Changes In Line Profiles Research Articles

Improving Earth-like planet detection in radial velocity using deep learning

Context. Many novel methods have been proposed to mitigate stellar activity for exoplanet detection as the presence of stellar activity in radial velocity (RV) measurements is the current major limitation. Unlike traditional methods that model stellar activity in the RV domain, more methods are moving in the direction of disentangling stellar activity at the spectral level. As deep neural networks have already been proven to be one of the most effective tools in data mining, in this work, we explore their potential in the context of Earth-like planet detection in RV measurements. Aims. The goal of this paper is to present a novel convolutional neural network-based algorithm that efficiently models stellar activity signals at the spectral level, enhancing the detection of Earth-like planets. Methods. Based on the idea that the presence of planets can only produce a Doppler shift at the spectral level while the presence of stellar activity can introduce a variation in the profile of spectral lines (asymmetry and depth change), we trained a convolutional neural network to build the correlation between the change in the spectral line profile and the corresponding RV, full width at half maximum (FWHM) and bisector span (BIS) values derived from the classical cross-correlation function. Results. This algorithm has been tested on three intensively observed stars: Alpha Centauri B (HD 128621), Tau ceti (HD 10700), and the Sun. By injecting simulated planetary signals at the spectral level, we demonstrate that our machine learning algorithm can achieve, for HD 128621 and HD 10700, a detection threshold of 0.5 m s−1 in semi-amplitude for planets with periods ranging from 10 to 300 days. This threshold would correspond to the detection of a ~4 M⊕ in the habitable zone of those stars. On the HARPS-N solar dataset, thanks to significantly more data, our algorithm is even more efficient at mitigating stellar activity signals and can reach a threshold of 0.2 m s−1, which would correspond to a 2.2 M⊕ planet on the orbit of the Earth. Conclusions. To the best of our knowledge, it is the first time that such low detection thresholds are reported for the Sun, but also for other stars, and therefore this highlights the efficiency of our convolutional neural network-based algorithm at mitigating stellar activity in RV measurements.

Long-Term Optical Monitoring of Broad-Line AGNs (LoTerm AGN): Case Study of NGC 3516

Properties of the broad line region (BLR) in active galactic nuclei (AGNs) are commonly used to estimate the mass of the supermassive black hole (SMBH) that powers an AGN. However, the understanding of the physics behind the BLR remains incomplete. The AGNs exhibit strong optical variability, observed in the change of the profiles and fluxes of broad emission lines. Utilizing this variability provides an opportunity to constrain the physics of the BLR, and understand the interplay of the BLR with SMBH and surrounding regions. Here, we present the long-term monitoring campaign of a sample of the known broad-line AGNs (identified as LoTerm AGN). The aim of this study is to show the importance of sustained and dedicated campaigns that continually collect spectroscopic data of the known AGNs over extended timescales, providing unique insight into the origin and structure of the BLR. LoTerm AGN is a collaborative network of seven moderate-size telescopes equipped for spectroscopy. We focus on the recent spectral data of the known changing-look AGN, NGC 3516. Specifically, we examine the broad hydrogen Balmer Hα line observed in the period 2020–2023, demonstrating that this AGN remains active with the BLR signatures observed in the spectra. No significant change in the broad line profile of Hα line is observed during this recent period.

Evidence for microlensing by primordial black holes in quasar broad emission lines

ABSTRACT With the detection of black hole mergers by the LIGO gravitational wave telescope, there has been increasing interest in the possibility that dark matter may be in the form of solar mass primordial black holes. One of the predictions implicit in this idea is that compact clouds in the broad emission line regions of high-redshift quasars will be microlensed, leading to changes in line structure and the appearance of new emission features. In this paper, the effect of microlensing on the broad emission line region is reviewed by reference to gravitationally lensed quasar systems where microlensing of the emission lines can be unambiguously identified. It is then shown that although changes in Seyfert galaxy line profiles occur on time-scales of a few years, they are too nearby for a significant chance that they could be microlensed, and are plausibly attributed to intrinsic changes in line structure. In contrast, in a sample of 53 high-redshift quasars, 9 quasars show large changes in line profile at a rate consistent with microlensing. These changes occur on a time-scale an order of magnitude too short for changes associated with the dynamics of the emission line region. The main conclusion of the paper is that the observed changes in quasar emission line profiles are consistent with microlensing by a population of solar mass compact bodies making up the dark matter, although other explanations like intrinsic variability are possible. Such bodies are most plausibly identified as primordial black holes.

