arXiv subjects

M. J. Martínez González

Publications and source records attributed to M. J. Martínez González.

At least 19 recordsLinked to original sources

arXiv2026

The Stellar Observations Network Group (SONG) -- A Legacy Archive of Stellar Time-Domain Spectroscopy

The Stellar Observations Network Group (SONG) network has operated for more than a decade, providing long-baseline, high-cadence spectroscopic observations of bright stars and the Sun. The observations, from 2014 through 2025, constitute a substantial archive of high-resolution spectra and precise radial velocities for a broad range of time-domain stellar astrophysics. We present an overview of the status, instrumentation, and scientific capabilities of the SONG network, and describe the scope and accessibility of the SONG Data Archive (SODA). We further illustrate the breadth of science enabled by SONG observations, including asteroseismology, stellar variability studies, binary-star characterisation, and exoplanet research. We summarise the operational status and observing strategies of the SONG facilities, describe the available data products and archive infrastructure, and outline procedures for accessing archival observations and proposing new observations within the SONG community framework. The SODA archive currently contains more than 580,000 spectra of 3091 stars obtained with SONG using either iodine-cell or Thorium-Argon wavelength calibration. The archive spans over a decade and includes extensive time-series data for bright targets across a wide range of stellar types and variability classes. Access to the archive is available to members of the SONG community, which remains open to new participants who agree to follow the community policies. The SONG archive has developed into a major long-baseline resource for stellar spectroscopy and radial-velocity time-series analysis. Continued expansion of the archive, together with coordinated observations obtained contemporaneously with TESS and future PLATO observations, is expected to enable new studies of stellar oscillations, variability, and exoplanet host stars through combined radial-velocity and photometric analyses.

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arXiv2025

Multi-wavelength spectropolarimetric observations of AR13724 performed by GRIS

Multi-line spectropolarimetric observations allow for the simultaneous inference of the magnetic field at different layers of the solar atmosphere and provide insight into how these layers are magnetically coupled. The new upgrade of the Gregor Infrared Spectrograph (GRIS) instrument offers such a possibility, allowing for the simultaneous observation of the Ca II line at 8542 A, the Si I line at 10827 A, and the He I triplet at 10830 A in addition to some additional weaker spectral lines that can probe deeper in the photosphere. Because these spectral lines are sensitive to the plasma properties at different regions of the solar atmosphere, their combined analysis can help understand the stratification of its thermal and magnetic properties from the photosphere to the chromosphere. This work showcases recent observations of the upgraded GRIS at the active region AR13724, which shows the instrument's potential for unravelling the most minute details of solar phenomena. In particular, we analyse the spatial distribution of the polarisation signals as well as the distribution of Stokes profiles for different regimes of the magnetic field strength. We also conduct a preliminary data analysis using relatively simple and approximate methods.

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arXiv2024

Non-Local Thermodynamic Equilibrium inversions of the Si I 10827 A spectral line

Inferring the coupling of different atmospheric layers requires observing spectral lines sensitive to the atmospheric parameters, particularly the magnetic field vector, at various heights. The best way to tackle this goal is to perform multi-line observations simultaneously. For instance, the new version of the Gregor Infrared Spectrograph instrument offers the possibility to observe the spectral lines at 8542 and 10830 A simultaneously for the first time. The first spectral window contains the Ca II 8542 A spectral line, while the Si I 10827 A transition and He I 10830 A triplet infrared lines can be found in the second spectral window. As the sensitivity to the atmospheric parameters and the height of formation of those transitions is different, combining them can help understand the properties of the solar photosphere and chromosphere and how they are magnetically coupled. Traditionally, the analysis of the Si I 10827 A transition assumes local thermodynamic equilibrium (LTE), which is not the best approximation to model this transition. Hence, in this work, we examine the potential of performing non-LTE (NLTE) inversions of the full Stokes vector of the Si I 10827 A spectral line. The results indicate that we properly infer the atmospheric parameters through an extended range of atmospheric layers in comparison with the LTE case (only valid for the spectral line wings, i.e., the low photosphere), with no impact on the robustness of the solution and just a minor increase in computational time. Thus, the NLTE assumption will help to accurately constrain the photospheric physical parameters when performing combined inversions with, e.g., the Ca II 8542 A spectral line.

