G. Peres

The Density of Coronal Plasma in Active Stellar Coronae

We have analyzed high-resolution X-ray spectra of a sample of 22 active stars observed with the High Energy Transmission Grating Spectrometer on Chandra in order to investigate their coronal plasma density. Densities were investigated using the lines of the He-like ions O VII, Mg XI, and Si XIII. Si XIII lines in all stars of the sample are compatible with the low-density limit (i.e., ne 1013 cm-3), casting some doubt on results based on lower resolution Extreme Ultraviolet Explorer (EUVE) spectra finding densities ne > 1013 cm-3. Mg XI lines betray the presence of high plasma densities up to a few times 1012 cm-3 for most of the sources with higher X-ray luminosity (1030 ergs s-1); stars with higher LX and LX/Lbol tend to have higher densities at high temperatures. Ratios of O VII lines yield much lower densities of a few times 1010 cm-3, indicating that the hot and cool plasma resides in physically different structures. In the cases of EV Lac, HD 223460, Canopus, ? Vel, TY Pyx, and IM Peg, our results represent the first spectroscopic estimates of coronal density. No trends in density-sensitive line ratios with stellar parameters effective temperature and surface gravity were found, indicating that plasma densities are remarkably similar for stars with pressure scale heights differing by up to 3 orders of magnitude. Our findings imply remarkably compact coronal structures, especially for the hotter (~7 MK) plasma emitting the Mg XI lines characterized by the coronal surface filling factor, f, ranging from 10-4 to 10-1, while we find f values from a few times 10-3 up to ~1 for the cooler (~2 MK) plasma emitting the O VII lines. We find that f approaches unity at the same stellar surface X-ray flux level as characterizes solar active regions, suggesting that these stars become completely covered by active regions. At the same surface flux level, f is seen to increase more sharply with increasing surface flux. These results appear to support earlier suggestions that hot 107 K plasma in active coronae arises from flaring activity and that this flaring activity increases markedly once the stellar surface becomes covered with active regions. Comparison of our measured line fluxes with theoretical models suggests that significant residual model inaccuracies might be present and, in particular, that cascade contributions to forbidden and intercombination lines resulting from dielectronic recombination might be to blame.

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

The Ultraluminous M81 X-9 Source: 20 Years’ Variability and Spectral States

The source X-9 was discovered with the Einstein Observatory in the —eld of M81 and is located in the dwarf galaxy Holmberg IX. X-9 has a 0.2¨4.0 keV luminosity in excess of the Eddington limit for a 1 compact accreting object, if it is at the same distance as Holmberg IX (3.4 Mpc). Past hypotheses on M _ the nature of this super-Eddington source included a supernova remnant or supershell, an accreting compact object, and a background QSO. To shed light on the nature of this source, we have analyzed archival data, including the Einstein data, 23 ROSAT observations, and BeppoSAX and ASCA pointings. Our analysis reveals that most of the emission of X-9 arises from a pointlike highly variable source (0.5¨ 2.4 keV ergs s~1) and that lower luminosity extended emission may be associated with L X D 2¨8 ] 1039 it. The spectrum of this source changes between low- and high-intensity states, in a way reminiscent of the spectra of galactic black hole candidates. Our result strongly suggest that X-9 is not a background QSO, but a bona —de ii super-Eddington ˇˇ source in Ho IX, a dwarf companion of M81.

