arXiv subjects

Vasiliki Pavlidou

Publications and source records attributed to Vasiliki Pavlidou.

At least 19 recordsLinked to original sources

arXiv2026

The Most Probable Outer Density Profile from Excursion Set Theory

Measurements of the turnaround radius around galaxy clusters can be used to break the degeneracy between measurements of the present day energy density of matter and dark energy. Korkidis & Pavlidou showed that the turnaround radius coincides with the first point of deviation between outer density profiles in N-body simulations and the analytic profile predicted by excursion set theory. However, their analytic profile relied on a number of simplifying assumptions, which may each introduce systematic error. We evaluate the effect of these assumptions on the shape of the analytic profile and its correspondence with simulated outer density profiles. We relax the key simplifying assumptions and re-derive the mode of the outer density profile from excursion set theory. We then numerically resolve the double distribution (DD) across a range of masses and clustering parameters, and compare the numerical mode estimate to the re-derived analytic profile. We find excellent agreement between our analytic profile and the numerically-realized DD. However, our analytic profiles diverge from N-body profiles, and this divergence grows as we relax successive assumptions. We relate this mismatch to the differing window functions used in the analytic and simulation-based approaches, which respectively yield Markovian and correlated density trajectories. We conclude that the analytic profile proposed by Korkidis & Pavlidou should only be used as a few-parameter effective description of the most-probable outer density profile, with parameters fitted to results of cosmological simulations.

astro-ph.CO↗

arXiv2026

WALOP-South: a four camera one shot imaging polarimeter for the PASIPHAE survey. Paper III -- PSF modelling

The two WALOP instruments, built for the PASIPHAE survey, will measure the linear polarization of large numbers of stars in the Galactic polar regions in the SDSS-$r$ band. They are designed with a wide field-of-view, enabling measurement of the Stokes parameters $I$, $q$, and $u$ for multiple stars simultaneously within a $35^\prime \times 35^\prime$ region of sky for WALOP-South and $30^\prime \times 30^\prime$ region for WALOP-North. In this paper, we present a polar shapelet-based PSF photometry framework for well-sampled stellar point sources applicable to WALOP-type wide-field polarimeters. Polar shapelets are a set of orthogonal basis functions, constructed from Gauss-Hermite or Gauss-Laguerre polynomials, that are well-suited to modelling localized PSF in a compact and efficient way. We developed an efficient PSF modelling method that uses polar shapelets as basis functions to reconstruct the spatial variation of the PSF shape across the CCD using Zemax-simulated images of one of the WALOP instruments, and show that a limited number of shapelet coefficients are sufficient to capture this variation consistently across different CCD locations. To simulate realistic star images, we introduce random sub-pixel shifts in the star centroids in the Zemax-simulated images, and account for this using a two-step iterative method that alternately estimates the PSF model and the sub-pixel centroid shift. Applying PSF photometry to the target faint stars, we demonstrate that the photometric accuracy of approximately 0.15% is achievable, and that the reconstructed PSF model can be incorporated into the photometry across different seeing conditions, meeting the polarimetric science requirements of the PASIPHAE survey.

astro-ph.IM↗

arXiv2026

Milky Way Atlas: A radial-velocity-resolved, three-dimensional map of H I within 1.25 kpc

We present a velocity-resolved three-dimensional map of local atomic hydrogen (HI) within 1.25 kpc of the Sun, tackling the challenge of converting emission from position-position-velocity space into true 3D structure. Our method combines the HI4PI full-sky survey with the Edenhofer et al. (2024) 3D dust map in the framework of Information Field Theory, enabling a joint reconstruction of the local HI density, radial velocity field, and effective line width while also separating emission arising inside the mapped local volume from more distant Galactic HI. The inference is driven by morphological matching between dust and HI structures together with kinematic coherence in 3D space. Synthetic data tests show that the method recovers the local density and velocity structure, even in the presence of substantial contamination from distant emission. The resulting map reveals a smoother, more diffuse local HI distribution than the dust, a declining HI-to-dust ratio toward high dust column densities consistent with the atomic-to-molecular transition, and a velocity field that captures both large-scale Galactic rotation and local non-circular velocities. Independent comparisons with maser and young stellar cluster velocities agree with the recovered kinematics. This HI map provides a new three-dimensional, kinematically resolved view of the nearby atomic interstellar medium and a foundation for localising other velocity-resolved Galactic emission in physical space.

