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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

THE KEPLER CATALOG OF STELLAR FLARES

A homogeneous search for stellar flares has been performed using every available Kepler light curve. An iterative light curve de-trending approach was used to filter out both astrophysical and systematic variability to detect flares. The flare recovery completeness has also been computed throughout each light curve using artificial flare injection tests, and the tools for this work have been made publicly available. The final sample contains 851,168 candidate flare events recovered above the 68% completeness threshold, which were detected from 4041 stars, or 1.9% of the stars in the Kepler database. The average flare energy detected is ∼10{sup 35} erg. The net fraction of flare stars increases with g − i color, or decreasing stellar mass. For stars in this sample with previously measured rotation periods, the total relative flare luminosity is compared to the Rossby number. A tentative detection of flare activity saturation for low-mass stars with rapid rotation below a Rossby number of ∼0.03 is found. A power-law decay in flare activity with Rossby number is found with a slope of −1, shallower than typical measurements for X-ray activity decay with Rossby number.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

QUASI-PERIODIC PULSATIONS IN SOLAR AND STELLAR FLARES: RE-EVALUATING THEIR NATURE IN THE CONTEXT OF POWER-LAW FLARE FOURIER SPECTRA

The nature of quasi-periodic pulsations (QPPs) in solar and stellar flares remains debated. Recent work has shown that power-law-like Fourier power spectra are an intrinsic property of solar and stellar flare signals, a property that many previous studies of this phenomenon have not accounted for. Hence a re-evaluation of the existing interpretations and assumptions regarding QPPs is needed. We adopt a Bayesian method for investigating this phenomenon, fully considering the Fourier power-law properties of flare signals. Using data from the PROBA2/Large Yield Radiometer, Fermi/Gamma-ray Burst Monitor, Nobeyama Radioheliograph, and Yohkoh/HXT instruments, we study a selection of flares from the literature identified as QPP events. Additionally, we examine optical data from a recent stellar flare that appears to exhibit oscillatory properties. We find that, for all but one event tested, an explicit oscillation is not required to explain the observations. Instead, the flare signals are adequately described as a manifestation of a power law in the Fourier power spectrum. However, for the flare of 1998 May 8, strong evidence for an explicit oscillation with P ≈ 14-16 s is found in the 17 GHz radio data and the 13-23 keV Yohkoh/HXT data. We conclude that, most likely, many previously analyzed events in the literature may be similarly described by power laws in the flare Fourier power spectrum, without invoking a narrowband, oscillatory component. Hence the prevalence of oscillatory signatures in solar and stellar flares may be less than previously believed. The physical mechanism behind the appearance of the observed power laws is discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

First Temperature Profile of a Stellar Flare Using Differential Chromatic Refraction

We present the first derivation of a stellar flare temperature profile from single-band photometry. Stellar flare DWF 030225.574−545707.45129 was detected in 2015 by the Dark Energy Camera as part of the Deeper, Wider, Faster program. The brightness (Δm g = −6.12) of this flare, combined with the high air mass (1.45 ≲ X ≲ 1.75) and blue filter (DES g, 398–548 nm) in which it was observed, provided ideal conditions to measure the zenithward apparent motion of the source due to differential chromatic refraction (DCR) and, from that, infer the effective temperature of the event. We model the flare’s spectral energy distribution as a blackbody to produce the constraints on flare temperature and geometric properties derived from single-band photometry. We additionally demonstrate how simplistic assumptions on the flaring spectrum, as well as on the evolution of flare geometry, can result in solutions that overestimate the effective temperature. Exploiting DCR enables studying chromatic phenomena with ground-based astrophysical surveys, and stellar flares on M dwarfs are a particularly enticing target for such studies due to their ubiquity across the sky and the heightened color contrast between their red quiescent photospheres and the blue flare emission. Our novel method will enable similar temperature constraints for a large sample of objects in upcoming photometric surveys like the Vera C. Rubin Legacy Survey of Space and Time.

