arXiv subjects

J. H. M. M. Schmitt

Publications and source records attributed to J. H. M. M. Schmitt.

209 records · Page 12Linked to original sources

arXiv2002

Coronal density diagnostics with Helium-like triplets: CHANDRA--LETGS observations of Algol, Capella, Procyon, Eps Eri, Alpha Cen A&B, UX Ari, AD Leo, YY Gem, and HR1099

We present an analysis of ten cool stars (Algol, Capella, Procyon, Eps Eri, Alpha Cen A&B, UX Ari, AD Leo, YY Gem, and HR1099) observed with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. This sample contains all cools stars observed with the LETGS presently available to us with integration times sufficiently long to warrant a meaningful spectral analysis. Our sample comprises inactive, moderately active, and hyperactive stars and samples the bulk part of activity levels encountered in coronal X-ray sources. We use the LETGS spectra to carry out density and temperature diagnostics with an emphasis on the H-like and the He-like ions. We find a correlation between line flux ratios of the Lyman-Alpha and He-like resonance lines with the mean X-ray surface flux. We determine densities using the He-like triplets. For active stars we find no significant deviations from the low-density limit for the ions of Ne, Mg, and Si, while the measured line ratios for the ions of C, N, and O do show evidence for departures from the low-density limit in the active stars, but not in the inactive stars. Best measurements can be made for the OVII triplet where we find significant deviations from the low-density limit for the stars Algol, Procyon, YY Gem, Eps Eri, and HR1099. We discuss the influence of radiation fields on the interpretation of the He-like triplet line ratios in the low-Z ions, which is relevant for Algol, and the influence of dielectronic satellite lines, which is relevant for Procyon. For the active stars YY Gem, Eps Eri, and HR1099 the low f/i ratios can unambiguously be attributed to high densties in the range 1--3 10^10 cm^-3 at OVII temperatures. We find our LETGS spectra to be an extremely useful tool for plasma diagnostics of stellar coronae.

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arXiv2002

Simultaneous X-ray spectroscopy of YY Gem with Chandra and XMM-Newton

We report on a detailed study of the X-ray spectrum of the nearby eclipsing spectroscopic binary YY Gem. Observations were obtained simultaneously with both large X-ray observatories, XMM-Newton and Chandra. We compare the high-resolution spectra acquired with the Reflection Grating Spectrometer onboard XMM-Newton and with the Low Energy Transmission Grating Spectrometer onboard Chandra, and evidence in direct comparison the good performance of both instruments in terms of wavelength and flux calibration. The strongest lines in the X-ray spectrum of YY Gem are from oxygen. Oxygen line ratios indicate the presence of a low-temperature component (1-4 MK) with density n_e < 2 10^{10} cm^-3. The X-ray lightcurve reveals two flares and a dip corresponding to the secondary eclipse. An increase of the density during phases of high activity is suggested from time-resolved spectroscopy. Time-resolved global fitting of the European Photon Imaging Camera CCD spectrum traces the evolution of temperature and emission measure during the flares. These medium-resolution spectra show that temperatures > 10^7 K are relevant in the corona of YY Gem although not as dominant as the lower temperatures represented by the strongest lines in the high-resolution spectrum. Magnetic loops with length on the order of 10^9 cm, i.e., about 5 % of the radius of each star, are inferred from a comparison with a one-dimensional hydrodynamic model. This suggests that the flares did not erupt in the (presumably more extended) inter-binary magnetosphere but are related to one of the components of the binary.

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arXiv2002

Chandra LETGS Observation of the Active Binary Algol

A high-resolution spectrum obtained with the low-energy transmission grating onboard the Chandra observatory is presented and analyzed. Our analysis indicates very hot plasma with temperatures up to T=15-20MK from the continuum and from ratios of hydrogen-like and helium-like ions of Si, Mg, and Ne. In addition lower temperature material is present since OVII and NVI are detected. Two methods for density diagnostics are applied. The He-like triplets from NVII to SiXIII are used and densities around 10^11 cm^-3 are found for the low temperature ions. Taking the UV radiation field from the B star companion into account, we find that the low-Z ions can be affected by the radiation field quite strongly, such that densities of 3 10^10 cm^-3 are also possible, but only assuming that the emitting plasma is immersed in the radiation field. For the high temperature He-like ions only low density limits are found. Using ratios of FeXXI lines produced at similar temperatures are sensitive to lower densities but again yield only low density limits. We thus conclude that the hot plasma has densities below 10^12 cm^-3. Assuming a constant pressure corona we show that the characteristic loop sizes must be small compared to the stellar radius and that filling factors below 0.1 are unlikely.

