Volume Emission Measure Research Articles

Enhanced Peak and Extended Cooling of the Extreme-ultraviolet Late Phase in a Confined Solar Flare

We present observations and analysis of an X1.8 noneruptive solar flare on 2012 October 23, which is characterized by an extremely large late-phase peak seen in the warm coronal extreme-ultraviolet (EUV) emissions (∼3 MK), with the peak intensity over 1.4 times that of the main flare peak. The flare is driven by a failed eruption of a magnetic flux rope, whose strong squeeze force acting on the overlying magnetic structures gives rise to an intense early heating of the late-phase loops. Based on differential emission measure analysis, it is found that the late-phase loops experience a “longer-than-expected” cooling, without the presence of any obvious additional heating, while their volume emission measure maintains a plateau for a long time before turning into an evident decay. Without the need for an additional heating, we propose that the special thermodynamic evolution of the late-phase loops revealed in this flare might arise from loop cross-sectional expansions with height, which are evidenced by both direct measurements from EUV images and by magnetic field extrapolation. By blocking the losses of both heat flux and mass from the corona, such an upward cross-sectional expansion not only elongates the loop-cooling time, but also more effectively sustains the loop density, therefore leading to a later-than-expected occurrence of the warm coronal late phase in combination with a sufficiently high late-phase peak. We further verify such a scenario by analytically solving the cooling process of a late-phase loop characterized by a variable cross section.

Latest Evolution of the X-Ray Remnant of SN 1987A: Beyond the Inner Ring

Based on our Chandra imaging-spectroscopic observations, we present the latest evolution of the X-ray remnant of SN 1987A. Recent changes in the electron temperatures and volume emission measures suggest that the blast wave in SN 1987A is moving out of the dense inner ring structure, also called the equatorial ring (ER). The 0.5–2.0 keV X-ray light curve shows a linearly declining trend (by ∼4.5% yr−1) between 2016 and 2020 as the blast wave heats the hitherto unknown circumstellar medium (CSM) outside the ER. While the peak X-ray emission in the latest 0.3–8.0 keV image is still within the ER, the radial expansion rate in the 3.0–8.0 keV images suggests an increasing contribution of the X-ray emission from less dense CSM since 2012, at least partly from beyond the ER. It is remarkable that, since 2020, the declining soft X-ray flux has stabilized around ∼7 × 10−12 erg s−1 cm−2, which may signal a contribution from the reverse-shocked outer layers of ejecta as predicted by the three-dimensional magnetohydrodynamic models. In the latest ACIS spectrum of supernova remnant 1987A in 2022 we report a significant detection of the Fe K line at ∼6.7 keV, which may be due to changing thermal conditions of the X-ray emitting CSM and/or the onset of reverse shock interactions with the Fe ejecta.

Estimations of Elemental Abundances during Solar Flares Observed in Soft X-Rays by the MinXSS-1 CubeSat Mission

Solar flares are complex phenomena emitting all types of electromagnetic radiation and accelerating particles on timescales of minutes, converting magnetic energy to thermal, radiative, and kinetic energy through magnetic reconnections. As a result, local plasma can be heated to temperatures in excess of 20 MK. During the soft X-ray (SXR) solar flare peak, the elemental abundance of low first ionization potential elements are typically observed to be depleted from coronal values. We explored the abundance variations using disk-integrated solar spectra from the Miniature X-ray Solar Spectrometer CubeSat-1 (MinXSS-1). MinXSS-1 is sensitive to the 1–12 keV energy range with an effective 0.25 keV FWHM resolution at 5.9 keV. During the year-long mission of MinXSS-1, between 2016 May and 2017 May, 21 flares with intermittent data downlinks were observed ranging from C to M class. We examine the time evolution of temperature, volume emission measure, and elemental abundances of Fe, Ca, Si, S, and Ar with CHIANTI spectral models near the peak SXR emission times observed in the MinXSS-1 data. We determined the average absolute abundance of A(Fe) = 7.81, A(Ca) = 6.84, A(S) = 7.28, A(Si) = 7.90, and A(Ar) = 6.56. These abundances are depleted from coronal values during the SXR peak compared to nonflaring times. The elemental abundance values that are depleted from their coronal values are consistent with the process of chromospheric evaporation, in which the lower atmospheric plasma fills the coronal loops.

