Observations Of X-ray Flares Research Articles

Intermittent heating in a model of solar coronal loops

X-ray and EUV observations of the solar corona reveal a very complex and dynamic environment where there are many examples of structures that are believed to outline the Sun's magnetic field. In this present study, the authors investigate the temporal response of the temperature, density and pressure of a solar coronal plasma contained within a magnetic loop to an intermittent heating source generated by Ohmic dissipation. The energy input is produced by a one-dimensional MHD flare model. This model is able to reproduce some of the statistical properties derived from X-ray flare observations. In particular the heat deposition consists of both a sub-flaring background and much larger, singular dissipative events. Two different heating profiles are investigated: (a) the spatial average of the square of the current along the loop and (b) the maximum of the square of the current along the loop. For case (a), the plasma parameters appear to respond more to the global variations in the heat deposition about its average value rather than to each specific event. For case (b), the plasma quantities are more intermittent in their evolution. In both cases the density response is the least bursty signal. It is found that the time-dependent energy input can maintain the plasma at typical coronal temperatures. Implications of these results upon the latest coronal observations are discussed.

X‐Ray Spectroscopy of Rapidly Rotating, Late‐Type Dwarf Stars

We present an X-ray spectroscopic analysis of two active dwarf stars with extremely short rotation periods, HD 197890 (Speedy Mic) and Gliese 890, using X-ray data acquired with the ASCA observatory. We analyze the X-ray spectrum of Speedy Mic in two separate time intervals, one corresponding to an apparent stellar flare, the other to quiescence or the nonflaring state. We also present a reanalysis of the ASCA spectrum of the M dwarf stellar binary YY Gem, during both quiescent and flaring states. We use the recently updated MEKAL plasma code to model these spectra and find that a minimum of two temperature components are required to obtain good fits for all three stars. The inferred emission measures of the hot components in YY Gem and Speedy Mic, and the temperature of the hot component in the case of YY Gem, increase significantly during the observed X-ray flares, in agreement with previous studies of similar stellar X-ray flares. In addition, metal abundances that are low compared to the solar photospheric values are preferred (but not required) for the quiescent coronae of Gliese 890 and Speedy Mic, while for the quiescent corona of YY Gem, subsolar metal abundances are required and solar abundance models can be ruled out with high confidence. We combine the results of this study with those of previous ASCA and ROSAT analyses of late-type dwarf stars with known rotation rates to obtain a sample of 17 G, K, and M dwarfs. We find a strong correlation of the X-ray to bolometric luminosity ratio, L X /L bol , both with the rotation period and the Rossby number (R o ), in agreement with previous studies. Using a homogeneous subset of 10 dwarfs for which we have purely ASCA-derived temperatures and elemental abundances, we find that the temperature of the hottest component present in their coronae is correlated with this luminosity ratio, as expected. Finally, we note that the Fe abundances for all active dwarf stars having normalized X-ray luminosities log (L X /L bol ) > −3.7 are significantly subsolar, whereas the Fe abundances in the less active stars are within a factor of 2 of the solar value.

Stereoscopic Observations of Solar Hard X‐Ray Flares Made byUlyssesandYohkoh

The Solar X-Ray/Cosmic Gamma-Ray Burst Experiment aboard the interplanetary spacecraft Ulysses has provided extensive observations of solar hard X-ray flares from a variety of angles with respect to the Sun-Earth line. During the period 1991 October-1993 June, Ulysses observed 13 flares that were also observed by the X-ray instruments aboard the Japanese satellite Yohkoh located near Earth. At least 12 flares were in full view of both the spacecraft. Eight flares, for which hard X-ray spectra were available, are examined to determine the directivity of the 20-125 keV hard X-ray sources in solar flares. They include one flare for which the view angles of Ulysses and Yohkoh were 80° and 25°, respectively. No evidence of systematic directivity was found within the uncertainty (a factor of ~2) of these measurements. These and other observations of directivity at higher energies are consistent with a nearly isotropic distribution of energetic electrons in most solar flares.

Interpreting Yohkoh hard and soft X-ray flare observations

A simple model is presented to account for theYohkoh flare observations of Feldmanet al. (1994), and Masuda (1994). Electrons accelerated by the flare are assumed to encounter the dense, small regions observed by Feldmanet al. at the tops of impulsively flaring coronal magnetic loops. The values of electron density and volume inferred by Feldmanet al. imply that these dense regions present an intermediate thick-thin target to the energised electrons. Specifically, they present a thick (thin) target to electrons with energy much less (greater) thanE c , where 15 keV <E c < 40 keV. The electrons are either stopped at the loop top or precipitate down the field lines of the loop to the footpoints. Collisional losses of the electrons at the loop top produce the heating observed by Feldmanet al. and also some hard X-rays. It is argued that this is the mechanism for the loop-top hard X-ray sources observed in limb flares by Masuda. Adopting a simple model for the energy losses of electrons traversing the dense region and the ambient loop plasma, hard X-ray spectra are derived for the loop-top source, the footpoint sources and the region between the loop top and footpoints. These spectra are compared with the observations of Masuda. The model spectra are found to qualitatively agree with the data, and in particular account for the observed steepening of the loop-top and footpoint spectra between 14 and 53 keV and the relative brightnesses of the loop-top and footpoint sources.