Clarke, Riley W. [University of Delaware, Newark, ↗
DOE OSTI2025

XRISM view of a stellar flare: High-resolution Fe K spectra of HR 1099, an RS CVn-type star

A high-resolution X-ray spectroscopic observation was made of the RS CVn-type binary star HR 1099 using the Resolve instrument onboard XRISM for its calibration purposes. During the $\sim$400 ks telescope time covering 1.5 binary orbit, a flare lasting for $\sim$100 ks was observed with a released X-ray radiation energy of ${\sim }10^{34}$ erg, making it the first stellar flare ever observed with an X-ray microcalorimeter spectrometer. The flare peak count rate is 6.4 times higher than that in quiescence and is distinguished clearly in time thanks to the long telescope time. Many emission lines were detected in the 1.7–10 keV range both in the flare and quiescent phases. Using the high spectral resolution of Resolve in the Fe K band (6.5–7.0 keV), we resolved the inner-shell lines of Fe XIX-XXVI as well as the outer-shell lines of Fe XXV-XXVI . These lines have peaks in the contribution functions at different temperatures over a wide range, allowing us to construct the differential emission measure (DEM) distribution over the electron temperature of 1–10 keV (roughly 10–100 MK) based only on Fe lines, thus without an assumption of the elemental abundance. The reconstructed DEM has a bimodal distribution, and only the hotter component increased during the flare. The elemental abundance was derived based on the DEM distribution thus constructed. A significant abundance increase was observed during the flare for Ca and Fe, which are some of the elements with the lowest first ionization potential among those analyzed, but not for Si, S, and Ar. This behavior is seen in some giant solar flares and the present result is a clear example in stellar flares.

Astronomy and AstroPhysics↗
DOE OSTI2012

THE DISCOVERY OF AN X-RAY/UV STELLAR FLARE FROM THE LATE-K/EARLY-M DWARF LMC 335

We report the discovery of an X-ray/UV stellar flare from the source LMC 335, captured by XMM-Newton in the field of the Large Magellanic Cloud. The flare event was recorded continuously in X-ray for its first 10 hr from the precursor to the late decay phases. The observed fluxes increased by more than two orders of magnitude at its peak in X-ray and at least one in the UV as compared to quiescence. The peak 0.1-7.0 keV X-ray flux is derived from the two-temperature APEC model to be {approx}(8.4 {+-} 0.6) Multiplication-Sign 10{sup -12} erg cm{sup -2} s{sup -1}. Combining astrometric information from multiple X-ray observations in the quiescent and flare states, we identify the NIR counterpart of LMC 335 as the Two Micron All Sky Survey source J05414534-6921512. The NIR color relations and spectroscopic parallax characterize the source as a Galactic K7-M4 dwarf at a foreground distance of (100-264) pc, implying a total energy output of the entire event of {approx}(0.4-2.9) Multiplication-Sign 10{sup 35} erg. This report comprises detailed analyses of this late-K/early-M dwarf flare event that has the longest time coverage yet reported in the literature. The flare decay can be modeled with two exponential components with timescales of {approx}28 minutes and {approx}4 hr, with a single-component decay firmly ruled out. The X-ray spectra during flare can be described by two components, a dominant high-temperature component of {approx}40-60 MK and a low-temperature component of {approx}10 MK, with a flare loop length of about 1.1-1.3 stellar radius.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

COMPARISON OF PHOTOMETRIC VARIABILITY BEFORE AND AFTER STELLAR FLARES

The energy in the solar acoustic spectrum is known to be correlated with flares, but it is not known if the same is true for stellar flares. In order to answer this question, we have analyzed 73 flares in 39 solar-like stars. These flares were identified in the 854 solar-like stars observed by the Kepler spacecraft that have stellar parameters measured with asteroseismology. Though we were not able to identify a statistically significant enhancement of the energy in the high-frequency part of the post-flare acoustic spectra compared to the pre-flare spectra of these stars, we did identify a larger variability between the energy in the high-frequency part of the post- and pre-flare acoustic spectra compared to spectra taken at random times.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