On Stellar Coronae and Solar Active Regions

Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun near peak activity level obtained on 1992 January 6, we search for coronal structures that have emission measure distributions EM(T ) that match the observed stellar coronal emission measure distributions derived for the intermediate-activity stars v Eri (K2 V) and m Boo A (G8 V) from Extreme Ultraviolet Explorer spectro- scopic observations. We —nd that the temperatures of the peaks of the observed stellar distributions EM(T ), as well as their slopes in the temperature range are very similar to those 6.0 ( log T ( 6.5, obtained for the brightest of the solar active regions in the 1992 January 6 SXT images. The observed slopes correspond approximately to EM P T b with b D 4, which is much steeper than predicted by static, uniformly heated loop models. Plasma densities in the coronae of v Eri and m Boo A are also observed to be essentially the same as the plasma densities typical of solar active regions. These data provide the best observational support yet obtained for the hypothesis that solar-like stars up to the activity levels of v Eri (K2 V) and m Boo A are dominated by active regions similar to, though possibly considerably larger than, those observed on the Sun. The surface —lling factor of bright active regions needed to explain the observed stellar emission measures is approximately unity. We speculate on the scenario in which small-scale ii nano—ares ˇˇ dominate the heating of active regions up to activity levels similar to those of v Eri (K2 V) and m Boo A. At higher activity levels still, the interactions of the active regions themselves may lead to increasing —aring on larger scales that is responsible for heating plasma to the observed coronal temperatures of on very active stars. Observations of X-ray and T Z 107 K EUV light curves using more sensitive instruments than are currently available, together with determi- nations of plasma densities over the full range of coronal temperatures (106¨107 K and higher), will be important to con—rm —are heating hypotheses and to elicit further details concerning coronal structures at solar-like active region temperatures and the temperatures that characterize the most (T ( 5 ) 106 K) active stars (T Z 107 K). Subject headings: stars: coronaestars: individual (v Eridani, m Bootis) ¨ Sun: corona ¨ Sun: X-rays, gamma raysX-rays: stars

The Importance of Magnetic-Field-Oriented Thermal Conduction in the Interaction of SNR Shocks with Interstellar Clouds

We explore the importance of magnetic-field-oriented thermal conduction in the interaction of supernova remnant (SNR) shocks with radiative gas clouds and in determining the mass and energy exchange between the clouds and the hot surrounding medium. We perform 2.5-dimensional MHD simulations of a shock impacting on an isolated gas cloud, including anisotropic thermal conduction and radiative cooling; we consider the representative case of a Mach 50 shock impacting on a cloud 10 times denser than the ambient medium. We consider different configurations of the ambient magnetic field and compare MHD models with or without thermal conduction. The efficiency of thermal conduction in the presence of a magnetic field is, in general, reduced with respect to the unmagnetized case. The reduction factor strongly depends on the initial magnetic field orientation, and it is at a minimum when the magnetic field is initially aligned with the direction of the shock propagation. Thermal conduction contributes to the suppression of hydrodynamic instabilities, reducing the mass mixing of the cloud and preserving the cloud from complete fragmentation. Depending on the magnetic field orientation, the heat conduction may determine a significant energy exchange between the cloud and the hot surrounding medium which, while remaining always at levels less than those in the unmagnetized case, leads to a progressive heating and evaporation of the cloud. This additional heating may offset the radiative cooling of some parts of the cloud, preventing the onset of thermal instabilities.

X-ray emission from MP Muscae : an old classical T Tauri star

Aims. We study the properties of X-ray emitting plasma of MP Mus, an old classical T Tauri star. We aim at checking whether an accretion process produces the observed X-ray emission and at deriving the accretion parameters and the characteristi cs of the shockheated plasma. We compare the properties of MP Mus with those of younger classical T Tauri stars to test whether age is rela ted to the properties of the X-ray emission plasma. Methods. XMM-Newton X-ray spectra allows us to measure plasma temperatures, abundances, and electron density. In particular th e density of cool plasma probes whether X-ray emission is produced by plasma heated in the accretion process. Results. X-ray emission from MP Mus originates from high density cool plasma but a hot flaring component is also present, suggestin g that both coronal magnetic activity and accretion contribute to the observed X-ray emission. We find a Ne/O ratio similar to that observed in the much younger classical T Tauri star BP Tau. From the soft part of the X-ray emission, mostly produced by plasma heated in the accretion shock, we derive a mass accretion rate of 5× 10 −11 M⊙ yr −1 .

TRACE-derived Temperature and Emission Measure Profiles along Long-lived Coronal Loops: The Role of Filamentation

In a recent Letter, Lenz et al. have shown evidence of uniform temperature along steady long coronal loops observed by TRACE in two different passbands (171 and 195 Å filters). We propose that such a piece of evidence can be explained by the subarcsecond structuring of the loops across the magnetic field lines. In this perspective, we present a model of a bundle of six thin parallel hydrostatic filaments with temperature stratification dictated by detailed energy balance and with temperatures at their apex ranging between 0.8 and 5 MK. If analyzed as a single loop, the bundle would appear isothermal along most of its length.

