Balmer Lines Research Articles

Determination of the Physical Parameters of AGNs in Seyfert 1 Galaxies LEDA 3095839 and VII Zw 244 Based on Spectropolarimetric Observations

Here we present the detailed investigation of AGNs in two Seyfert 1 galaxies, LEDA 3095839 and VII Zw 244. Both of them were observed within the photometric reverberation mapping project in Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS), during which we earlier obtained the SMBHs masses. After that, both galaxies were observed in spectropolarimetric and polarimetric modes on the BTA 6 m telescope of the SAO RAS with the focal reducer SCORPIO-2. The linear polarization of the continuum and broad Balmer lines has been measured. It was found that (i) there were no signs of equatorial scattering in the LEDA 3095839 galaxy in the broad Hα line, and we were able to estimate the value of SMBH spin and the magnetic field strength in the disk from the level of continuum polarization; (ii) for the galaxy VII Zw 244, the presence of equatorial scattering was shown, due to which the mass of the SMBH was independently measured, the inclination angle of the system was obtained, and the value of the spin was estimated.

The GAPS programme at TNG

Context. Active-region evolution plays an important role in the generation and variability of magnetic fields on the surface of lower main sequence stars. However, determining the lifetime of active-region growth and decay as well as their evolution is a complex task. Most previous studies of this phenomenon are based on optical light curves, while little is known about the chromosphere and the transition region. Aims. We aim to test whether or not the lifetime of active-region evolution shows any dependency on stellar parameters, particularly stellar age. Methods. We identified a sample of stars with well-defined ages via their kinematics and membership to young stellar associations and moving groups. We made use of high-resolution échelle spectra from HARPS at La Silla 3.6m-telescope and HARPS-N at TNG to compute rotational velocities, activity levels, and emission excesses. We used these data to revisit the activity-rotation-age relationship. The time-series of the main optical activity indicators, namely Ca II H and K, Balmer lines, Na I D1, D2, and He I D3, were analysed together with the available photometry using state-of-the-art Gaussian processes to model the stellar activity of these stars. Autocorrelation functions of the available photometry were also analysed. We used the derived lifetimes of active-region evolution to search for correlations with stellar age, spectral type, and activity level. We also used the pooled variance technique to characterise the activity behaviour of our targets. Results. Our analysis confirms the decline of activity and rotation as a star ages. We also confirm that the rotation rate decays with age more slowly for cooler stars and that, for a given age, cooler stars show higher levels of activity. We show that F- and G-type young stars also depart from the inactive stars in the flux–flux relationship. The Gaussian process analysis of the different activity indicators does not seem to provide any useful information on the lifetime and evolution of active regions. On the other hand, the lifetimes of active regions derived from the light-curve analysis might correlate with stellar age and temperature. Conclusions. Although we suggest caution because of small number statistics, our results suggest that active regions seem to live longer on younger, cooler, and more active stars.

The Host Galaxy and Rapidly Evolving Broad-line Region in the Changing-look Active Galactic Nucleus 1ES 1927+654

Changing-look active galactic nuclei (AGNs) present an important laboratory to understand the origin and physical properties of the broad-line region (BLR). We investigate follow-up optical spectroscopy spanning ∼500 days after the outburst of the changing-look AGN 1ES 1927+654. The emission lines displayed dramatic, systematic variations in intensity, velocity width, velocity shift, and symmetry. Analysis of optical spectra and multiband images indicates that the host galaxy contains a pseudobulge and a total stellar mass of . Enhanced continuum radiation from the outburst produced an accretion disk wind, which condensed into BLR clouds in the region above and below the temporary eccentric disk. Broad Balmer lines emerged ∼100 days after the outburst, together with an unexpected, additional component of narrow-line emission. The newly formed BLR clouds then traveled along a similar eccentric orbit (e ≈ 0.6). The Balmer decrement of the BLR increased by a factor of ∼4–5 as a result of secular changes in cloud density. The drop in density at late times allowed the production of He i and He ii emission. The mass of the black hole cannot be derived from the broad emission lines because the BLR is not virialized. Instead, we use the stellar properties of the host galaxy to estimate . The nucleus reached near or above its Eddington limit during the peak of the outburst. We discuss the nature of the changing-look AGN 1ES 1927+654 in the context of other tidal disruption events.