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arXiv2023

Estimating the longitudinal magnetic field in the chromosphere of quiet-Sun magnetic concentrations

Details of the magnetic field in the quiet Sun chromosphere are key to our understanding of essential aspects of the solar atmosphere. We aim to determine the longitudinal magnetic field component (B_lon) of quiet Sun regions depending on their size. We estimated B_lon by applying the weak-field approximation (WFA) to high-spatial-resolution Ca II 854.2 nm data taken with the Swedish 1m Solar Telescope. Specifically, we analyzed the estimates inferred for different spectral ranges using the data at the original cadence and temporally integrated signals. The longitudinal magnetic field in each considered plasma structure correlates with its size. Using a spectral range restricted to the line core leads to chromospheric longitudinal fields varying from 50 G at the edges to 150-500 G at the center of the structure. These values increase as the spectral range widens due to the photospheric contribution. However, the difference between this contribution and the chromospheric one is not uniform for all structures. Small and medium-sized concentrations show a steeper height gradient in B_lon compared to their chromospheric values, so estimates for wider ranges are less trustworthy. Signal addition does not alleviate this situation as the height gradients in B_lon are consistent with time. Finally, despite the amplified noise levels that deconvolving processes may cause, data restored with the destretching technique show similar results, though are affected by smearing. We obtained B_lon estimates similar to those previously found, except for large concentrations and wide spectral ranges. In addition, we report a correlation between the height variation of B_lon compared to the chromospheric estimates and the concentration size. This correlation affects the difference between the photospheric and chromospheric magnetic flux values and the reliability of the estimates for wider spectral ranges.

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arXiv2021

Mapping the hidden magnetic field of the quiet Sun

The Sun is the only star where we can resolve the intricate magnetism that all convective stars harbor. Yet, more than 99% of its visible surface along the solar cycle (the so-called quiet Sun) is filled with a tangled, unresolved magnetism. These "hidden" fields are thought to store enough magnetic energy to play a role in the heating of the Sun's outer atmosphere, but its field strength is still not constrained. Previous investigations based on the Hanle effect in atomic lines claim a strong magnetization of about 100 G, while the same effect in molecules show a factor of 10 weaker fields. The discrepancy disappears if the magnetic field strength of the hidden is not homogeneous in the solar surface. In this letter, we prove using magnetohydrodynamical simulations that it is possible to infer the average field strength of the hidden quiet Sun magnetic fields using multi-line inversions of intensity profiles in the Zeeman regime. Using this technique with 15 spectral lines in the 1.5 $μ$m spectral range, we reveal that the spatial distribution of the hidden field is strongly correlated with convection motions, and that the average magnetization is about 46 G. Reconciling our findings with the Hanle ones is not obvious and will require future work on both sides, since it implies an increase of the field strength with height, something that is physically questionable.

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arXiv2021

Empirical determination of atomic line parameters of the 1.5 $μ$m spectral region

Both the quality and amount of astrophysical data are steadily increasing over time owing to the improvement of telescopes and their instruments. This requires corresponding evolution of the techniques used for obtaining and analyzing the resulting data. The infrared spectral range at 1.56 $μ$m usually observed by the GRegor Infrared Spectrograph (GRIS) at the GREGOR solar telescope has a width of around 30 $Å$ and includes at least 15 spectral lines. Normally, only a handful of spectral lines (five at most) are used in studies using GRIS because of the lack of atomic parameters for the others. Including more spectral lines may alleviate some of the known ambiguities between solar atmospheric parameters. We used inversion techniques to infer both solar atmospheric models and the atomic parameters of spectral lines which, under the local thermodynamic equilibrium (LTE) approximation, reproduce spectropolarimetric observations. We present accurate atomic parameters for 15 spectral lines within the spectral range from 15644 $Å$ to 15674 $Å$. This spectral range is commonly used in solar studies because it enables the study of the low photosphere. Moreover, the infrared spectral lines are better tracers of the magnetic fields than the optical ones.

