arXiv subjects

Réka Könyves-Tóth

Publications and source records attributed to Réka Könyves-Tóth.

At least 19 recordsLinked to original sources

arXiv2026

Fast and furious -- High-velocity features of Ca and Si in the spectra of Type Ia supernovae

High velocity absorption lines of Ca II and Si II (high-velocity features, HVFs) are common in the early spectra of Type Ia supernovae (SNe Ia), but their physical origin is still poorly understood. We used 181 optical spectra of 56 SNe Ia that show HVFs in at least the Ca II NIR triplet to better understand the nature of HVFs. To model the profiles of HVFs we applied Gaussian fitting as well as spectrum synthesis modeling with the SYNOW code. We found that Gaussian fitting usually underestimates the velocity of HVFs compared to SYNOW, and derived a linear formula for correction. In accord with many previous studies, the observed properties of HVFs (velocity, strength, duration) were found to correlate with the general observables of SNe Ia (decline rate, spectral subtype). Brighter, slow-declining SNe usually exhibit stronger, higher velocity HVFs, while fainter objects typically show weak, low velocity HVFs. SNe Ia in early-type galaxies showed weaker and slower HVFs compared to events in late-type galaxies in our sample. Comparing these observational findings with theoretical predictions, we conclude that the overall physical picture of HVFs is quite complex, and could be explained by multiple models. A double detonation explosion model is promising as it is inherently capable of explaining the creation of high-velocity shells that may produce HVFs, but an asymmetric delayed detonation model or interaction with circumstellar material cannot be ruled out.

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arXiv2026

SN 2026dix: a nearby, transitional Type IIb/Ib supernova consistent with a warm supergiant progenitor

Understanding the final evolution stages of (very) massive stars and their explosive outcomes, stripped-envelope supernovae (SESNe), represent a long-term challenge for astrophysics. The latest results support a continuous distribution within the traditional SESN subclasses (IIb, Ib, Ic). The nearby ($D \approx 17.5$ Mpc) SN 2026dix seems to be another member of the recently identified group of transitional Type IIb/Ib explosions. A point source is located at the SN position within the uncertainties on multiple pre-explosion {\it HST} images. Together with post-explosion photometry and spectroscopy, it provides a good opportunity to study a rare type of SN in detail. We carried out a thorough comparative light-curve (LC) and spectral analysis of SN 2026dix. We also constructed its bolometric LC and modeled it semi-analytically. In addition, we constructed the spectral energy distribution of the presumed progenitor and compared this to model stellar atmospheres and binary stellar evolution tracks. We infer that SN 2026dix arose from an explosion in an interacting binary system, consistent with the properties of a warm ($T_\textrm{eff} \approx 6750-$7750 K; spectral type F2 to A7), luminous ($\log(L_{\rm bol}/L_{\odot})\approx 4.9-5.3$) supergiant primary and a less luminous, less massive hot dwarf companion. The nature of the identified progenitor closely resembles that of some other known cases of SNe IIb and also well aligns with both the results of our semi-analytical LC modeling and the comparison of the LCs and spectra of SN 2026dix with the output of radiative-transfer models of an exploding star with $M_\textrm{ini}$ = 18 $M_{\odot}$.

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arXiv2026

AT 2016blu: Accretion-Powered Outbursts in a Luminous Blue Variable and Compact Object Binary

We present the first X-ray detection of the supernova (SN) impostor AT 2016blu in NGC 4559. AT 2016blu exhibited 27 detected quasiperiodic (~113 day) outbursts between 2012 and June 2026, presumably triggered by periastron passages in an eccentric binary system in which the primary is a $\gtrsim 33~M_{\odot}$ luminous blue variable (LBV). AT 2016blu was serendipitously observed by Chandra in 2001-2002, prior to its first documented outburst in 2012, but the stacked archival data show no X-ray detection. We monitored the source's visual-wavelength variability around its predicted outburst time and triggered a Chandra Target of Opportunity program in March 2026, obtaining observations with detections over five closely spaced epochs. These data indicate an X-ray luminosity of $\log L_{\rm X} \approx 38.64 \pm 0.11$~erg~s$^{-1}$ and an accretion rate of $\dot{M} \gtrsim 8\times10^{-8}~M_{\odot}~\mathrm{yr^{-1}}$, consistent with the presence of a compact companion. We therefore conclude that AT 2016blu is the first known case of an LBV SN impostor whose outbursts are driven by intermittent accretion onto a compact object. Given that the system consists of a massive star and a compact companion, AT 2016blu is a high-mass X-ray binary, similar to SN 2010da, although the donor star in SN 2010da is a red supergiant.