Long-term optical spectral monitoring of a changing-look active galactic nucleus NGC 3516

Context.We analyze the broad Hβline profile variability of a “changing look” active galactic nucleus (CL-AGN) NGC 3516 over an extensive period of 25 years (from 1996 to 2021). The observed change in the broad line profile may indicate a change in the geometry of the broad line region (BLR). The main objective is to follow and understand the change in the BLR over a long period as well as its connection to the CL mechanism.Aims.Using spectral line profiles, we aim to explore changes in the kinematics and dimensions of the BLR in NGC 3516. We consider two possible scenarios: the changes in the broad-line emission are either caused by a decrease of ionisation continuum emission or by the BLR obscuration by outer dusty regions. With this investigation, we aim to clarify the CL mechanism of this AGN.Methods.We analyzed the spectral band around the Hβline as well as the broad Hβline parameters and how they change over time. We modelled the broad-line profiles, assuming that there is an emission from the accretion disc superposed with emission from a surrounding region that is outside the disc.Results.We find that in the type 1 activity phase occurring when the strong broad emission lines are observed, the BLR is very complex. There is a clear disc-like BLR that contributes to the broad line wings and an additional intermediate line region (ILR) that contributes to the line core. In the high-activity phase, the ILR emission is close to the center of the line, although in some cases, it is slightly shifted to the red. In the low-activity phase (i.e. type 2 phase), the ILR component has a significant shift to the blue, indicating an outflow.Conclusions.We propose that the changing-look mechanism in NGC 3516 is rather connected with the intrinsic effects than with an outer obscuring region. It may still be possible that the dust plays an important role in the low-activity phase when it is coming from within the BLR, leading to a dusty BLR. In this way, it would cause a decrease in the ionisation and recombination rates.

Evolution of spectral lines under time-dependent illumination of an absorbing and scattering atmosphere

This is a continuation of the author’s research on the temporal variations in the spectral line profiles formed in absorbing and scattering atmospheres illuminated by non-stationary energy sources. The line radiation transfer in an atmosphere of finite optical thickness is considered under assumption of complete frequency redistribution. We treat two types of illumination of the medium: the sources of the form of Dirac δ(t) and Heaviside H(t) unit jump functions. The goal of the work is to identify the dependence of the dynamics of changes in the spectral line profiles on frequency redistribution and the values of various optical parameters such as the optical thickness of the medium and the value of scattering coefficient. It is shown that in some cases this dependence allows to use observational material on dynamics of the burst phenomena to estimate some of these characteristics.

Accretion variability from minute to decade timescales in the classical T Tauri star CR Cha

Context. Classical T Tauri stars are pre-main-sequence stars that are surrounded by a circumstellar disk from which they accrete material. The mass accretion process is essential in the formation of Sun-like stars. Although often described with simple and static models, the accretion process is inherently time variable. Aims. We examine the accretion process of the low-mass young stellar object CR Cha on a wide range of timescales from minutes to a decade by analyzing both photometric and spectroscopic observations from 2006, 2018, and 2019. Methods. We carried out a period analysis of the light curves of CR Cha from the TESS mission and the ASAS-SN and the ASAS-3 databases. We studied the color variations of the system using I, J, H, K-band photometry obtained contemporaneously with the TESS observing window. We analyzed the amplitude, timescale, and the morphology of the accretion tracers found in a series of high-resolution spectra obtained in 2006 with the AAT/UCLES, in 2018 with the HARPS, and in 2019 with the ESPRESSO and the FEROS spectrographs. Results. All photometric data reveal periodic variations compatible with a 2.327-day rotational period. In addition, the ASAS-SN and ASAS-3 data indicate a long-term brightening by 0.2 mag between 2001 and 2008, and a slightly lower brightening than 0.1 mag in the 2015–2018 period. The near-infrared photometry indicates a short-term brightening trend during the observations in 2019. The corresponding color variations can be explained either by a changing accretion rate or changes in the inner disk structure. The Hα line profile variability suggests that the amplitude variations of the central peak, likely due to accretion, are most significant on daily or hourly timescales. On yearly timescales, the line morphology also changes significantly. Conclusions. The photometric variability shows that the period of about 2.3 days is stable in the system over decades. Our results show that the amplitude of the variations in the Hα emission increases on timescales from hours to days or weeks, after which it remains similar even at decadal timescales. On the other hand, we found significant morphological variations on yearly or decadal timescales, indicating that the different physical mechanisms responsible for the line profile changes, such as accretion or wind, are present to varying degrees at different times.