astro-ph.GA↗

arXiv2026

Evidence for cloud-to-cloud variations in the ratio of polarized thermal dust emission to starlight polarization

The correlation between optical starlight polarization and polarized thermal dust emission can be used to infer intrinsic dust properties. This correlation is quantified by the ratio Rp/p, which has been measured to be 5.42 +/- 0.05 MJy sr^-1 at 353 GHz when averaged over large areas of the sky. We investigate this correlation using newly published stellar polarimetric data densely sampling a continuous sky region of ~4 square degrees at intermediate Galactic latitude. We combine RoboPol optical polarization measurements for 1,430 stars with submillimeter data from the Planck satellite at 353 GHz. We perform linear fits between the Planck (Qs, Us) and optical (qv, uv) Stokes parameters, accounting for the differences in resolution between the two datasets as well as the distribution of clouds along the line of sight. We find that in this region of the sky the Rp/p value is 3.67 +/- 0.05 MJy sr^-1, indicating a significantly shallower slope than that found previously using different stellar samples. We also find significant differences in the fitted slopes when fitting the Qs-qv and Us-uv data separately. We explore two explanations using mock data: miscalibration of polarization angle and variations in Rp/p along the line of sight due to multiple clouds. We show that the former can produce differences in the correlations of Qs-qv and Us-uv, but large miscalibration angles would be needed to reproduce the magnitude of the observed differences. Our simulations favor the interpretation that Rp/p differs between the two dominant clouds that overlap on the sky in this region. The difference in Rp/p suggests that the two clouds may have distinct dust polarimetric properties. With knowledge from the tomographic decomposition of the stellar polarization, we find that one cloud appears to dominate the correlation of Us-uv, while both clouds contribute to the correlation of the Qs-qv data.

astro-ph.GA↗

arXiv2025

What can cosmic-ray knees reveal about source populations?

Cosmic ray (CR) knees (spectral steepenings) encode information on CR accelerator populations. We seek population features that imprint onto knee observables in a manner that is robust enough to be discernible even in the presence of significant systematics in CR data. In particular, we explore how diversity among population members could imprint on the knee phenomenology, under the assumption that a knee is due to a fixed-rigidity cutoff in the source spectra. We use a simple theoretical model for a population of CR accelerators. Each population member accelerates CR to a power-law spectrum, up to a cutoff rigidity. We allow for variance among members, in cutoff rigidity and power-law slope. We find that: (a) the slope step of the spectrum is $\sim 0.5$, decreasing weakly with increasing spread in either property; (b) composition always breaks first; (c) the difference between the break energies in composition and flux increases with increasing diversity; (d) composition and flux break together only if population diversity is minimal. These trends are robust under our assumptions; deviations from them would indicate more complex physics than encoded in our simple model. Comparing these trends with observed CR knees, we conclude that: (i) the primary knee at $\sim 4\times10^{15}$ eV is consistent with a constant-rigidity cutoff according to KASCADE-Grande data processed with post-LHC hadronic models, but not according to other datasets; (ii) the second knee at $\sim 5 \times 10^{17}$ eV requires more complexity than our model; (iii) the spectral feature identified by Auger at $\sim 10^{19}$ eV is consistent with a constant-rigidity source cutoff only if there is a substantial spread in both cutoff rigidity and slope. Interestingly, a significant spread in slope would also result in spectral curvature before the break, which would in turn be contributing to the ankle feature.

astro-ph.HE↗

arXiv2025

Searching for a signature of turnaround in galaxy clusters with convolutional neural networks

