arXiv subjects

Stefan Cikota

Publications and source records attributed to Stefan Cikota.

17 recordsLinked to original sources

arXiv2026

Towards High-Throughput Visible Photonic Lanterns for the EMARCOT Project

Photonic EMARCOT is an innovative project involving Spanish, German and Australian research institutes that aims to integrate multiple Optical Tube Assemblies (OTAs) using photonic lanterns. The "Pathfinder" prototype, featuring seven OTAs with a 1.1-meter effective aperture, will feed a spectrograph at the Calar Alto Observatory, with first light expected in 2026. We report the fabrication and experimental evaluation of a custom 7x1 multi-mode photonic lantern (MMPL) developed for this framework, featuring seven 25 um core multi-mode inputs merging into a single 50 um core multi-mode output optimized for the visible wavelength range (400-700 nm). Optical characterization centered at 600 nm reveals exceptional channel-to-channel uniformity, with statistical variations close to zero across both bare-fiber and connectorized MMPL configurations. However, the total baseline throughput of this initial device was limited to below 4%. From the refractive index studies, this low throughput is attributed to severe refractive index mismatch between the internal fiber cladding geometry and the structural capillary, which suppresses total internal reflection during the tapering transition. This work establishes an important diagnostic baseline that highlights the necessary fabrication tolerances needed to improve future high-throughput manufacturing processes for precision radial-velocity astronomy.

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arXiv2026

Demonstration of a multimode-to-multimode photonic lantern for astronomy

Photonic lanterns have been widely used in astronomy as low-loss multiplexing devices, typically coupling light from a multimode input into several single-mode outputs. In this work, we present the first multimode-to-multimode photonic lantern specifically designed to combine light from several multimode fibers into a single multimode waveguide. We fabricated and characterized the devices at multiple wavelengths to evaluate the performance of the adiabatic multimode transition. The measured efficiencies exceed $90\ \%$, demonstrating low-loss multimode propagation and efficient modal transfer through the lantern structure. This architecture enables efficient multimode beam combination and represents a significant step toward scalable modular telescope concepts without requiring diffraction-limited injection.

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arXiv2026

The ORCA-TWIN qCMOS Experiment I. Science case and commissioning at Calar Alto Observatory

We describe a pilot study to explore a new generation of fast and low noise CMOS image sensors for time domain astronomy, using two remote telescopes with a baseline of 1635 km. The experiment involves direct imaging with novel qCMOS image sensor technology that combines fast readout with sub-electron readout noise. Moreover, synchronized observations from two remote telescope sites will be used to explore new approaches for measuring Solar System bodies, precision stellar photometry, and speckle imaging. A fast-track installation of an ORCA-Quest2 camera at the Calar Alto Observatory 1.23m telescope has demonstrated the potential of the qCMOS technology for time domain astronomy. Numerical simulations suggest that owing to sub-electron readout noise, qCMOS sensors outperform classical CCDs for high-cadence imaging on 1m-class telescopes. The small penalty for post-readout binning, that is almost insignificant in comparison to higher readout noise detectors, opens interesting applications for scene-dependent data processing in direct imaging, and potentially even for spectroscopy.

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arXiv2026

Observing Orbital Decay in the Ultracompact Hot Subdwarf Binary System ZTFJ213056.71+442046.5

Ultracompact Galactic binary systems (UCBs) emit low-frequency gravitational waves (GWs). The emission of GWs is causing these systems to lose angular momentum, which is detectable by observing an orbital period decay. ZTFJ213056.71+442046.5 (ZTFJ2130) is an UCB with a period of 39.3401(1) minutes consisting of a Roche lobe-filling hot subdwarf and a white dwarf companion. We attempt to measure the orbital decay rate $\dot{P}$ caused by GW emission of ZTFJ2130 and predict the expected GW signal for LISA. High-speed photometry was conducted using the FLI Kepler KL4040FI CMOS camera, mounted to the 1.2-meter Oskar Lühning telescope at the Hamburg Observatory as well as the Hamamatsu ORCA-Quest 2 qCMOS camera at the 1.23-meter telescope at CAHA in Spain. ZTFJ2130 was observed on six nights between August 2024 and September 2025. The obtained lightcurves combined with previous high-cadence observations were used to conduct an O-C timing analysis. Additionally, we employed the LISA data analysis tool ldasoft to model the expected GW data. We measure a period change of $(-1.97\pm0.05)\times10^{-12}\,\mathrm{ss^{-1}}$. Assuming only GW emission, this result was used to calculate a chirp mass of $(0.408\pm0.006)\,\mathrm{M_{\odot}}$. From ldasoft we predict that LISA will be able to measure the chirp mass with an uncertainty of 5%. We measure $\dot{P}$ with an uncertainty of only 2% and show that modern (q)CMOS detectors are well suited to provide precise timing measurements, enabling the measurement of the orbital decay of UCBs with high precision with modest size telescopes. The derived orbital decay is fully consistent with predictions from spectral and lightcurve modeling. We show that future observations with LISA can potentially provide a deviation from only gravitational wave effects, e.g. due to accretion, if the effect is sufficiently large.

