Coronal X-ray Emission Research Articles

A scenario of planet erosion by coronal radiation

Context: According to theory, high-energy emission from the coronae of cool stars can severely erode the atmospheres of orbiting planets. No observational tests of the long term effects of erosion have yet been made. Aims: To analyze the current distribution of planetary mass with X-ray irradiation of the atmospheres in order to make an observational assessment of the effects of erosion by coronal radiation. Methods: We study a large sample of planet-hosting stars with XMM-Newton, Chandra and ROSAT; make a careful identification of X-ray counterparts; and fit their spectra to make accurately measurements of the stellar X-ray flux. Results: The distribution of the planetary masses with X-ray flux suggests that erosion has taken place: most surviving massive planets, (M_p sin i >1.5 M_J), have been exposed to lower accumulated irradiation. Heavy erosion during the initial stages of stellar evolution is followed by a phase of much weaker erosion. A line dividing these two phases could be present, showing a strong dependence on planet mass. Although a larger sample will be required to establish a well-defined erosion line, the distribution found is very suggestive. Conclusions: The distribution of planetary mass with X-ray flux is consistent with a scenario in which planet atmospheres have suffered the effects of erosion by coronal X-ray and EUV emission. The erosion line is an observational constraint to models of atmospheric erosion.

TESTING MODELS OF ACCRETION-DRIVEN CORONAL HEATING AND STELLAR WIND ACCELERATION FOR T TAURI STARS

Classical T Tauri stars are pre-main-sequence objects that undergo simultaneous accretion, wind outflow, and coronal X-ray emission. The impact of plasma on the stellar surface from magnetospheric accretion streams is likely to be a dominant source of energy and momentum in the upper atmospheres of these stars. This paper presents a set of models for the dynamics and heating of three distinct regions on T Tauri stars that are affected by accretion: (1) the shocked plasmas directly beneath the magnetospheric accretion streams, (2) stellar winds that are accelerated along open magnetic flux tubes, and (3) closed magnetic loops that resemble the Sun's coronal active regions. For the loops, a self-consistent model of coronal heating was derived from numerical simulations of solar field-line tangling and turbulent dissipation. Individual models are constructed for the properties of 14 well-observed stars in the Taurus-Auriga star-forming region. Predictions for the wind mass loss rates are, on average, slightly lower than the observations, which suggests that disk winds or X-winds may also contribute to the measured outflows. For some of the stars, however, the modeled stellar winds do appear to contribute significantly to the measured mass fluxes. Predictions for X-ray luminosities from the shocks and loops are in general agreement with existing observations. The stars with the highest accretion rates tend to have X-ray luminosities dominated by the high-temperature (5-10 MK) loops. The X-ray luminosities for the stars having lower accretion rates are dominated by the cooler accretion shocks.

Do solar decimetric spikes originate in coronal X-ray sources?

In the standard solar flare scenario, a large number of particles are accelerated in the corona. Nonthermal electrons emit both X-rays and radio waves. Thus, correlated signatures of the acceleration process are predicted at both wavelengths, coinciding either close to the footpoints of a magnetic loop or near the coronal X-ray source. We attempt to study the spatial connection between coronal X-ray emission and decimetric radio spikes to determine the site and geometry of the acceleration process. The positions of radio-spike sources and coronal X-ray sources are determined and analyzed in a well-observed limb event. Radio spikes are identified in observations from the Phoenix-2 spectrometer. Data from the Nan\c{c}ay radioheliograph are used to determine the position of the radio spikes. RHESSI images in soft and hard X-ray wavelengths are used to determine the X-ray flare geometry. Those observations are complemented by images from GOES/SXI. We find that decimetric spikes do not originate from coronal X-ray flare sources contrary to previous expectations. However, the observations suggest a causal link between the coronal X-ray source, related to the major energy release site, and simultaneous activity in the higher corona.