Flare Activity in Algol-type Binaries

Two sites of flare activity have been identified in Algol-type binaiies. One is associated with the shock region from the accretion on B-A type primary, another, predominant one is apparently connected with coronal mass ejections (CME) from the late-type, Roche lobe filling secondary. The relative contribution from both sources of activity is still to be found. According to Peters &amp; Polidan (1984), a moderately hot (105 K) and low density ne ≃ 109 cm−3 high turbulent accretion region (HTAR) surrounding the hot component has been discovered. Typical dimensions of HTAR agree with the scale height of the X-ray source HX ≃ 1.2 · 1011 · T7 cm (Harnden et al 1977), where T7 is the temperature in units of 107 K. But the decay time of the HTAR, comparable to the orbital period, more favourably agrees with the idea of CME as deduced from Ginga observations of X-ray flares on Algol (Stern et al. 1992). One solution of the controversy as to the contribution from HTAR would be to concentrate on X-ray observations of systems like u Her and U CrB, where the secondary is possibly earlier than FO (thus, is not expected to possess an extensive convection zone) and on the other hand on systems like U Cep, where during periods of high activity Ṁ ≃ 10−6M⊙ per year. Thus, the accretion luminosity must be at least by two orders of magnitude higher than in Algol. At present the analysis of X-ray flares is based upon the solar type CME assuming different scenarios of radiative and/or conductive cooling (see Stern et al. 1992). However, the current models of CME do not take into account the binary nature of the flaring object.

Electron acceleration and heating in solar flares: Interpretation of H? signatures

Vector magnetogram, Hα, and hard X-ray observations of flares are reviewed which show that nonthermal electron signatures in Hα are never cospatial with regions of maximum current density for the small number of flares analyzed, but lie to the sides of these regions. By considering electron acceleration and transport requirements, four conditions are found that must be fulfilled to observe nonthermal electron signatures in Hα: (1) The plasma betaβ ≤ 0.3 in the acceleration region. (2) The energy flux of electrons above 20 keV is greater than 1010 erg cm−2 s−1. (3) The column densityN ≤ 1020 cm−2 between the electron source and the chromosphere. (4) The coronal pressure in the flux tube connecting to the Hα layerp ≤ 100 dyne cm−2. Condition 2 can be most easily met in the initial stages of flares. In contrast, the only condition for a high-pressure Hα signature isp ≥ 1000 dyne cm−2, which is most easily met in a region of maximum current density or heating and far enough into the flare for significant heating to have occurred. Thus, high-pressure signatures should be expected to occur more frequently than nonthermal electron signatures and to occur generally later in time.

GINGA observations of X-ray flares on Algol

The Ginga X-ray satellite observed Algol (Beta Per) for 2 days in 1989 January, including both the primary optical eclipse and most of the secondary eclipse. We derive upper limits of about 20 and 10 percent, respectively, for the eclipsed flux fraction during the two eclipses. A large flare lasting over 12 hr was seen prior to and during secondary eclipse. High-temperature Fe line emission is clearly detected in the proportional counter data. The Fe line equivalent width is variable during the flare, ranging from 0.4-1.0 keV. Except for two intervals during the flare rise, the observed equivalent width is lower than predicted using solar abundances and an optically thin plasma model. Similar behavior has also been observed by Ginga in a large flare on UX Ari: in both events, opacity effects at line center may be playing a significant role. Loop model analysis of the large flare suggests that it involves a substantially longer loop or loops than a shorter duration Algol flare seen with Exosat.

X-ray Spectral Diagnostics for Coronal Loops in the Active K Dwarf AB Doradus

AbstractWe discuss theoretical X-ray spectra for coronal loop models in the rapidly rotating young K dwarf AB Doradus (HD 36705), as a typical representative of active X-ray bright stars. The loop models are based on EXOSAT and IUE observations, and further motivated by a possible connection between the observed X-ray flares and co-rotating clouds of neutral hydrogen (a few/day). The resulting synthetic spectra between 0.5 – 7 keV can be approximated by a linear combination of three distinct temperature components. Two components are sufficient between 0.5 – 2.5 keV. Below 0.1 keV the loop spectra deviate significantly from the few component fits. To test some basic assumptions (dynamic vs. static, constant vs. variable cross-sectional area), useful constraints on the DEM(T)-distribution could be obtained with the grating-spectrometer of XMM with 103 – 104sec exposure times. The ratio of the He-type (O VII) forbidden and intercombination lines at 0.56 keV will provide sufficient density diagnostics, to distinguish e.g.between compact and large loops. The crystal-spectrometers of XMM and XSPECT could achieve the same but with longer (105 sec) observing time. The strong Ly α line of O VIII at 0.65 keV can be observed with the crystals in 104 sec, and used even for rotational modulation and flare studies, and giving additional information about flows in flaring loops. At the Iron 6.7 keV lines, where the gratings do not work, the crystals should be used together with low resolution devices, to set constraints on the hottest gas at loop summits. Our discussion applies also to several brighter cool stars but with shorter observing time (like Capella and HR 1099, which are over 3 times X-ray brighter than AB Dor).