The Deeper, Wider, Faster programme: exploring stellar flare activity with deep, fast cadenced DECam imaging via machine learning

ABSTRACT We present our 500 pc distance-limited study of stellar flares using the Dark Energy Camera as part of the Deeper, Wider, Faster programme. The data were collected via continuous 20-s cadence g-band imaging and we identify 19 914 sources with precise distances from Gaia DR2 within 12, ∼3 deg2, fields over a range of Galactic latitudes. An average of ∼74 min is spent on each field per visit. All light curves were accessed through a novel unsupervised machine learning techniques designed for anomaly detection. We identify 96 flare events occurring across 80 stars, the majority of which are M dwarfs. Integrated flare energies range from ∼1031–1037 erg, with a proportional relationship existing between increased flare energy with increased distance from the Galactic plane, representative of stellar age leading to declining yet more energetic flare events. In agreement with previous studies we observe an increase in flaring fraction from M0 to M6 spectral types. Furthermore, we find a decrease in the flaring fraction of stars as vertical distance from the galactic plane is increased, with a steep decline present around ∼100 pc. We find that $\sim 70{{\ \rm per\ cent}}$ of identified flares occur on short time-scales of <8 min. Finally, we present our associated flare rates, finding a volumetric rate of 2.9 ± 0.3 × 10−6 flares pc−3 h−1.

Webb, S.↗
DOE OSTI2019

Identification of Stellar Flares Using Differential Evolution Template Optimization

In this paper, we explore methods for the identification of stellar flare events in irregularly sampled data of ground-based time domain surveys. In particular, we describe a new technique for identifying flaring stars, which we have implemented in a publicly available Python module called "PyVAN." The approach uses the Differential Evolution algorithm to optimize parameters of empirically derived light curve templates for different types of stars to fit a candidate light curve. The difference of the likelihoods that these best-fit templates produced the observed data is then used to delineate targets that are well-explained by a flare template but simultaneously poorly explained by templates of common contaminants. By testing on light curves of known identity and morphology, we show that our technique is capable of recovering flaring status in 69% of all light curves containing a flare event above thresholds drawn to include <1% of any contaminant population. By applying to Palomar Transient Factory data, we show consistency with prior samples of flaring stars, and identify a small selection of candidate flaring G-type stars for possible follow-up.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Stellar flares from blended and neighbouring stars in Kepler short cadence observations

ABSTRACT We present the results of a search for stellar flares from stars neighbouring the target sources in the Kepler short cadence data. These flares have been discarded as contaminants in previous surveys and therefore provide an unexplored resource of flare events, in particular high-energy events from faint stars. We have measured M dwarf flare energies up to 1.5 × 1035 erg, pushing the limit for flare energies measured using Kepler data. We have used our sample to study the flaring activity of wide binaries, finding that the lower mass counterpart in a wide binary flares more often at a given energy. Of the 4430 flares detected in our original search, 298 came from a neighbouring star, a rate of 6.7 ± 0.4 per cent for the Kepler short cadence light curves. We have used our sample to estimate a 5.8 ± 0.1 per cent rate of false positive flare events in studies using Transiting Exoplanet Survey Satellite short cadence data.