Velocity monitoring of γ Cas stars reveals their binarity status

ABSTRACT The binary status of γ Cas stars has been discussed while theoretically examining the origin of their peculiar X-ray emission. However, except in two cases, no systematic radial velocity monitoring of these stars had been undertaken yet to clarify their status. We now fill this gap using TIGRE, CARMENES, and UVES high-resolution spectroscopy. Velocities were determined for 16 stars, revealing shifts and/or changes in line profiles. The orbit of six new binaries could be determined: the long periods (80–120 d) and small velocity amplitudes (5–7 km s−1) suggest low mass companions (0.6–1 M⊙). The properties of the known γ Cas binaries appear similar to those of other Be systems, with no clear-cut separation between them. One of the new systems is a candidate for a rare case of quadruple system involving a Be star. Five additional γ Cas stars display velocity variations compatible with the presence of companions, but no orbital solution could yet be formally established for them hence they only receive the status of ‘binary candidate’.

Zodiacal exoplanets in time – XIII. Planet orbits and atmospheres in the V1298 Tau system, a keystone in studies of early planetary evolution

ABSTRACT Studies of planetary systems of stars in star-forming regions and young clusters open a window on the formative stages of planetary evolution. We obtained high-cadence high-resolution infrared spectroscopy of the solar-mass Taurus association-member V1298 Tau during a transit of its 10R⊕-size ‘b’ planet. We measured the systemic radial velocity (RV) and find that the kinematics of V1298 Tau suggest an affiliation with a ≳6 Myr-old subgroup. A comparison of V1298 Tau and the nearby, co-moving star 2M0405 with stellar evolution models suggests an age of ∼10–25 Myr. We measured the projected spin-orbit angle of ‘b’ as $\lambda =15_{-16}^{+15}$ and $\lambda = 2_{-4}^{+12}$ degrees using the apparent RV shift and change in line profile, respectively, induced by the transient occultation of the rotating star by the planet. These values indicate a prograde orbit like that of the interior ‘c’ planet of V1298 Tau and point to a co-planar multiplanet system that formed within a disc. We also measured variation in the strength of the 1083 nm triplet of neutral orthohelium as a probe of any extended/escaping atmosphere around ‘b’. We detect a steady decrease in absorption over the transit that appears to arise from the star or its planetary system. While this variation could be ascribed to ‘b’ or possibly to the immediately preceding transit of ‘d’, we cannot rule out that this is due to rapid variation in the stellar disc-integrated flux in the triplet. The amplitude of variation (∼0.04 nm) is consistent with moderate estimates of atmospheric escape driven by XUV radiation from the central star.

Anomalous broad-line region responses to continuum variability in active galactic nuclei – I. Hβ variability

ABSTRACT In the standard active galactic nucleus (AGN) reverberation-mapping model, variations in broad-line region (BLR) fluxes are predicted from optical continuum variability (taken as a proxy for the ionizing continuum) convolved with a response function that depends on the geometry. However, it has long been known that BLR variability can deviate from these predictions. We analyse both extensive long-term Hβ and continuum monitoring of NGC 5548 and a large sample of high-quality Hβ light curves of other AGNs to investigate the frequency and characteristics of the anomalous responses of the BLR. We find that anomalies are very common and probably occur in every object. Onsets can be on a time-scale only slightly longer than the light-crossing time and durations are of the order of the characteristic time-scale of variability of the optical continuum to several times longer. Anomalies are larger when NGC 5548 is in a low state, but otherwise there is no correlation with continuum variability. There is abundant evidence for the optical continuum of AGNs varying independently of the higher-energy continua and this is sufficient to explain the anomalous responses of the total BLR flux. There are good reasons for believing that the frequent lack of correlation between spectral regions is due to anisotropic and non-axisymmetric emission. Rapid changes in line profiles and velocity-dependent lags are consistent with this. Motion of compact absorbing clouds across the line of sight is another possible cause of anomalies. The prevalence of anomalies should be considered when planning reverberation-mapping campaigns.