X-Ray Emission from the Exoplanet Hosting LTT 1445 Triple Star System

JWST will be able to observe the atmospheres of rocky planets transiting nearby M dwarfs. The M-dwarf triple star system LTT 1445, at a distance of 6.86 pc, hosts some of the nearest rocky terrestrial planets. These planets most likely orbit the M 3.5V star LTT 1445A. During a 28.6 ks Chandra ACIS-S3 observation we have: (i) spatially resolved and detected all three stars in the LTT 1445 system; (ii) measured the X-ray luminosity of the individual stars, including LTT 1445A, for the first time; (iii) studied the flux variability of the X-ray sources and found strong variability from the A and C components; and (iv) investigated how the coronal luminosities, temperatures, and volume emission measures vary at different activity levels. Combining these X-ray data with upcoming HST ultraviolet observations will allow a differential emission measure estimation of the star’s extreme-ultraviolet spectrum, thereby facilitating modeling of the rocky planets’ atmospheres.

X-RAY OBSERVATION OF AM HERCULIS IN A VERY LOW STATE WITHSUZAKU

The X-ray observation of AM Her in a very low state was performed with {\it Suzaku} in October 2008. One flare event with a time scale of $\sim$ 3700 sec was detected at the X-ray luminosity of $6.0 \times 10^{29} {\rm ~erg ~sec}^{-1}$ in the 0.5 -- 10 keV band assuming at a distance of 91 pc. The X-ray spectrum is represented by a thermal plasma emission model with a temperature of $8.67_{-1.14}^{+1.31}$ keV. During the quiescence out of the flare interval, {\it Suzaku} also detected significant X-rays at a luminosity of $1.7 \times 10^{29} {\rm ~erg ~sec}^{-1}$ in the 0.5 -- 10 keV band, showing a clear spin modulation at a period of 0.1289273(2) days at BJD 2454771.581. The X-ray spectra in the quiescence were represented by a MEKAL + Power Law (PL) model or a single CEMEKL model, which are also supported by phase-resolved analyses. A correlation between the temperature and the volume emission measure was found together with historical X-ray measurements of AM Her in various states. In order to account for a possible non-thermal emission from AM Her, particle acceleration mechanisms in the AM Her system are also discussed, including a new proposal of a shock acceleration process on the top of the accretion column.

CAVITY OF MOLECULAR GAS ASSOCIATED WITH SUPERNOVA REMNANT 3C 397

3C 397 is a radio and X-ray bright Galactic supernova remnant (SNR) with an unusual rectangular morphology. Our CO observation obtained with the Purple Mountain Observatory at Delingha reveals that the remnant is well confined in a cavity of molecular gas, and embedded at the edge of a molecular cloud (MC) at the local standard of rest systemic velocity of ~32 km/s. The cloud has a column density gradient increasing from southeast to northwest, perpendicular to the Galactic plane, in agreement with the elongation direction of the remnant. This systemic velocity places the cloud and SNR 3C 397 at a kinematic distance of ~10.3 kpc. The derived mean molecular density (~10-30 cm^-3) explains the high volume emission measure of the X-ray emitting gas. A 12CO line broadening of the ~32 km/s component is detected at the westmost boundary of the remnant, which provides direct evidence of the SNR-MC interaction and suggests multi-component gas there with dense (~10^4 cm^-3) molecular clumps. We confirm the previous detection of a MC at ~38 km/s to the west and south of the SNR and argue, based on HI self-absorption, that the cloud is located in the foreground of the remnant. A list of Galactic SNRs presently known and suggested to be in physical contact with environmental MCs is appended in this paper.