Probing TYC 3315-1807-1, An sdB+dM Binary Displaying Strong Period Variation and Reflection Effect

Subdwarf (sdB) stars include core helium-burning stars with a very thin hydrogen envelope that lies at the blue end of the horizontal branch (or extreme horizontal branch). Among them, short-period sdB binaries especially with cool companions are significant to test and constrain binary evolution. We discuss one such sdB+dM type binary, TYC 3315-1807-1 (V1), which was first reported by Kawka. Results of the photometric analysis on Transiting Exoplanet Survey Satellite data are being reported. Light variation in the light curve suggests that the system displays a large reflection effect with no eclipses. Spectroscopic observations of the object were also carried out to probe into the nature of secondary companion as well as to understand the post-common-envelope evolution of such objects. The variability in Balmer, He, and Na line profiles as a function of phase, probably caused by observed reflection effect was identified and studied. Period variation study of the object was done using times of minima obtained from the literature and the O − C plot was produced, which points to a decrease in the period (dp/dt = −1.36315 × 10−7 day yr−1), and the possible scenario of evolution is discussed. From the evolutionary models, we constrain the possible mass of the sdB to be 0.274 M ⊙ and that of the secondary is 0.113 M ⊙, and we conclude that V1 may evolve directly as a helium-core white dwarf.

First optical identification of the SRG/eROSITA-detected supernova remnant G 116.6 − 26.1. I. Preliminary results

ABSTRACT The supernova remnant (SNR) candidate G 116.6 − 26.1 is one of the few high Galactic latitude (|b| > 15°) remnants detected so far in several wavebands. It was discovered recently in the SRG/eROSITA all-sky X-ray survey and also displays a low-frequency weak radio signature. In this study, we report the first optical detection of G 116.6 − 26.1 through deep, wide-field, and higher resolution narrowband imaging in H$\alpha$, $[\rm S\,{\small{\rm II}}]$ and $[\rm O\,{\small{\rm III}}]$ light. The object exhibits two major and distinct filamentary emission structures in a partial shell-like formation. The optical filaments are found in an excellent positional match with available X-ray, radio, and UV maps, can be traced over a relatively long angular distance (38 and 70 arcmin) and appear unaffected by any strong interactions with the ambient interstellar medium. We also present a flux-calibrated, optical emission spectrum from a single location, with Balmer and several forbidden lines detected, indicative of emission from shock excitation in a typical evolved SNR. Confirmation of the most likely SNR nature of G 116.6 − 26.1 is provided from the observed value of the line ratio [S ii] / H$\alpha$ = $0.56\, \pm \, 0.06$, which exceeds the widely accepted threshold 0.4, and is further strengthened by the positive outcome of several diagnostic tests for shock emission. Our results indicate an approximate shock velocity range 70–100 km s−1 at the spectroscopically examined filament, which, when combined with the low emissivity in H$\alpha$ and other emission lines, suggest that G 116.6 − 26.1 is a SNR at a mature evolutionary stage.

Analysis of the ion collisional contribution over the Stark profile in Hα line

Laboratory plasmas can be characterized by Optical Emission Spectroscopy techniques by using Hydrogen Balmer series lines. The line shape is the result of the convolution of the several internal process occuring in plasma. The Stark effect over the Hydrogen emitter atom has been studied in a limited range of plasma parameters, obtaining a profile which is the result of a sum of a given number of Lorentzian functions for the Hα and Hβ line. In this paper, we have extended this analysis to cover a wide range of plasma conditions for the Hα line. The number of Lorentzian functions depends on the contribution of ion collisions around the Hydrogen emitter atom and their mobility in comparison with that of electrons.

