Peak Emission Measure Research Articles

AstroSat Investigation of X-ray Flares on Two Active K-M Systems: CC Eri and AB Dor

We present an X-ray and UV investigation of five X-ray flares detected on two active systems, CC Eri and AB Dor, using the AstroSat observatory. The peak X-ray luminosities of the flares in the 0.3–7.0 keV band are found to be within 1031−33 erg s−1. Preliminary spectral analysis indicates the presence of three and four-temperature corona for CC Eri and AB Dor, respectively, where the highest temperature is found to vary with flare. The flare temperatures peaked at 51–59 MK for CC Eri and 29–44 MK for AB Dor. The peak emission measures of the flaring loops are estimated to be ∼1054 for CC Eri and ∼1055 cm−3 for AB Dor. Global metallic abundances were also found to increase during flares.

A Database of Magnetic and Thermodynamic Properties of Confined and Eruptive Solar Flares

Solar flares sometimes lead to coronal mass ejections that directly affect Earth's environment. However, a large fraction of flares, including on solar-type stars, are confined flares. What are the differences in physical properties between confined and eruptive flares? For the first time, we quantify the thermodynamic and magnetic properties of hundreds of confined and eruptive flares of GOES class C5.0 and above, 480 flares in total. We first analyze large flares of GOES class M1.0 and above observed by the Solar Dynamics Observatory, 216 flares in total, including 103 eruptive and 113 confined flares, from 2010 until 2016 April; we then look at the entire data set of 480 flares above class C5.0. We compare GOES X-ray thermodynamic flare properties, including peak temperature and emission measure, and active-region (AR) and flare-ribbon magnetic field properties, including reconnected magnetic flux and peak reconnection rate. We find that for fixed peak X-ray flux, confined and eruptive flares have similar reconnection fluxes; however, for fixed peak X-ray flux confined flares have on average larger peak magnetic reconnection rates, are more compact, and occur in larger ARs than eruptive flares. These findings suggest that confined flares are caused by reconnection between more compact, stronger, lower-lying magnetic fields in larger ARs that reorganizes a smaller fraction of these regions’ fields. This reconnection proceeds at faster rates and ends earlier, potentially leading to more efficient flare particle acceleration in confined flares.

Study of the energetic X-ray superflares from the active fast rotator AB doradus

ABSTRACT We present the analyses of intense X-ray flares detected on the active fast rotator AB Dor using observations from the XMM–Newton. A total of 21 flares are detected, and 13 flares are analysed in detail. The total X-ray energy of these flares is found to be in the range of 1034−36 erg, in which the peak flare flux increased up to 34 times from the pre-/post-flaring states for the strongest observed flare. The duration of these flaring events is found to be 0.7 to 5.8 h. The quiescent state X-ray spectra are found to be explained by a three-temperature plasma with average temperatures of 0.29, 0.95, and 1.9 keV, respectively. The temperatures, emission measures, and abundances are found to be varying during the flares. The peak flare temperature was found in the 31–89 MK range, whereas the peak emission measure was 1052.5–54.7 cm−3 . The abundances vary during the flares and increase by a factor of ∼3 from the quiescent value for the strongest detected flare. The variation in individual abundances follows the inverse-FIP effect in quiescent and flare phases. The X-ray light curves of AB Dor are found to exhibit rotational modulation. The semi-loop lengths of the flaring events are derived in the range of 109.9−10.7 cm, whereas the minimum magnetic field to confine the plasma in the flaring loop is estimated between 200 and 700 G.

AstroSat observations of long-duration X-ray superflares on active M-dwarf binary EQ Peg