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arXiv2020

Photospheric magnetic topology of a north polar region

Aims. We aim to characterise the magnetism of a large fraction of the north polar region close to a maximum of activity, when the polar regions are reversing their dominant polarity. Methods. We make use of full spectropolarimetric data from the CRisp Imaging Spectro-Polarimeter installed at the Swedish Solar Telescope. The data consist of a photospheric spectral line, which is used to infer the various physical parameters of different quiet Sun regions by means of the solution of the radiative transfer equation. We focus our analysis on the properties found for the north polar region and their comparison to the same analysis applied to data taken at disc centre and low-latitude quiet Sun regions for reference. We also analyse the spatial distribution of magnetic structures throughout the north polar region. Results. We find that the physical properties of the polar region (line-of-sight velocity, magnetic flux, magnetic inclination and magnetic azimuth) are compatible with those found for the quiet Sun at disc centre and are similar to the ones found at low latitudes close to the limb. Specifically, the polar region magnetism presents no specific features. The structures for which the transformation from a line-of-sight to a local reference frame was possible harbour large magnetic fluxes ($>10^{17}$ Mx) and are in polarity imbalance with a dominant positive polarity, the largest ones ($>10^{19}$ Mx) being located below $73^{\circ}$ latitude.

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arXiv2019

Spectropolarimetric analysis of an active region filament. II. Evidence of the limitations of a single component model

Our aim is to demonstrate the limitations of using a single component model to study the magnetic field of an active region filament. For that we have analyzed the polarimetric signals of the He I 10830 A multiplet acquired with the infrared spectrograph GRIS of the GREGOR telescope (Tenerife, Spain). After a first analysis of the general properties of the filament using Hazel under the assumption of a single component model atmosphere, in this second part we focus our attention on the observed Stokes profiles and the signatures which cannot be explained with this model. We have found an optically thick filament where the blue and the red components have the same sign in the linear polarization as an indication of radiative transfer effects. Moreover, the circular polarization signals inside the filament show the presence of strong magnetic field gradients. We have also shown that even a filament with such high absorption still shows signatures of the circular polarization generated by the magnetic field below the filament. The reason is that the absorption of the spectral line decays very quickly towards the wings, just where the circular polarization has a larger amplitude. In order to separate both contributions, we explore the possibility of a two component model but the inference becomes impossible to overcome as a high number of solutions is compatible with the observations.

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arXiv2019

Spectropolarimetric analysis of an active region filament. I. Magnetic and dynamical properties from single component inversions

The determination of the magnetic filed vector in solar filaments is possible by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral lines like those of the He I multiplet at 10830 A. We study the vector magnetic field of an active region filament (NOAA 12087). Spectropolarimetric data of this active region was acquired with the GRIS instrument at the GREGOR telescope and studied simultaneously in the chromosphere with the He I 10830 A multiplet and in the photosphere with the Si I 10827 A line. As it is usual from previous studies, only a single component model is used to infer the magnetic properties of the filament. The results are put into a solar context with the help of the Solar Dynamic Observatory images. Some results clearly point out that a more complex inversion had to be done. Firstly, the Stokes $V$ map of He I does not show any clear signature of the presence of the filament. Secondly, the local azimuth map follows the same pattern than Stokes $V$ as if the polarity of Stokes $V$ were conditioning the inference to very different magnetic field even with similar linear polarization signals. This indication suggests that the Stokes $V$ could be dominated by the below magnetic field coming from the active region, and not, from the filament itself. Those and more evidences will be analyzed in depth and a more complex inversion will be attempted in the second part of this series.

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arXiv2019

Diagnostic potential of the Ca II 8542A line for solar filaments

In this study we explore the diagnostic potential of the chromospheric Ca II line at 8542A for studying the magnetic and dynamic properties of solar filaments. We have acquired high spatial resolution spectropolarimetric observations in the Ca II 8542A line using the CRISP instrument at the Swedish 1-m Solar Telescope. We use the NICOLE inversion code to infer physical properties from observations of a solar filament. We discuss the validity of the results due to the assumption of hydrostatic equilibrium. We have used observations from other telescopes such as CHROTEL and SDO, in order to study large scale dynamics and the long term evolution of the filament. We show that the Ca II 8542A line encodes information of the temperature, line-of-sight velocity and magnetic field vector from the region where the filament is located. The current noise level only allow us to estimate an upper limit of 260G for the total magnetic field of the filament. Our study also reveals that if we only consider information from the aforementioned spectral line, the geometric height, the temperature and the density can be degenerated parameters outside the hydrostatic equilibrium approach.