Galaxy clusters are important cosmological probes that have helped to establish the $\mathrmΛ$CDM paradigm as the standard model of cosmology. However, recent tensions between different types of high-accuracy data highlight the need for novel probes of the cosmological parameters. Such a probe is the turnaround density: the mass density on the scale where galaxies around a cluster join the Hubble flow. To measure it, one must locate the distance from the cluster center where turnaround occurs. Earlier work has shown that a turnaround radius can be readily identified in simulations by analyzing the 3D dark matter velocity field. However, measurements using realistic data face challenges due to projection effects. This study aims to assess the feasibility of measuring the turnaround radius using machine learning techniques applied to simulated idealized observations of galaxy clusters. We employed N-body simulations across various cosmologies to generate galaxy cluster projections. Utilizing convolutional neural networks, we assessed the predictability of the turnaround radius based on galaxy line-of-sight velocity, number density, and mass profiles. We find a strong correlation between the turnaround radius and the central mass of a galaxy cluster, rendering the mass distribution outside the virial radius of little relevance to the model's predictive power. The velocity dispersion among galaxies also contributes valuable information concerning the turnaround radius. Importantly, the accuracy of a line-of-sight velocity model remains robust even when the data within the $\mathrm{R_{200}}$ of the central overdensity are absent. Single-cluster turnaround radius inference from projected observables seems to be highly challenging. Future progress is likely to require statistical approaches, especially stacking, to exploit cosmological information encoded at turnaround scales.

astro-ph.CO↗

arXiv2025

TRIShUL: Technique for Reconstructing magnetic Interstellar Structure Using starLight polarization

We present a novel technique to decompose line-of-sight (LOS) stellar polarization as a function of distance, aimed at reconstructing three dimensional (3D) plane-of-sky (POS) magnetic structures in the interstellar medium (ISM). The method assumes that the observed polarization arises from discrete, thin dust layers located at varying distances along the LOS. Using a simple frequentist framework, it identifies structural changes in the distance-sorted cumulative Mahalanobis distance of Stokes parameters (q and u) to detect the locations of dust layers and estimate their associated physical properties (parallax and Stokes parameters) necessary to construct 3D maps. We benchmark the method using mock datasets representative of high-Galactic-latitude regions, incorporating realistic Gaia parallax uncertainties and polarization expected from the upcoming Pasiphae survey. Tests show that the method reliably recovers dust cloud distances and polarization properties when the polarization exceeds 0.1%, and the effective background-star fraction is greater than 10% in samples of about 345 stars. The dependence on background fraction decreases as the intrinsic polarization amplitude of the dust field increases. We apply our method to existing polarization data from two illustrative sightlines, one at intermediate-high Galactic latitude and one near the Galactic plane, with known tomographic solutions, finding excellent agreement with the literature and demonstrating its accuracy across both regions. Comparing with the BISP-1 approach, both methods effectively recover dust cloud properties, but our approach is prior-free and computationally more efficient in determining the optimal number of clouds along the LOS. These advantages make it flexible and broadly applicable for multi-layer dust cloud reconstruction for the upcoming era of large-scale stellar polarization surveys.

astro-ph.GA↗

arXiv2025

Wide Area Linear Optical Polarimeter Control Software

The WALOPControl software is designed to facilitate comprehensive control and operation of the WALOP (Wide Area Linear Optical Polarimeter) polarimeters, ensuring safe and concurrent management of various instrument components and functionalities. This software encompasses several critical requirements, including control of the filter wheel, calibration half-wave plate, calibration polarizer, guider positioning, focusers, and 4 concurrent CCD cameras. It also manages the host telescope and dome operations while logging operational parameters, user commands, and environmental conditions for troubleshooting and stability. It provides a user-friendly graphical user interface, secure access control, a notification system for errors, and a modular configuration for troubleshooting are integral to the software's architecture. It is accessible over the internet with the backend developed using NodeJS and ExpressJS, featuring a RESTful API that interacts with a MongoDB database, facilitating real-time status updates and data logging. The frontend utilizes the React.JS framework, with Redux for state management and Material UI for the graphical components. The system also allows for automatic observations based on user-defined schedules. A Continuous Integration and Continuous Deployment (CI CD) pipeline ensures the software's reliability through automated testing and streamlined deployment. The WALOPControl software is a key component of the PASIPHAE (Polar-Areas Stellar Imaging in Polarimetry High Accuracy Experiment) project, which aims to study the dust and magnetic field of the Milky Way by observing the polarization of starlight.

astro-ph.IM↗

arXiv2025

Reconstructing the Magnetic Field in an Arbitrary Domain via Data-driven Bayesian Methods and Numerical Simulations