ABSTRACT We present a comprehensive study of three large long-duration flares detected on an active M-dwarf binary EQ Peg using the Soft X-Ray Telescope of the AstroSat observatory. The peak X-ray luminosities of the flares in the 0.3–7-keV band are found to be within ∼5–10 × 1030$\rm {erg}~\rm {s}^{-1}$. The e-folding rise- and decay-times of the flares are derived to be in the range of 3.4–11 and 1.6–24 ks, respectively. Spectral analysis indicates the presence of three temperature corona with the first two plasma temperatures remain constant during all the flares and the post-flare observation at ∼3 and ∼9 MK. The flare temperature peaked at 26, 16, and 17 MK, which are 2, 1.3, and 1.4 times more than the minimum value, respectively. The peak emission measures are found to be 3.9–7.1 × 1053 cm−3, whereas the abundances peaked at 0.16–0.26 times the solar abundances. Using quasi-static loop modelling, we derive loop lengths for all the flares as 2.5 ± 0.5 × 1011, 2.0 ± 0.5 × 1011, and 2.5 ± 0.9 × 1011 cm, respectively. The density of the flaring plasma is estimated to be 4.2 ± 0.8 × 1010, 3.0 ± 0.7 × 1010, 2.2 ± 0.8 × 1010 cm−3 for flares F1, F2, and F3, respectively. Whereas the magnetic field for all three flares is estimated to be <100 G. The estimated energies of all three flares are ≳ 1034–1035erg, putting them in a category of superflare. All three superflares are also found to be the longest duration flares ever observed on EQ Peg.

X-Ray Superflares from Pre-main-sequence Stars: Flare Modeling

Abstract Getman et al. report the discovery, energetics, frequencies, and effects on environs of >1000 X-ray superflares with X-ray energies E X ∼ 1034–1038 erg from pre-main-sequence (PMS) stars identified in the Chandra MYStIX and SFiNCs surveys. Here we perform detailed plasma evolution modeling of 55 bright MYStIX/SFiNCs superflares from these events. They constitute a large sample of the most powerful stellar flares analyzed in a uniform fashion. They are compared with published X-ray superflares from young stars in the Orion Nebula Cluster, older active stars, and the Sun. Several results emerge. First, the properties of PMS X-ray superflares are independent of the presence or absence of protoplanetary disks inferred from infrared photometry, supporting the solar-type model of PMS flaring magnetic loops with both footpoints anchored in the stellar surface. Second, most PMS superflares resemble solar long-duration events that are associated with coronal mass ejections. Slow-rise PMS superflares are an interesting exception. Third, strong correlations of superflare peak emission measure and plasma temperature with the stellar mass are similar to established correlations for the PMS X-ray emission composed of numerous smaller flares. Fourth, a new correlation of loop geometry is linked to stellar mass; more massive stars appear to have thicker flaring loops. Finally, the slope of a long-standing relationship between the X-ray luminosity and magnetic flux of various solar-stellar magnetic elements appears steeper in PMS superflares than for solar events.

Detection of spectral variations of Anomalous Microwave Emission with QUIJOTE and C-BASS

ABSTRACT Anomalous Microwave Emission (AME) is a significant component of Galactic diffuse emission in the frequency range 10–$60\, \mathrm{GHz}$ and a new window into the properties of sub-nanometre-sized grains in the interstellar medium. We investigate the morphology of AME in the ≈10○ diameter λ Orionis ring by combining intensity data from the QUIJOTE experiment at 11, 13, 17, and $19\, \mathrm{GHz}$ and the C-Band All Sky Survey (C-BASS) at $4.76\, \mathrm{GHz}$, together with 19 ancillary data sets between 1.42 and $3000\, \mathrm{GHz}$. Maps of physical parameters at 1○ resolution are produced through Markov chain Monte Carlo (MCMC) fits of spectral energy distributions (SEDs), approximating the AME component with a lognormal distribution. AME is detected in excess of $20\, \sigma$ at degree-scales around the entirety of the ring along photodissociation regions (PDRs), with three primary bright regions containing dark clouds. A radial decrease is observed in the AME peak frequency from $\approx 35\, \mathrm{GHz}$ near the free–free region to $\approx 21\, \mathrm{GHz}$ in the outer regions of the ring, which is the first detection of AME spectral variations across a single region. A strong correlation between AME peak frequency, emission measure and dust temperature is an indication for the dependence of the AME peak frequency on the local radiation field. The AME amplitude normalized by the optical depth is also strongly correlated with the radiation field, giving an overall picture consistent with spinning dust where the local radiation field plays a key role.