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arXiv2026

SN 2022erq: A Superluminous Thermonuclear Supernova with Escalating Preexplosion Mass Loss

We present a photometric and spectroscopic study of the superluminous Type Ia supernova SN 2022erq. Its early spectra, dominated by iron-group elements with weak intermediate-mass features, might indicate highly efficient nuclear burning, broadly similar to that inferred for some overluminous SNe Ia. The rapid emergence and persistence of narrow Balmer emission lines superposed on this iron-rich spectrum provide clear evidence of long-lived interaction with a hydrogen-rich circumstellar medium (CSM), establishing SN 2022erq as a member of the rare Ia-CSM class. SN 2022erq reached a peak bolometric luminosity of about 8 x 10^43 erg/s and exhibited an exceptionally slow post-peak decline, indicating that its light curve is dominated by long-duration ejecta-CSM interaction. By combining H-alpha diagnostics with bolometric light-curve modeling, we reconstruct the pre-explosion mass-loss history of the progenitor. The mass-loss rate escalated by one order of magnitude over the final decades, rising from about 0.04 to about 0.6 solar masses per year. This surge produced a massive, extended CSM shell of about 3 solar masses out to about 3.5 x 10^16 cm. The young stellar environment (about 100 Myr) together with this substantial, extensive CSM points to a progenitor system consisting of a white dwarf and an intermediate-mass companion that underwent increasing mass loss prior to explosion.

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arXiv2026

JWST Spectroscopy of SN Ia 2022aaiq and 2024gy: Evidence for Enhanced Central Stable Ni Abundance and a Deflagration-to-Detonation Transition

We present optical + near-infrared (NIR) + mid-infrared (MIR) observations of the normal Type Ia supernovae (SN Ia) 2022aaiq and 2024gy in the nebular phase, continuously spanning 0.35-28 microns. Medium-resolution JWST spectroscopy reveals novel narrow ($v_{\mathrm{FWHM}}<1500$ km s$^{-1}$) [Ni II] 1.94 and 6.64 micron cores in both events. The MIR [Ni II] 6.64 micron line exhibits a distinct narrow core atop a broader base, indicating a central enhancement of stable Ni. This structure points to high central densities consistent with a near-Chandrasekhar-mass ($M_{\text{Ch}}$) progenitor or a high-metallicity sub-$M_{\text{Ch}}$ progenitor. From detailed line-profile inversions of SN 2024gy, we derive emissivity profiles for stable iron-group elements (IGEs), radioactive material, and intermediate-mass elements (IMEs), revealing spatially distinct ejecta zones. The [Ni III] 7.35 micron line shows a shallow-to-steep slope transition - a "broken-slope" morphology - that matches predictions for delayed detonation explosions with separated deflagration and detonation ashes. We also reanalyze and compare to archival JWST spectra of SN 2021aefx and the subluminous SN 2022xkq. From the stable Ni luminosities, we infer that SN 2024gy produced ~5-10 times more stable Ni mass than SN 2022xkq, favoring a near-$M_{\text{Ch}}$ scenario for SN 2024gy and sub-$M_{\text{Ch}}$ scenario for SN 2022xkq. These results demonstrate that resolved line profiles, now accessible with JWST, provide powerful diagnostics of explosion geometry, central density, and progenitor mass in SN Ia.

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arXiv2026

Long-term optical and near-infrared photometric evolution of SN 2019vxm, an interacting Type IIn supernova

The diversity of Type IIn supernovae is largely driven by the properties of the circumstellar material (CSM) they explode into. We examine the temporal evolution of SN 2019vxm, an interacting supernova that belongs to the class of long-lasting Type IIn events, using multicolor photometry spanning the ultraviolet, optical and near-infrared wavelengths, including over 650 days of optical and 1500 days of IR coverage. The evolution of the spectral energy distribution and bolometric luminosity, as well as the effective temperature and radius of the photosphere, indicates that the supernova was initially surrounded by an optically thick CSM, which was heated and pushed outward by the forward shock of the impacting ejecta. About 80-100 days after the explosion the forward shock and the photosphere decouples, and we observe the receding photosphere of the H-recombination front within the now thinned CSM. Near-IR measurements reveal long-lasting, slowly cooling emission from circumstellar dust around SN 2019vxm and an IR rebrightening about one year after explosion, which we tentatively identify as a signature of an outer CSM region. We find that due to the moving photosphere and the transition from optically thick to partially thin inner CSM, modeling the explosion and subsequent interaction of the ejecta with the CSM to infer progenitor and CSM masses faces difficulties. Nevertheless, the inferred high masses and extremely high mass-loss rates point to a massive progenitor undergoing intense pre-supernova mass loss.