Stellar labels for hot stars from low-resolution spectra

We set out to determine stellar labels from low-resolution survey spectra of hot stars, specifically OBA stars with Teff ≳ 7500 K. This fills a gap in the scientific analysis of large spectroscopic stellar surveys such as LAMOST, which offers spectra for millions of stars at R ~ 1800 and covers 3800 Å ≤ λ ≤ 9000 Å. We first explore the theoretical information content of such spectra to determine stellar labels via the Cramér-Rao bound. We show that in the limit of perfect model spectra and observed spectra with signal-to-noise ratio ~50–100, precise estimates are possible for a wide range of stellar labels: not only the effective temperature, Teff, surface gravity, log g, and projected rotation velocity, vsin i, but also the micro-turbulence velocity,vmic, helium abundance, NHe/Ntot, and the elemental abundances [C/H], [N/H], [O/H], [Si/H], [S/H], and [Fe/H]. Our analysis illustrates that the temperature regime of Teff ~ 9500 K is challenging as the dominant Balmer and Paschen line strengths vary little with Teff. We implement the simultaneous fitting of these 11 stellar labels to LAMOST hot-star spectra using the Payne approach, drawing on Kurucz’s ATLAS12/SYNTHE local thermodynamic equilibrium spectra as the underlying models. We then obtain stellar parameter estimates for a sample of about 330 000 hot stars with LAMOST spectra, an increase by about two orders of magnitude in sample size. Among them, about 260 000 have good Gaia parallaxes (ω/σω > 5), and their luminosities imply that ≳95% of them are luminous stars, mostly on the main sequence; the rest are evolved lower luminosity stars, such as hot subdwarfs and white dwarfs. We show that the fidelity of the results, particularly for the abundance estimates, is limited by the systematics of the underlying models as they do not account for nonlocal thermodynamic equilibrium effects. Finally, we show the detailed distribution of vsin i of stars with 8000–15 000 K, illustrating that it extends to a sharp cutoff at the critical rotation velocity, vcrit, across a wide range of temperatures.

Identification of New Classical Be Stars from the LAMOST Medium Resolution Survey

Be stars are B-type main-sequence stars that display broad Balmer emission lines in their spectra. Identification of the Be population is essential to further examine the formation and evolutionary models. We report the detection of classical Be (CBe) stars from observations with the Large sky Area Multi-Object fiber Spectroscopic Telescope Medium Resolution Survey Data Release 7 (LAMOST MRS DR7). We used a deep convolutional neural network, ResNet, with an 18 layer module to examine the morphology of the Hα profile. We identified 1162 candidate Be stars from the collection of 2,260,387 spectra for 789,918 stars in the database. The ResNet network achieves a Be-star classification accuracy of 99.5%. Among the detections, 151 of these are prior known Be stars crossmatched from the literature. By applying a three-step test, we identified 183 new CBe stars. We find that 41 CBe stars are members of known open clusters. Based on an investigation of the kinematics of the identified CBe stars from the Gaia EDR3 astrometric solutions, we identified 16 new runaways. These new identifications will provide a reference for future follow-ups to further investigate their physical properties.

Dependence of Narrow-line Region Sizes on [O iii] Luminosity in Low-redshift Active Galactic Nuclei with Double-peaked Broad Balmer Emission Lines

In this paper, simple but interesting results are reported on the upper limits of narrow-line region (NLR) sizes of a small sample of 38 low-redshift (z < 0.1) active galactic nuclei (AGN) with double-peaked broad emission lines (double-peaked BLAGN), in order to check whether the NLR sizes in type-1 AGN (broad line) and type-2 AGN (narrow line) obey a similar empirical dependence on [O iii] luminosity. In order to correct the inclination effects on projected NLR sizes of type-1 AGN, the accretion disk origin is commonly applied to describe the double-peaked broad Hα line, leading to the determined inclination angles of central disk-like broad-line regions of 38 double-peaked BLAGN. Then, considering the fixed Sloan Digital Sky Survey (SDSS) fiber radius, the upper limits of the NLR sizes of the 38 double-peaked BLAGN can be estimated. Meanwhile, a strong linear correlation between continuum luminosity and [O iii] luminosity is applied to confirm that the [O iii] emissions of the 38 double-peaked BLAGN are totally covered in the SDSS fibers. Considering the reddening-corrected measured [O iii] luminosity, the upper limits of the NLR sizes of the 38 double-peaked BLAGN are within a 99.9999% confidence interval of the expected results from the empirical relation between NLR size and [O iii] luminosity in type-2 AGN. In the current understanding, there are no challenges to the unified model of AGN through the space properties of NLRs.

