arXiv subjects

Robert Falewicz

Publications and source records attributed to Robert Falewicz.

8 recordsLinked to original sources

arXiv2026

New Approach to Superflare Energy Determination

We present a new method for estimating the total energy radiated by stellar flares in broad-band continua, which assumes a constant emitting area but incorporates a time-dependent temperature evolution. This physically motivated approach offers an alternative to the commonly used method that assumes a fixed flare temperature about of 10 000\,K and variable area. By allowing the temperature to vary over time while keeping the emitting area constant, our method captures more realistic flare behaviour. This time-dependent treatment of the flare temperature is supported by numerous solar observations, numerical simulations, and multiwavelength studies of active stars. We demonstrate that using peak flare temperatures estimated from a semi-empirical model grid, rather than assuming an ad-hoc flare temperature value, improves the accuracy of total energy estimates. Although the most precise results still require a multi-band photometry/spectroscopy or independently constrained flare temperatures, our method offers a practical and scalable solution for single-band observations. It is particularly well suited for main-sequence stars of spectral types K4 and later with known effective temperatures. Finally we discuss how the flare continuum behaves under varying chromospheric conditions. Our method improves flare energy estimates by incorporating a physically relevant time-dependent temperature evolution and empirically derived peak temperatures, rather than assuming a constant 10 000\,K value. This modification reduces systematic errors that can reach factors up to ten as compared to previous estimates. We proved this on a sample of 50,320 TESS flares.

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arXiv2024

Analysis of the Stellar Occultations During the Unprecedented Long-Duration Flare

In strong stellar and solar flares flare loops typically appear during the decay phase, providing an additional contribution to the flare emission and, possibly, obscuring the flare emission. Super-flares, common in active, cool stars, persist mostly from minutes to several hours and alter the star's luminosity across the electromagnetic spectrum. Recent observations of a young main-sequence star reveal a distinctive cool loop arcade forming above the flaring region during a 27-hour superflare event, obscuring the region multiple times. Analysis of these occultations enables the estimation of the arcade's geometry and physical properties. The arcade's size expanded from 0.213 to 0.391 R$_*$ at a speed of approximately 3.5$\,$km/s. The covering structure exhibited a thickness below 12$\,$200$\,$km, with electron densities ranging from 10$^{13}$ to 10$^{14}\,$cm$^{-3}$ and temperatures below 7$\,$600$\,$K, 6$\,$400$\,$K, and 5$\,$077$\,$K for successive occultations. Additionally, the flare's maximum emission temperature has to exceed 12$\,$000$\,$K for the occultations to appear. Comparing these parameters with known values from other stars and the Sun suggests the structure's nature as an arcade of cool flare loops. For the first time, we present the physical parameters and the reconstructed geometry of the cool flare loops that obscure the flaring region during the gradual phase of a long-duration flare on a star other than the Sun.

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arXiv2023

Unveiling the spectacular over 24-hour flare of star CD-36 3202

We studied the light curve of the star CD-36 3202, observed by TESS for the presence of stellar spots and to analyze the rotationally modulated flare. We mainly wanted to model the light curve of this flare and estimate its location regarding stellar spots. The flare lasted approximately 27$\,$h. Using our tool new \texttt{findinc\_mc} we managed to estimate the inclination angle of the star to $70^\circ\pm8^\circ$. With \texttt{BASSMAN} we modeled the light curve of the CD-36 3202 and we estimated that three spots are present on the surface of this star. The mean temperature of the spots was about $4000\pm 765\,$K, and the total spottedness was on average $11.61\%\pm0.13\,$\%. We created a new tool named \texttt{MFUEA} to model rotationally modulated flares. Using this software we estimated the latitude of the flare long-duration event equal to $69^{+2}_{-1}\,$deg in latitude. Our estimation of the flare's location was the first recreation of the exact position of a flare compared with the spots. The flare is placed 12$^\circ$ from the center of the coolest spot. This makes the flare related to the magnetic processes above the active region represented by the spot. Removing the effects of rotational modulation from the flare light curve allowed us to correct the estimation of bolometric energy released during the event from $(1.15\pm 0.35)\times 10^{35}\,$erg to $(3.99\pm 1.22)\times 10^{35}\,$erg.

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arXiv2023

Analysis of Solar-like X-Class Flare on Wolf 359 Observed Simultaneously with TESS and XMM-Newton

We present an analysis of a flare on the Wolf 359 star based on simultaneous observations of TESS and XMM-Newton. A stellar flare with energy comparable to an X-class solar flare is analyzed on this star for the first time. The main goal of the study was to determine whether the same physical processes drive and occur in stellar flares as in the solar flares. We tried to estimate the flare class by various direct and indirect methods. Light curves and spectra in different energy ranges were used to determine the parameters and profiles of the flare. From the XMM-Newton EPIC-pn X-ray data, we estimated the temperature and emission measure during the flare. The thermodynamical timescale and the loop semi-length were also determined with two different methods. The RGS spectra enabled us to calculate the differential emission measure (DEM) distributions. The obtained DEM distributions have three components at temperature values of 3 MK, 7 MK, and 16-17 MK. The analysis of the line ratio in helium-like triplets allowed us to determine the plasma electron density. Our results for the flare loop on Wolf 359 were compared to typical parameters for solar flares observed with GOES and RHESSI. This supports our conclusion that the processes taking place in stellar flares are like those in solar flares. The determined geometrical parameters of the phenomenon do not differ from the values of analogs occurring on the Sun.