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arXiv2026

Distance estimate to NGC 6951 from supernova siblings Type IIP SN 2020dpw and Type Ib SN 2021sjt

{Supernova (SN) siblings are powerful tools used to calibrate and improve distance measurement methods, and to make the systematic uncertainty to distances to their host galaxies considerably lower compared to other techniques.} {In this paper we present distance estimates to NGC6951, a galaxy that hosted the Type IIP SN~2020dpw, the Type Ib SN~2021sjt, and three other SNe.} {Photometric observations of the two objects were carried out using two 80cm RC telescopes located in Hungary, while spectra were obtained from the LCO and the WiseRep database. For the distance estimates, we applied the expanding photosphere method (EPM), which connects the observed angular radius ($θ$) of a SN to its physical radius and is related to the velocity of the photosphere ($v_{\rm ph}$). Although the EPM is mostly applied to derive the distance of Type IIP SNe, in the literature there are several examples of this technique being used for Type IIn and stripped-envelope SNe as well. Therefore, we made another attempt to infer the distance of the Type Ib SN~2021sjt by applying the EPM together with its Type IIP sibling SN~2020dpw. } {Our analysis resulted in a distance of $25.76 \pm 0.34 (\rm random) \pm 5.51$ (systematic) Mpc and $24.57 \pm 1.27 (\rm random) \pm 4.64$ (systematic) Mpc for SN~2020dpw and SN~2021sjt, respectively. Systematic errors were estimated with respect to the used dilution factor, the interstellar reddening, and the date of the explosion (which was fixed to a value between the last non-detection and the first detection for each object).} {The obtained distance values agree with each other and with the literature, which shows the validity of the methods used. In this way, new and perhaps improved distance estimates to NGC 6951 were obtained, and the applicability of the EPM for Type Ib SNe was tested.}

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arXiv2026

Interferometric Images of the Starspot Evolution of $ζ$ Andromedae

The evolution of starspots of the giant primaries of RS CVn systems is typically detected indirectly with photometric and spectroscopic monitoring. These observations suggest slowly-evolving stellar surfaces and can constrain differential rotation as starspots move with respect to one another. However, starspot latitudes are difficult to constrain without resolved images of the stellar surfaces from which the unambiguous locations of starspots are determined. We imaged the active RS CVn primary $ζ$ And with the 330-m-baseline Center for High Angular Resolution Astronomy Array for three epochs over approximately six rotations of the star. The resultant images show a more complicated picture of stellar activity than expected from the contemporaneous photometry and earlier Doppler images. The spot structures change on the timescale of rotation, making differential rotation difficult to study. Our observations show changes in the polar spot, growing over time. We do not detect the secondary star in the interferometric data, though the observations are sensitive to the predicted 0.75 $M_\odot$ main-sequence star, and we suggest the companion may be a white dwarf.

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arXiv2025

SN 2023taz: Implications for the UV Diversity of Superluminous Supernovae

Superluminous supernovae (SLSNe) are some of the brightest explosions in the Universe representing the extremes of stellar deaths. At the upper end of their distribution is SN\,2023taz, one of the most luminous SLSNe discovered to date with a peak absolute magnitude of $M_{g,\rm{peak}}=-22.75 \pm 0.03$ and a lower limit for energy radiated of $E=2.9 \times 10^{51}$\,erg. Magnetar model fits reveal individual parameter values typical of the SLSN population, but the combination of a low $B$-field and ejecta mass with a short spin period places SN\,2023taz in a unusual region of parameter space, accounting for its extreme luminosity. The optical data around peak are consistent with a temperature of $\sim$17\,000\,K but SN\,2023taz shows a surprising deficit in the UV compared to other events in this temperature range. We find no indication of dust extinction that could plausibly explain the UV deficit. The lower level of UV flux is reminiscent of the absorption seen in lower-luminosity events like SN\,2017dwh, where Fe-group elements are responsible for the effect. However, in the case of SN\,2023taz, there is no evidence for a larger amount of Fe-group elements which could contribute to line blanketing. Comparing to SLSNe with well-observed UV spectra, an underlying temperature of $8000-9000$\,K would match the UV spectral slope, but is not consistent with the optical colour temperatures of these events. The most likely explanation is enhanced absorption by intermediate-mass elements, challenging previous findings that SLSNe exhibit similar UV absorption line equivalent widths. This highlights the need for expanded UV spectroscopic coverage of SLSNe, especially at early times, to build a framework for interpreting their diversity and to enable classification at higher redshifts where optical observations will exclusively probe rest-frame UV emission.