Probing star formation and ISM properties using galaxy disk inclination

Attenuation by dust severely impacts our ability to obtain unbiased observations of galaxies, especially as the amount and wavelength dependence of the attenuation varies with the stellar mass M*, inclination i, and other galaxy properties. In this study, we used the attenuation – inclination models in ultraviolet, optical, and near-infrared bands designed by Tuffs and collaborators to investigate the average global dust properties in galaxies as a function of M*, the stellar mass surface density μ*, the star-formation rate SFR, the specific star-formation rate sSFR, the star-formation main-sequence offset dMS, and the star-formation rate surface density ΣSFR at redshifts z ∼ 0 and z ∼ 0.7. We used star-forming galaxies from the Sloan Digital Sky Survey (∼20 000) and Galaxy And Mass Assembly (∼2000) to form our low-z sample at 0.04 &lt; z &lt; 0.1 and star-forming galaxies from Cosmological Evolution Survey (∼2000) for the sample at 0.6 &lt; z &lt; 0.8. We found that galaxies at z ∼ 0.7 have a higher optical depth τBf and clumpiness F than galaxies at z ∼ 0. The increase in F hints that the stars of z ∼ 0.7 galaxies are less likely to escape their birth cloud, which might indicate that the birth clouds are larger. We also found that τBf increases with M* and μ*, independent of the sample and therefore redshift. We found no clear trends in τBf or F with the SFR, which could imply that the dust mass distribution is independent of the SFR. In turn, this would imply that the balance of dust formation and destruction is independent of the SFR. Based on an analysis of the inclination dependence of the Balmer decrement, we found that reproducing the Balmer line emission requires not only a completely optically thick dust component associated with star-forming regions, as in the standard model, but an extra component of an optically thin dust within the birth clouds. This new component implies the existence of dust inside H II regions that attenuates the Balmer emission before it escapes through gaps in the birth cloud and we found it is more important in high-mass galaxies. These results will inform our understanding of dust formation and dust geometry in star-forming galaxies across redshift.

The Large Dispersion and Scattering of FRB 20190520B Are Dominated by the Host Galaxy

The repeating fast radio burst FRB 20190520B is localized to a galaxy at z = 0.241, much closer than expected given its dispersion measure DM = 1205 ± 4 pc cm−3. Here we assess implications of the large DM and scattering observed from FRB 20190520B for the host galaxy’s plasma properties. A sample of 75 bursts detected with the Five-hundred-meter Aperture Spherical radio Telescope shows scattering on two scales: a mean temporal delay τ(1.41 GHz) = 10.9 ± 1.5 ms, which is attributed to the host galaxy, and a mean scintillation bandwidth Δν d(1.41 GHz) = 0.21 ± 0.01 MHz, which is attributed to the Milky Way. Balmer line measurements for the host imply an Hα emission measure (galaxy frame) EMs = 620 pc cm−6 × (T/104 K)0.9, implying DMHα of order the value inferred from the FRB DM budget, pc cm−3 for plasma temperatures greater than the typical value 104 K. Combining τ and DMh yields a nominal constraint on the scattering amplification from the host galaxy , where describes turbulent density fluctuations and G represents the geometric leverage to scattering that depends on the location of the scattering material. For a two-screen scattering geometry where τ arises from the host galaxy and Δν d from the Milky Way, the implied distance between the FRB source and dominant scattering material is ≲100 pc. The host galaxy scattering and DM contributions support a novel technique for estimating FRB redshifts using the τ–DM relation, and are consistent with previous findings that scattering of localized FRBs is largely dominated by plasma within host galaxies and the Milky Way.