Jackman, James A. G.↗
DOE OSTI2011

A SURVEY OF HIGH-CONTRAST STELLAR FLARES OBSERVED BY CHANDRA

The X-ray light curves of pre-main-sequence (PMS) stars can show variability in the form of flares altering a baseline characteristic activity level; the largest X-ray flares are characterized by a rapid rise to more than 10 times the characteristic count rate, followed by a slower quasi-exponential decay. Analysis of these high-contrast X-ray flares enables the study of the innermost magnetic fields of PMS stars. We have scanned the ANCHORS database of Chandra observations of star-forming regions to extend the study of flare events on PMS stars both in sky coverage and in volume. We developed a sample of 30 high-contrast flares out of the 14,000 stars of various ages and masses available in ANCHORS at the start of our study. Applying methods of time-resolved spectral analysis, we obtain the temperatures, confining magnetic field strengths, and loop lengths of these bright, energetic flares. The results of the flare analysis are compared to the Two Micron All Sky Survey and Spitzer data available for the stars in our sample. We find that the longest flare loop lengths (of order several stellar radii) are only seen on stars whose IR data indicates the presence of disks. This suggests that the longest flares may stretch all the way to the disk. Such long flares tend to be more tenuous than the other large flares studied. A wide range of loop lengths are observed, indicating that different types of flares may occur on disked young stellar objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

INFLUENCE OF STELLAR FLARES ON THE CHEMICAL COMPOSITION OF EXOPLANETS AND SPECTRA

More than three thousand exoplanets have been detected so far, and more and more spectroscopic observations of exoplanets are performed. Future instruments ( James Webb Space Telescope ( JWST ), E-ELT, PLATO, Ariel, etc.,) are eagerly awaited, as they will be able to provide spectroscopic data with greater accuracy and sensitivity than what is currently available. This will allow more accurate conclusions to be drawn regarding the chemistry and dynamics of exoplanetary atmospheres, provided that the observational data are carefully processed. One important aspect to consider is temporal stellar atmospheric disturbances that can influence the planetary composition, and hence spectra, and potentially can lead to incorrect assumptions about the steady-state atmospheric composition of the planet. In this paper, we focus on perturbations coming from the host star in the form of flare events that significantly increase photon flux impingement on the exoplanets atmosphere. In some cases, particularly for M stars, this sudden increase may last for several hours. We aim to discover to what extent a stellar flare is able to modify the chemical composition of the planetary atmosphere and, therefore, influence the resulting spectra. We use a one-dimensional thermo-photochemical model to study the neutral atmospheric composition of two hypothetical planets located around the star AD Leo. We place the two planets at different distances from the star, which results in effective atmospheric temperatures of 412 and 1303 K. AD Leo is an active star that has already been observed during a flare. Therefore, we use the spectroscopic data from this flare event to simulate the evolution of the chemical composition of the atmospheres of the two hypothetical planets. We compute synthetic spectra to evaluate the implications for observations. The increase in the incoming photon flux affects the chemical abundances of some important species (such as H and NH{sub 3}), down to altitudes associated with an atmospheric pressure of 1 bar, which can lead to variations in planetary spectra (up to 150 ppm) if performed during transit. We find that each exoplanet has a post-flare steady-state composition that is significantly different from the pre-flare steady-state. We predict that these variations could be detectable with both current and future spectroscopic instruments, if sufficiently high signal-to-noise spectra are obtained.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

PRODUCTION OF {sup 9}Be THROUGH THE {alpha}-FUSION REACTION OF METAL-POOR COSMIC RAYS AND STELLAR FLARES

Spectroscopic observations of metal-poor stars have indicated possible {sup 6}Li abundances that are much larger than the primordial abundance predicted in the standard big bang nucleosynthesis model. Possible mechanisms of {sup 6}Li production in metal-poor stars include pregalactic and cosmological cosmic-ray (CR) nucleosynthesis and nucleosynthesis by flare-accelerated nuclides. We study {sup 9}Be production via two-step {alpha}-fusion reactions of CR or flare-accelerated {sup 3,4}He through {sup 6}He and {sup 6,7}Li, in pregalactic structure, intergalactic medium, and stellar surfaces. We solve transfer equations of CR or flare particles and calculate nuclear yields of {sup 6}He, {sup 6,7}Li, and {sup 9}Be taking account of probabilities of processing {sup 6}He and {sup 6,7}Li into {sup 9}Be via fusions with {alpha} particles. Yield ratios, i.e., {sup 9}Be/{sup 6}Li, are then calculated for the CR and flare nucleosynthesis models. We suggest that the future observations of {sup 9}Be in metal-poor stars may find enhanced abundances originating from metal-poor CR or flare activities.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