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arXiv2018

High-resolution spectroscopy of Boyajian's star during optical dimming events

Boyajian's star is an apparently normal main sequence F-type star with a very unusual light curve. The dipping activity of the star, discovered during the Kepler mission, presents deep, asymmetric, and aperiodic events. Here we present high resolution spectroscopic follow-up during some dimming events recorded post-Kepler observations, from ground-based telescopes. We analise data from the HERMES, HARPS-N and FIES spectrographs to characterise the stellar atmosphere and to put some constraints on the hypotheses that have appeared in the literature concerning the occulting elements. The star's magnetism, if existing, is not extreme. The spots on the surface, if present, would occupy 0.02% of the area, at most. The chromosphere, irrespective of the epoch of observation, is hotter than the values expected from radiative equilibrium, meaning that the star has some degree of activity. We find no clear evidence of the interstellar medium nor exocoments being responsible for the dimmings of the light curve. However, we detect at 1-2 sigma level, a decrease of the radial velocity of the star during the first dip recorded after the \emph{\emph{Kepler}} observations. We claim the presence of an optically thick object with likely inclined and high impact parameter orbits that produces the observed Rossiter-McLaughlin effect.

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arXiv2018

Where are the solar magnetic poles?

Regardless of the physical origin of stellar magnetic fields - fossil or dynamo induced - an inclination angle between the magnetic and rotation axes is very often observed. Absence of observational evidence in this direction in the solar case has led to generally assume that its global magnetic field and rotation axes are well aligned. We present the detection of a monthly periodic signal of the photospheric solar magnetic field at all latitudes, and especially near the poles, revealing that the main axis of the Sun's magnetic field is not aligned with the surface rotation axis. This result reinforces the view of our Sun as a common intermediate-mass star. Furthermore this detection challenges and imposes a strong observational constraint to modern solar dynamo theories.

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arXiv2018

Magnetic topology of the north solar pole

We study the polar magnetism near an activity maximum when these regions change their polarity, from which it is expected that its magnetism should be less affected by the global field. To fully characterise the magnetic field vector, we use deep full Stokes polarimetric observations of the 15648.5 Å and 15652.8 Å FeI lines. We observe the north pole as well as a quiet region at disc centre to compare their field distributions. In order to calibrate the projection effects, we observe an additional quiet region at the east limb. We find that the two limb datasets share similar magnetic field vector distributions. This means that close to a maximum, the poles look like typical limb, quiet-Sun regions. However, the magnetic field distributions at the limbs are different from the distribution inferred at disc centre. At the limbs, we infer a new population of magnetic fields with relatively strong intensities ($\sim$600-$\sim$800 G), inclined by 30 deg with respect to the line of sight, and with an azimuth aligned with the solar disc radial direction. We propose that this new population at the limbs is due to the observation of unresolved magnetic loops as seen close to the limb. These loops have typical granular sizes as measured in the disc centre. At the limbs, where the spatial resolution decreases, we observe them spatially unresolved, which explains the new population of magnetic fields that is inferred. This is the first (indirect) evidence of small-scale magnetic loops outside the disc centre and would imply that these small-scale structures are ubiquitous on the entire solar surface. This result has profound implications for the energetics not only of the photosphere, but also of the outer layers since these loops have been reported to reach the chromosphere and the low corona.

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arXiv2016

Probing deep photospheric layers of the quiet Sun with high magnetic sensitivity

Context. Investigations of the magnetism of the quiet Sun are hindered by extremely weak polarization signals in Fraunhofer spectral lines. Photon noise, straylight, and the systematically different sensitivity of the Zeeman effect to longitudinal and transversal magnetic fields result in controversial results in terms of the strength and angular distribution of the magnetic field vector. Aims. The information content of Stokes measurements close to the diffraction limit of the 1.5 m GREGOR telescope is analyzed. We took the effects of spatial straylight and photon noise into account. Methods. Highly sensitive full Stokes measurements of a quiet-Sun region at disk center in the deep photospheric Fe I lines in the 1.56 μm region were obtained with the infrared spectropolarimeter GRIS at the GREGOR telescope. Noise statistics and Stokes V asymmetries were analyzed and compared to a similar data set of the Hinode spectropolarimeter (SOT/SP). Simple diagnostics based directly on the shape and strength of the profiles were applied to the GRIS data. We made use of the magnetic line ratio technique, which was tested against MHD simulations. Results. About 80% of the GRIS spectra of a very quiet solar region show polarimetric signals above a 3σ level. Area and amplitude asymmetries agree well with small-scale surface dynamo MHD simulations. The magnetic line ratio analysis reveals ubiquitous magnetic regions in the ten to hundred Gauss range with some concentrations of kilo-Gauss fields. Conclusions. The GRIS spectropolarimetric data at a spatial resolution of 0.40" are so far unique in the combination of high spatial resolution scans and high magnetic field sensitivity. Nevertheless, the unavoidable effect of spatial straylight and the resulting dilution of the weak Stokes profiles means that inversion techniques still bear a high risk of misinterpretating the data.

