arXiv subjects

Aleksandra Dragović

Publications and source records attributed to Aleksandra Dragović.

3 recordsLinked to original sources

arXiv2026

Quantifying Sky Map Resolution Requirements for Beam Chromaticity Correction in Sky-Averaged 21 cm Experiments

The 21 cm hyperfine transition of neutral hydrogen provides one of the few direct probes of the early cosmic history. High-redshift detections of this signal could shine light on the poorly understood epochs of the Dark Ages and Cosmic Dawn, periods that remain among the least understood in the Universe's history. However, this signal is masked by bright foregrounds and is distorted by the chromaticity of the antenna used to detect it. Correcting for the chromaticity requires an accurate representation of the radio sky across relevant frequencies. However, base sky maps used for this correction, such as instances of the Global Sky Model (GSM), are limited by resolution, calibration and extrapolation uncertainties, especially when scaled to lower frequencies relevant for Cosmic Dawn studies. This work quantifies how accurately the base map must represent the true sky to produce a reliable beam correction. Using simulated sky data, we generate beam chromaticity corrections generated from progressively degraded versions of the base map, and compare how well they do in comparison to a correction based on the full-resolution map. We find that degradation in map resolution of $\sim 1^\circ$ does not introduce measurable increases in the amplitude of the residuals when correcting for the chromaticity and subtracting three different foreground models (two polynomials and a power law expansion). These results provide practical constraints on the required resolution and accuracy of sky models used in beam correction pipelines, informing future design and calibration strategies for global 21 cm experiments, and future efforts to map the low frequency sky.

astro-ph.CO↗

arXiv2025

Heat Transport in Ionic Liquids

Heat transfer in liquids is a very challenging problem as it combines the competing effect of high frequency oscillations, which dominate liquid heat capacity, and diffusive motion, which enables transport macroscopic flow. This issue is compounded by the relatively junior state of dynamical theories of liquid thermodynamics. Nevertheless, molten salts are playing an increasingly important role in industrial and energy applications and there is a pressing need to understand the mechanisms behind their irreversible transport processes. Here we use molecular dynamics simulations to investigate the heat transport of three different molten salts: LiCl, KCl, and the eutectic point of their mixture. While all simulations consider the properties of the liquid within the frame of its centre of mass, we calculate different susceptibilities which implicitly include and explicitly exclude the heat carried by partial mass currents within this frame. We find that, while the heat advected by partial mass currents in the mixture increases with increasing temperature, the heat transferred by collective vibrational motion (phonons) decreases with increasing temperature. This causes a maximum in the heat conductance with temperature in the mixtures only - in pure salts each contribution decreases monotonically with temperature. We attribute this anomaly to the extra freedom afforded to ionic motion in mixtures - in pure salts the motion of cations and anions is bound due to conservation of linear momentum. In mixtures, a coherent but diffusive collective motion is enabled by the release of Li ions from this condition by the introduction of a third species. We tentatively ascribe this coherent collective motion to the ``diffusive" phonons that have been used to explain a similar anomaly in the thermal conductivity of solids.

cond-mat.mtrl-sci↗

arXiv2024

Configurational and Vibrational Entropy in Hydrogen-bonded Fluids

The thermodynamics of liquids and supercritical fluids is notorious for eluding a general theory, as can be done for crystalline solids on the basis of phonons and crystal symmetry. The extension of solid state notions such as configurational entropy and phonons to the liquid state remains an intriguing but challenging topic. This is particularly true for liquids like water whose many structural anomalies give it unique properties. Here, for simple fluids, we specify the thermodynamics across the liquid, supercritical, and gaseous states using the spectrum of propagating phonons, thereby determining the non-ideal entropy of the fluid using a single parameter arising from this phonon spectrum. This identifies a marked distinction between these "simple" fluids, and hydrogen bonded fluids whose non-ideal entropy cannot be determined by the phonon spectrum alone. We relate this phonon theory of thermodynamics to the previously observed excess entropy scaling in liquids and how the phonon spectrum creates corresponding states across the fluid phase diagram. These results provide important theoretical understanding to supercritical fluid, whose properties are still poorly understood despite widespread deployment in environmental and energy applications.

cond-mat.soft↗