Inverse problems are prevalent in numerous scientific and engineering disciplines, where the objective is to determine unknown parameters within a physical system using indirect measurements or observations. The inherent challenge lies in deducing the most probable parameter values that align with the collected data. This study introduces an algorithm for reconstructing parameters by addressing an inverse problem formulated through differential equations underpinned by uncertain boundary conditions or variant parameters. We adopt a Bayesian approach for parameter inference, delineating the establishment of prior, likelihood, and posterior distributions, and the subsequent resolution of the maximum a posteriori problem via numerical optimization techniques. The proposed algorithm is applied to the task of magnetic field reconstruction within a conical domain, demonstrating precise recovery of the true parameter values.

physics.comp-ph↗

arXiv2024

Hubble Expansion Signature on Simulated Halo Density Profiles: A Path to Observing the Turnaround Radius

Density profiles are important tools in galaxy cluster research, offering insights into clusters dynamical states and their relationship with the broader Universe. While these profiles provide valuable information about the matter content of the Universe, their utility in understanding its dark energy component has remained limited due a lack of tools allowing us to study the transition from cluster portions that are relaxed and infalling, to those that are merging with the Hubble flow. In this work we investigate signatures of this transition in stacked density profiles of simulated cluster-sized halos at different redshifts. To highlight the Hubble flow around clusters we use their turnaround radius to normalize stacked simulated density profiles and calculate their logarithmic slope. Then, we complement our analysis by modeling the outer portions of these profiles assuming Gaussian early Universe statistics and spherical collapse without shell-crossing. We find the logarithmic slope of median cluster density profiles beyond the turnaround radius - where the Hubble flow dominates - to be Universal and well described by our model. Importantly, we find the slope of the profiles to diverge from the SCM prediction from within the turnaround radius where the actual profiles exhibit caustics which give rise to the splashback feature. We suggest utilizing this divergence from the spherical collapse model as a method to identify the turnaround radius in stacked cluster density profiles, offering a new perspective on understanding cluster dynamics and their cosmological implications.

astro-ph.CO↗

arXiv2024

The WALOP-North Instrument I: Optical Design, Filter Design, Calibration

The Wide Area Linear Optical Polarimeter North (WALOP-North) is an optical polarimeter designed for the needs of the PASIPHAE survey. It will be installed on the 1.3m telescope at the Skinakas Observatory in Crete, Greece. After commissioning, it will measure the polarization of millions of stars at high Galactic latitude, aiming to measure hundreds of stars per $deg^2$. The astronomical filter used in the instrument is a modified, polarimetrically-neutral broadband SDSS-r. This instrument will be pioneering one due to its large field-of-view (FoV) of $30\times 30$ $arcmin^2$ and high accuracy polarimetry measurements. The accuracy and sensitivity of the instrument in polarization fraction will be at the 0.1\% and 0.05\% level, respectively. Four separate 4k$\times$4k CCDs will be used as the instrument detectors, each imaging one of the $0°, 45°, 90°$ and $135°$ polarized FoV separately, therefore making the instrument a four-channel, one-shot polarimeter. Here, we present the overall optical design of the instrument, emphasizing on the aspects of the instrument that are different from WALOP-South. We also present a novel design of filters appropriate for polarimetry along with details on the management of the instrument size and its polarimetric calibration.

astro-ph.IM↗

arXiv2024

Mass scaling relations for dark halos from an analytic universal outer density profile

The average matter density within the turnaround scale, which demarcates where galaxies shift from clustering around a structure to joining the expansion of the Universe, is an important cosmological probe. However, a measurement of the mass enclosed by the turnaround radius is difficult. Analyses of the turnaround scale in simulated galaxy clusters place the turnaround radius at about three times the virial radius in a $ΛCDM$ universe and at a (present-day) density contrast with the background matter density of the Universe of $δ\sim 11$. Assessing the mass at such extended distances from a cluster's center is a challenge for current mass measurement techniques. Consequently, there is a need to develop and validate new mass-scaling relations, to connect observable masses at cluster interiors with masses at greater distances. Our research aims to establish an analytical framework for the most probable mass profile of galaxy clusters, leading to novel mass scaling relations, allowing us to estimate masses at larger scales. We derive such analytical mass profiles and compare them with those from cosmological simulations. We use excursion set theory, which provides a statistical framework for the density and local environment of dark matter halos, and complement it with the spherical collapse model to follow the non-linear growth of these halos. The profile we developed analytically shows a good agreement (better than 30\%, and dependent on halo mass) with the mass profiles of simulated galaxy clusters. Mass scaling relations are obtained from the analytical profile with offset better than 15\% from the simulated ones. This level of precision highlights the potential of our model for probing structure formation dynamics at the outskirts of galaxy clusters.