X-RAY-EMITTING STARS IDENTIFIED FROM THEROSATALL-SKY SURVEY AND THE SLOAN DIGITAL SKY SURVEY

The ROSAT All-Sky Survey (RASS) was the first imaging X-ray survey of the entire sky. While X-ray source counterparts are known to range from distant quasars to nearby M dwarfs, the RASS data alone are often insufficient to determine the nature of an X-ray source. As a result, large-scale follow-up programs are required to construct samples of known X-ray emitters. We use optical data produced by the Sloan Digital Sky Survey (SDSS) to identify 709 stellar X-ray emitters cataloged in the RASS and falling within the SDSS Data Release 1 footprint. Most of these are bright stars with coronal X-ray emission unsuitable for SDSS spectroscopy, which is designed for fainter objects (g > 15 mag). Instead, we use SDSS photometry, correlations with the Two Micron All Sky Survey and other catalogs, and spectroscopy from the Apache Point Observatory 3.5 m telescope to identify these stellar X-ray counterparts. Our sample of 707 X-ray-emitting F, G, K, and M stars is one of the largest X-ray-selected samples of such stars. We derive distances to these stars using photometric parallax relations appropriate for dwarfs on the main sequence, and use these distances to calculate LX. We also identify a previously unknown cataclysmic variable (CV) as a RASS counterpart. Separately, we use correlations of the RASS and the SDSS spectroscopic catalogs of CVs and white dwarfs (WDs) to study the properties of these rarer X-ray-emitting stars. We examine the relationship between (fX/fg) and the equivalent width of the Hbeta emission line for 46 X-ray-emitting CVs and discuss tentative classifications for a subset based on these quantities. We identify 17 new X-ray-emitting DA (hydrogen) WDs, of which three are newly identified WDs. (abridged)

Hard X-ray emission from the solar corona

This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even γ-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual “footpoint” emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration.

1-8 Å Coronal Background X-Ray Emission and the Associated Indicators of Photospheric Magnetic Activity

Consistent correlations between X-ray background flux (XBF) and sunspot area over three solar cycles suggest a strong relationship between the flux emergence at the locations of strong field regions, sunspots, and the coronal heating processes that ultimately result in coronal X-ray emission. We find that the pattern of variation of sunspot numbers shows noticeable deviations from the pattern depicted by the sunspot areas when compared to XBF. A weight factor, such as Wi, assigned to individual spot groups in R = k(Wigi + n) may provide better estimation of the true strength of solar cycles.

The X-ray cycle in the solar-type star HD 81809

Context. The 11-yr cycle is the best known manifestation of the Sun's activity. While chromospheric cycles have been studied in a number of solar-like stars, very little is known about how these are reflected in the cyclical behavior of the coronal X-ray emission in stars other than the Sun.

Coronal structure of the classical T Tauri star V2129 Oph

The nature of the magnetic coupling between T Tauri stars and their discs determines not only the mass accretion process but possibly the spin evolution of the central star. We have taken a recently published surface magnetogram of one moderately accreting T Tauri star (V2129 Oph) and used it to extrapolate the geometry of its large-scale field. We determine the structure of the open (wind-bearing) field lines, the closed (X-ray bright) field lines and the relatively small subset of field lines that pass through the equatorial plane inside the Keplerian corotation radius and which are therefore available to accrete. We consider a series of models in which the stellar magnetic field is opened up by the outward pressure of the hot coronal gas at a range of radii or source surfaces. As the source surface is increased, accretion takes place along progressively simpler field structures and impacts on progressively fewer sites at the stellar surface. This is consistent with the observed variation in the Ca ii IRT and He i lines which suggests that accretion in the visible hemisphere is confined to a single high-latitude spot. By determining the density and velocity of the accretion flows, we find that in order to have most of the total mass accretion rate impacting on a single high-latitude region we need disc material to accrete from approximately 7R⊙, close to the Keplerian corotation radius at 6.8R⊙. We also calculate the coronal density and X-ray emission measure. We find that both the magnitude and rotational modulation of the emission measure increase as the source surface is increased. For the field structure of V2129 Oph which is dominantly octupolar, the emission forms a bright, high-latitude ring that is always in view as the star rotates. Since the accretion funnels are not dense enough to cause significant scattering of coronal X-ray photons, they provide only a low rotational modulation of around 10 per cent at most.

Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM 513−46546

We present the first simultaneous radio, X-ray, ultraviolet, and optical spectroscopic observations of the M8.5 dwarf TVLM 513–46546, with a duration of 9 hr. These observations are part of a program to study the origin of magnetic activity in ultracool dwarfs, and its impact on chromospheric and coronal emission. Here we detect steady quiescent radio emission superposed with multiple short-duration, highly polarized flares; there is no evidence for periodic bursts previously reported for this object, indicating their transient nature. We also detect soft X-ray emission, with -->LX/Lbol ≈ 10−5.1, the faintest to date for any object later than M5, and a possible X-ray flare. TVLM 513–46546 continues the trend of severe violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4 orders of magnitude. From the optical spectroscopy we find that the Balmer line luminosity exceeds the X-ray luminosity by a factor of a few, ruling out chromospheric heating by coronal X-ray emission. More importantly, we detect sinusoidal Hα and Hβ equivalent width light curves with a period of 2 hr, matching the rotation period of TVLM 513–46546. This behavior points to a corotating chromospheric hot spot or an extended magnetic structure, with a covering fraction of about 50%. This feature may be transitory based on the apparent decline in light-curve peak during the four observed maxima. From the radio data we infer a large-scale and steady magnetic field of ~102 G. A large-scale field is also required by the sinusoidal Balmer line emission. The radio flares, on the other hand, are produced in a component of the field with a strength of ~3 kG and a likely multipolar configuration. The overall lack of correlation between the various activity indicators suggests that the short-duration radio flares do not have a strong influence on the chromosphere and corona, and that the chromospheric emission is not the result of coronal heating.

Hard X‐Ray Emissions from Partially Occulted Solar Flares

Observations of solar flares partially occulted by the solar limb provide diagnostics of coronal hard X-ray (HXR) emissions in the absence of generally much brighter emissions from footpoints of flare loops. In this paper, a statistical survey of 55 partially occulted flares observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) is presented, revealing the existence of two different components of coronal HXR emissions. Below ~15 keV thermal HXR emission with a gradual time profile is generally dominant, while at higher energies an additional component is seen in 50 out of 55 events. This additional component shows faster time variations in the order of tens of seconds and is most prominent during the rise of the thermal emission. A comparison of the centroid positions of these two emissions shows that they are most often cospatial within ~2000 km, although for a few events clear separations are observed as well. The spectra of the high-energy component show a rather steep (soft) power law with indices mostly between ~4 and ~6. Thin target emission in the corona from flare-accelerated electrons is discussed as a possible origin of the fast time variation component.

Searching for Decaying Axionlike Dark Matter from Clusters of Galaxies

We constrain the lifetime of radiatively decaying dark matter in clusters of galaxies inspired by generic Kaluza-Klein axions, which have been invoked as a possible explanation for the solar coronal x-ray emission. These particles can be produced inside stars and remain confined by the gravitational potential of clusters. By analyzing x-ray observations of merging clusters, where gravitational lensing observations have identified massive, baryon poor structures, we derive the first cosmological lifetime constraint on this kind of particles of tau > or = 10(23) sec.