Characterization of X-ray flare properties of AB Dor

AbstractThe strong similarities between the flares observed on the Sun and in low mass stars has raised question regarding dynamo in these stars. Using the Sun as a prototype, one may be able to address this. In this paper, we present an analysis of 30 intense X-ray flares observed from AB Dor. These flares detected in XMM-Newton data show a rapid rise (500-3000 s) and a slow decay (1000-6000 s). Our studies suggest that the scaling law between the flare peak emission measure and the flare peak temperature for all the flares observed on AB Dor is very similar to the relationship followed by solar flares. Furthermore, we obtain the frequency distribution of flare energies which is a crucial diagnostic to calculate the overall energy residing in a flare. Our results of this study indicate that the large flare (1033 ≤ E ≤ 1034 erg) may not contribute to the heating of the corona.

Forecast of solar proton flux profiles for well‐connected events

AbstractWe have developed a forecast model of solar proton flux profiles (> 10 MeV channel) for well‐connected events. Among 136 solar proton events (SPEs) from 1986 to 2006, we select 49 well‐connected ones that are all associated with single X‐ray flares stronger than M1 class and start to increase within 4 h after their X‐ray peak times. These events show rapid increments in proton flux. By comparing several empirical functions, we select a modified Weibull curve function to approximate a SPE flux profile. The parameters (peak flux, rise time, and decay time) of this function are determined by the relationship between X‐ray flare parameters (peak flux, impulsive time, and emission measure) and SPE parameters. For 49 well‐connected SPEs, the linear correlation coefficient between the predicted and the observed proton peak fluxes is 0.65 with the RMS error of 0.55 log10(pfu). In addition, we determine another forecast model based on flare and coronal mass ejection (CME) parameters using 22 SPEs. The used CME parameters are linear speed and angular width. As a result, we find that the linear correlation coefficient between the predicted and the observed proton peak fluxes is 0.83 with the RMS error of 0.35 log10(pfu). From the relationship between error of model and CME acceleration, we find that CME acceleration is an important factor for predicting proton flux profiles.

MULTI-WAVELENGTH OBSERVATIONS OF THE SPATIO-TEMPORAL EVOLUTION OF SOLAR FLARES WITH AIA/SDO. II. HYDRODYNAMIC SCALING LAWS AND THERMAL ENERGIES

In this study we measure physical parameters of the same set of 155 M and X-class solar flares observed with AIA/SDO as analyzed in Paper I, by performing a {\sl differential emission measure (DEM)} analysis to determine the flare peak emission measure $EM_p$, peak temperature $T_p$, electron density $n_p$, and thermal energy $E_{th}$, in addition to the spatial scales $L$, areas $A$, and volumes $V$ measured in Paper I. The parameter ranges for M and X-class flares are: $\log(EM_p)=47.0-50.5$, $T_p=5.0-17.8$ MK, $n_p=4 \times 10^9-9 \times 10^{11}$ cm$^{-3}$, and thermal energies of $E_{th}=1.6 \times 10^{28}-1.1 \times 10^{32}$ erg. We find that these parameters obey the Rosner-Tucker-Vaiana (RTV) scaling law $T_p^2 \propto n_p L$ and $H \propto T^{7/2} L^{-2}$ during the peak time $t_p$ of the flare density $n_p$, when energy balance between the heating rate $H$ and the conductive and radiative loss rates is achieved for a short instant, and thus enables the applicability of the RTV scaling law. The application of the RTV scaling law predicts powerlaw distributions for all physical parameters, which we demonstrate with numerical Monte-Carlo simulations as well as with analytical calculations. A consequence of the RTV law is also that we can retrieve the size distribution of heating rates, for which we find $N(H) \propto H^{-1.8}$, which is consistent with the magnetic flux distribution $N(\Phi) \propto \Phi^{-1.85}$ observed by Parnell et al.(2009) and the heating flux scaling law $F_H \propto H L \propto B/L$ of Schrijver et al.(2004). The fractal-diffusive self-organized criticality model in conjunction with the RTV scaling law reproduces the observed powerlaw distributions and their slopes for all geometrical and physical parameters and can be used to predict the size distributions for other flare datasets, instruments, and detection algorithms.