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arXiv2016

Synthetic polarimetric spectra from stellar prominences

Stellar prominences detected in rapidly rotating stars serve as probes of the magnetism in the corona of cool stars. We have synthesized the temporal evolution of the Stokes profiles generated in the He I 10830 and 5876 A triplets during the rotation of a prominence around a star. The synthesis was performed with the HAZEL code using a cloud model in which the prominence is characterized by a slab located at a fixed latitude and height. It accounts for the scattering polarization and Zeeman and Hanle effects. Several cases with different prominence magnetic field strengths and orientations have been analyzed. The results show an emission feature that drifts across the profile while the prominence is out of the stellar disk. When the prominence eclipses the star, the intensity profile shows an absorption. The scattering induced by the prominence generates linear polarization signals in Stokes Q and U profiles, which are modified by the Hanle effect when a magnetic field is present. Due to the Zeeman effect, Stokes V profiles show a signal with very low amplitude when the magnetic field along the line-of-sight is different from zero. The estimated linear polarization signals could potentially be detected with the future spectropolarimeter MIRADAS, to be attached to GTC telescope.

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arXiv2016

Active region filaments might harbor weak magnetic fields

Recent spectropolarimetric observations of active region filaments have revealed polarization profiles with signatures typical of the strong field Zeeman regime. The conspicuous absence in those observations of scattering polarization and Hanle effect signatures was then pointed out by some authors. This was interpreted either as a signature of mixed "turbulent" field components or as a result of optical thickness. In this article, we present a natural scenario to explain these Zeeman-only spectro-polarimetric observations of active region filaments. We propose a two-component model, one on top of the other. Both components have horizontal fields, the azimuth difference between them being close to 90 degrees. The component that lies lower in the atmosphere is permeated by a strong field of the order of 600 G, while the upper component has much weaker fields, of the order of 10 G. The ensuing scattering polarization signatures of the individual components have opposite signs, so that its combination along the line of sight reduces --and even can cancel out-- the Hanle signatures, giving rise to an apparent only-Zeeman profile. This model is also applicable to other chromospheric structures seen in absorption above active regions.

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arXiv2014

Hierarchical analysis of the quiet Sun magnetism

Standard statistical analysis of the magnetic properties of the quiet Sun rely on simple histograms of quantities inferred from maximum-likelihood estimations. Because of the inherent degeneracies, either intrinsic or induced by the noise, this approach is not optimal and can lead to highly biased results. We carry out a meta-analysis of the magnetism of the quiet Sun from Hinode observations using a hierarchical probabilistic method. This model allows us to infer the statistical properties of the magnetic field vector over the observed field-of-view consistently taking into account the uncertainties in each pixel due to noise and degeneracies. Our results point out that the magnetic fields are very weak, below 275 G with 95% credibility, with a slight preference for horizontal fields, although the distribution is not far from a quasi-isotropic distribution.

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arXiv2014

A search for magnetic fields on central stars in planetary nebulae

One of the possible mechanisms responsible for the panoply of shapes in planetary nebulae is the presence of magnetic fields that drive the ejection of ionized material during the proto-planetary nebula phase. Therefore, detecting magnetic fields in such objects is of key importance for understanding their dynamics. Still, magnetic fields have not been detected using polarimetry in the central stars of planetary nebulae. Circularly polarized light spectra have been obtained with the Focal Reducer and Low Dispersion Spectrograph at the Very Large Telescope of the European Southern Observatory and the Intermediate dispersion Spectrograph and Imaging System at the William Herschel Telescope. Nineteen planetary nebulae spanning very different morphology and evolutionary stages have been selected. Most of central stars have been observed at different rotation phases to point out evidence of magnetic variability. In this paper, we present the result of two observational campaigns aimed to detect and measure the magnetic field in the central stars of planetary nebulae on the basis of low resolution spectropolarimetry. In the limit of the adopted method, we can state that large scale fields of kG order are not hosted on the central star of planetary nebulae.

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