Experimental distinction of the molecularly induced Balmer emission contribution and its application for inferring molecular divertor density with 2D filtered camera measurements during detachment in JET L-mode plasmas

A previously presented model for generating 2D estimates of the divertor plasma conditions at JET from deuterium Balmer line intensity ratios, obtained from tomographic reconstructions of divertor camera images, was amended to consider also the Balmer emission arising from molecular processes. Utilizing the AMJUEL and H2VIBR atomic and molecular databases of EIRENE enabled also inference of the molecular divertor density from the distinguished molecularly induced emission. Analysis of a JET L-mode density scan suggests the molecularly induced emission accounting for up to 60%–70% and 10%–20% of the Balmer D and D intensities, respectively, at the onset of detachment, while electron-ion recombination becomes increasingly dominant with deepening detachment. Similar observations were made by post-processing EDGE2D-EIRENE simulations, which indicated significant roles of molecular D ions and vibrational excitation of the D2 molecules as precursors for the molecularly induced emission. The experimentally inferred molecular density at the outer strike point was found to increase monotonously with decreasing strike point temperature, reaching approximately 30%–50% of the local electron density at 1–2 m−3 at 0.7 eV. A further steep increase by a factor of 3–5 was observed with decrease of to 0.5 eV. The observations are in qualitative and reasonable quantitative agreement with EDGE2D-EIRENE predictions of within the uncertainties of the experimental data.

Slow convection and fast rotation in crystallization-driven white dwarf dynamos

ABSTRACT It has been recently suggested that white dwarfs generate magnetic fields in a process analogous to the Earth. The crystallization of the core creates a compositional inversion that drives convection, and combined with rotation, this can sustain a magnetic dynamo. We reanalyse the dynamo mechanism, arising from the slow crystallization of the core, and find convective turnover times tconv of weeks to months – longer by orders of magnitude than previously thought. With white dwarf spin periods P ≪ tconv, crystallization-driven dynamos are almost always in the fast-rotating regime, where the magnetic field B is at least in equipartition with the convective motion and is possibly further enhanced by a factor of B ∝ (tconv/P)1/2, depending on the assumed dynamo scaling law. We track the growth of the crystallized core using mesa and compute the magnetic field B(Teff) as a function of the white dwarf’s effective temperature Teff. We compare this prediction with observations and show that crystallization-driven dynamos can explain some – but not all – of the ∼MG magnetic fields measured for single white dwarfs, as well as the stronger fields measured for white dwarfs in cataclysmic variables, which were spun up by mass accretion to short P. Our B(Teff) curves might also explain the clustering of white dwarfs with Balmer emission lines around Teff ≈ 7500 K.

Modeling Balmer line signatures of stellar CMEs

Abstract From the Sun we know that coronal mass ejections (CMEs) are a transient phenomenon, often correlated with flares. They have an impact on solar mass- and angular momentum loss, and therefore solar evolution, and make a significant part of space weather. The same is true for stars, but stellar CMEs are still not well constrained, although new methodologies have been established, and new detections presented in the recent past. So far, probable detections of stellar CMEs have been presented, but their physical parameters which are not directly accessible from observations, such as electron density, optical thickness, temperature, etc., have been so far not determined for the majority of known events. We apply cloud modeling, as commonly used on the Sun, to a known event from the literature, detected on the young dMe star V374 Peg. This event manifests itself in extra emission on the blue side of the Balmer lines. By determining the line source function from 1D NLTE modeling together with the cloud model formulation we present distributions of physical parameters of this event. We find that except for temperature and area all parameters are at the upper range of typical solar prominence parameters. The temperature and the area of the event were found to be higher than for typical solar prominences observed in Balmer lines. We find more solutions for the filament than for the prominence geometry. Moreover we show that filaments can appear in emission on dMe stars contrary to the solar case.