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arXiv2001

The joint XMM-Newton and Chandra view of YY Gem

We have observed the flare star YY Gem simultaneously with XMM-Newton and Chandra as part of a multi-wavelength campaign aiming at a study of variability related to magnetic activity in this short-period eclipsing binary. Here we report on the first results from the analysis of the X-ray spectrum. The vicinity of the star provides high enough S/N in the CCD cameras onboard XMM-Newton to allow for time-resolved spectroscopy. Since the data are acquired simultaneously they allow for a cross-calibration check of the performance of the XMM-Newton RGS and the LETGS on Chandra.

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arXiv2001

Detection of differential rotation in psi Cap with profile analysis

We report detection of differential rotation on the F5 dwarf psi Cap using line profile analysis. The Fourier transform of both FeI lambda 5775 and SiI lambda 5772 are used to obtain a projected rotational velocity of v sini = (42+-1)km/s. Modelling of the Fourier transformed profiles shows that the combined effects of equatorial velocity, inclination and differential rotation dominate the line profile while limb darkening and turbulence velocities have only minor effects. Rigid rotation is shown to be inconsistent with the measured profiles. Modelling the line profiles analogous to solar differential rotation we find a differential rotation parameter of alpha = 0.15+-0.1 (15+-10%) comparable to the solar case. To our knowledge this is the first successful measurement of differential rotation through line profile analysis.

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arXiv2001

Microwave plasma emission of a flare on AD Leo

An intense radio flare on the dMe star AD Leo, observed with the Effelsberg radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85 GHz is analysed. A lower limit of the brightness temperature of the totally right handed polarized emission is estimated as T_b ~ 5x10^10 K (with values T_b > ~3x10^13 K considered to be more probable), which requires a coherent radio emission process. In the interpretation we favour fundamental plasma radiation by mildly relativistic electrons trapped in a hot and dense coronal loop above electron cyclotron maser emission. This leads to densities and magnetic field strengths in the radio source of n ~ 2x10^11 cm^-3 and B ~ 800 G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of r ~ 7x10^8 cm. A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature T >= 2x10^7 K and density n_ext <= 10^-2 n. Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons.

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arXiv2000

Helium-like triplet density diagnostics: Applications to CHANDRA--LETGS X-ray observations of Capella and Procyon

Electron density diagnostics based on the triplets of Helium-like CV, NVI, and OVII are applied to the X-ray spectra of Capella and Procyon measured with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory. New theoretical models for the calculation of the line ratios between the forbidden (f), intercombination (i), and the resonance (r) lines of the helium-like triplets are used. The derived densities are quite typical of densities found in the solar active regions, and also pressures and temperatures in Procyon's and Capella's corona at a level of T=10^6K are quite similar. We find no evidence for densities as high as measured in solar flares. Comparison of our Capella and Procyon measurements with the Sun shows little difference in the physical properties of the layers producing the CV, NVI, and OVII emission. Assuming the X-ray emitting plasma to be confined in magnetic loops, we obtain typical loop length scales of L_Capella > 8 L_Procyon from the loop scaling laws, implying that the magnetic structures in Procyon and Capella are quite different. The total mean surface fluxes emitted in the helium- and hydrogen-like ions are quite similar for Capella and Procyon, but exceed typical solar values by one order of magnitude. We thus conclude that Procyon's and Capella's coronal filling factors are larger than corresponding solar values.