The onset of jets in classical novae

ABSTRACT We present two further classical novae, V906 Car and V5668 Sgr, which show jets and accretion disc spectral signatures in their H α complexes throughout the first 1000 d following their eruptions. From extensive densely time-sampled spectroscopy, we measure the appearance of the first high-velocity absorption component in V906 Car, and the duration of the commencement of the main H α emission. We constrain the time taken for V5668 Sgr to transition to the nebular phase using [N ii] 6584 Å. We find these timings to be consistent with the jet and accretion disc model for explaining optical spectral line profile changes in classical novae, and discuss the implications of this model for enrichment of the interstellar medium.

Solar activity and responses observed in Balmer lines

Context. Many stars show Sun-like magnetic activity cycles, which are frequently observed by tracking changes in the chromospherically sensitive CaII H&K doublet. However, relationships between the line profile changes related to the magnetic activity seen in strong spectral transitions in other portions of a stellar spectrum are yet to be understood. Aims. We follow variability patterns in various solar lines in order to relate them to the emergence, passage, and decay of active solar regions. Methods. The line activity indices (core-to-wing ratio) for the upper Balmer lines – Hβ, Hγ, and Hδ – are constructed from the near-daily solar measurements acquired by the Ozone Monitoring Instrument and the TROPOspheric Monitoring Instrument. Results. On solar rotation timescales, the upper Balmer line activity indices closely follow variations in the total solar irradiance, r ∼ −(0.6 − 0.7), and thus frequently deviate from the behavior of the line activity indices that track chromospheric activity levels (e.g., the CH 430 nm band used in this study), specifically during passages of big sunspot groups.

Superfast Line Profile Variability in the Spectra of $$\boldsymbol{\rho}$$ Leo: New Results

The results of ultrahigh-time-resolution ( $$\Delta T\sim 12$$ s) observations of the B1 Iab supergiant $$\rho$$ Leo with the 1.25-m telescope at the Crimean Astronomical Station of the SAI MSU are presented. Regular line profile variations with periods of 15–30 min have been revealed. Significant changes in the average line profiles on time intervals of 1–3 days have been detected. Evidence for the existence of short-period line profile variations on time scales of 15–25 min is provided. The nature of the short-period line profile variations is discussed. The possibility of studying the fast variations in the profiles of broad lines (mostly the Balmer hydrogen and He I lines) using low-resolution spectrographs with $$R=1000{-}2000$$ at 1–2-m telescopes is pointed out.

Chromospheric activity in bright contact binary stars

Context. Studying chromospheric activity of contact binaries is an effective way of revealing the magnetic activity processes of these systems. One efficient but somewhat neglected method for this purpose is to follow the changes of the Hα line profiles via optical spectroscopy. Aims. Our goal is to perform a comprehensive preliminary analysis based on the optical spectral signs of chromospheric activity on the largest sample of contact binaries to date. Methods. We collected optical echelle spectra on 12 bright contact binaries in 17 nights. We derived new radial velocity curves from our observations. For quantifying the apparent chromospheric activity levels of the systems, we subtracted self-constructed synthetic spectra from the observed ones and measured the equivalent widths of the residual Hα-profiles at each observed epoch. Our well-sampled data set allowed us to study the short-term variations of chromospheric activity levels as well as to search for correlations between them and some basic physical parameters of the systems. Results. Fitting the radial velocity curves, we re-determined the mass ratios and systemic velocities of all observed objects. We found that chromospheric activity levels of the studied systems show various changes during the orbital revolution: we see either flat, one-peaked, or two-peaked distributions of equivalent width vs. the orbital phase. The first case means that the activity level is probably constant, while the latter two cases suggest the presence of one or two active longitudes at the stellar surfaces. Our correlation diagrams show that mean chromospheric activity levels may be related to the orbital periods, B−V color indices, inverse Rossby numbers, and temperature differences of the components. At the same time, no clear trend is visible with respect to the mass ratios, inclinations, or fill-out factors of the systems. A- and W-type contact binaries in our sample show similar distributions on each of the studied correlation diagrams.

Doppler shifts and spectral line profile changes in the starlight scattered from an exoplanet

ABSTRACT Scattered starlight from an exoplanet, commonly called reflected light, can be used to characterize the planet including the determination of its albedo and inclination of orbit. The relatively low flux ratio between directly observed starlight and starlight scattered off hot Jupiters make these systems the prime candidates for detection of reflected light using high-resolution spectroscopy. The first detections have been claimed for 51 Peg b. In a first calculation, we derive the Doppler shift of reflected light measured by a remote observer for a planet modelled as a point-like particle in orbit around a star. We find that the Doppler shift of reflected light from planets have a different Doppler shift to that of light emitted directly from the planet with magnitude equivalent to the radial motion of the planet with respect to the star. This only occurs for non-circular orbits. Secondly, restricting our investigation to planets that are tidally locked and orbit in circular orbits we account for the finite size of the star and planet by integrating the contribution to a simulated spectral line across both their surfaces. Since exact analytical expressions cannot be easily derived as a function of all free parameters of the problem, we have developed a software tool called REflected STARlighT (restart) that produces the resulting line profiles. By applying it to study cases found in the literature, we explicitly show that hot Jupiters such as WASP-19b and 51 Peg b should show substantial broadening and asymmetric distortions compared to the nominal stellar line.