The Long-term Spectroscopic and Photometric Behavior of the Carbon Star RS Cygni

RS Cygni is a semiregular variable carbon star that has a distinctive light curve. Twenty-one years of optical spectroscopic and photometric observations of this star made at Grinnell College have been used not only to produce a comprehensive survey of the behavior of this star but also to compare these two sets of data. A number of spectral features vary in strength with the cycle of the star and some, but not all, of this variation is correlated with the dips that often occur near the maxima of the light curve. In particular, the strengths of the Balmer emission lines often have two peaks per cycle. Such correlation might be consistent with a mechanism for producing the dips, such as dust formation, that reduces the brightness in a way that does not involve rapid changes in photospheric characteristics. The large amount of data and the precision of the photometry presented give an unusually detailed view of how the light of RS Cyg varies over time.

Isotope Effect for Plasma Detachment in Helium and Hydrogen/Deuterium Mixture Plasmas

To investigate the isotope effect on the plasma detachment in helium (He) and hydrogen (H)/deuterium (D) mixture plasmas, we performed H2, D2 or He gas puffing into He plasma in the linear plasma device NAGDIS-II. Axial distributions of electron density (ne) and electron temperature (Te) were obtained using a movable Langmuir probe. Additionally, optical emission spectroscopy (OES) was applied to measure axial distributions of Balmer lines and He I lines. When the neutral gas pressure was high (ΔPn = 7 ∼10 mTorr), ne distribution in He-D2 mixture plasma was similar to that in pure He plasma, showing a sharp decrease at the downstream region where Te < 1 eV. In contrast, in He-H2 mixture plasma, a decrease in ne was confirmed from upstream region where Te > 1 eV. The upstream Hα/Hγ in He-H2 plasma was significantly larger than the Dα/Dγ in He-D2 plasma at the same Te. This result indicated that molecular activated recombination (MAR) processes significantly occurred in He-H2 plasma, while electron-ion recombination (EIR) processes were dominant in He-D2 and pure He plasmas.

X-ray emission associated with radiative recombination for Pb82+ ions at threshold energies

For decelerated bare lead ions at a low beam energy of 10 MeV/u, the x-ray emission associated with radiative recombination (RR) at threshold energies has been studied at the electron cooler of CRYRING@ESR at GSI, Darmstadt. In our experiment, we observed the full x-ray emission pattern by utilizing dedicated x-ray detection chambers installed at 0∘ and 180∘ observation geometry. Most remarkably, no line distortion effects due to delayed emission are present in the well-defined x-ray spectra, spanning a wide range of x-ray energies (from about 5 to 100 keV), which enables us to identify fine-structure resolved Lyman, Balmer, and Paschen x-ray lines along with the RR transitions into the K, L, and M shells of the ions. For comparison with theory, an elaborate theoretical model is established taking into account the initial population distribution via RR for all atomic levels up to Rydberg states with principal quantum number n=165 in combination with time-dependent feeding transitions. Within the statistical accuracy, the experimental data are in very good agreement with the results of rigorous relativistic predictions. Most notably, this comparison sheds light on the contribution of prompt and delayed x-ray emission (up to 70 ns) to the observed x-ray spectra, originating in particular from yrast transitions into inner shells.2 MoreReceived 19 January 2022Accepted 31 March 2022DOI:https://doi.org/10.1103/PhysRevA.105.052804Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAtomic & molecular collisionsAtomic spectraElectronic transitionsTechniquesEnergy-dispersive x-ray spectroscopyAtomic, Molecular & OpticalAccelerators & Beams

THA 15−31: Discovery with VLT/X-shooter and Swift/UVOT of a new symbiotic star of the accreting-only variety