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arXiv2026

Towards a Comprehensive Understanding of Planetary Systems through Population-Level, Large-Scale Surveys

Over the past three decades, exoplanet research has delivered an extensive census of planets spanning a wide range of masses, sizes, and orbital configurations. Despite this progress, the physical interpretation of these populations remains severely limited, as precise constraints on planetary masses, interior structures, and atmospheres are available only for a small, highly selected subset of targets. As a result, most known exoplanets remain physically ambiguous, preventing the construction of robust population-level trends and limiting our understanding of planet formation, evolution, and habitability. In the coming decades, missions such as PLATO, Earth 2.0, and the Nancy Grace Roman Space Telescope will dramatically expand the number of exoplanets detected. However, without a corresponding capability to characterise planetary masses and atmospheres at scale, these discoveries will remain largely detection-driven. Current and planned facilities, including JWST and ELT-class instruments, excel at detailed studies of individual systems but are intrinsically unsuited for large, homogeneous surveys. This white paper identifies population-level physical characterisation as a fundamental science challenge for the 2040s and motivates the need for a new observational paradigm. We outline how photonics-enabled, modular telescope architectures can deliver the survey speed, stability, and scalability required to jointly probe planetary interiors and atmospheres across statistically meaningful samples, thereby enabling a comprehensive and physically grounded understanding of planetary systems.

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arXiv2025

Revisiting the atmosphere of HAT-P-70b with CARMENES high-resolution transmission spectroscopy

Owing to hot and inflated envelopes that facilitate atmospheric studies, ultra-hot Jupiters (UHJs) have attracted much attention. Significant progress has been achieved, from enlarging the sample size to broadening the studies to encompass diverse stellar types and ages. Here, we present a transmission spectroscopy study of HAT-P-70b, an UHJ orbiting a young A-type star, through high-resolution observations with CARMENES at the 3.5m Calar Alto telescope. By using the line-by-line technique, we confirm the previous detections of Ha, Na I, and Ca II, report a new tentative detection of K I, and impose an upper limit on the He triplet absorption. Through cross-correlation analysis, we identify the Ca II and Fe I absorptions, both blue-shifted by approximately 5 km/s, indicating a day-to-night side atmospheric wind. Additionally, we find a new tentative detection of K I. We do not see any significant atmospheric molecular signal in the near-infrared data. Putting HAT-P-70b in the context of UHJs from the literature, it turns out that (1) Ha absorption is more common on gas giants orbiting stars younger than 1 Gyr, with a relative detection probability of $P_{\rm Age<1\,Gyr}({\rm Ha})/P_{\rm Age\geq1\,Gyr}({\rm Ha})\sim 3$; (2) any UHJ is likely to exhibit Fe I absorption if it has Ca II.

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arXiv2023

DESI-253.2534+26.8843: A New Einstein Cross Spectroscopically Confirmed with VLT/MUSE and Modeled with GIGA-Lens

Gravitational lensing provides unique insights into astrophysics and cosmology, including the determination of galaxy mass profiles and constraining cosmological parameters. We present spectroscopic confirmation and lens modeling of the strong lensing system DESI-253.2534+26.8843, discovered in the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys data. This system consists of a massive elliptical galaxy surrounded by four blue images forming an Einstein Cross pattern. We obtained spectroscopic observations of this system using the Multi Unit Spectroscopic Explorer (MUSE) on ESO's Very Large Telescope (VLT) and confirmed its lensing nature. The main lens, which is the elliptical galaxy, has a redshift of $z_{L1} = 0.636\pm 0.001$, while the spectra of the background source images are typical of a starburst galaxy and have a redshift of $z_s = 2.597 \pm 0.001$. Additionally, we identified a faint galaxy foreground of one of the lensed images, with a redshift of $z_{L2} = 0.386$. We employed the GIGA-Lens modeling code to characterize this system and determined the Einstein radius of the main lens to be $θ_{E} =2.520{''}_{-0.031}^{+0.032}$, which corresponds to a velocity dispersion of $σ$ = 379 $\pm$ 2 km s$^{-1}$. Our study contributes to a growing catalog of this rare kind of strong lensing systems and demonstrates the effectiveness of spectroscopic integral field unit observations and advanced modeling techniques in understanding the properties of these systems.