The broadening of the hydrogen lines during flares is thought to result from increased charge (electron, proton) density in the flare chromosphere. However, disagreements between theory and modeling prescriptions have precluded an accurate diagnostic of the degree of ionization and compression resulting from flare heating in the chromosphere. To resolve this issue, we have incorporated the unified theory of electric pressure broadening of the hydrogen lines into the non-LTE radiative-transfer code RH. This broadening prescription produces a much more realistic spectrum of the quiescent, A0 star Vega compared to the analytic approximations used as a damping parameter in the Voigt profiles. We test recent radiative-hydrodynamic (RHD) simulations of the atmospheric response to high nonthermal electron beam fluxes with the new broadening prescription and find that the Balmer lines are overbroadened at the densest times in the simulations. Adding many simultaneously heated and cooling model loops as a “multithread” model improves the agreement with the observations. We revisit the three-component phenomenological flare model of the YZ CMi Megaflare using recent and new RHD models. The evolution of the broadening, line flux ratios, and continuum flux ratios are well-reproduced by a multithread model with high-flux nonthermal electron beam heating, an extended decay phase model, and a “hot spot” atmosphere heated by an ultrarelativistic electron beam with reasonable filling factors: ∼0.1%, 1%, and 0.1% of the visible stellar hemisphere, respectively. The new modeling motivates future work to understand the origin of the extended gradual phase emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

MULTI-WAVELENGTH CHARACTERIZATION OF STELLAR FLARES ON LOW-MASS STARS USING SDSS AND 2MASS TIME-DOMAIN SURVEYS

We present the first rates of flares from M dwarf stars in both red optical and near-infrared (NIR) filters. We have studied {approx}50,000 M dwarfs from the Sloan Digital Sky Survey (SDSS) Stripe 82 area and 1321 M dwarfs from the Two Micron All Sky Survey (2MASS) Calibration Scan Point Source Working Database that overlap SDSS imaging fields. We assign photometric spectral types from M0 to M6 using (r - i) and (i - z) colors for every star in our sample. Stripe 82 stars each have 50-100 epochs of data, while 2MASS Calibration stars have {approx}1900 epochs. From these data we estimate the observed rates and theoretical detection thresholds for flares in eight photometric bands as a function of spectral type. Optical flare rates are found to be in agreement with previous studies, while the frequency per hour of NIR flare detections is found to be more than two orders of magnitude lower. An excess of small-amplitude flux increases in all bands exhibits a power-law distribution, which we interpret as the result of flares below our detection thresholds. In order to investigate the recovery efficiency for flares in each filter, we extend a two-component flare model into the NIR. Quiescent M0-M6 spectral templates were used with the model to predict the photometric response of flares from u to K{sub s} . We determine that red optical filters are sensitive to flares with u-band amplitudes {approx}>2 mag, and NIR filters to flares with {Delta}u {approx}> 4.5 mag. Our model predicts that M0 stars have the best color contrast for J-band detections, but M4-M6 stars should yield the highest rate of NIR flares with amplitudes of {Delta}J {>=} 0.01 mag. Characterizing flare rates and photometric variations at longer wavelengths is important for predicting the signatures of M dwarf variability in next-generation surveys, and we discuss their impact on surveys such as the Large Synoptic Survey Telescope.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A UNIFIED COMPUTATIONAL MODEL FOR SOLAR AND STELLAR FLARES