THE THERMAL PROPERTIES OF SOLAR FLARES OVER THREE SOLAR CYCLES USING GOES X-RAY OBSERVATIONS

Solar flare X-ray emission results from rapidly increasing temperatures and emission measures in flaring active region loops. To date, observations from the X-Ray Sensor (XRS) onboard the Geostationary Operational Environmental Satellite (GOES) have been used to derive these properties, but have been limited by a number of factors, including the lack of a consistent background subtraction method capable of being automatically applied to large numbers of flares. In this paper, we describe an automated temperature and emission measure-based background subtraction method (TEBBS), which builds on the methods of Bornmann (1990). Our algorithm ensures that the derived temperature is always greater than the instrumental limit and the pre-flare background temperature, and that the temperature and emission measure are increasing during the flare rise phase. Additionally, TEBBS utilizes the improved estimates of GOES temperatures and emission measures from White et al. (2005). TEBBS was successfully applied to over 50,000 solar flares occurring over nearly three solar cycles (1980-2007), and used to create an extensive catalog of the solar flare thermal properties. We confirm that the peak emission measure and total radiative losses scale with background subtracted GOES X-ray flux as power-laws, while the peak temperature scales logarithmically. As expected, the peak emission measure shows an increasing trend with peak temperature, although the total radiative losses do not. While these results are comparable to previous studies, we find that flares of a given GOES class have lower peak temperatures and higher peak emission measures than previously reported. The resulting TEBBS database of thermal flare plasma properties is publicly available on Solar Monitor (www.solarmonitor.org/TEBBS/) and will be available on Heliophysics Integrated Observatory (www.helio-vo.eu).

Bright X‐Ray Flares in Orion Young Stars from COUP: Evidence for Star‐Disk Magnetic Fields?

We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 day observation of the Orion Nebula Cluster (ONC), concentrating on the events with the highest statistics (in terms of photon flux and event duration). Analysis of the flare decay allows to determine the physical parameters of the flaring structure, particularly its size and (using the peak temperature and emission measure of the event) the peak density, pressure, and minimum confining magnetic field. A total of 32 events, representing the most powerful 1% of COUP flares, have sufficient statistics and are sufficiently well resolved to grant a detailed analysis. A broad range of decay times are present in the sample of flares, with τlc (the 1/e decay time) ranging from 10 to 400 ks. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures that are found, from the analysis of the flare's decay, to confine the plasma are in a number of cases very long, with semilengths up to 1012 cm, implying the presence of magnetic fields of hundreds of G (necessary to confine the hot flaring plasma) extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures (length L R*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L ≤ R*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures that confine the flaring plasma are actually the same type of structures that channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.

Solar active regions: The footpoints of 1 MK loops

On-disc SOHO and TRACE observations of the footpoints of 1 MK quiescent loops in a solar active region are presented. These types of loops are long-lived and high-lying features that are best seen in the TRACE 173 A passband, sensitive to 1 MK temperatures. SOHO/CDS data are used to show the clear association between the high-lying 1 MK coronal emission and emission formed at lower heights and temperatures, in the transition region. The 1 MK loops are rooted in strong unipolar regions located at the supergranular network boundaries. These loops have near-isothermal distributions at each location along their length, with peak emission measures at T = 0.7 MK, where electron densities are ∼2 x 10 9 cm - 3 . The loops showed photospheric abundances, at odds with previous results based on Skylab data.

The peculiar nebula Simeis 57

We present high resolution radio continuum maps of the Galactic nebula Simeis 57 (= HS 191 = G 80.3+4.7) made with the WSRT and the DRAObservatory at frequencies of 609, 1412 and 1420 MHz. At optical and at radio wavelengths, the nebula has a peculiar ``S'' shape, crossed by long, thin and straight filaments. The radio maps, combined with other maps from existing databases, show essentially all radio emission from the peculiar and complex nebula to be thermal and optically thin. Although neither the distance nor the source of excitation of Simeis 57 are known, the nebula can only have a moderate electron density of typically n(e) = 100 cm-3. Its mass is also low, not exceeding some tens of solar masses. Peak emission measures are 5000 pc cm-6.Obscuring dust is closely associated with the nebula, but seems to occur mostly in front of it. Extinctions vary from A(V) = 1.0 mag to A(V) = 2.8 mag with a mean of about 2 mag. The extinction and the far-infrared emission at 100 microns are well-correlated.

