We investigate the long-term evolution of X-ray coronae of solar analogs based on high-resolution X-ray spectroscopy and photometry with XMM-Newton. Six nearby main-sequence G stars with ages between ≈ 0.1 Gyr and ≈ 1.6 Gyr and rotation periods between ≈ 1 d and 12.4 d have been observed. We use the X-ray spectra to derive coronal element abundances of C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measure distribution (EMD). We find that the abundances change from an inverse-First Ionization Potential (FIP) distribution in stars with ages around 0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr and beyond. This transformation is coincident with a steep decline of non-thermal radio emission. The results are in qualitative agreement with a simple model in which the stream of electrons in magnetic fields suppresses diffusion of low-FIP ions from the chromosphere into the corona. The coronal emission measure distributions show shapes characterized by power-laws on each side of the EMD peak. The latter shifts from temperatures of about 10 MK in the most rapidly rotating, young stars to temperatures around 4 MK in the oldest target considered here. The power-law index on the cooler side of the EMD exceeds expected slopes for static loops, with typical values being 1.5-3. We interpret this slope with a model in which the coronal emission is due to a superposition of stochastically occurring flares, with an occurrence rate that is distributed in radiated energy E as a power-law, dN/dE ∝ E −α , as previously found for solar and stellar flares. We obtain the relevant power-law index α from the slope of the high-temperature tail of the EMD. Our EMDs indicate α ≈ 2.2 − 2.8, in excellent agreement with values previously derived from light curves of magnetically active stars. Modulation with time scales reminiscent of flares is found in the light curves of all our targets. Several strong flares are also observed. We use our α values to simulate light curves and compare them with the observed light curves. We thus derive the range of flare energies required to explain the light-curve modulation. More active stars require a larger range of flare energies than less active stars within the framework of this simplistic model. In an overall scenario, we propose that flaring activity plays a larger role in more active stars. In this model, the higher flare rate is responsible both for the higher average coronal temperature and the high coronal X-ray luminosity, two parameters that are indeed found to be correlated.
The Swift GRB satellite is an excellent facility for studying novae. Its rapid response time and sensitive X-ray detector provides an unparalleled opportunity to investigate the -2previously poorly sampled evolution of novae in the X-ray regime. This paper presents Swift observations of 52 Galactic/Magellanic Cloud novae. We included the XRT (0.3-10 keV) X-ray instrument count rates and the UVOT (1700-8000Å) filter photometry. Also included in the analysis are the publicly available pointed observations of 10 additional novae the X-ray archives. This is the largest X-ray sample of Galactic/Magellanic Cloud novae yet assembled and consists of 26 novae with super soft X-ray emission, 19 from Swift observations. The data set shows that the faster novae have an early hard X-ray phase that is usually missing in slower novae. The Super Soft X-ray phase occurs earlier and does not last as long in fast novae compared to slower novae. All the Swift novae with sufficient observations show that novae are highly variable with rapid variability and different periodicities. In the majority of cases, nuclear burning ceases less than 3 years after the outburst begins. Previous relationships, such as the nuclear burning duration vs. t 2 or the expansion velocity of the eject and nuclear burning duration vs. the orbital period, are shown to be poorly correlated with the full sample indicating that additional factors beyond the white dwarf mass and binary separation play important roles in the evolution of a nova outburst. Finally, we confirm two optical phenomena that are correlated with strong, soft X-ray emission which can be used to further increase the efficiency of X-ray campaigns.
Abstract.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 175Å 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 fitted 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-effect, confirming earlier EUVE results.
Swift X-ray observations of the ∼60 day supersoft phase of the recurrent nova RS Ophiuchi (RS Oph) 2006 show the progress of nuclear burning on the white dwarf (WD) in exquisite detail. First seen 26 days after the optical outburst, this phase started with extreme variability likely due to variable absorption, although intrinsic WD variations are not excluded. About 32 days later, a steady decline in count rate set in. NLTE model atmosphere spectral fits during the supersoft phase show that the effective temperature of the WD increases from ∼65 eV to ∼90 eV during the extreme variability phase, falling slowly after about day 60 and more rapidly after day 80. The bolometric luminosity is seen to be approximately constant and close to Eddington from day 45 up to day 60, the subsequent decline possibly signaling the end of extensive nuclear burning. Before the decline, a multiply-periodic ∼35 s modulation of the soft X-rays was present and may be the signature of a nuclear fusion driven instability. Our measurements are consistent with a WD mass near the Chandrasekhar limit; combined with a deduced accumulation of mass transferred from its binary companion, this leads us to suggest that RS Oph is a strong candidate for a future supernova explosion. The main uncertainty now is whether the WD is the CO type necessary for a Type Ia supernova. This may be confirmed by detailed abundance analyses of spectroscopic data from the outbursts.
RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts have indicated that high-velocity ejecta interact with a preexisting red giant wind, setting up shock systems analogous to those seen in supernova remnants. However, in the previous outburst in 1985, X-ray observations did not commence until 55 days after the initial explosion. Here we report on Swift observations covering the first month of the 2006 outburst with the Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14Y25 keV band from t ¼ 0 to t $ 6 days. XRT observations from 0.3 to 10 keV started 3.17 days after outburst. The rapidly evolving XRT spectra clearly show the presence of both line and continuum emission, which can be fitted by thermal emission from hot gas whose characteristic temperature, overlying absorbing column (N H ) W , and resulting unabsorbed total flux decline monotonically after the first few days. Derived shock velocities are in good agreement with those found from observations at other wavelengths. Similarly, (N H ) W is in accord with that expected from the red giant wind ahead of the forward shock. We confirm the basic models of the 1985 outburst and conclude that standard phase I remnant evolution terminated by t $ 6 days and the remnant then rapidly evolved to display behavior characteristic of phase III. Around t ¼ 26 days, however, a new, luminous, and highly variable soft X-ray source began to appear, whose origin will be explored in a subsequent paper.
Abstract. Spatial information from stellar X-ray coronae cannot be assessed directly, but scaling laws from the solar corona make it possible to estimate sizes of stellar coronae from the physical parameters temperature and density. While coronal plasma temperatures have long been available, we concentrate on the newly available density measurements from line fluxes of X-ray lines measured for a large sample of stellar coronae with the Chandra and XMM-Newton gratings. We compiled a set of 64 grating spectra of 42 stellar coronae. Line counts of strong H-like and He-like ions and Fe lines were measured with the CORA single-purpose line fitting tool by Ness & Wichmann (2002). Densities are estimated from He-like f /i flux ratios of O and Ne representing the cooler (1-6 MK) plasma components. The densities scatter between log n e ≈ 9.5−11 from the O triplet and between log n e ≈ 10.5−12 from the Ne triplet, but we caution that the latter triplet may be biased by contamination from Fe and Fe lines. We find that low-activity stars (as parameterized by the characteristic temperature derived from H-and He-like line flux ratios) tend to show densities derived from O of no more than a few times 10 10 cm −3 , whereas no definitive trend is found for the more active stars. Investigating the densities of the hotter plasma with various Fe line ratios, we found that none of the spectra consistently indicates the presence of very high densities. We argue that our measurements are compatible with the low-density limit for the respective ratios (≈5 × 10 12 cm −3 ). These upper limits are in line with constant pressure in the emitting active regions. We focus on the commonly used Rosner et al. (1978) scaling law to derive loop lengths from temperatures and densities assuming loop-like structures as identical building blocks. We derive the emitting volumes from direct measurements of ionspecific emission measures and densities. Available volumes are calculated from the loop-lengths and stellar radii, and are compared with the emitting volumes to infer filling factors. For all stages of activity we find similar filling factors up to 0.1.
V4743 Sgr (Nova Sgr 2002 No. 3) was discovered on 20 September 2002. We obtained a 5 ks ACIS-S spectrum in November 2002 and found that the nova was faint in X-rays. We then obtained a 25 ks CHANDRA LETGS observation on 19 March 2003. By this time, it had evolved into the Super Soft X-ray phase exhibiting a continuous spectrum with deep absorption features. The light curve from the observation showed large amplitude oscillations with a period of 1325 s (22 min) followed by a decline in total count rate after ∼ 13 ks of observations. The count rate dropped from ∼ 40 cts s −1 to practically zero within ∼ 6 ks and stayed low for the rest of the observation (∼ 6 ks. The spectral hardness ratio changed from maxima to minima in correlation with the oscillations, and then became significantly softer during the decay. Strong H-like and He-like lines of oxygen, nitrogen, and carbon were found in absorption during the bright phase, indicating temperatures between 1-2 MK, but they were shifted in wavelength corresponding to a Doppler velocity of -2400 km s −1 . The spectrum obtained after the decline in count rate showed emission lines of C vi, N vi, and N vii suggesting that we were seeing expanding gas ejected during the outburst, probably originating from CNO-cycled material. An XMM-Newton ToO observation, obtained on 4 April 2003 and a later LETGS observation from 18 July 2003 also showed oscillations, but with smaller amplitudes.
Abstract. Electron density diagnostics based on the triplets of helium-like C v, N vi, and O vii 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 (logarithmic) electron densities (in cgs units) derived from the f/i ratios for Capella are < 9.38 cm −3 for O vii (2σ upper limit) (f/i = 4.0 ± 0.25), 9.86 ± 0.12 cm −3 for N vi (f/i = 1.78 ± 0.25), and 9.42 ± 0.21 cm −3 for C v (f/i = 1.48 ± 0.34), while for Procyon we obtain 9.28for N vi (f/i = 1.33 ± 0.28), and < 8.92 cm −3 for C v (f/i = 0.48 ± 0.12). These 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 6 K 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 C v, N vi, and O vii emission. Assuming the X-ray emitting plasma to be confined in magnetic loops, we obtain typical loop length scales of L Capella ≥ 8 LProcyon 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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