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arXiv2000

An X-ray Flare Detected on the M8 Dwarf VB 10

We have detected an X-ray flare on the very low mass star VB 10 (GL 752 B; M8 V) using the ROSAT High Resolution Imager. VB 10 is the latest type (lowest mass) main sequence star known to exhibit coronal activity. X-rays were detected from the star during a single 1.1-ksec segment of an observation which lasted 22 ksec in total. The energy released by this flare is on the order of 10^27 ergs/sec. This is at least two orders of magnitude greater than the quiescent X-ray luminosity of VB 10, which has yet to be measured. This X-ray flare is very similar in nature to the far ultraviolet flare which was observed by Linsky et al. (1995) using the Goddard High Resolution Spectrograph onboard Hubble Space Telescope. We discuss reasons for the extreme difference between the flare and quiescent X-ray luminosities, including the possibility that VB 10 has no quiescent (10^6 K) coronal plasma at all.

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arXiv2000

First Light Measurements of Capella with the Low Energy Transmission Grating Spectrometer aboard the Chandra X-ray Observatory

We present the first X-ray spectrum obtained by the Low Energy Transmission Grating Spectrometer (LETGS) aboard the Chandra X-ray Observatory. The spectrum is of Capella and covers a wavelength range of 5-175 A (2.5-0.07 keV). The measured wavelength resolution, which is in good agreement with ground calibration, is $Δλ\simeq$ 0.06 A (FWHM). Although in-flight calibration of the LETGS is in progress, the high spectral resolution and unique wavelength coverage of the LETGS are well demonstrated by the results from Capella, a coronal source rich in spectral emission lines. While the primary purpose of this letter is to demonstrate the spectroscopic potential of the LETGS, we also briefly present some preliminary astrophysical results. We discuss plasma parameters derived from line ratios in narrow spectral bands, such as the electron density diagnostics of the He-like triplets of carbon, nitrogen, and oxygen, as well as resonance scattering of the strong Fe XVII line at 15.014 A.

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arXiv1999

Spectroscopic analysis of a super-hot giant flare observed on Algol by BeppoSAX on 30 August 1997

We present an X-ray observation of the eclipsing binary Algol, obtained with the BeppoSAX observatory. During the observation a huge flare was observed, exceptional both in duration as well as in peak plasma temperature and total energy release. The wide spectral response of the different BeppoSAX instruments, together with the long decay time scale of the flare, allowed us to perform a detailed time-resolved X-ray spectroscopic analysis of the flare. We derive the physical parameters of the emitting region together with the plasma density applying different methods to the observed flare decay. The X-ray emission from the flare is totally eclipsed during the secondary optical eclipse, so that the size of the emitting region is strongly constrained (as described in a companion paper) on purely geometrical arguments. The size of the flare thus derived is much smaller than the size derived from the analysis of the evolution of the spectral parameters using the quasi-static cooling formalism, showing that the time evolution of the flare is determined essentially from the temporal profile of the heating, with the intrinsic decay of the flaring loop having little relevance. The very high signal-to-noise of the individual spectra strongly constrains some of the derived physical parameters. In particular, very significant evidence for a three-fold increase in coronal abundance and for a large increase in absorbing column density during the initial phases of the flare evolution is present.

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arXiv1999

Continuous heating of a giant X-ray flare on Algol

Giant flares can release large amounts of energy within a few days: X-ray emission alone can be up to ten percent of the star's bolometric luminosity. These flares exceed the luminosities of the largest solar flares by many orders of magnitude, which suggests that the underlying physical mechanisms supplying the energy are different from those on the Sun. Magnetic coupling between the components in a binary system or between a young star and an accretion disk has been proposed as a prerequisite for giant flares. Here we report X-ray observations of a giant flare on Algol B, a giant star in an eclipsing binary system. We observed a total X-ray eclipse of the flare, which demonstrates that the plasma was confined to Algol B, and reached a maximum height of 0.6 stellar radii above its surface. The flare occurred around the south pole of Algol B, and energy must have been released continously throughout its life. We conclude that a specific extrastellar environment is not required for the presence of a flare, and that the processes at work are therefore similar to those on the Sun.

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