Spectroscopic characteristics of polar plumes

Extreme ultraviolet observations of plumes in polar coronal holes are presented and their spectroscopic signatures discussed. The study focuses on the base of plumes seen on the disk of the Sun with the Coronal DiagnosticSpectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) satellite. Spectroscopic diagnostic techniques are applied to characterise the plumes in terms of density, temperature, emission measure and element abundance. Attention is drawn to the particular limitations of some of the techniques when applied to plume structures. In particular, we revisit the Widing & Feldman (1992) findings of a plume having a large first ionization potential (FIP) effect of 10, showing that instead the Skylab data are consistent with no FIP effect. We present for the first time CDS-GIS (grazing incidence spectrometer) observations of a plume. These observations have been used to confirm the results obtained from normal incidence (NIS) observations. We find that polar plumes exhibit the same characteristics as the Elephant's Trunk equatorial plume. The most striking characteristic of the plume bases is that they are near-isothermal with a peak emission measure at transition region temperatures ≃8 x 10 5 K. At these temperatures, plumes have averaged densities N e ≃ 1.2 x 10 9 cm-3, about twice the value of the surrounding coronal hole region. Element abundances in the plumes are found to be close to photospheric, with the exception of neon which appears to be depleted by 0.2 dex relative to oxygen. The absence of a significant FIP effect in plumes is consistent with fast solar wind plasma, although it is not sufficient to prove a link between the two. Finally, we present a comparison between GIS spectra and the SOHO EIT (EUV Imaging Telescope) broad-band images, showing that temperatures derived from the EIT ratio technique are largely overestimated, for plumes and coronal holes. This is partly due to the fact that the so called Fe XII 195 A and Fe XV 284 A filters are not isothermal, and in coronal holes and plumes lower-temperature lines dominate the EIT signal.

Extreme Ultraviolet ExplorerObservations of the W Ursae Majoris Contact Binary 44i Bootis: Coronal Structure and Variability

EUVE observations of 44i Boo (HD 133640), the first W UMa-type active binary observed with EUVE, give evidence for localized coronal structure. An EUV light curve has been obtained with the EUVE Deep Survey (DS) detector and Lexan/boron filter during the 6 day observation (19.5 continuous epochs) in 1994. A period search on the DS data stream reveals a period corresponding to 0.272652 ± 0.00141 days, longer than the optical period (0.267817 days) by 428 s. Since the DS flux originates predominantly from high-temperature plasma (Te ~ 107 K), this observation demonstrates for the first time an orbital phase-dependent component of the coronal emission of a contact binary, indicates the presence of an active region on the primary star, and signals a different rotation rate of the corona with respect to the photosphere. The high-temperature plasma shows no correspondence with the two partial eclipses per orbit found in the optical and in the chromospheric and transition-region emission. A flaring enhancement appears associated with maximum brightness. The EUV spectrum shows strong emission from highly ionized Fe. Like the emission measure distributions of Capella and other RS CVn binaries observed with EUVE, the spectrum of 44i Boo requires a continuous distribution of plasma temperatures to account for the relative Fe emission line fluxes. A local enhancement of emission measure at about Te ~ 8 × 106 K is reminiscent of the narrow feature found in Capella at Te ~ 6 × 106 K, although substantially broader. Line ratio diagnostics indicate extremely high densities, Ne ~ 2 × 1013 cm-3—higher than has been observed for other EUVE coronal sources. We place a hard lower limit on the Fe/H abundance ratio of Fe/H > 0.46 with respect to the solar photospheric value. We compare our results with those derived from the ROSAT PSPC and conclude that the soft X-ray band in ROSAT observations does not give reliable abundances or low-temperature emission measures. The high electron density and peak emission measure together imply a small emitting volume, L3 (L ~ 0.004R). If all the EUV emission arises from such a volume, the EUV light curve places tight constraints on the possible location of a hot spot, namely that it must be located at high latitudes (>72°) on the primary star.

