arXiv subjects

N. Loudas

Publications and source records attributed to N. Loudas.

3 recordsLinked to original sources

arXiv2026

Through a glass, darkly: a combined framework for estimating fast radio burst host galaxy and population properties in an era of uncertain host identification

The identification of fast radio burst (FRB) host galaxies, and subsequently their redshifts ($z$), has allowed the FRB dispersion measure (DM) to be used to probe the cosmological distribution of ionised gas, and study the properties of the FRB population itself. However, current methods cannot account for FRBs with uncertain host galaxy associations, leading to underutilisation of data, and potential biases towards nearby, bright hosts. In this work, we develop a methodology which can. We do so by combining three ingredients - the zDM code, for modelling the Macquart relation; PATH, for statistical host galaxy identification; and a set of models able to describe the intrinsic FRB host galaxy distribution - into a single formalism. We prove the fidelity of our formalism by using a synthetic set of FRB observations, with simulated hosts sampled from galaxy catalogues, and show that it reproduces intrinsic host galaxy properties even for FRB localisation uncertainties of 30", where a traditional PATH analysis produces no confidant host associations. When applied to a sample of FRBs localised by the Australian Square Kilometre Array Pathfinder, we confirm previous results showing that FRBs prefer host galaxies fainter than that predicted by star-formation, consistent with an exponential surface density scaling as $0.41^{+0.13}_{-0.09}$ times the half-light radius. We encourage the application of this formalism to data-sets from other FRB-hunting instruments.

astro-ph.HE↗

arXiv2026

Updating the PATH framework with FRB host galaxy models

Over a hundred fast radio burst (FRB) host galaxies have now been identified, enabling both comparisons of host redshift with FRB dispersion measure to study the cosmological distribution of ionised gas, and analyses of host properties in order to identify FRB progenitors. The standard method for determining the most likely FRB host galaxy in an optical image is the Bayesian framework Probabilistic Association of Transients to their Hosts (PATH), which accounts for uncertainties in the radio localisation, and simplified prior distributions on the host being observable. In this work we extend PATH, incorporating physically-motivated priors that are based on expectations about FRB host galaxy magnitudes. We develop three different models for the apparent r-band magnitude distribution based on an FRB's expected host galaxy redshift, $P(m_r|z)$ and combine these with expectations for redshift based on an FRB's dispersion measure, $P(z|DM)$. We fit the parameters of these prior models using host galaxy candidates for 32 FRBs detected by the Australian SKA Pathfinder (ASKAP) in incoherent sum (ICS) mode by the Commensal Real-time ASKAP Fast Transients (CRAFT) survey. Employing PATH with the new priors on the host magnitudes, we find increased confidence in the most probable hosts of all ASKAP ICS FRB host galaxies. All three models predict similar distributions of FRB host magnitudes at low redshift $(z \sim 0.1)$, and we confirm previous results that the true FRB host galaxy distribution is fainter than expected for a star-formation-weighted distribution (p-value of 0.12%). However, a mass-weighted distribution provides an even worse fit (p-value of $10^{-9}$). Tests against more FRBs in the $z > 0.5$ range, where the models differ, and extensions of the models to account for e.g. host metallicity, may help to resolve these uncertainties in the FRB host distribution.

astro-ph.HE↗

arXiv2024

SMILE: Discriminating milli-lens systems in a VLBI pilot project

Dark Matter (DM) remains poorly probed on critical, sub-galactic scales, where predictions from different models diverge in terms of abundance and density profiles of halos. Gravitational lens systems on milli-arcsecond scales (milli-lenses) are expected for a population of dense DM halos (free-floating or sub-halos) and free-floating supermassive black holes in the mass range of $10^6$ to $10^9\,M_\odot$. In this paper, we aim to look for milli-lens systems via a systematic search in a large sample of radio-loud AGN observed with very-long-baseline interferometry (VLBI). We present the observational strategy to discriminate milli-lenses from contaminant objects mimicking a milli-lens morphology. In a pilot project, we have investigated VLBI images from 13,828 sources from the Astrogeo VLBI image database and reduced the number of candidates to 40 in a first step. We present here the images and analysis of new sensitive follow-up observations with the EVN at 5 and 22 GHz and streamline our analysis to reject milli-lens candidates. By using constraints such as the surface brightness ratio, conservation of spectral shape, stability of flux ratios over time, and changes in morphology, we can confidently discriminate between milli-lenses and contaminant objects that mimick them. Using the above constraints, we rule out 31 out of our initial 40 candidates of milli-lens systems, demonstrating the power of our approach. Also, we found many new candidate compact symmetric objects, which are thought to be primarily short-lived jetted radio sources. This serves as a pathfinder for the final sample used for the Search for MIlli-LEnses (SMILE) project, which will allow us to constrain DM models by comparing the results to theoretical predictions. This SMILE sample will consist of $\sim$5,000 sources based on the VLA CLASS survey, including many observations obtained for this project specifically.

astro-ph.GA↗