astro-ph.GA↗

arXiv2023

An independent determination of the distance to supernova SN 1987A by means of the light echo AT 2019xis

Accurate distance determination to astrophysical objects is essential for the understanding of their intrinsic brightness and size. The distance to SN 1987A has been previously measured by the expanding photosphere method, and by using the angular size of the circumstellar rings with absolute sizes derived from light curves of narrow UV emission lines, with reported distances ranging from 46.77 kpc to 55 kpc. In this study, we independently determined the distance to SN 1987A using photometry and imaging polarimetry observations of AT 2019xis, a light echo of SN 1987A, by adopting a radiative transfer model of the light echo developed in Ding et al. (2021). We obtained distances to SN 1987A in the range from 49.09 $\pm$ 2.16 kpc to 59.39 $\pm$ 3.27 kpc, depending on the interstellar polarization and extinction corrections, which are consistent with the literature values. This study demonstrates the potential of using light echoes as a tool for distance determination to astrophysical objects in the Milky Way, up to kiloparsec level scales.

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arXiv2022

MARCOT Pathfinder at Calar Alto Progress Report

MARCOT Pathfinder is a precursor for MARCOT (Multi Array of Combined Telescopes) at Calar Alto Observatory (CAHA) in Spain. MARCOT is intended to provide CARMENES, currently fiber-fed from the CAHA 3.5m Telescope, with a 5-15m light collecting area from a battery of several tens of small telescopes that are incoherently fed into the final joint single fiber feed of the spectrograph. The modular concept, based on commercially available telescopes, results in cost estimates that are a fraction of the ones for extremely large telescopes (ELT). As a novel approach, MARCOT will employ Multi-Mode Photonic Lanterns (MM-PL) that are being developed as a variant of classical photonic lanterns, to combine the light from the individual telescopes to a single fiber feed to the instrument. This progress report presents the overall concept of MARCOT, the pathfinder telescope and enclosure that is being commissioned at CAHA, the concept of MM-PL, and the next step of installing the Potsdam Multiplex Raman Spectrograph (MRS). MARCOT Pathfinder will be used to validate the conceptual design and predicted performance of MM-PL on sky with a 7 unit telescope prototype.

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arXiv2019

Linear spectropolarimetry of 35 Type Ia Supernovae with VLT/FORS: An analysis of the Si II line polarization

Spectropolarimetry enables us to measure the geometry and chemical structure of the ejecta in supernova explosions, which is fundamental for the understanding of their explosion mechanism(s) and progenitor systems. We collected archival data of 35 Type Ia Supernovae (SNe Ia), observed with FORS on the Very Large Telescope at 127 epochs in total. We examined the polarization of the Si II $λ$6355 $Å$ line (p$_{\rm Si II}$) as a function of time which is seen to peak at a range of various polarization degrees and epochs relative to maximum brightness. We reproduced the $Δ$m$_{15}$-p$_{\rm Si II}$ relationship identified in a previous study, and show that subluminous and transitional objects display polarization values below the $Δ$m$_{15}$-p$_{\rm Si II}$ relationship for normal SNe Ia. We found a statistically significant linear relationship between the polarization of the Si II $λ$6355 $Å$ line before maximum brightness and the Si II line velocity and suggest that this, along with the $Δ$m$_{15}$-p$_{\rm Si II}$ relationship, may be explained in the context of a delayed-detonation model. In contrast, we compared our observations to numerical predictions in the $Δ$m$_{15}$-v$_{\rm Si II}$ plane and found a dichotomy in the polarization properties between Chandrasekhar and sub-Chandrasekhar mass explosions, which supports the possibility of two distinct explosion mechanisms. A subsample of SNe display evolution of loops in the $q$-$u$ plane that suggests a more complex Si structure with depth. This insight, which could not be gleaned from total flux spectra, presents a new constraint on explosion models. Finally, we compared our statistical sample of the Si II polarization to quantitative predictions of the polarization levels for the double-detonation, delayed-detonation, and violent-merger models.