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arXiv2025

One Year of SN 2023ixf: Breaking Through the Degenerate Parameter Space in Light-Curve Models with Pulsating Progenitors

We present and analyze the extensive optical broadband photometry of the Type II SN 2023ixf up to one year after explosion. We find that, when compared to two pre-existing model grids, the bolometric light curve is consistent with drastically different combinations of progenitor and explosion properties. This may be an effect of known degeneracies in Type IIP light-curve models. We independently compute a large grid of $\texttt{MESA+STELLA}$ single-star progenitor and light-curve models with various zero-age main-sequence masses, mass-loss efficiencies, and convective efficiencies. Using the observed progenitor variability as an additional constraint, we select stellar models consistent with the pulsation period and explode them according to previously established scaling laws to match plateau properties. Our hydrodynamic modeling indicates that SN 2023ixf is most consistent with a moderate-energy ($E_{\rm exp}\approx7\times10^{50}$ erg) explosion of an initially high-mass red supergiant progenitor ($\gtrsim 16.5\ M_{\odot}$) that lost a significant amount of mass in its prior evolution, leaving a low-mass hydrogen envelope ($\lesssim 3\ M_{\odot}$) at the time of explosion, with a radius $\gtrsim 950\ R_{\odot}$ and a synthesized $^{56}$Ni mass of $\approx0.068\ M_{\odot}$. We posit that previous mass transfer in a binary system may have stripped the envelope of SN 2023ixf's progenitor. The analysis method with pulsation period presented in this work offers a way to break degeneracies in light-curve modeling in the future, particularly with the upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time, when a record of progenitor variability will be more common.

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arXiv2025

SN 2021hpr: A Normal Type Ia Supernova Showing Excess Emission in the Early Rising Phase

We present extensive optical observations of a nearby Type Ia supernova (SN Ia), SN 2021hpr, located in the spiral galaxy NGC 3147 at a distance of $\sim$ 45 Mpc. Our observations cover a phase within $\sim 1-2$ days to $\sim 290$ days after the explosion. SN 2021hpr is found to be a spectroscopically normal SN Ia, with an absolute B-band peak magnitude of $M_{max}(B) \approx -19.16 \pm 0.14$ mag and a post-peak decline rate of $Δm_{15}(B)= 1.00 \pm 0.01 $ mag. Early-time light curves showed a $\sim 7.0 \%$ excess emission compared to a homogeneously expanding fireball model, likely due to SN ejecta interacting with a companion or immediate circumstellar matter. The optical spectra of SN 2021hpr are overall similar to those of normal SNe Ia, but characterized by prominent detached high-velocity features (HVFs) of Si {\sc ii} and Ca {\sc ii} in the early phase. After examining a small sample of well-observed normal SNe Ia, we find that the HVFs are likely common for the subgroup with early-excess emission. The association of early bump feature with the HVFs could be attributed to density or abundance enhancement at the outer layer of the exploding star, likely as a result of interactions with companion$/$CSM or experiencing more complete burning. Nevertheless, the redshifted Fe {\sc ii} and Ni {\sc ii} lines in the nebular-phase spectra of SN 2021hpr, contrary to the blueshift trend seen in other SNe Ia showing early bump features, indicate its peculiarity in the explosion that remains to be understood.

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arXiv2025

The bolometric light curve modeling of 98 Type I superluminous supernovae using the magnetar- and the circumstellar interaction models reveals surprisingly high ejecta masses