We present a unified computational framework that can be used to describe impulsive flares on the Sun and on dMe stars. The models assume that the flare impulsive phase is caused by a beam of charged particles that is accelerated in the corona and propagates downward depositing energy and momentum along the way. This rapidly heats the lower stellar atmosphere causing it to explosively expand and dramatically brighten. Our models consist of flux tubes that extend from the sub-photosphere into the corona. We simulate how flare-accelerated charged particles propagate down one-dimensional flux tubes and heat the stellar atmosphere using the Fokker–Planck kinetic theory. Detailed radiative transfer is included so that model predictions can be directly compared with observations. The flux of flare-accelerated particles drives return currents which additionally heat the stellar atmosphere. These effects are also included in our models. We examine the impact of the flare-accelerated particle beams on model solar and dMe stellar atmospheres and perform parameter studies varying the injected particle energy spectra. We find the atmospheric response is strongly dependent on the accelerated particle cutoff energy and spectral index.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

EVIDENCE OF MULTIPLE SLOW ACOUSTIC OSCILLATIONS IN THE STELLAR FLARING LOOPS OF PROXIMA CENTAURI

We present the first observational evidence of multiple slow acoustic oscillations in the post-flaring loops of the corona of Proxima Centauri using XMM-Newton observations. We find the signature of periodic oscillations localized in the decay phase of the flare in its soft (0.3-10.0 keV) X-ray emissions. Using the standard wavelet tool, we find multiple periodicities of 1261 s and 687 s. These bursty oscillations persist for durations of 90 minutes and 50 minutes, respectively, for more than three cycles. The intensity oscillations with a period of 1261 s may be the signature of the fundamental mode of slow magnetoacoustic waves with a phase speed of 119 km s{sup –1} in a loop of length 7.5 × 10{sup 9} cm, which is initially heated, producing the flare peak temperature of 33 MK and later cooled down in the decay phase and maintained at an average temperature of 7.2 MK. The other period of 687 s may be associated with the first overtone of slow magnetoacoustic oscillations in the flaring loop. The fundamental mode oscillations show dissipation with a damping time of 47 minutes. The period ratio P {sub 1}/P {sub 2} is found to be 1.83, indicating that such oscillations are most likely excited in longitudinal density stratified stellar loops. We estimate the density scale height of the stellar loop system as ∼23 Mm, which is smaller than the hydrostatic scale height of the hot loop system, and implies the existence of non-equilibrium conditions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Does the stellar distribution flare? A comparison of stellar scale heights with LAB H I data

The question of whether the stellar populations in the Milky Way take part in the flaring of scale heights as observed for the H I gas is a matter of debate. Standard mass models for the Milky Way assume a constant scale height for each of the different stellar distributions. However, there is mounting evidence that at least some of the stellar distributions reach, at large galactocentric distances, high altitudes, which are incompatible with a constant scale height. We discuss recent observational evidence for stellar flaring and compare it with H I data from the Leiden/Argentine/Bonn survey. Within the systemic and statistical uncertainties we find a good agreement between both.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

A macroscopic description of a generalized self-organized criticality system: Astrophysical applications

We suggest a generalized definition of self-organized criticality (SOC) systems: SOC is a critical state of a nonlinear energy dissipation system that is slowly and continuously driven toward a critical value of a system-wide instability threshold, producing scale-free, fractal-diffusive, and intermittent avalanches with power law-like size distributions. We develop here a macroscopic description of SOC systems that provides an equivalent description of the complex microscopic fine structure, in terms of fractal-diffusive transport (FD-SOC). Quantitative values for the size distributions of SOC parameters (length scales L, time scales T, waiting times Δt, fluxes F, and fluences or energies E) are derived from first principles, using the scale-free probability conjecture, N(L)dL∝L {sup –d}, for Euclidean space dimension d. We apply this model to astrophysical SOC systems, such as lunar craters, the asteroid belt, Saturn ring particles, magnetospheric substorms, radiation belt electrons, solar flares, stellar flares, pulsar glitches, soft gamma-ray repeaters, black-hole objects, blazars, and cosmic rays. The FD-SOC model predicts correctly the size distributions of 8 out of these 12 astrophysical phenomena, and indicates non-standard scaling laws and measurement biases for the others.

79 ASTRONOMY AND ASTROPHYSICS↗