Beginning of the super‐soft phase of the classical nova V2491 Cygni

AbstractWe present the results of soft X‐ray studies of the classical nova V2491 Cygni using the Suzaku observatory. On day 29 after outburst, a soft X‐ray component with a peak at ∼0.5 keV has appeared, which is tantalising evidence for the beginning of the super‐soft X‐ray emission phase. We show that an absorbed blackbody model can describe the observed spectra, yielding a temperature of 57 eV, neutral hydrogen column density of 2 × 1021 cm–2, and a bolometric luminosity of ∼1036 erg s–1. However, at the same time, we also found a good fit with an absorbed thin‐thermal plasma model, yielding a temperature of 0.1 keV, neutral hydrogen column density of 4 × 1021 cm–2, and a volume emission measure of ∼1058 cm–3. Owing to low spectral resolution and low signal‐to‐noise ratio below 0.6 keV, the statistical parameter uncertainties are large, but the ambiguity of the two very different models demonstrates that the systematic errors are the main point of concern. The thin‐thermal plasma model implies that the soft emission originates from optically thin ejecta, while the blackbody model suggests that we are seeing optically thick emission from the white dwarf (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

SUZAKU DISCOVERY OF THE STRONG RADIATIVE RECOMBINATION CONTINUUM OF IRON FROM THE SUPERNOVA REMNANT W49B

We present a hard X-ray spectrum of unprecedented quality of the Galactic supernova remnant W49B obtained with the Suzaku satellite. The spectrum exhibits an unusual structure consisting of a saw-edged bump above 8 keV. This bump cannot be explained by any combination of high-temperature plasmas in ionization equilibrium. We firmly conclude that this bump is caused by the strong radiative recombination continuum (RRC) of iron, detected for the first time in a supernova remnant. The electron temperature derived from the bremsstrahlung continuum shape and the slope of the RRC is 1.5 keV. On the other hand, the ionization temperature derived from the observed intensity ratios between the RRC and K-alpha lines of iron is 2.7 keV. These results indicate that the plasma is in a highly overionized state. Volume emission measures independently determined from the fluxes of the thermal and RRC components are consistent with each other, suggesting the same origin of these components.

Solar Flare Geometries. II. The Volume Fractal Dimension

Based on the area fractal dimension D2 of solar flares measured in Paper I, we carry out modeling of the three-dimensional (3D) flare volume here and derive an analytical relation between the volume fractal scaling V(L) ∝ LD3 and the area fractal scaling A(L) ∝ LD2. The 3D volume model captures a flare arcade with a variable number of flare loops; its fractal structure is not isotropic, but consists of aligned one-dimensional substructures. The geometry of the arcade model has three free parameters and makes some simplifying assumptions, such as semicircular loops, east-west orientation, location near the equator, and no magnetic shear. The analytical model predicts the scaling of the area filling factor qA(nloop) and volumetric filling factor qV(nloop) as a function of the number of loops nloop, and allows one to predict the volume filling factor qV(qA) and volume fractal dimension D3(D2) from the observationally measured parameters qA and D2. We also corroborate the analytical model with numerical simulations. We apply this fractal model to the 20 flares analyzed in Paper I and find maximum volume filling factors with a median range of qV ≈ 0.03–0.08 (assuming solid filling for loop widths of 1 Mm). The fractal nature of the flare volume has important consequences for correcting electron densities determined from flare volume emission measures and density-dependent physical quantities, such as the thermal energy or radiative cooling time. The fractal scaling has also far-reaching consequences for frequency distributions and scaling laws of solar and stellar flares.

Scaling Laws of Solar and Stellar Flares

In this study we compile for the first time comprehensive data sets of solar and stellar flare parameters, including flare peak temperatures Tp, flare peak volume emission measures EMp, and flare durations τf from both solar and stellar data, as well as flare length scales L from solar data. Key results are that both the solar and stellar data are consistent with a common scaling law of EMp ∝ T4.7p, but the stellar flares exhibit ≈250 times higher emission measures (at the same flare peak temperature). For solar flares we observe also systematic trends for the flare length scale L(Tp) ∝ T0.9p and the flare duration τF(Tp) ∝ T0.9p as a function of the flare peak temperature. Using the theoretical RTV scaling law and the fractal volume scaling observed for solar flares, i.e., V(L) ∝ L2.4, we predict a scaling law of EMp ∝ T4.3p, which is consistent with observations, and a scaling law for electron densities in flare loops, np ∝ T2p/L ∝ T1.1p. The RTV-predicted electron densities were also found to be consistent with densities inferred from total emission measures, np = (EMp/qVV)1/2, using volume filling factors of qV = 0.03–0.08 constrained by fractal dimensions measured in solar flares. Solar and stellar flares are expected to have similar electron densities for equal flare peak temperatures Tp, but the higher emission measures of detected stellar flares most likely represent a selection bias of larger flare volumes and higher volume filling factors, due to low detector sensitivity at higher temperatures. Our results affect also the determination of radiative and conductive cooling times, thermal energies, and frequency distributions of solar and stellar flare energies.