astro-ph.CO↗

arXiv2024

Improving polarimetric accuracy of RoboPol to $<$ 0.05 % using a half-wave plate calibrator system

RoboPol is a four-channel, one-shot linear optical polarimeter that has been successfully operating since 2013 on the 1.3 m telescope at Skinakas Observatory in Crete, Greece. Using its unique optical system, it measures the linear Stokes parameters $q$ and $u$ in a single exposure with high polarimetric accuracy of 0.1% - 0.15% and 1 degree in polarization angle in the R broadband filter. Its performance marginally degrades in other broadband filters. The source of the current instrumental performance limit has been identified as unaccounted and variable instrumental polarization, most likely originating from factors such as temperature and gravity-induced instrument flexure. To improve the performance of RoboPol in all broadband filters, including R, we have developed a rotating half-wave plate calibrator system. This calibrator system is placed at the beginning of the instrument and enables modulation of polarimetric measurements by beam swapping between all four channels of RoboPol. Using the new calibrator system, we observed multiple polarimetric standard stars over two annual observing seasons with RoboPol. This has enabled us to achieve a polarimetric accuracy of better than 0.05 % in both $q$ and $u$, and 0.5 degrees in polarization angle across all filters, enhancing the instrument's performance by a factor of two to three.

astro-ph.IM↗

arXiv2024

Systems design, assembly, integration and lab testing of WALOP-South Polarimeter

Wide-Area Linear Optical Polarimeter (WALOP)-South is the first wide-field and survey-capacity polarimeter in the optical wavelengths. On schedule for commissioning in 2024, it will be mounted on the 1 m SAAO telescope in Sutherland Observatory, South Africa to undertake the PASIPHAE sky survey. PASIPHAE program will create the first polarimetric sky map in the optical wavelengths, spanning more than 2000 square degrees of the southern Galactic region. The innovative design of WALOP-South will enable it to measure the linear polarization (Stokes parameters $q$ and $u$), in a single exposure, of all sources in a field of view (FoV) of $35\times35$ arcminutes-squared in the SDSS-r broadband and narrowband filters between 500-750 nm with 0.1 % polarization accuracy. The unique goals of the instrument place very stringent systems engineering goals, including on the performance of the optical, polarimetric, optomechanical, and electronic subsystems. All the subsystems have been designed carefully to meet the overall instrument performance goals. As of May 2024, all the instrument optical and mechanical subsystems have been assembled and are currently getting tested and integrated. The complete testing and characterization of the instrument in the lab is expected to be completed by August 2024. In this paper, we will present (a) the design and development of the entire instrument and its major subsystems, focusing on the opto-mechanical design which has not been reported before, and (b) assembly and integration of the instrument in the lab and early results from lab characterization of the instrument.

astro-ph.IM↗

arXiv2024

Getting the tiger by the tail: Probing the turnaround radius of structures with outer halo density profiles

In this paper we develop a new semianalytical approach to quantifying the density profile of outer dark matter halos, motivated by the remarkable universality those profiles, for a wide range of dynamical parameters of the inner halos. We show that our minimalist spherical collapse based model is robust under significant variations of its input parameters around the currently known values and we identify the turnaround radius as the most important scale of the problem. Based on that observation, we argue that the turnaround radius accurately represents the transition to the external density profile, and we provide a universal test based on geometrical characteristics of the outer profile as a proposal for measuring the turnaround radius of structures.

astro-ph.CO↗

arXiv2024

Non-parametric Bayesian reconstruction of Galactic magnetic fields using Information Field Theory: The inclusion of line-of-sight information in ultra-high energy cosmic ray backtracking

