial inference, we are able to create the basis of an MCMC algorithm, which can be specified to parametric models with minimal effort. The appeal of this novel approach is the wide class of models which it permits, ease of implementation and the posterior-like fiducial distribution without the need for a prior. We provide background information for the derivation of the relevant fiducial quantities, and a proof that the proposed MCMC algorithm targets the correct fiducial distribution. We need not assume independence nor identical distribution of the data, which makes the method attractive for application to time series and spatial data. Well-performing simulation results of the MA(1) and Mat\'ern models are presented.

stat.ME↗

arXiv2024

Variable Stars in M31 Stellar Clusters from the Panchromatic Hubble Andromeda Treasury

Variable stars in stellar clusters can offer key constraints on stellar evolution and pulsation models, utilising estimates of host cluster properties to constrain stellar physical parameters. We present a catalogue of 86 luminous (F814W<19) variable stars in M31 clusters identified by mining the archival Panchromatic Hubble Andromeda Treasury (PHAT) survey using a combination of statistical analysis of sparse PHAT light curves and difference imaging. We determine the evolutionary phases and initial masses of these variable stars by matching them with theoretical isochrones generated using host cluster properties from the literature. We calculate the probability of PHAT photometry being blended due to the highly crowded nature of cluster environments for each cluster-variable star, using these probabilities to inform our level of confidence in the derived properties of each star. Our 86 cluster-variable stars have initial masses between 0.8--67 $M_{\odot}$. Their evolutionary phases span the main sequence, more evolved hydrogen- and helium-burning phases, and the post-asymptotic giant branch. We identify numerous candidate variable star types: RV Tauri variables, red supergiants and slowly pulsating B-type supergiants, along with Wolf Rayet stars, $\alpha$ Cygni and Mira variables, a classical Cepheid and a possible super-asymptotic giant. We characterise 12 cluster-variable stars at higher confidence based on their difference image quality and lower blending probability. Ours is the first systematic study of variable stars in extragalactic stellar clusters leveraging the superior resolution of the Hubble Space Telescope and demonstrating the unique power of stellar clusters in constraining the fundamental properties of variable stars.

astro-ph.SR↗

arXiv2024

Asymptotic Theory for Estimation of the Husler-Reiss Distribution via Block Maxima Method

The H\"usler-Reiss distribution describes the limit of the pointwise maxima of a bivariate normal distribution. This distribution is defined by a single parameter, $\lambda$. We provide asymptotic theory for maximum likelihood estimation of $\lambda$ under a block maxima approach. Our work assumes independent and identically distributed bivariate normal random variables, grouped into blocks where the block size and number of blocks increase simultaneously. With these assumptions our results provide conditions for the asymptotic normality of the Maximum Likelihood Estimator (MLE). We characterize the bias of the MLE, provide conditions under which this bias is asymptotically negligible, and discuss how to choose the block size to minimize a bias-variance trade-off. The proofs are an extension of previous results for choosing the block size in the estimation of univariate extreme value distributions (Dombry and Ferreria 2019), providing a potential basis for extensions to multivariate cases where both the marginal and dependence parameters are unknown. The proofs rely on the Argmax Theorem applied to a localized loglikelihood function, combined with a Lindeberg-Feller Central Limit Theorem argument to establish asymptotic normality. Possible applications of the method include composite likelihood estimation in Brown-Resnick processes, where it is known that the bivariate distributions are of H\"usler-Reiss form.

math.ST↗

arXiv2022

A Targeted Search for Main Belt Comets

Main Belt Comets (MBCs) exhibit sublimation-driven activity while occupying asteroid-like orbits in the Main Asteroid Belt. MBCs and candidates show stronger clustering of their longitudes of perihelion around 15{\deg} than other objects from the Outer Main Belt (OMB). This potential property of MBCs could facilitate the discovery of new candidates by observing objects in similar orbits. We acquired deep r-band images of 534 targeted asteroids using the INT/WFC between 2018 and 2020. Our sample is comprised of OMB objects observed near perihelion, with longitudes of perihelion between 0{\deg} and 30{\deg} and orbital parameters similar to knowns MBCs. Our pipeline applied activity detection methods to 319 of these objects to look for tails or comae, and we visually inspected the remaining asteroids. Our activity detection pipeline highlighted a faint anti-solar tail-like feature around 2001 NL19 (279870) observed on 2018 November 07, six months after perihelion. This is consistent with cometary activity but additional observations of this object will be needed during its next perihelion to investigate its potential MBC status. If it is active our survey yields a detection rate of $\sim$1:300, which is higher than previous similar surveys, supporting the idea of dynamical clustering of MBCs. If not, it is consistent with previously estimated abundance rates of MBCs in the OMB (<1:500).

astro-ph.EP↗

arXiv2020

Combining interdependent climate model outputs in CMIP5: A spatial Bayesian approach

Projections of future climate change rely heavily on climate models, and combining climate models through a multi-model ensemble is both more accurate than a single climate model and valuable for uncertainty quantification. However, Bayesian approaches to multi-model ensembles have been criticized for making oversimplified assumptions about bias and variability, as well as treating different models as statistically independent. This paper extends the Bayesian hierarchical approach of Sansom et al. (2017) by explicitly accounting for spatial variability and inter-model dependence. We propose a Bayesian hierarchical model that accounts for bias between climate models and observations, spatial and inter-model dependence, the emergent relationship between historical and future periods, and natural variability. Extensive simulations show that our model provides better estimates and uncertainty quantification than the commonly used simple model mean. These results are illustrated using data from the CMIP5 model archive. As examples, for Central North America our projected mean temperature for 2070--2100 is about 0.8 K lower than the simple model mean, while for East Asia it is about 0.5 K higher; however, in both cases, the widths of the 90% credible intervals are of the order 3--6 K, so the uncertainties overwhelm the relatively small differences in projected mean temperatures.