X-Ray Emission from Stellar Flares: Exosat Results

AbstractWe present an overview of recent observations of stellar X-ray flares obtained with the EXOSAT Observatory. We discuss a few examples of flares from M dwarf flare stars, from RS CVn and Algol-type binaries, from single late-type stars (including a G0 dwarf and an A-type visual binary), and from pre-main-sequence objects. We also draw some general conclusions from the pieliminary analysis of the EXOSAT data sample.

X-ray observations of solar flares with the Einstein Observatory

The first Einstein Observatory Imaging Proportional Counter (IPC) observations of solar flares are presented. These flares were detected in scattered X-ray light when the X-ray telescope was pointed at the sunlit earth. The propagation and scattering of solar X-rays in the earth's atmosphere are discussed in order to be able to deduce the solar X-ray flux incident on top of the atmosphere from scattered X-ray intensity measurements. After this correction, the scattered X-ray data are interpreted as full-disk observations of the sun obtained with the same instrumentation used for observations of flares on other stars. Employing the same data analysis and interpretation techniques, extremely good agreement is found between the physical flare parameters deduced from IPC observations and 'known' properties of compact loop flares. This agreement demonstrates that flare observations with the IPC can reveal physical parameters such as temperature and density quite accurately in the solar case and therefore suggests that the interpretations of stellar X-ray flare observations are on a physically sound basis.

X-ray flaring activity of flare stars and RS CVn binaries

In recent times, X-ray flares have been observed in several flare stars and RS CVn binaries. From an analysis of the X-ray flare observations we find a correlation between the average energy emitted in the X-ray band in flares and the bolometric luminosity of the stars. This relation is similar to the relation obtained in the optical band specifically for the flare stars and hence we suggest a similarity in the flaring nature of the flare stars and the RS CVn binaries. Further, a correlation is also found between the X-ray flare decay time and the peak X-ray luminosity for the flare stars and the RS CVn binaries. We show that this relation can be explained by the solar flare loop models.

X-Ray Observations Of Stellar Flares

ABSTRACTThe history of stellar X-ray flare observations prior to EINSTEIN is reviewed. X-ray light curves as measured by the IPC are then presented for all time resolved flare events discovered as of July 1982 in the EINSTEIN data set. These light curves are analyzed in terms of solar-like loop models to derive densities, temperatures, loop lengths, magnetic field strength lower limits, etc. The failure of the model to adequately represent the observations in the case of the YZ CMi flares is discussed. The relationship of X-ray to optical emission and X-ray to UV emission is considered from both an observational and a theoretical viewpoint. It is concluded that the characterization of a flare by a single, time averaged ratio, Lx /Lopt , is not physically significant.

Solar abundances from X-ray flare observations

X-ray line and continuum observations are used to derive the absolute coronal abundances of Si, S, Ar, and Ca from lines of hydrogenor helium-like ions. For Si (7.7+0.2, -0.3) and Ca (6.5+0.1, -0.3) the measured coronal values are in good agreement with the photospheric values, but the value for S (6.9+0.1, -0.3) is approximately half of the photospheric value. The abundance of Ar (6.4+0.2, -0.3) also appears to be lower in the corona by a factor of 2-3 than would be expected. It is suggested that these differences are due to ambipolar and magnetic diffusion effects.

Does the emission measure decrease during the start of a soft X-ray flare?

Soft X-ray flare observations, interpreted as the emission from a single temperature plasma, frequently indicate a significant decrease in the inferred emission measure. It is shown that this effect results naturally from the isothermal assumption, and is eliminated when the preflare contribution to the total emission is removed.

Extreme-ultraviolet observations of a flare on Proxima Centauri and implications concerning flare-star scaling theory

We report the first detection of EUV radiation from a UV Ceti-type flare star, Proxima Centauri, and derive an emission measure of 4.6 x 10/sup 52/ cm3/sup . ThisimpliesatotalthermalXhyphen/(1-250 A) of L/sub x/=7-18 x 10/sup 29/ ergs s/sup -1/. Using past X-ray flare observations of YZ CMi and UV Ceti and a recombination model, we derive limits on L/sub x//L/sub opt/ to test flare-star scaling theories. We conclude that L/sub x//L/sub opt/ in the observed flare is at variance with Mullan's scaling law theory and may be highly variable among stellar flares.

Periodic Heating Mechanism in Solar Flares

Observations of periodic X-ray flares are described. The flare origin is suggested to be energy transfer from oscillating layers low in the chromosphere.