On the spatial and temporal characteristics and formation mechanisms of soft X-ray emission in the solar corona

Our main goal is to show that the spatial and temporal dynamics of the temperature content for plasma structures in the solar corona can be described quantitatively in principle, which is necessary for understanding the formation mechanisms of soft X-ray emission. An approach based on a consistent modeling of complex data from the CORONAS-F, GOES, and RHESSI satellites is suggested. A basically new element of this approach is the use of time series of monochromatic full-Sun images in the X-ray MgXII 8.42 A line and EUV lines obtained in the SPIRIT experiment onboard CORONAS-F. Two inversion procedures have been used to determine the volume and column differential emission measures defined by the Stieltjes integral: an optimization one based on a multitemperature parametric model and an iterative one based on the Bayesian theorem, respectively. The calculations with coronal abundances agree with the RHESSI data within the experimental error limits, while those with photospheric abundances give no satisfactory agreement. The relatively cold (with temperature 2–4 MK) and transient (4–10 MK) plasmas are shown to play a significant role in producing soft X-ray emission during flare events and in their energy budget. The spatial electron density and temperature distributions and their time evolution have been obtained for long-duration events that were first observed in the monochromatic MgXII channel and were previously called “spiders.” The method used has allowed us to verify the absolute intercalibration of the fluxes recorded in all experiments and to reference the SPIRIT MgXII images to the solar disk. We also consider possible flare plasma heating mechanisms for impulsive and long-duration (spider) flare events.

ChandraHigh‐Resolution X‐Ray Spectroscopy of AM Herculis

We present the results of high-resolution spectroscopy of the prototype Polar AM Herculis observed with Chandra High Energy Transmission Grating . The X-ray spectrum contains hydrogen-like and helium-like lines of Fe, S, Si, Mg, Ne, and O, with several Fe L-shell emission lines. The forbidden lines in the spectrum are generally weak, whereas the hydrogen-like lines are stronger suggesting that emission from a multitemperature, collisionally ionized plasma dominates. The helium-like line flux ratios yield a plasma temperature of 2 MK and a plasma density 1-9 × 1012 cm-3, whereas the line flux ratio of Fe XXVI to Fe XXV gives an ionization temperature of 12.4 keV. We present the differential emission measure distribution of AM Her, whose shape is consistent with the volume emission measure obtained by a multitemperature APEC model. The multitemperature plasma model fit to the average X-ray spectrum indicates the mass of the white dwarf to be ~1.15 M☉. From phase-resolved spectroscopy, we find the line centers of Mg XII, S XVI, resonance line of Fe XXV, and Fe XXVI emission modulated by a few hundred to 1000 km s-1 from the theoretically expected values, indicating bulk motion of ionized matter in the accretion column of AM Her. The observed velocities of Fe XXVI ions are close to the expected shock velocity for a 0.6 M☉ white dwarf. The observed velocity modulation is consistent with that expected from a single pole accreting binary system.

Chromospheric Evaporation in Solar Flares Revisited

We investigate the initial stage of chromospheric evaporation in flares using soft X-ray spectra obtained by the Bragg Crystal Spectrometer (BCS) experiment on Yohkoh. We find that the centroid wavelength of the Ca XIX line in spectra with the first detectable emission is within about 8.5 × 10-4 A of the rest wavelength, which corresponds to a Doppler shift of no more than 80 km s-1. We also determine the minimum detectable soft X-ray flare volume emission measure from BCS Ca XIX flare spectra. We find that the minimum detectable emission measured by BCS is produced by an X-ray flux that is about equal to the peak intensity of a class A6 flare. These results are difficult to reconcile with one-dimensional hydrodynamic simulations of an impulsively heated flare loop, which predict large Doppler shifts during the initial stage of the heating. Furthermore, inspection of high spatial resolution TRACE images of flare plasma indicate significant differences between the observed morphology and the predictions of hydrodynamic models. The evolution of the intensity and the Doppler shifts are more consistent with models that assume the sequential heating of small-scale threads rather than the heating of an individual loop. However, the bright knots of emission and asymmetrical intensity distributions seen in flare images cannot be explained by current numerical models of chromospheric evaporation.