We report the discovery and characterization of a new symbiotic star of the accreting-only variety, which we observed in the optical/near-infrared (NIR) with VLT/X-shooter and in the X-rays/ultraviolet with Swift/UVOT+XRT. The new symbiotic star, THA 15−31, was previously described as a pre-main sequence star belonging to the Lupus 3 association. Our observations, ancillary data, and Gaia EDR3 parallax indicate that THA 15−31 is a symbiotic star composed of an M6III red giant and an accreting companion, is subject to EB − V = 0.38 reddening, and is located at a distance of ∼12 kpc and at 1.8 kpc above the Galactic plane in the outskirts of the Bulge. The luminosity of the accreting companion is ∼100 L⊙, placing THA 15−31 among the symbiotic stars accreting at a high rate (2.5 × 10−8 M⊙ yr−1 if the accretion is occurring on a white dwarf of 1 M⊙). The observed emission lines originate primarily from H I, He I, and Fe II, with no He II or other high-excitation lines observed; a sharp central absorption superimposed on the Balmer emission lines is observed, while all other lines have a simple Gaussian-like profile. The emission from the companion dominates over the M6III red giant at U and B-band wavelengths, and is consistent with an origin primarily in an optically thick accretion disk. No significant photometric variability is observed at optical or NIR wavelengths, suggesting either a face-on orbital orientation and/or that the red giant is far from Roche-lobe filling conditions. The profile of emission lines supports a low orbital inclination if they form primarily in the accretion disk. An excess emission is present in AllWISE W3 (12 μm) and W4 (22 μm) data, radiating a luminosity ≥35 L⊙, consistent with thermal emission from optically thin circumstellar dust.

Discovery of eRASSt J192932.9–560346: A bright, two-pole accreting, eclipsing polar

We report the discovery of a bright (V ~ 15), eclipsing, two-pole accreting magnetic cataclysmic variable (CV), a polar, as counterpart of the SRG/eROSITA and Gaia transients eRASSt J192932.9–560346 and Gaia21bxo. Frequent large-amplitude changes of its brightness at X-ray and optical wavelengths by more than four magnitudes are indicative of a CV nature of the source. Identification spectra obtained with the 10m SALT telescope revealed the typical features of a magnetic CV: strong, broad HeI, HeII, and hydrogen Balmer emission lines superposed on a blue continuum. Time-resolved photoelectric polarimetry revealed that the circular polarization varies from −20 to +20% and the linear polarization varies from 0 to 10%, confirming the system to be a magnetic CV of the polar subclass. High-cadence photometry revealed deep, structured eclipses, indicating that the system is a two-pole accretor. The orbital period determined from the eclipse times is 92.5094 ± 0.0002 min. The X-ray spectrum is thermal only, and the implied luminosity is LX = 2.2 × 1031 erg s−1 at the Gaia-determined distance of 376 pc.

Low luminosity Type II supernovae – IV. SN 2020cxd and SN 2021aai, at the edges of the sub-luminous supernovae class

ABSTRACT Photometric and spectroscopic data for two Low Luminosity Type IIP Supernovae (LL SNe IIP) 2020cxd and 2021aai are presented. SN 2020cxd was discovered 2 d after explosion at an absolute magnitude of Mr = −14.02 ± 0.21 mag, subsequently settling on a plateau which lasts for ∼120 d. Through the luminosity of the late light curve tail, we infer a synthesized 56Ni mass of (1.8 ± 0.5) × 10−3 M⊙. During the early evolutionary phases, optical spectra show a blue continuum ($T\, \gt $8000 K) with broad Balmer lines displaying a P Cygni profile, while at later phases, Ca ii, Fe ii, Sc ii, and Ba ii lines dominate the spectra. Hydrodynamical modelling of the observables yields $R\, \simeq$ 575 R⊙ for the progenitor star, with Mej = 7.5 M⊙ and $E\, \simeq$ 0.097 foe emitted during the explosion. This low-energy event originating from a low-mass progenitor star is compatible with both the explosion of a red supergiant (RSG) star and with an Electron Capture Supernova arising from a super asymptotic giant branch star. SN 2021aai reaches a maximum luminosity of Mr = −16.57 ± 0.23 mag (correcting for AV = 1.92 mag), at the end of its remarkably long plateau (∼140 d). The estimated 56Ni mass is (1.4 ± 0.5) × 10−2 M⊙. The expansion velocities are compatible with those of other LL SNe IIP (few 103 km s−1). The physical parameters obtained through hydrodynamical modelling are $R\, \simeq$ 575 R⊙, Mej = 15.5 M⊙, and E = 0.4 foe. SN 2021aai is therefore interpreted as the explosion of an RSG, with properties that bridge the class of LL SNe IIP with standard SN IIP events.