The impact of spin temperature fluctuations on the 21-cm moments

This paper considers the impact of Lyman-alpha coupling and X-ray heating on the 21-cm brightness-temperature one-point statistics (as predicted by semi-numerical simulations). The X-ray production efficiency is varied over four orders of magnitude and the hardness of the X-ray spectrum is varied from that predicted for high-mass X-ray binaries, to the softer spectrum expected from the hot inter-stellar medium. We find peaks in the redshift evolution of both the variance and skewness associated with the efficiency of X-ray production. The amplitude of the variance is also sensitive to the hardness of the X-ray SED. We find that the relative timing of the coupling and heating phases can be inferred from the redshift extent of a plateau that connects a peak in the variance's evolution associated with Lyman-alpha coupling to the heating peak. Importantly, we find that late X-ray heating would seriously hamper our ability to constrain reionization with the variance. Late X-ray heating also qualitatively alters the evolution of the skewness, providing a clean way to constrain such models. If foregrounds can be removed, we find that LOFAR, MWA and PAPER could constrain reionization and late X-ray heating models with the variance. We find that HERA and SKA (phase 1) will be able to constrain both reionization and heating by measuring the variance using foreground-avoidance techniques. If foregrounds can be removed they will also be able to constrain the nature of Lyman-alpha coupling.Comment: 16 pages, 13 figure, 1 table. Accepted for publication in MNRA

Distinguishing models of reionization using future radio observations of 21-cm 1-point statistics

We explore the impact of reionization topology on 21-cm statistics. Four reionization models are presented which emulate large ionized bubbles around over-dense regions (21CMFAST/ global-inside- out), small ionized bubbles in over-dense regions (local-inside-out), large ionized bubbles around under-dense regions (global-outside-in) and small ionized bubbles around under-dense regions (local-outside-in). We show that first-generation instruments might struggle to distinguish global models using the shape of the power spectrum alone. All instruments considered are capable of breaking this degeneracy with the variance, which is higher in outside-in models. Global models can also be distinguished at small scales from a boost in the power spectrum from a positive correlation between the density and neutral-fraction fields in outside-in models. Negative skewness is found to be unique to inside-out models and we find that pre-SKA instruments could detect this feature in maps smoothed to reduce noise errors. The early, mid and late phases of reionization imprint signatures in the brightness-temperature moments, we examine their model dependence and find pre-SKA instruments capable of exploiting these timing constraints in smoothed maps. The dimensional skewness is introduced and is shown to have stronger signatures of the early and mid-phase timing if the inside-out scenario is correct.Comment: 18 pages, 13 figures, updated to agree with published versio

Coloniality and the Politicisation of Literary Heritage Conservation

In his autobiography The Life and Rhymes of Benjamin Zephaniah, the dub poet reflects on the relationship between architecture, the built environment, the writing process, and the politics that underpin them. In particular, he notes the impact of colonial legacies in urban planning, his experience as patron of the Ujima Housing Co-operative Group, and the inspiration gained from the diversity of the built environment in Newham (where Zephaniah has resided since 1980). This paper uses these reflections as a starting point to explore the politics of literary heritage conservation in Britain and the colonialism implicit within it. There has been a wave of recent interest in the politics underpinning literary production in the UK sparked by Bernardine Evaristo’s ‘Black Britain, Writing Back’ campaign to correct ‘historical bias in publishing’ and the colonial legacies that foster it. However, far less attention has been paid to the politics behind the conservation of literary heritage. A 2020 report by the National Trust pointed to the role played by ‘conserved’ writers’ houses, such as Bateman’s, Shaw’s Corner, and Monk’s House, in sparking the republication of previous out of print works by writers. The conservation of literary heritage therefore clearly expresses a cultural power, articulating who is left in, or out, of an ever-changing ‘canon’ while providing access points to the literary legacy of writers. Yet, despite attempts to ‘decolonise’ heritage sites, such as the National Trust’s Colonialism and Historic Slavery Report, there is only limited reflection of the coloniality of the values that underpin what is conserved and how we conserve it. In France, the fact that the renovation of Andre Gide’s house in Cuverville sparked a government heritage conservation debate, while the destruction of James Baldwin’s house in St Paul de Vence (2014) failed to elicit similar protests, has led to a rethinking of the politics that facilitate literary heritage conservation. Yet, in Britain, there has been a significant lack of debate on the legacies of Empire and the destruction of black British writers’ houses. This paper reflects on Toni Morrison’s sense of writing as ‘literary archaeology’ to argue for the conservation of writers’ houses as a form of ‘literary architexture’ utilising the semiotics of lived experience to conserve and interpret writers’ homes. First, it applies this methodology to case studies of writers’ homes to examine the coloniality built into the process of heritage conservation. Noting, for example, that the conservation plans for Bloomsbury Group heritage sites dwell on their networks with white literati while failing to mention the significance of global majority figures like Pat Nelson and Berto Pasuka. Secondly, it asks that we re-examine the coloniality behind heritage conservation to fully explore the power structures that override whose literary heritage is conserved and the consequences of this for future