Variations on a theme: the puzzling behaviour of Schulte 12

One of the first massive stars detected in X-rays, Schulte 12 has remained a puzzle in several aspects. In particular, its extreme brightness both in the visible and X-ray ranges is intriguing. Thanks to Swift and XMM-Newton observations covering ∼5000 d, we report the discovery of a regular 108 d modulation in X-ray flux of unknown origin. The minimum in the high-energy flux appears due to a combination of increased absorption and decreased intrinsic emission. We examined in parallel the data from a dedicated spectroscopic and photometric monitoring in the visible and near-IR domains, complemented by archives. While a similar variation timescale is found in those data, they do not exhibit the strict regular clock found at high energies. Changes in line profiles cannot be related to binarity but rather correspond to non-radial pulsations. Considering the substantial revision of the distance of Schulte 12 from the second Gaia data release, the presence of such oscillations agrees well with the evolutionary status of Schulte 12, as it lies in an instability region of the HR diagram.

A New Look into Putative Duplicity and Pulsations of the Be Star β CMi*

Abstract Bright Be star β CMi has been identified as a nonradial pulsator on the basis of space photometry with the Microvariability and Oscillations of Stars (MOST) satellite and also as a single-line spectroscopic binary with a period of 170.ͩ4. The purpose of this study is to re-examine both these findings using numerous electronic spectra from the Dominion Astrophysical Observatory, Ondřejov Observatory, Universitätssterwarte Bochum, archival electronic spectra from several observatories, as well as the original MOST satellite photometry. We measured the radial velocity of the outer wings of the double Hα emission in all spectra at our disposal, and were not able to confirm significant radial-velocity changes. We also discuss the problems related to the detection of very small radial-velocity changes and conclude that while it is still possible that the star is a spectroscopic binary, there is currently no convincing proof of it from the radial-velocity measurements. Wavelet analysis of the MOST photometry shows that there is only one persistent (and perhaps slightly variable) periodicity of 0.ͩ617 of the light variations, with a double-wave light curve; all other short periods having only transient character. Our suggestion that this dominant period is the star’s rotational period agrees with the estimated stellar radius, projected rotational velocity, and with the orbital inclination derived by two teams of investigators. New spectral observations obtained in the whole-night series would be needed to find out whether some possibly real, very small radial-velocity changes cannot, in fact, be due to rapid line-profile changes.

Inner disk structure of the classical T Tauri star LkCa 15

Context. Magnetospheric accretion has been thoroughly studied in young stellar systems with full non-evolved accretion disks, but it is poorly documented for transition disk objects with large inner cavities. Aims. We aim at characterizing the star-disk interaction and the accretion process onto the central star of LkCa 15, a prototypical transition disk system with an inner dust cavity that is 50 au wide. Methods. We obtained quasi-simultaneous photometric and spectropolarimetric observations of the system over several rotational periods. We analyzed the system light curve and associated color variations, as well as changes in spectral continuum and line profile to derive the properties of the accretion flow from the edge of the inner disk to the central star. We also derived magnetic field measurements at the stellar surface. Results. We find that the system exhibits magnetic, photometric, and spectroscopic variability with a period of about 5.70 days. The light curve reveals a periodic dip, which suggests the presence of an inner disk warp that is located at the corotation radius at about 0.06 au from the star. Line profile variations and veiling variability are consistent with a magnetospheric accretion model where the funnel flows reach the star at high latitudes. This leads to the development of an accretion shock close to the magnetic poles. All diagnostics point to a highly inclined inner disk that interacts with the stellar magnetosphere. Conclusions. The spectroscopic and photometric variability on a timescale of days to weeks of LkCa 15 is remarkably similar to that of AA Tau, the prototype of periodic dippers. We therefore suggest that the origin of the variability is a rotating disk warp that is located at the inner edge of a highly inclined disk close to the star. This contrasts with the moderate inclination of the outer transition disk seen on the large scale and thus provides evidence for a significant misalignment between the inner and outer disks of this planet-forming transition disk system.