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arXiv2019

Pointing System for the Large Size Telescopes Prototype of the Cherenkov Telescope Array

The pointing system of the prototype of the Large Size Telescope (LST-1) for the Cherenkov Telescope Array observatory, should ensure mapping of the gamma-ray image of a point-like source in the Cherenkov camera to the sky coordinates with a precision better than 14 arcseconds. Detailed studies of the telescope deformations are performed in order to disentangle different deformations and quantify their contributions to the miss-pointing, to learn how to correct for them, and finally how to design the system for offline and online pointing corrections. The LST-1 pointing precision system consist of several devices mounted at the center of the dish: Starguider Camera (SG), Camera Displacement Monitor (CDM), two inclinometers, four distance meters, and an Optical Axis Reference Laser (OARL), working together with the LEDs mounted in a circle around the Cherenkov camera. The online pointing corrections are based on a bending model as currently done by existing IACTs. The offline corrections will be performed combining measurements done by the SG and CDM cameras. SG will provide the position of the Cherenkov camera center with respect to the sky coordinates with a precision of 5 arcseconds, while CDM will provide the deviation of the telescope optical axis defined by the OARL spots with respect to the Cherenkov camera center with a precision better than 5 arcseconds. Laboratory measurements on dedicated test benches showed that the required pointing precision can be achieved for SG, CDM and inclinometer.

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arXiv2018

Testing the magnetar scenario for superluminous supernovae with circular polarimetry

Superluminous supernovae (SLSNe) are at least $\sim$5 times more luminous than common supernovae (SNe). Especially hydrogen-poor SLSN-I are difficult to explain with conventional powering mechanisms. One possible scenario that might explain such luminosities is that SLSNe-I are powered by an internal engine, such as a magnetar or an accreting black hole. Strong magnetic fields or collimated jets can circularly polarize light. In this work, we measured circular polarization of two SLSNe-I with the FOcal Reducer and low dispersion Spectrograph (FORS2) mounted at the ESO's Very Large Telescope (VLT). PS17bek, a fast evolving SLSN-I, was observed around peak, while OGLE16dmu, a slowly evolving SLSN-I, was observed 100 days after maximum. Neither SLSN shows evidence of circularly polarized light, however, these non-detections do not rule out the magnetar scenario as the powering engine for SLSNe-I. We calculate the strength of the magnetic field and the expected circular polarization as a function of distance from the magnetar, which decreases very fast. Additionally, we observed no significant linear polarization for PS17bek at four epochs, suggesting that the photosphere near peak is close to spherical symmetry.

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arXiv2018

Spectropolarimetry of Galactic stars with anomalous extinction sightlines

Highly reddened type Ia Supernovae (SNe Ia) with low total-to-selective visual extinction ratio values, $R_V$, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths ($λ_{max} \lesssim 0.4 μm$). It is not clear why sightlines to SNe Ia display such different continuum polarization profiles from interstellar sightlines in the Milky Way with similar $R_V$ values. We investigate polarization profiles of a sample of Galactic stars with low $R_V$ values, along anomalous extinction sightlines, with the aim to find similarities to the polarization profiles that we observe in SN Ia sightlines. We undertook spectropolarimetry of 14 stars, and used archival data for three additional stars, and run dust extinction and polarization simulations to infer a simple dust model that can reproduce the observed extinction and polarization curves. Our sample of Galactic stars with low $R_V$ values and anomalous extinction sightlines displays normal polarization profiles with an average $λ_{max} \sim 0.53 {μm}$, and is consistent within 3$σ$ to a larger coherent sample of Galactic stars from literature. Despite the low $R_V$ values of dust towards the stars in our sample, the polarization curves do not show any similarity to the continuum polarization curves observed towards SNe Ia with low $R_V$ values. There is a correlation between the best-fit Serkowski parameters $K$ and $λ_{max}$, but we did not find any significant correlation between $R_V$ and $λ_{max}$. Our simulations show that the $K-λ_{max}$ relationship is an intrinsic property of polarization. Furthermore, we have shown that in order to reproduce polarization curves with normal $λ_{max}$ and low $R_V$ values, a population of large (a $\geq 0.1 μm$) interstellar silicate grains must be contained in the dust's composition.

astro-ph.GA↗

arXiv2018

Activity of (2060) Chiron possibly caused by impacts?