Ultraviolet Spectroscopy of AB Doradus with the [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]Telescope[/ITAL]: Impulsive Flares and Bimodal Profiles of C [CSC]iv[/CSC] λ1549 in a Young Star

We observed AB Doradus, a young and active late-type star (K0–K2 IV–V, P = 0.514 days), with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with time and spectral resolutions of 27 s and 15 km s-1, respectively (1994 November 14.08–14.30 UT). The wavelength band (1531–1565 A) included the strong C IV doublet (1548.202 and 1550.774 A, formed in the transition region at 105 K), the chromospheric Si II 1533.432 A line, and the blend of Si I, C I, and Fe II lines at 1561 A. The mean quiescent C IV flux state was characterized by FC IV = (7.80 ± 0.34) × 105 ergs cm-2 s-1, close to the saturated value and 100 times the solar one. The line profile (after removing the rotational and instrumental profiles) is bimodal, consisting of two Gaussians, one narrow (FWHM = 70 km s-1) and the other broad (FWHM = 330 km s-1). This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that transition-region transient events (random multiple velocities), with large surface coverage, give rise to the broadening of the narrow component while true microflaring is responsible for the broad one, as suggested by Wood, Linsky, & Ayres. The transition region was observed to flare frequently with different timescales and magnitudes. The largest impulsive flare seen in the C IV λ1549 emission, at day 14.22, reached in less than 1 minute a peak differential emission measure N V (104.85–105.15 K) = 1051.2 cm-3 and returned exponentially in 5 minutes to the 7 times lower quiescent level. The 3 minute average line profile of the flare was blueshifted (-190 km s-1) and broadened (FWHM = 800 km s-1). This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam, accelerated (by reconnection) at the apex of a coronal loop.

Electron Trapping Times and Trap Densities in Solar Flare Loops Measured withCOMPTONandYOHKOH

We measure energy-dependent time delays of ≈ 20-200 keV hard X-ray (HXR) emission from 78 flares observed simultaneously with the Compton Gamma Ray Observatory and Yohkoh. Fast time structures ( 1 s) are filtered out, because their time delays have been identified in terms of electron time-of-flight (TOF) differences from directly precipitating electrons (Aschwanden et al.). For the smooth HXR flux, we find systematic time delays in the range of τS = t50 keV-t200 keV ≈ -(1 ... 10) s, with a sign opposite to TOF delays, i.e., the high-energy HXRs lag the low-energy HXRs. We interpret these time delays of the smooth HXR flux in terms of electron trapping, and we fitted a model of the collisional deflection time tDefl(E) E -->3/2n -->−1e to the observed HXR delays in order to infer electron densities n${r Trap}e$ --> in the trap. Independently, we determine the electron density n${r SXR}e$ --> in flare loops from soft X-ray (SXR) peak emission measures EM= -->∫ n -->2e dh, using loop width (w) measurements to estimate the column depth dh ≈ w. Comparing the two independent density measurements in HXR and SXR, we find a mean ratio of q -->e=n${r Trap}e$ -->/n${r SXR}e$ --> ≈ 1, with a relatively small scatter by a factor of ≈ 2. Generally, it is likely that the SXR-bright flare loops have a higher density than the trapping regions (when qe 1). Our measurements provide comprehensive evidence that electron trapping in solar flares is governed in the weak-diffusion limit, i.e., that the trapping time corresponds to the collisional deflection time, while pitch-angle scattering by resonant waves seems not to be dominant in the 20-200 keV energy range. The measurements do not support a second-step acceleration scenario for energies ≤200 keV.

Are Coronae of Magnetically Active Stars Heated by Flares?