Mapping EK Draconis with PEPSI

Aims. We present the first temperature surface map of EK Dra from very-high-resolution spectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope. Methods. Changes in spectral line profiles are inverted to a stellar surface temperature map using our iMap code. The long-term photometric record is employed to compare our map with previously published maps. Results. Four cool spots were reconstructed, but no polar spot was seen. The temperature difference to the photosphere of the spots is between 990 and 280 K. Two spots are reconstructed with a typical solar morphology with an umbra and a penumbra. For the one isolated and relatively round spot (spot A), we determine an umbral temperature of 990 K and a penumbral temperature of 180 K below photospheric temperature. The umbra to photosphere intensity ratio of EK Dra is approximately only half of that of a comparison sunspot. A test inversion from degraded line profiles showed that the higher spectral resolution of PEPSI reconstructs the surface with a temperature difference that is on average 10% higher than before and with smaller surface areas by ~10–20%. PEPSI is therefore better suited to detecting and characterising temperature inhomogeneities. With ten more years of photometry, we also refine the spot cycle period of EK Dra to 8.9 ± 0.2 yr with a continuing long-term fading trend. Conclusions. The temperature morphology of spot A so far appears to show the best evidence for the existence of a solar-like penumbra for a starspot. We emphasise that it is more the non-capture of the true umbral contrast rather than the detection of the weak penumbra that is the limiting factor. The relatively small line broadening of EK Dra, together with the only moderately high spectral resolutions previously available, appear to be the main contributors to the lower-than-expected spot contrasts when comparing to the Sun.

Constraining sub-parsec binary supermassive black holes in quasars with multi-epoch spectroscopy – III. Candidates from continued radial velocity tests

Quasars whose broad emission lines show temporal, bulk radial velocity (RV) shifts have been proposed as candidate sub-parsec (sub-pc), binary supermassive black holes (BSBHs). We identified a sample of 16 BSBH candidates based on two-epoch spectroscopy among 52 quasars with significant RV shifts over a few rest-frame years. The candidates showed consistent velocity shifts independently measured from two broad lines (H$\beta$ and H$\alpha$ or Mg${\rm \,II}$) without significant changes in the broad-line profiles. Here in the third paper of the series, we present further third- and fourth-epoch spectroscopy for 12 of the 16 candidates for continued RV tests, spanning $\sim$5--15 yr in the quasars' rest frames. Cross-correlation analysis of the broad H$\beta$ calibrated against [O${\rm\,III}]\,\lambda 5007$ suggests that 5 of the 12 quasars remain valid as BSBH candidates. They show broad H$\beta$ RV curves that are consistent with binary orbital motion without significant changes in the broad line profiles. Their broad H$\alpha$ (or Mg${\rm \,II}$) lines display RV shifts that are either consistent with or smaller than those seen in broad H$\beta$. The RV shifts can be explained by a $\sim$0.05--0.1 pc BSBH with an orbital period of $\sim$40--130 yr, assuming a mass ratio of 0.5--2 and a circular orbit. However, the parameters are not well constrained given the few epochs that sample only a small portion of the hypothesized binary orbital cycle. The apparent occurrence rate of sub-pc BSBHs is $\lesssim$13$\pm$5% among all SDSS quasars, with no significant difference in the subsets with and without single-epoch broad line velocity offsets. Dedicated long-term spectroscopic monitoring is still needed to further confirm or reject these BSBH candidates.

Development of a spectroscopic diagnostic tool for electric field measurements in IShTAR (Ion cyclotron Sheath Test ARrangement).

IShTAR, Ion cyclotron Sheath Test ARrangement, is a linear device dedicated to the investigation of the edge plasma-ICRF (Ion Cyclotron Range of Frequencies) antenna interactions in tokamak edge-like conditions and serves as a platform for a diagnostic development for measuring the electric fields in the vicinity of ICRF antennas. We present here our progress in the development of an optical emission spectroscopy method for measuring the electric fields which concentrates on the changes in the helium spectral line profiles introduced by the external electrical field, i.e., the Stark effect. To be able to fully control the operating parameters, at the first stage of the study, the measurements are conducted on a planar electrode installed in the centre of the plasma column in IShTAR's helicon plasma source. At the second stage, the measurements are performed in the vicinity of IShTAR's ICRF antenna.