We present the bolometric light curve modeling of 98 hydrogen-poor superluminous supernovae (SLSNe-I) using three types of power inputs: the magnetar model and two kinds of circumstellar interaction models, applying the constant density and the steady wind scenario. The quasi-bolometric luminosities of the objects were calculated from the ZTF g- and r-band data using the methodology of \citet{chen23b}, and then they were modeled with the Minim code. It was found that the light curves of 45 SLSNe-I can be fitted equally well with both the magnetar and the CSM models, 14 objects prefer the magnetar model and 39 SLSNe-I favor the CSM model. The magnetar modeling yielded a mean spin period of $P~=~4.1 \pm 0.20$ ms and a magnetic field of $B~=~5.65 \pm 0.43 \cdot 10^{14}$ G, consistently with the literature. However, the ejected mass was estimated to be significantly larger compared to previous studies presenting either multi-color light curve modeling with MOSFiT or bolometric light curve modeling: we obtained a mean value and standard error of 34.26 and 4.67 $M_\odot$, respectively. The circumstellar interaction models resulted in even larger ejecta masses with a mean and standard error of 116.82 and 5.97 $M_\odot$ for the constant density model, and 105.99 and 4.50 $M_\odot$ for the steady wind model. Although the ejected mass depends strongly on the electron scattering opacity (assumed to be $κ~=~$0.2 in this work) and the ejecta velocity, which were estimated to be globally larger compared to earlier studies, our results suggest that SLSNe-I are indeed the explosions of the most massive stars.

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arXiv2024

Ejecta masses in Type Ia Supernovae -- Implications for the Progenitor and the Explosion Scenario

The progenitor system(s) as well as the explosion mechanism(s) of thermonuclear (Type Ia) supernovae are long-standing issues in astrophysics. Here we present ejecta masses and other physical parameters for 28 recent Type Ia supernovae inferred from multiband photometric and optical spectroscopic data. Our results confirm that the majority of SNe Ia show {\it observable} ejecta masses below the Chandrasekhar-limit (having a mean $M_{\rm ej} \approx 1.1 \pm 0.3$ M$_\odot$), consistent with the predictions of recent sub-M$_{\rm Ch}$ explosion models. They are compatible with models assuming either single- or double-degenerate progenitor configurations. We also recover a sub-sample of supernovae within $1.2 $ M$_\odot$ $< M_{\rm {ej}} < 1.5$ M$_\odot$ that are consistent with near-Chandrasekhar explosions. Taking into account the uncertainties of the inferred ejecta masses, about half of our SNe are compatible with both explosion models. We compare our results with those in previous studies, and discuss the caveats and concerns regarding the applied methodology.

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arXiv2024

The Type I Superluminous Supernova Catalog I: Light Curve Properties, Models, and Catalog Description

We present the most comprehensive catalog to date of Type I Superluminous Supernovae (SLSNe), a class of stripped envelope supernovae (SNe) characterized by exceptionally high luminosities. We have compiled a sample of 262 SLSNe reported through 2022 December 31. We verified the spectroscopic classification of each SLSN and collated an exhaustive data set of UV, optical and IR photometry from both publicly available data and our own FLEET observational follow-up program, totaling over 30,000 photometric detections. Using these data we derive observational parameters such as the peak absolute magnitudes, rise and decline timescales, as well as bolometric luminosities, temperature and photospheric radius evolution for all SLSNe. Additionally, we model all light curves using a hybrid model that includes contributions from both a magnetar central engine and the radioactive decay of $^{56}$Ni. We explore correlations among various physical and observational parameters, and recover the previously found relation between ejecta mass and magnetar spin, as well as the overall progenitor pre-explosion mass distribution with a peak at $\approx 6.5$ M$_\odot$. We find no significant redshift dependence for any parameter, and no evidence for distinct sub-types of SLSNe. We find that $< 3$\% of SLSNe are best fit with a significant contribution from radioactive decay $\gtrsim 50$\%, representing a set of relatively dim and slowly declining SNe. We provide several analytical tools designed to simulate typical SLSN light curves across a broad range of wavelengths and phases, enabling accurate K-corrections, bolometric scaling calculations, and inclusion of SLSNe in survey simulations or future comparison works. The complete catalog, including all of the photometry, models, and derived parameters, is made available as an open-source resource on GitHub.