stat.AP↗

arXiv2020

Modeling the Invariance of Virtual Pointers in LLVM

Devirtualization is a compiler optimization that replaces indirect (virtual) function calls with direct calls. It is particularly effective in object-oriented languages, such as Java or C++, in which virtual methods are typically abundant. We present a novel abstract model to express the lifetimes of C++ dynamic objects and invariance of virtual table pointers in the LLVM intermediate representation. The model and the corresponding implementation in Clang and LLVM enable full devirtualization of virtual calls whenever the dynamic type is statically known and elimination of redundant virtual table loads in other cases. Due to the complexity of C++, this has not been achieved by any other C++ compiler so far. Although our model was designed for C++, it is also applicable to other languages that use virtual dispatch. Our benchmarks show an average of 0.8% performance improvement on real-world C++ programs, with more than 30% speedup in some cases. The implementation is already a part of the upstream LLVM/Clang and can be enabled with the -fstrict-vtable-pointers flag.

cs.PL↗

arXiv2017

Reaction paths of alane dissociation on the Si(001) surface

Building on our earlier study, we examine the kinetic barriers to decomposition of alane, AlH$_3$, on the Si(001) surface, using the nudged elastic band (NEB) approach within DFT. We find that the initial decomposition to AlH with two H atoms on the surface proceeds without a significant barrier. There are several pathways available to lose the final hydrogen, though these present barriers of up to 1 eV. Incorporation is more challenging, with the initial structures less stable in several cases than the starting structures, just as was found for phosphorus. We identify a stable route for Al incorporation following selective surface hydrogen desorption (e.g. by STM tip). The overall process parallels PH$_3$, and indicates that atomically precise acceptor doping should be possible.

cond-mat.mtrl-sci↗

arXiv2016

A Security Evaluation Framework for U.K. E-Goverment Services Agile Software Development

This study examines the traditional approach to software development within the United Kingdom Government and the accreditation process. Initially we look at the Waterfall methodology that has been used for several years. We discuss the pros and cons of Waterfall before moving onto the Agile Scrum methodology. Agile has been adopted by the majority of Government digital departments including the Government Digital Services. Agile, despite its ability to achieve high rates of productivity organized in short, flexible, iterations, has faced security professionals disbelief when working within the U.K. Government. One of the major issues is that we develop in Agile but the accreditation process is conducted using Waterfall resulting in delays to go live dates. Taking a brief look into the accreditation process that is used within Government for I.T. systems and applications, we focus on giving the accreditor the assurance they need when developing new applications and systems. A framework has been produced by utilizing the Open Web Application Security Project (OWASP) Application Security Verification Standard (ASVS). This framework will allow security and Agile to work side by side and produce secure code.

cs.CR↗

arXiv2014

Hydrogen adsorption and diffusion around Si(001)/Si(110) corners in nanostructures

While the diffusion of hydrogen on silicon surfaces has been relatively well characterised both experimentally and theoretically, the diffusion around corners between surfaces, as will be found on nanowires and nanostructures, has not been studied. Motivated by nanostructure fabrication by Patterned Atomic Layer Epitaxy (PALE), we present a density functional theory (DFT) study of the diffusion of hydrogen around the edge formed by the orthogonal (001) and (110) surfaces in silicon. We find that the barrier from (001) to (110) is approximately 0.3 eV lower than from (110) to (001), and that it is comparable to diffusion between rows on the clean surface, with no significant effect on the hydrogen patterns at the growth temperatures used.

cond-mat.mtrl-sci↗

arXiv2007

Survival of a diffusing particle in an expanding cage

We consider a Brownian particle, with diffusion constant D, moving inside an expanding d-dimensional sphere whose surface is an absorbing boundary for the particle. The sphere has initial radius L_0 and expands at a constant rate c. We calculate the joint probability density, p(r,t|r_0), that the particle survives until time t, and is at a distance r from the centre of the sphere, given that it started at a distance r_0 from the centre.

cond-mat.stat-mech↗

arXiv2007

Survival probability of a diffusing particle constrained by two moving, absorbing boundaries

We calculate the exact asymptotic survival probability, Q, of a one-dimensional Brownian particle, initially located located at the point x in (-L,L), in the presence of two moving absorbing boundaries located at \pm(L+ct). The result is Q(y,λ) = \sum_{n=-\infty}^\infty (-1)^n \cosh(ny) \exp(-n^2λ), where y=cx/D, λ= cL/D and D is the diffusion constant of the particle. The results may be extended to the case where the absorbing boundaries have different speeds. As an application, we compute the asymptotic survival probability for the trapping reaction A + B -> B, for evanescent traps with a long decay time.

cond-mat.stat-mech↗

arXiv2005

Survival probabilities in the double trapping reaction A +B -> B, B + C -> C

We consider the double trapping reaction A + B -> B, B + C -> C in one dimension. The survival probability of a given A particle is calculated under various conditions on the diffusion constants of the reactants, and on the ratio of initial B and C particle densities. The results are of more general form than those obtained in previous work on the problem.

cond-mat.stat-mech↗