arXiv subjects

Santiago Hernández-Díaz

Publications and source records attributed to Santiago Hernández-Díaz.

5 recordsLinked to original sources

arXiv2026

A geometrical-optics analogy for gravitational lensing using axicon-type lenses

Gravitational lensing is often introduced through the bending of light by mass, but its geometrical nature can be difficult to visualize without invoking the full machinery of general relativity. We present a geometrical-optics analogy in which selected lensing-like image morphologies are generated by ray tracing through axicon-type lenses. In contrast with analogies based on a spatially varying refractive index, the present model uses optical elements with a constant refractive index; the redistribution of rays is produced by the geometry of the refracting surfaces. After deriving the ray-tracing equations for a general two-surface axicon, we apply the model to conical and exponential profiles. Axially symmetric configurations generate ring-like images, misaligned incidence produces partial arcs, and broken axial symmetry leads to four-image patterns reminiscent of an Einstein cross. The model is not intended as a physical substitute for a relativistic lens equation, but as a computational and pedagogical tool for exploring how surface geometry and symmetry control ray deflection, redistribution, and image multiplicity.

gr-qc↗

arXiv2026

Balmer decrements as a new diagnostic for period-bounce Cataclysmic Variable stars

Cataclysmic variable stars (CVs) evolve toward shorter orbital periods ($P_{\rm orb}$) until they reach a minimum $P_{\rm orb}$ near $P_{\rm orb}\sim80$ min. Beyond this point, the donor star becomes out of thermal equilibrium or increasingly degenerate, causing the system to "bounce back" to longer $P_{\rm orb}$ values. Such highly evolved systems are known as period-bouncers. Although 40-80\% of all CVs are expected to have reached this stage, period-bouncers come up for only 3-25\% of the observed CV population. This is likely a consequence of their intrinsic faintness associated with lower mass-transfer rates. Establishing new diagnostics to unveil this missing population is therefore crucial. Two samples of non-magnetic CVs with public SDSS optical spectra were constructed: one of short-period pre-bounce CVs and another of period-bounce CVs. For systems showing Balmer absorption from the white dwarf (WD), hydrogen-dominated atmosphere models were fitted and subtracted to correct for the WD component. H$α$, H$β$, and H$γ$ fluxes were measured. We then investigated statistical relations between the Balmer decrements, the H$β$ line luminosity, and $P_{\rm orb}$, and compared the measured Balmer decrements with theoretical predictions from accretion disc models. Short-period pre-bounce CVs show flat Balmer decrements, that is Balmer line ratios close to unity. In contrast, systems near and beyond the period minimum exhibit progressively steeper decrements (H$α$/H$β$>1 and H$γ$/H$β$<1). This behaviour is attributed to their lower mass accretion rates, as inferred from the H$β$ line luminosity. We fitted a linear logistic regression model to the diagram of H$γ$/H$β$ versus H$α$/H$β$. We establish that this diagnostic diagram effectively separates period-bouncers from pre-bounce CVs.

astro-ph.SR↗

arXiv2026

A Catalogue of Orbital Periods of Cataclysmic Variables and Candidates from TESS Observations

We present a systematic analysis of 2544 cataclysmic variable systems and related candidates observed by the Transiting Exoplanet Survey Satellite (TESS), with the aim of compiling a comprehensive catalogue of orbital periods. Using 2-minute photometric time-series data, we applied an automated algorithm to generate Lomb-Scargle periodograms and identify the most significant coherent periodic signals, which were subsequently verified through visual inspection. This process yielded a confident sample of 910 sources exhibiting at least one periodic signal, hereafter referred to as the Cataclysmic Variable Confident Catalogue (CCC). For each object, we report the most likely orbital period together with additional periodic features such as spin modulations and/or superhump signals when present. To assess consistency with previously published measurements, we cross-matched the CCC with the Ritter \& Kolb catalogue, identifying 300 overlapping systems, of which 215 showed full agreement with the R\&K orbital periods, while 39 displayed discrepancies for which the R\&K values were revised based on our TESS measurements and supporting evidence from the literature. Overall, the CCC provides a means to validate known orbital periods, propose corrections where necessary, and offer new determinations for systems with previously unknown periods, thereby supporting a more complete and reliable characterisation of the cataclysmic variable population.

astro-ph.SR↗

arXiv2025

A systematic search for orbital periods of polars with TESS. Methods, detection limits, and results

Context. Determining the orbital periods of cataclysmic variable stars (CVs) is essential for confirming candidates and for the understanding of their evolutionary state. The Transiting Exoplanet Survey Satellite (TESS) provides month-long photometric data across nearly the entire sky that can be used to search for periodic variability in such systems. Aims. This study aims to identify and confirm the orbital periods for members of a recent compilation of magnetic CVs (known as polars) using TESS light curves. In addition, we set out to investigate their reliability, and hence the relevance of TESS for variability studies of CVs. Results. Ninety-five of the 217 polars in our sample have pipeline-produced TESS two-minute cadence light curves available. The results from our period search are overall in good agreement with the previously reported values. Out of the 95 analysed systems, 85 exhibit periods consistent with the literature values. Among the remaining ten objects, four are asynchronous polars, where TESS light curves resolve the orbital period, the white dwarf's spin period, and additional beat frequencies. For four systems, the periods detected from the TESS data differ from those previously reported. For two systems, a period detection was not possible. Our analysis of the flattened TESS light curves reveals a positive correlation between noise levels and TESS magnitude. Our noise level estimates resemble the rmsCDPP, a measure of white noise provided with the TESS pipeline products. However, our values for the noise level are systematically higher than the rmsCDPP indicating red noise and high-frequency signals hidden in the flattened light curves. Additionally, we present a statistical methodology to assess the reliability of period detections in TESS light curves. We find that for TESS magnitudes $\gtrsim$ 17, period detections become increasingly unreliable.

astro-ph.SR↗

arXiv2025

eROSITA-selection of new period-bounce Cataclysmic Variables: First follow-up confirmation using TESS and SDSS

Between 40$\%$ and 80$\%$ of cataclysmic variables (CVs) are expected to have evolved past the period-minimum and contain a degenerate donor. However, observational surveys for CVs have only been able to detect a few of these highly evolved "period-bouncers", most likely due to the intrinsic faintness associated with their predicted low mass accretion rates. We have produced an initial selection of 137 high-likelihood period-bounce candidates from WD catalog based on our multiwavelength period-bouncer scorecard and selection cuts including X-ray data from the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Spektrum-Roentgen-Gamma spacecraft (SRG). We have laid out three main requirements (classification as a CV, determination of an orbital period and detection of a very late-type donor) that should result in the confirmation of several of these candidates. Our path for confirming these candidates has already produced its first successful result with the confirmation of GALEX J125751.4-283015 as a new period-bouncer. Several other candidates have already fulfilled at least one of our three requirements making their future confirmation likely. Our search for period-bouncers using the X-ray eROSITA emission of objects in optical WD catalogs has led to the confirmation of six new period-bouncers identified from the Gaia DR3 WD catalog (five previously known CVs and one WD candidate), a 18$\%$ increase that brings the present population to 39 systems. Both the selection method for period-bounce candidates and the confirmation path that we outlined will aid in future searches for new period-bounce candidates, contributing to the goal of resolving the discrepancy between the predicted high number of period-bouncers and the low number of these systems successfully observed to date.

astro-ph.SR↗