Coronal flares are promising candidates as agents for plasma heating in coronae of magnetically active stars. This Letter confronts recently reconstructed coronal emission measure distributions of solar-like stars with results from a statistical, hydrodynamic approach to flare simulations. The time evolution of a statistical set of flare energy release events, distributed in total energy as a power law as observed in the Sun or in active stars, is simulated in a family of magnetically rigid loops. The resulting time-averaged emission measure distribution is double peaked, with the hotter peak due to the cooling plasma of energetic flares in the larger loops, while the cooler peak can be understood as a dispersion effect of a quiescent isothermal plasma induced by the large number of low-energy flares (microflares). The intervening minimum occurs around 10 MK and can be explained as being due to the combined effects of the flare decay of energetic flares, the power-law increase of the flare frequency toward low-energy flares, and a nearly exponential dependence between flare peak emission measure and simultaneous temperature in a given loop. The required simulation flare frequency decreases with decreasing average coronal temperature, in agreement with the observation that the average coronal temperature is correlated with magnetic activity indicators.

An exceptional X-ray flare on the dMe star EQ1839.6 + 8002

A large impulsive flare was detected serendipitously during observations of the radio galaxy 3C390.3 made with the Ginga satellite on 1991 February 14, between 22:30 and 23:00 (UT). The dMe star EQ1839.6 + 8002 is the most likely source of this flare. The rise of the flare was observed and the spectra obtained are of sufficient quality to determine the temperature and emission measure as a function of time. The flare is exceptional in its high peak temperature (Te ∼ 108 K) and emission measure ( ∼ 9 × 1053 cm−3), the largest recorded for a flare on a dMe star, and comparable with those for flares in RS CVn systems. The normalized X-ray luminosity is also very large (Lx/Lbol ∼ 0.25). The flare geometry and plasma parameters are derived by making various assumptions concerning the dominant terms in the internal energy equation, which takes account of a varying mass and volume. The initial heating appears to be proportional to the gas pressure. At the peak Te the electron density is ∼ 1.7 × 1012 cm−3, and the hot plasma has a length ∼ 1010 cm. The flare plasma then cools initially by ‘evaporative conduction’. Two alternative simple models are made of the flare decay beyond this time (cooling with constant mass or constant volume). Both require continued heating, and the latter gives results similar to those expected in quasi-static conditions. The evolution of the flare temperature and density broadly resembles that predicted by numerical simulations. Observations with the Einstein IPC, the EXOSAT LE and the ROSAT PSPC instruments show flaring and quiescent properties similar to those of the flare stars discussed by Pallavicini et al.

Spectral and Temporal Characteristics of X-Ray--bright Stars in the Pleiades

We follow up our deep ROSAT imaging survey of the Pleiades (Stauffer et al. 1994) with an analysis of the spectral and temporal characteristics of the X-ray-bright stars in the Pleiades. Raymond &amp; Smith (1977) one-and two-temperature models have been used to fit the position-sensitive proportional counter (PS PC) pulse-height spectra of the dozen or so brightest sources associated with late-type Pleiades members. The best-fit temperatures suggest hot coronal temperatures for K, M, and rapidly rotating G stars, and cooler temperatures for F and slowly rotating G stars. In order to probe the many less X-ray-luminous stars, we have generated composite spectra by combining net counts from all Pleiades members according to spectral type and rotational velocity. Model fits to the composite spectra confirm the trend seen in the individual spectral fits. Particularly interesting is the apparent dependence of coronal temperature on L<SUB>X</SUB>/L<SUB>bol</SUB>. A hardness-ratio analysis also confirms some of these trends. The PSPC data have also revealed a dozen or so strong X-ray flares with peak X-ray luminosities in excess of ∼10<SUP>30</SUP> ergs s<SUP>-1</SUP>. We have modeled the brightest of these flares with a simple quasi-static cooling loop model. The peak temperature and emission measure and the inferred electron density and plasma volume suggest a very large scale flaring event. The PSPC data were collected over a period of ∼18 months, allowing us to search for source variability on timescales ranging from less than a day (in the case of flares) to more than a year between individual exposures. On approximately year-long timescales, roughly 25% of the late-type stars are variable. Since the Pleiades was also intensively monitored by the imaging instruments on the Einstein Observatory, we have examined X-ray luminosity variations on the 10 yr timescale between Einstein and ROSAT and find that up to 40% of the late-type stars are X-ray variable. Since there is only marginal evidence for increased variability on decade-long timescales, the variability observed on long and short timescales may have a common physical origin.