The quasi-persistent neutron star soft X-ray transient 1M 1716-315 in quiescence

We report on our analysis of a 20 ks Chandra X-ray observation of the quasi-persistent neutron star soft X-ray transient (SXT) 1M 1716−315 in quiescence. Only one source was detected in the HEAO-1 error region. Its luminosity is 1.6 × 1032–1.3 × 1033 erg s−1. In this, the range is dominated by the uncertainty in the source distance. The source spectrum is well described by an absorbed soft spectrum, e.g. a neutron star atmosphere or blackbody model. No optical or near-infrared counterpart is present at the location of the X-ray source, down to a magnitude limit of I≳ 23.5 and Ks≳ 19.5. The positional evidence, the soft X-ray spectrum together with the optical and near-infrared non-detections provide strong evidence that this source is the quiescent neutron star SXT. The source is 10–100 times too bright in X-rays in order to be explained by stellar coronal X-ray emission. Together with the interstellar extinction measured in outburst and estimates for the source distance, the reported optical and near-infrared limit give an upper limit on the absolute magnitude of the counterpart of I > 8.6 and Ks > 5.1. This implies that the system is either an ultra-compact X-ray binary having Porb < 1 h or the companion star is an M-dwarf. We reconstructed the long-term X-ray light curve of the source. 1M 1716−315 has been active for more than 12 yr before returning to quiescence, the reported Chandra observation started 16.9 ± 4.1 yr after the outburst ended.

Hard X‐Ray Production in a Failed Filament Eruption

We revisit the failed filament eruption of 2002 May 27, first studied in detail by Ji et al. We investigate the temporal and spatial relationship between the filament dynamics and the production of hard X-ray emission using spatially resolved high-cadence data from TRACE and RHESSI. We confirm the presence of a hard X-ray source in the corona above the filament prior to the main activation phase and identify a second coronal hard X-ray source, not considered by earlier studies, that occurs under the apex of the filament during the erupting phase when the filament is clearly strongly kinked. We argue that this second source of coronal hard X-ray emission implies ongoing magnetic reconnection in a current sheet formed via a kink instability resulting from the interaction of the two adjacent legs underneath the writhing filament, in agreement with simulation results. The presence of this second energy release site has important implications for models of solar eruptions.

A simple model for the saturation of coronal X-ray emission of rapidly rotating late-type stars

Received ; accepted Abstract. An explanation of the observed saturation of X-ray emission with increasing rotation rate of late-type stars is suggested. The suggestion is based on a simple model for the closed field line region of the magnetized stellar corona (stellar magnetosphere). The model assumes a magnetic dipole field aligned with the rotation axis confining a co-rotating, isothermal plasma. The e ects of a stellar wind and of toroidal fields are neglected in the model. The closed field line region in the model is defined by those field lines on which the ratio of plasma pressure to magnetic pressure (plasma ) does not exceed one. Using scaling relations of surface magnetic field strength, surface density and coronal temperature with angular velocity, the model is used to calculate the stellar X-ray emission measure as a function of rotation rate. The results are in good agreement with the saturation in X-ray emission with rotation that is observed in G-K stars. The dependence of the results on the parameters of the model will be discussed.

NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars

We present a final summary of all ROSAT X-ray observations of nearby stars. All available ROSAT observations with the ROSAT PSPC, HRI and WFC have been matched with the CNS4 catalog of nearby stars and the results gathered in the Nearby X-ray and XUV-emitting Stars data base, available via www from the Home Page of the Hamburger Sternwarte at the URL http://www.hs.uni-hamburg.de/DE/For/Gal/Xgroup/nexxus. New volume-limited samples of F/G-stars (dlim = 14 pc), K-stars (dlim = 12 pc), and M-stars (dlim = 6 pc) are constructed within which detection rates of more than 90% are obtained; only one star (GJ 1002) remains undetected in a pointed follow-up observation. F/G-stars, K-stars and M-stars have indistinguishable surface X-ray flux distributions, and the lower envelope of the observed distribution at FX ≈ 10 4 erg/cm 2 /s is the X-ray flux level observed in solar coronal holes. Large amplitude variations in X-ray flux are uncommon for solar-like stars, but maybe more common for stars near the bottom of the main sequence; a large amplitude flare is reported for the M star LHS 288. Long term X-ray light curves are presented for α Cen A/B and Gl 86, showing variations on time scales of weeks and demonstrating that α Cen B is a flare star.

Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton

We report results from a comprehensive study of the nearby M dwarf Proxima Centauri with the XMM-Newton satellite, using simultaneously its X-ray detectors and the Optical Monitor with its U band filter. We find strongly variable coronal X-ray emission, with flares ranging over a factor of 100 in peak flux. The low-level emission is found to be continuously variable on at least three time scales (a slow decay of several hours, modulation on a time scale of 1 hr, and weak flares with time scales of a few minutes). Several weak flares are characteristically preceded by an optical burst, compatible with predictions from standard solar flare models. We propose that the U band bursts are proxies for the elusive stellar non-thermal hard X-ray bursts suggested from solar observations. In the course of the observation, a very large X-ray flare started and was observed essentially in its entirety. Its peak luminosity reached erg s-1 [0.15–10 keV], and the total X-ray energy released in the same band is derived to be ergs. This flare has for the first time allowed to measure significant density variations across several phases of the flare from X-ray spectroscopy of the O vii He-like triplet; we find peak densities reaching up to cm-3 for plasma of about 1–5 MK. Abundance ratios show little variability in time, with a tendency of elements with a high first ionization potential to be overabundant relative to solar photospheric values. Using Fe xvii lines with different oscillator strengths, we do not find significant effects due to opacity during the flare, indicating that large opacity increases are not the rule even in extreme flares. We model the large flare in terms of an analytic 2-Ribbon flare model and find that the flaring loop system should have large characteristic sizes (1) within the framework of this simplistic model. These results are supported by full hydrodynamic simulations. Comparing the large flare to flares of similar size occurring much more frequently on more active stars, we propose that the X-ray properties of active stars are a consequence of superimposed flares such as the example analyzed in this paper. Since larger flares produce hotter plasma, such a model also explains why, during episodes of low-level emission, more active stars show hotter plasma than less active stars.

Magnetic Loop Models: from Sun to Stars

I review recent progress in determining the nature of the loop structures that form the coronae of solar-like stars. This progress has been driven by observational advances, in particular the new results from X-ray satellites (Chandra and XMM-Newton) and the availability of surface magnetograms from Zeeman-Doppler imaging. It is now clear that stars that are similar to the Sun in mass, but which rotate more rapidly, have a very different magnetic field structure. Their surfaces are more heavily spotted, with spots appearing at all latitudes, extending all the way up to the rotation pole. Their coronae are correspondingly much brighter in X-rays, containing plasma that is hotter and denser than on the Sun. In addition, stellar coronae can support massive co-rotating prominences out to many stellar radii. Recent efforts in modelling these magnetic structures are now bringing together both the surface magnetograms and also the coronal X-ray emission. The resulting coronal loop models show complex loop structures on all scales, with much of the X-ray emission coming from high latitudes where is does not suffer rotational self-eclipse. The observed high densities and X-ray emission measures are a natural consequence of the high magnetic flux density at the surface. The stripping of the corona due to centrifugal effects at high rotation rates can also explain the saturation and supersaturation of X-ray emission with increasing rotation rates, and the recent observation of a high rotational modulation in a supersaturated star.

Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars

The maximum amplitude (A m a x ) of spot-induced brightness variations from long-term V-band photometry and the ratio L x /L ? o l between X-ray and bolometric luminosities are suitable indicators of the level of magnetic activity in the photosphere and in the corona of late-type stars, respectively. By using these activity indicators we investigate the dependence of coronal X-ray emission on the level of photospheric starspot activity in a homogeneous sample of low mass main sequence field and cluster stars of different ages (IC 2602, IC 4665, IC 2391, a Persei, Pleiades and Hyades). First, the activity-rotation connection at the photospheric level is re-analysed, as well as its dependence on spectral type and age. The upper envelope of A m a x increases monotonically with decreasing rotational period (P) and Rossby number (R 0 ) showing a break around 1.1 d that separates two rotation regimes where the starspot activity shows different behaviours. The A m a x -P and A m a x -R 0 relations are fitted with linear, exponential and power laws to look for the function which best represents the trend of the data. The highest values of A m a x are found among K-type stars and at the ages of α Persei and Pleiades. We also analyse the activity-rotation connection at the coronal level as well as its dependence on spectral type. The level of X-ray emission increases with increasing rotation rate up to a saturation level. The rotational period at which saturation occurs is colour-dependent and increases with advancing spectral type. Also the Lx/L b o l -P and Lx/L b o l -R 0 relations are fitted with linear, exponential and power laws to look for the best fitting function. Among the fastest rotating stars (P ≤ 0.3 d) there is evidence of super-saturation. Also the highest values of L x /L b o l are found among K-type stars. Finally, the photospheric-coronal activity connection is investigated by using for the first time the largest ever sample of light curve amplitudes as indicators of the magnetic filling factor. The activity parameters L x /L b o l and A m a x are found to be correlated with each other, thus confirming the dependence of coronal activity on photospheric magnetic fields. More precisely, the L x /L b o l -A m a x distribution shows the presence of an upper envelope, which is constant at the L x /L b o l ≃ -3.0 saturation level, and of a lower envelope. The best fit to the lower envelope is given by a power law with steepness decreasing from F-G to M spectral types. However, it is considered a tentative result, since the fit reduced chi-squares are large. Such spectral-type dependence may be related to a colour dependence of A m a x on the total starspot filling factor, as well as to the coronal emission being possibly more sensitive to starspot activity variations in F- and G-type than in M-type stars. The L x /L b o l -A m a x mean values for each cluster in our sample decrease monotonically with increasing age, showing that the levels of photospheric and coronal activity evolve in time according to a single power law till the Sun's age.

ChandraObservations of Magnetic White Dwarfs and Their Theoretical Implications

Observations of cool DA and DB white dwarfs have not yet been successful in detecting coronal X-ray emission, but observations of late-type dwarfs and giants show that coronae are common for these stars. To produce coronal X-rays, a star must have dynamo-generated surface magnetic fields and a well-developed convection zone. There is some observational evidence that the DA star LHS 1038 and the DB star GD 358 have weak and variable surface magnetic fields. It has been suggested that such fields can be generated by dynamo action, and since both stars have well-developed convection zones, theory predicts detectable levels of coronal X-rays from these white dwarfs. However, we present analysis of Chandra observations of both stars showing no detectable X-ray emission. The derived upper limits for the X-ray fluxes provide strong constraints on theories of formation of coronae around magnetic white dwarfs. Another important implication of our negative Chandra observations is the possibility that the magnetic fields of LHS 1038 and GD 358 are fossil fields.

Coronal X-Ray Emission from an Intermediate-Age Brown Dwarf

We report the X-ray detection of the brown dwarf (BD) companion TWA 5B in an 12 Myr old pre-main-sequence binary system. We clearly resolve the faint companion (35 photons) separated from the X-ray luminous primary by 2'' in a Chandra ACIS image. TWA 5B shows a soft X-ray spectrum with a low plasma temperature of only 0.3 keV and a constant flux during the 3 hr observation, of which the characteristics are commonly seen in the solar corona. The X-ray luminosity is 4 ? 1027 ergs s-1 (0.1-10 keV band) or log = -3.4. Comparing these properties to both younger and older BDs, we discuss the evolution of the X-ray emission in BDs. During their first few megayears, they exhibit high levels of X-ray activity, as seen in higher mass pre-main-sequence stars. The level in TWA 5B is still high at t 12 Myr in log LX/Lbol, while kT has already substantially cooled.