21-cm signatures of residual HI inside cosmic HII regions during reionization

We investigate the impact of sinks of ionizing radiation on the reionization-era 21-cm signal, focusing on 1-point statistics. We consider sinks in both the intergalactic medium and inside galaxies. At a fixed filling factor of HII regions, sinks will have two main effects on the 21-cm morphology: (i) as inhomogeneous absorbers of ionizing photons they result in smaller and more widespread cosmic HII patches; and (ii) as reservoirs of neutral gas they contribute a non-zero 21-cm signal in otherwise ionized regions. Both effects damp the contrast between neutral and ionized patches during reionization, making detection of the epoch of reionization with 21-cm interferometry more challenging. Here we systematically investigate these effects using the latest semi-numerical simulations. We find that sinks dramatically suppress the peak in the redshift evolution of the variance, corresponding to the midpoint of reionization. As previously predicted, skewness changes sign at midpoint, but the fluctuations in the residual HI suppress a late-time rise. Furthermore, large levels of residual HI dramatically alter the evolution of the variance, skewness and power spectrum from that seen at lower levels. In general, the evolution of the large-scale modes provides a better, cleaner, higher signal-to-noise probe of reionization.Comment: Minor edits to agree with MNRAS published versio

Comparison of Crude Oil Fouling Using Two Different Probes

A variety of different fouling rigs, each with its own advantages and disadvantages is available to assess crude oil fouling. In this research, fouling of three crude oils are compared, using two electrically heated annular probes with strikingly different modes of operation. The Hot Liquid Process Simulator (HLPS) was operated in single-pass mode, under creeping flow conditions for time durations of a few hours at constant surface temperature. The Portable Fouling Research Unit (PFRU) was operated at velocities a factor of 250 higher, and followed the fouling process over two or more days at constant heat flux, using re-circulation of the crude oil. It was of interest to determine whether the two devices led to the same relative ranking of extent or rate of fouling, and exhibited similar responses to changes in surface temperature. Two conventional light crude oils, and a synthetic crude oil derived from heavy oil were tested at average surface temperatures in the range 225-380°C, and bulk temperatures roughly 100°C lower. With the low velocity probe, typical fouling resistances after four hours were over an order of magnitude higher than for the high velocity probe after 48 hours. Fouling rates were two orders of magnitude greater in the low velocity unit. Some differences in relative ranking of the fouling potentials for the three oils were noted

Fouling of Some Canadian Crude Oils

A thermal fouling study was undertaken using three sour Canadian crude oils. Experiments were carried out in a re-circulation fouling loop, equipped with an annular (HTRI) electrically heated probe. Fluids at pressures of about 1000-1340 kPa under a nitrogen atmosphere were re-circulated at a velocity of 0.75 m/s for periods of 48 hours, and the decline in heat transfer coefficient followed under conditions of constant heat flux. Bulk temperatures were varied over the range 200-285°C, and initial surface temperatures from 300 to 380°C. Heat fluxes were in the range of 265-485 kW/m2

PRECIPITATION AND FOULING IN HEAVY OIL–DILUENT BLENDS

Heavy oil fractions rich in asphaltenes were mixed with diluents containing from 0.6-25% aromatics, and the resulting blends subjected to batch precipitation experiments at 85°C, and thermal fouling tests at surface temperatures in the range 230-310ºC. Deposit compositions were determined, and are compared with suspended asphaltene composition. When the heavy oils were blended with the most aromatic diluents, precipitation and fouling were negligible. As the aromaticity of the diluents was decreased, the extent of asphaltene precipitation and the fouling rates increased. The solubility parameter of the blends, δmix, and the flocculation solubility parameter for asphaltenes, δf , were determined from measurements of the asphaltene flocculation onset by titration with heptane at temperatures from 25 to 50°C. Literature models predict no asphaltene precipitation (and presumably little fouling) will occur when [δmix- δf ] \u3e0 . Both the amount of asphaltene precipitated in the batch experiments, and the rate of thermal fouling decreased as the solubility parameter difference [δmix - δf] increased from negative to positive values. However some precipitation and fouling was observed at the expected condition for mixture stability δmix ≥ δf . As the temperature of the flocculation titration was raised towards the bulk temperatures of the experiments, values of [δmix-δf] decreased, and the agreement of the data with prediction of the point for zero precipitation and fouling improved. For unstable oil blends, the solubility parameter provides a good predictive measure of the tendency for asphaltene precipitation, and for heat exchanger fouling