(abridged) Ultra-high energy cosmic rays (UHECRs) are extremely energetic charged particles with energies surpassing $10^{18}$ eV. Their sources remain elusive, obscured by deflections caused by the Galactic magnetic field (GMF). This challenge is further complicated by our limited understanding of the three-dimensional structure of the GMF, as current GMF observations consist primarily of quantities integrated along the line-of-sight (LOS). Nevertheless, data from upcoming stellar polarisation surveys along with Gaia's stellar parallax data are expected to yield local GMF measurements.. In this work, we employ methods of Bayesian statistical inference in order to sample the posterior distribution of the GMF within part of the Galaxy. By assuming a known rigidity and arrival direction of an UHECR, we backtrack its trajectory through various GMF configurations drawn from the posterior distribution. Our objective is to rigorously evaluate our algorithm's performance in scenarios that closely mirror the setting of expected future applications. In pursuit of this, we condition the posterior to synthetic integrated LOS measurements of the GMF, in addition to synthetic local POS-component measurements. In this proof of concept work, we assume the ground truth to be a magnetic field produced by a dynamo simulation of the Galactic ISM. Our results demonstrate that for all locations of the observed arrival direction on the POS, our algorithm is able to substantially update our knowledge on the original arrival direction of UHECRs with rigidity $E/Z = 5 \times 10^{19}$ eV, even in the case of complete absence of LOS information. If integrated data is included in the inference, then the regions of the celestial sphere where the maximum error occurs diminishes greatly. Even in those regions the maximum error is diminished by a factor of about $3$ in the specific setting studied.

astro-ph.HE↗

arXiv2023

Reconstructing Galactic magnetic fields from local measurements for backtracking ultra-high-energy cosmic rays

(abridged) Ultra-high energy cosmic rays (UHECRs) are highly energetic charged particles with energies exceeding $10^{18}$ eV. Identifying their sources and production mechanism can provide insight into many open questions in astrophysics and high energy physics. However, the Galactic magnetic field (GMF) deflects UHECRs, and the high uncertainties in our current understanding of the $3$-dimensional structure of the GMF does not permit us to accurately determine their true arrival direction on the plane of the sky (PoS). This difficulty arises from the fact that currently all GMF observations are integrated along the line-of-sight (LoS). Upcoming stellar optopolarimetric surveys as well as Gaia data on stellar parallaxes, are expected to provide local measurements of the GMF in the near future. In this paper, we evaluate the reconstruction of the GMF in a limited region of the Galaxy given sparse and local GMF measurements within that region, through Bayesian inference using principles of Information Field Theory. We backtrack UHECRs through GMF configurations drawn from the posterior to improve our knowledge of their true arrival directions. We show that, for a weakly turbulent GMF, it is possible to correct for its effect on the observed arrival direction of UHECRs to within $\sim 3^\circ$. For completely turbulent fields, we show that our procedure can still be used to significantly improve our knowledge on the true arrival direction of UHECRs.

astro-ph.HE↗

arXiv2023

Turnaround density evolution encodes cosmology in simulations

The mean matter density within the turnaround radius, which is the boundary that separates a nonexpanding structure from the Hubble flow, was recently proposed as a novel cosmological probe. According to the spherical collapse model, the evolution with cosmic time of this turnaround density, $\rm ρ_{ta}(z)$, can be used to determine both $\rm Ω_m$ and $Ω_Λ$, independently of any other currently used probe. The properties of $\rm ρ_{ta}$ predicted by the spherical collapse model were also shown to persist in the presence of full three-dimensional effects in $\rm Λ$CDM N-body cosmological simulations when considering galaxy clusters at the present time, $z=0$. However, a small offset was discovered between the spherical-collapse prediction of the value of $ρ_{ta}$ at $z=0$ and its value measured in simulations. In this letter, we explore whether this offset evolves with cosmic time; whether it differs in different cosmologies; whether its origin can be confidently identified; and whether it can be corrected. We found that the offset does evolve slightly with redshift, and that it correlates strongly with the deviation from spherical symmetry of the dark matter halo distribution inside and outside of the turnaround radius. We used an appropriate metric to quantify deviations in the environment of a structure from spherical symmetry. We found that using this metric, we can construct a sphericity-selected sample of halos for which the offset of $ρ_{ta}$ from the spherical collapse prediction is zero, independently of redshift and cosmology. We found that a sphericity-selected halo sample allows us to recover the simulated cosmology, and we conclude that the turnaround density evolution indeed encodes the cosmology in N-body simulations.

astro-ph.CO↗