The centaur 95P/(2060) Chiron is showing comet-like activity since its discovery, but the mass-loss mechanisms triggering its activity remained unexplained. Although the collision rates in the centaur region are expected to be very low, and impacts are thought not to be responsible for the mass-loss, since the recent indications that Chiron might possess a ring similar to Chariklo's, and assuming that there is debris orbiting around, the impact triggered mass-loss mechanism should not be excluded as a possible cause of its activity. From time series observations collected on Calar Alto Observatory in Spain between 2014 and 2016, we found that the photometric scatter in Chiron's data is larger than a control star's scatter, indicating a possible microactivity, possibly caused by debris falling back to Chiron's surface and lifting small clouds of material. We also present rotational light curves, and measurements of Chiron's absolute magnitudes, that are consistent with the models supporting the presumption that Chiron possesses rings. By co-adding the images acquired in 2015, we have detected a $ \sim $5 arcsec long tail, showing a surface brightness of 25.3 mag(V)/arcsec$^{2}$.

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arXiv2017

Common continuum polarization properties: a possible link between proto-planetary nebulae and Type Ia Supernovae progenitors

The lines-of-sight to highly reddened SNe Ia show peculiar continuum polarization curves, growing toward blue wavelengths and peaking at $λ_{max} \lesssim 0.4 μm$, like no other sight line to any normal Galactic star. We examined continuum polarization measurements of a sample of asymptotic giant branch (AGB) and post-AGB stars from the literature, finding that some PPNe have polarization curves similar to those observed along SNe Ia sight lines. Those polarization curves are produced by scattering on circumstellar dust. We discuss the similarity and the possibility that at least some SNe Ia might explode during the post-AGB phase of their binary companion. Furthermore, we speculate that the peculiar SNe Ia polarization curves might provide observational support to the core-degenerate progenitor model.

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arXiv2017

Spatially resolved analysis of Superluminous Supernovae PTF~11hrq and PTF~12dam host galaxies

Superluminous supernovae (SLSNe) are the most luminous supernovae in the universe. They are found in extreme star-forming galaxies and are probably connected with the death of massive stars. One hallmark of very massive progenitors would be a tendency to explode in very dense, UV-bright, and blue regions. In this paper we investigate the resolved host galaxy properties of two nearby hydrogen-poor SLSNe, PTF~11hrq and PTF~12dam. For both galaxies \textit{Hubble Space Telescope} multi-filter images were obtained. Additionally, we performe integral field spectroscopy of the host galaxy of PTF~11hrq using the Very Large Telescope Multi Unit Spectroscopic Explorer (VLT/MUSE), and investigate the line strength, metallicity and kinematics. Neither PTF~11hrq nor PTF~12dam occurred in the bluest part of their host galaxies, although both galaxies have overall blue UV-to-optical colors. The MUSE data reveal a bright starbursting region in the host of PTF~11hrq, although far from the SN location. The SN exploded close to a region with disturbed kinematics, bluer color, stronger [OIII], and lower metallicity. The host galaxy is likely interacting with a companion. PTF~12dam occurred in one of the brightest pixels, in a starbursting galaxy with a complex morphology and a tidal tail, where interaction is also very likely. We speculate that SLSN explosions may originate from stars generated during star-formation episodes triggered by interaction. High resolution imaging and integral field spectroscopy are fundamental for a better understanding of SLSNe explosion sites and how star formation varies across their host galaxies.

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arXiv2016

Linear spectropolarimetry of polarimetric standard stars with VLT/FORS2

We reduced ESO's archival linear spectropolarimetry data (4000-9000Å) of 6 highly polarized and 8 unpolarized standard stars observed between 2010 and 2016, for a total of 70 epochs, with the FOcal Reducer and low dispersion Spectrograph (FORS2) mounted at the Very Large Telescope. We provide very accurate standard stars polarization measurements as a function of wavelength, and test the performance of the spectropolarimetric mode (PMOS) of FORS2. We used the unpolarized stars to test the time stability of the PMOS mode, and found a small ($\leq$0.1%), but statistically significant, on-axis instrumental polarization wavelength dependency, possibly caused by the tilted surfaces of the dispersive element. The polarization degree and angle are found to be stable at the level of $\leq$0.1% and $\leq$0.2 degrees, respectively. We derived the polarization wavelength dependence of the polarized standard stars and found that, in general, the results are consistent with those reported in the literature, e.g. Fossati et al. (2007) who performed a similar analysis using FORS1 data. The re-calibrated data provide a very accurate set of standards that can be very reliably used for technical and scientific purposes. The analysis of the Serkowski parameters revealed a systematic deviation from the width parameter $K$ reported by Whittet et al. (1992). This is most likely explained by incorrect effective wavelengths adopted in that study for the R and I bands.

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