Dust content in compact HII regions (NGC 7538-IRS1, IRS2, and IRS3)

The luminosity of the central star of the compact HII regions of NGC 7538 was estimated from the solid angle of the IR sources subtended relative to the central star, and was found to be 5 ∼ 10 times as intense as that of IR sources. Under the single central star approximation, the luminosity gives a stellar UV photon rate NU(∗)(s −1 )o f∼3.0×10 48 , ∼1.5×10 49 , ∼5.1×10 49 , and ∼1.7 × 10 47 for the compact HII regions of NGC 7538-IRS1(A/2), B, IRS2, and IRS3, respectively. NU(∗) and the observed electron density, ne, provide the dust opacity of the ionizing photon, τSd, for the optical path out to the Stromgren sphere radius rS, assuming a gas with standard dust content. Ionizing photon opacity over the same optical path but with the actual dust content τSda is also derived from ri/rS ,w hereri is the radius of the ionized sphere, which is estimated from NU(∗ )a nd the observed volume emission measure n 2(4πr 3 /3) (Spitzer 1978). An observational trend of γ{NU(∗)/4πr 2 } 1/2 ∼ constant, where γ = τSda/τSd, was obtained for the 4 compact HII regions of the NGC 7538(N). Fourteen selected compact HII regions from data catalogued by VLA observations were examined for this trend, and a similar result was obtained. A limit of γ as 15 ≥ γ ≥ 0.1 was given for the 14 selected sources. The size of the dust-depleted cavity of the NGC 7538(N) suggested by Chini et al. (1986) coincides with that of the ionized sphere of the IRS2 of the region.

The X-ray spectra of the flaring and quiescent states of AT Microscopii observed by XMM-Newton

The X-ray spectrum of the late-type M-dwarf binary AT Mic (dM4.5e+dM4.5e) is observed in the wavelength range 1–40 Å by means of rgs and epic-mos on board XMM-Newton. During the exposure a flare occured. We have performed a 3-temperature fit and a DEM-modeling to the flaring and quiescent part of the spectrum. We report the coronal temperature distribution, emission measures, and abundances of the flaring and quiescent state of this bright X-ray source. The temperature range stretches from about 1 to 60 MK. The total volume emission measure in this temperature interval is cm-3 for the quiescent state and cm-3 for the flare state. This difference is due to the contribution of the hot temperature component. The high-resolution spectrum of AT Mic, obtained by rgs, is dominated by the H- and He-like transitions of C, N, O, and Ne and by Fe xvii lines, produced by the plasma with temperatures from 1 to 10 MK. The epic-mos spectrum below 10 Å shows H- and He-like Ne, Si and the iron K-shell transitions. They are produced by the hot component (30 MK). The iron K-shell is more prominent in the flare state. The abundance pattern in the quiescent state of AT Mic shows the depletion of low-FIP elements relative to high-FIP elements, indicating the presence of an I(nverse)FIP effect in this active star. In the flare state, however, some flattening of this IFIP effect is present.

The Coronae of AR Lacertae

We observed the coronally active eclipsing binary AR Lac with the High Energy Transmission Grating on Chandra for a total of 97 ks, spaced over five orbits, at quadratures and conjunctions. Contemporaneous and simultaneous EUV spectra and photometry were also obtained with the Extreme Ultraviolet Explorer. Significant variability in both X-ray and EUV fluxes were observed, dominated by at least one X-ray flare and one EUV flare. We saw no evidence of primary or secondary eclipses, but exposures at these phases were short and intrinsic variability compromised detection of any geometric modulation. X-ray flux modulation was largest at high temperature, indicative of flare heating of coronal plasma rather than changes in emitting volume or global emission measure. Analysis of spectral line widths interpreted in terms of Doppler broadening suggests that both binary stellar components are active. On the basis of line fluxes obtained from total integrated spectra, we have modeled the emission measure and abundance distributions. The EUV spectral line fluxes were particularly useful for constraining the parameters of the cool (≤2 × 106 K) plasma. A strong maximum was found in the differential emission measure, characterized by two apparent peaks at log T = 6.9 and 7.4, together with a weak but significant cooler maximum near log T = 6.2 and a moderately strong hot tail from log T = 7.6-8.2. Coronal abundances have a broad distribution and show no simple correlation with first ionization potential. While the resulting model spectrum generally agrees very well with the observed spectrum, there are some significant discrepancies, especially among the many Fe L lines. Both the emission measure and abundance distributions are qualitatively similar to prior determinations from other X-ray and ultraviolet spectra, indicating some long-term stability in the overall coronal structure.