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arXiv2024

SN 2022oqm: A Bright and Multi-peaked Calcium-rich Transient

We present the photometric and spectroscopic evolution of SN 2022oqm, a nearby multi-peaked hydrogen- and helium-weak calcium-rich transient (CaRT). SN 2022oqm was detected 13.1 kpc from its host galaxy, the face-on spiral galaxy NGC 5875. Extensive spectroscopic coverage reveals an early hot (T >= 40,000 K) continuum and carbon features observed $\sim$1~day after discovery, SN Ic-like photospheric-phase spectra, and strong forbidden calcium emission starting 38 days after discovery. SN 2022oqm has a relatively high peak luminosity (MB = -17 mag) for (CaRTs), making it an outlier in the population. We determine that three power sources are necessary to explain the light curve (LC), with each corresponding to a distinct peak. The first peak is powered by an expanding blackbody with a power law luminosity, suggesting shock cooling by circumstellar material (CSM). Subsequent LC evolution is powered by a double radioactive decay model, consistent with two sources of photons diffusing through optically thick ejecta. From the LC, we derive an ejecta mass and 56Ni mass of ~0.6 solar masses and ~0.09 solar masses. Spectroscopic modeling suggests 0.6 solar masses of ejecta, and with well-mixed Fe-peak elements throughout. We discuss several physical origins for SN 2022oqm and find either a surprisingly massive white dwarf progenitor or a peculiar stripped envelope model could explain SN 2022oqm. A stripped envelope explosion inside a dense, hydrogen- and helium-poor CSM, akin to SNe Icn, but with a large 56Ni mass and small CSM mass could explain SN 2022oqm. Alternatively, helium detonation on an unexpectedly massive white dwarf could also explain SN 2022oqm.

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arXiv2023

Initial 56Ni Masses in Type Ia Supernovae

We infer initial masses of the synthesized radioactive nickel-56 in a sample of recent Type Ia supernovae applying a new formalism introduced recently by Khatami & Kasen (2019). It is shown that the nickel masses we derive do not differ significantly from previous estimates based on the traditional Arnett-model. We derive the $β$ parameter for our sample SNe and show that these are consistent with the fiducial value of $\sim 1.6$ given by Khatami & Kasen (2019) from SN Ia hydrodynamical simulations.

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arXiv2023

Type W and Type 15bn subgroups of hydrogen-poor superluminous supernovae: pre-maximum diversity, post-maximum homogeneity?

In this study, we analyze the post-maximum spectra of a sample of 27 Type I superluminous supernovae (SLSNe-I) in order to search for physical differences between the so-called Type W and Type 15bn sub-types. This paper is a continuation of \citet{ktr21} and \citet{ktr22}. In the former, it was revealed that not all SLSNe-I show the W-shaped absorption feature between 4000 and 5000 Å in the pre-maximum spectra, and two new SLSN-subgroups were disclosed: Type W, where the W-shaped feature is present, and Type 15bn, where it is missing. In the latter, it was shown that the pre-maximum photosphere of Type W SLSNe-I tend to be hotter compared to Type 15bn objects, and they are different regarding their ion composition, their early light curves and their geometry as well. For completeness, post-maximum data are analyzed in this paper. It is concluded that in terms of photospheric temperature and velocity, Type W and Type 15bn SLSNe decrease to a similar value by the post-maximum phases, and their pseudo-nebular spectra are nearly uniform. Pseudo-equivalent width calculations show that the pEW of the wavelength range between 4166 and 5266 Å evolve differently in case of the two sub-types, while the other parts of the spectra seem to evolve similarly. It was found that the host galaxies of the studied objects do not differ significantly in their star formation rate, morphology, stellar mass and absolute brightness. The main difference behind the bimodality of Type W and Type 15bn SLSNe-I therefore is in their pre-maximum evolution.

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arXiv2023

Optical polarization and spectral properties of the H-poor superluminous supernovae SN 2021bnw and SN 2021fpl

New optical photometric, spectrocopic and imaging polarimetry data are combined with publicly available data to study some of the physical properties of the two H-poor superluminous supernovae (SLSN) SN 2021bnw and SN 2021fpl. For each SLSN, the best-fit parameters obtained from the magnetar model with \texttt{MOSFiT} do not depart from the range of parameter obtained on other SLSNe discussed in the literature. A spectral analysis with \texttt{SYN++} shows that SN 2021bnw is a W Type, Fast evolver, while SN 2021fpl is a 15bn Type, Slow evolver. The analysis of the polarimetry data obtained on SN 2021fpl at four epochs (+1.8, +20.6, +34.1 and +43.0 days, rest-frame) shows $> 3σ$ polarization detections in the range 0.8--1 $\%$. A comparison of the spectroscopy data suggests that SN 2021fpl underwent a spectral transition a bit earlier than SN 2015bn, during which, similarly, it could have underwent a polarization transition. The analysis of the polarimetry data obtained on SN 2021bnw do not show any departure from symmetry of the photosphere at an empirical diffusion timescale of $\approx$ 2 (+81.1 days rest-frame). This result is consistent with those on the sample of W Type SLSN observed at empirical diffusion timescale $\le$ 1 with that technique, even though it is not clear the effect of limited spectral windows varying from one object to the other. Measurements at higher empirical diffusion timescale may be needed to see any departure from symmetry as it is discussed in the literature for SN 2017egm.

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