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arXiv2022

Statistical Analysis of Stellar Flares from the First Three Years of TESS Observations

In this paper, we study stellar light curves from the TESS satellite (Transiting Exoplanet Survey Satellite) for the presence of stellar flares. The main aim is to detect stellar flares using two-minutes cadence data and to perform statistical analysis. To find and analyze stellar flares we prepared automatic software WARPFINDER. We implemented three methods described in this paper: trend, difference, and profile fitting. Automated search for flares was accompanied by visual inspection. Using our software we analyzed two-minute cadence light curves of 330,000 stars located in the first 39 sectors of TESS observations. As a result, we detected over 25,000 stars showing flare activity with the total number of more than 140,000 flares. This means that about 7.7% of all the analyzed objects are flaring stars. The estimated flare energies range between $10^{31}$ and $10^{36}$ erg. We prepared a preliminary preview of the statistical distribution of parameters such as a flare duration, amplitudes and energy, and compared it with previous results. The relationship between stellar activity and its spectral type, temperature and mass was also statistically analyzed. Based on the scaling laws, we estimated the average values of the magnetic field strength and length of the flare loops. In our work, we used both single (about 60%), and double (about 40%) flare profiles to fit the observational data. The components of the double profile are supposed to be related to the direct heating of the photosphere by non-thermal electrons and back warming processes.

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arXiv2022

Starspots Modelling and Flare Analysis on Selected MV Stars

We studied light curves of GJ 1243, YZ CMi, and V374 Peg, observed by TESS for the presence of stellar spots and stellar flares. One of the main goals was to model light curves of spotted stars to estimate the number of spots along with their parameters using our original BASSMAN software. The modeled light curves were subtracted from the observations to increase efficiency of flare detection. Flares were detected automatically with our new dedicated software WARPFINDER. We estimated the presence of two spots on GJ 1243 with mean temperature about 2800$\,$K and spottedness varying between $3\%-4\%$ of the stellar surface and two spots on V374 Peg with a mean temperature of about 3000$\,$K and spottedness about 6$\%$ of the stellar surface. On YZ CMi we found two different models for two light curves separated in time by one and a half year. One of them is three-spot model with mean temperature of about 3000$\,$K and spottedness of star about 9$\%$ of the stellar surface. The second is a four-spot model with mean temperature about 2800$\,$K and spottedness about 7$\%$ of the stellar surface. We tested whether the flares are distributed homogeneously in phase and if there is any correlation between the presence of spots and the distribution of flares. For YZ CMi one spot is in anticorrelation with the distribution of the flares and for GJ 1243 shows non-homogeneous distribution of flares.

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arXiv2020

The depth and the vertical extent of the energy deposition layer in a medium-class solar flare

We analyze here variations of the position and the vertical extent of the energy deposition layer (EDL) inthe C1.6 GOES-class solar flare observed at 10:20 UT on 2012 September 10. The variations of the EDL arecontrasted with the variations of the spectra and emission intensities recorded in the H-alpha line with the very high time resolution using the MSDP spectrograph at Bialkow Observatory. The flare radiated hard X-rays (HXR) detectable up to a energy of 70 keV. A numerical model of the flare used in the analysis assumes that the non-thermal electrons (NTEs) carried the external energy to the flare. The NTEs energy flux was derived from a non-thermal component seen in RHESSI spectra. The main geometrical parameters of the flare were derived using restored RHESSI imaging data. We found that the variations of the X-ray fluxes recorded in various energy bands and the variations of the H-alpha intensities were well correlated in time during the pre-impulsive and impulsive phases of the flare and they agreed with the variations of the calculated position and vertical extent of the EDL. The variations of the emission noticed in various parts of the H-alpha line profile were caused by individual episodes of energy depositionby the beams of NTEs of various energy spectra on various depths in the chromospheric plasma. These results supplement our previous findings for the solar flare on 21 June2013, having nearly the same GOES-class of C1.1 but HXR emission below 34 keV only (Falewicz et al. 2017) (hereafter Paper I).

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arXiv2018

Chromospheric response during the precursor and the main phase of a B6.4 flare on August 20, 2005

Solar flare precursors depict constrained rate of energy release contrasting the imminent rapid energy release which calls for different regime of plasma processes to be at play. Due to subtle emission during the precursor phase, its diagnostics remain delusive, revealing either the non-thermal electrons (NTEs) or the thermal conduction to be the driver. In this regard, we investigate the chromospheric response during various phases of a B6.4 flare on August 20, 2005. Spatio-temporal investigation of flare ribbon enhancement during the precursor phase, carried out using spectra-images recorded in several wavelength positions on the H-alpha line profile, revealed its delayed response (180 seconds) compared to the X-ray emission, as well as sequential increment in the width of the line-profile which are indicative of a slow heating process. However, energy contained in the H-alpha emission during the precursor phase reach as high as 80% of that estimated during the main phase. Additionally, the plasma hydrodynamics during the precursor phase, as resulted from the application of a single-loop one-dimensional model, revealed the presence of power-law extension in the model generated X-ray spectra, with flux lower than the RHESSI background. Therefore, our multi-wavelength diagnostics and hydrodynamical modeling of the precursor emission indicates the role of a two-stage process. Firstly, reconnection triggered NTEs, although too small in flux to overcome the observational constraints, thermalize in the upper chromosphere. This leads to the generation of a slow conduction front which causes plasma heating during the precursor phase.

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