A transient event in lines of Ne V, VI and VII

We report observations of a transition region brightening made with the Coronal Diagnostic Spectrometer onboard the Solar and Heliospheric Observatory. We observed a region of the quiet Sun in 12 spectral lines with temperatures of formation from K to K. The transient event occured in a network boundary region and was most pronounced in the Ne vi 562.8-Å line. Although the Ne v 572.3-Å and Ne vii 561.7-Å lines also show increases in intensity, the changes in the lines of helium, oxygen and magnesium, formed at lower or higher temperatures, are smaller or not significant. Thus the event is most significant in the relatively narrow temperature range of K to K. The event lasted at least 53 min. In the many data sets we have obtained, only one other region shows an obviously high Ne vi to O iv intensity ratio, so the observed event is clearly unusual. Previous studies of blinkers have not included the lines of Ne v, Ne vi or Ne vii; the highest temperature transition region line used has been the O v 627.9-Å line. Future studies of blinkers should include these higher temperature lines. We derive volume emission measures in the event from the various line intensities, estimate the electron densities and discuss the energy budget and possible origins of the event.

XMM-Newton Detection of Extended X-ray Emission from the Saturn Nebula

XMM-Newton EPIC observations of the planetary nebula NGC 7009, the Saturn Nebula, have detected extended X-ray emission from its central cavity. The diffuse X-ray emission must originate in the shocked fast stellar wind. Spectral analyses show that the temperature of the hot gas is 1.7 x 106 K. The RMS density derived from the volume emission measure is a few tens H-atom cm-3. The hot gas does not appear over-pressurized with respect to the nebular shell. The Saturn Nebula may represent an evolutionary stage at which the dynamic effects of the hot gas in the central cavity on the cold nebular shell starts to decline due to the diminishing strength of the fast stellar wind and the expansion of the central cavity.

Analysis of the Temperature and Emission Measure of Solar Coronal Arcades and Test of a Scaling Law of Flare/Arcade Loop Length

We analyze 17 arcades to study the relations between solar flares and arcades. Soft X-ray images taken with Yohkoh's soft X-ray telescope are used to derive T0, EM0, and temporal variation of Tarc and EMarc, where Tarc and T0 are the temperatures of an arcade and prearcade region and EMarc and EM0 are the volume emission measures of an arcade and prearcade region. It is found that T0 ~ 2 MK and Tarc ~ 4 MK. We also estimate prearcade coronal electron density n0 and arcade electron density narc to find that narc is comparable to n0 (narc ~ n0 ~ 108 cm-3). Using these observed EM, T, and n0, we calculate the theoretical loop length Ltheor based on the scaling law for solar and stellar flares derived by Shibata & Yokoyama and compare it with observed flare/arcade loop length Lobs. The result shows a good correlation between them (Ltheor ~ Lobs) and indicates the need of plasma β for the scaling law (Ltheor ∝ EM3/5T-8/5nβ-6/5). This supports the theory of the scaling law and is indirect evidence that flares and arcades are heated by the same magnetic reconnection mechanism.

High-resolution X-ray spectroscopy of Procyon by Chandra and XMM-Newton

We report the analysis of the high-resolution soft X-ray spectrum of the nearby F-type star Procyon in the wavelength range from 5 to 175 A obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board Chandra and with the Reflection Grating Spectrometers (RGS) and the EPIC-MOS CCD spectrometers on board XMM-Newton. Line fluxes have been measured separately for the RGS and LETGS. Spectra have been tted globally to obtain self-consistent temperatures, emission measures, and abundances. The total volume emission measure is 4:1 10 50 cm 3 with a peak between 1 and 3 MK. No indications for a dominant hot component (T > 4 MK) were found. We present additional evidence for the lack of a solar-type FIP-eect, conrming earlier EUVE results.