X-ray emitting MHD accretion shocks in classical T Tauri stars Case for moderate to high plasma-β values

Context. Plasma accreting onto classical T Tauri stars (CTTS) is believed to impact the stellar surface at free-fall velocities, generating a shock. Current time-dependent models describing accretion shocks in CTTSs are one-dimensional, assuming that the plasma moves and transports energy only along magnetic field lines (β � 1). Aims. We investigate the stability and dynamics of accretion shocks in CTTSs, considering the case of β > 1 in the post-shock region. In these cases the 1D approximation is not valid and a multi-dimensional MHD approach is necessary. Methods. We model an accretion stream propagating through the atmosphere of a CTTS and impacting onto its chromosphere by performing 2D axisymmetric MHD simulations. The model takes into account the stellar magnetic field, the gravity, the radiative cooling, and the thermal conduction (including the effects of heat flux saturation). Results. The dynamics and stability of the accretion shock strongly depend on the plasma β. In the case of shocks with β> 10, violent outflows of shock-heated material (and possibly MHD waves) are generated at the base of the accretion column and intensely perturb the surrounding stellar atmosphere and the accretion column itself (therefore modifying the dynamics of the shock). In shocks with β ≈ 1, the post-shock region is efficiently confined by the magnetic field. The shock oscillations induced by cooling instability are strongly influenced by β :f or β> 10, the oscillations may be rapidly dumped by the magnetic field, approaching a quasi-stationary state, or may be chaotic with no obvious periodicity due to perturbation of the stream induced by the post-shock plasma itself; for β ≈ 1 the oscillations are quasi-periodic, although their amplitude is smaller and the frequency higher than those predicted by 1D models.

X-ray emission from dense plasma in classical T Tauri stars: hydrodynamic modeling of the accretion shock

Context. High spectral resolution X-ray observations of classical T Tauri stars (CTTSs) demonstrate the presence of plasma at temperature T ∼ 2−3 × 10 6 K and density ne ∼ 10 11 −10 13 cm −3 , which are unobserved in non-accreting stars. Stationary models suggest that this emission is due to shock-heated accreting material, but do not allow us to analyze the stability of the material and its position in the stellar atmosphere. Aims. We investigate the dynamics and stability of shock-heated accreting material in classical T Tauri stars and the role of the stellar chromosphere in determining the position and thickness of the shocked region. Methods. We perform one-dimensional hydrodynamic simulations of the impact of an accretion flow on the chromosphere of a CTTS, including the effects of gravity, radiative losses from optically thin plasma, thermal conduction and a well tested detailed model of the stellar chromosphere. We present the results of a simulation based on the parameters of the CTTS MP Mus. Results. We find that the accretion shock generates an hot slab of material above the chromosphere with a maximum thickness of 1.8 × 10 9 cm, density ne ∼ 10 11 −10 12 cm −3 , temperature T ∼ 3 × 10 6 K, and uniform pressure equal to the ram pressure of the accretion flow (∼450 dyn cm −2 ). The base of the shocked region penetrates the chromosphere and remains at a position at which the ram pressure is equal to the thermal pressure. The system evolves with quasi-periodic instabilities of the material in the slab

Morphology and spectral characteristics of the X-ray emission of M33

A previous analysis of the X-ray data on M33 has been extended to include a detailed study of the morpholgoy and spectral characteristics of the X-ray emission, and the results are reported. A low surface brightness, extended emission in the plane of the galaxy is detected. The X-ray luminosity of this component, about 10 to the 38th egs/s, is comparable to the total luminosity of the bright sources observed in the same region. Its radial distribution is similar to that of the blue light. The spectrum of the extended emission shows two distinct components: a hard one, with a temperature above 3 keV and a soft one with a temperature below 1 keV. The X-ray spectrum of the nuclear source, which is inconsistent with any of the known spectra of X-ray binary sources, can be fitted with either a low-temperature thermal emission or a steep power law model. 71 references.