arXiv subjects

J. Robrade

Publications and source records attributed to J. Robrade.

At least 73 records · Page 4Linked to original sources

arXiv2010

eROSITA on SRG: a X-ray all-sky survey mission

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission which is scheduled for launch in late 2012. eROSITA is fully approved and funded by the German Space Agency DLR and the Max-Planck-Society. The design driving science is the detection of 50 - 100 thousands Clusters of Galaxies up to redshift z ~ 1.3 in order to study the large scale structure in the Universe and test cosmological models, especially Dark Energy. This will be accomplished by an all-sky survey lasting for four years plus a phase of pointed observations. eROSITA consists of seven Wolter-I telescope modules, each equipped with 54 Wolter-I shells having an outer diameter of 360 mm. This would provide and effective area at 1.5 keV of ~ 1500 cm2 and an on axis PSF HEW of 15" which would provide an effective angular resolution of 25"-30". In the focus of each mirror module, a fast frame-store pn-CCD will provide a field of view of 1 deg in diameter for an active FOV of ~ 0.83 deg^2. At the time of writing the instrument development is currently in phase C/D.

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arXiv2010

X-ray emission from the remarkable A-type star HR 8799

We present a Chandra observation of the exceptional planet bearing A5V star HR 8799, more precisely classified as a kA5hF0mA5 star and search for intrinsic X-ray emission. We clearly detect HR 8799 at soft X-ray energies with the ACIS-S detector in a 10 ks exposure; minor X-ray brightness variability is present during the observation. The coronal plasma is described well by a model with a temperature of around 3 MK and an X-ray luminosity of about Lx = 1.3 x 10^28 erg/s in the 0.2-2.0 keV band, corresponding to an activity level of log Lx/Lbol ~ -6.2. Altogether, these findings point to a rather weakly active and given a RASS detection, long-term stable X-ray emitting star. The X-ray emission from HR 8799 resembles those of a late A/early F-type stars, in agreement with its classification from hydrogen lines and effective temperature determination and thus resolving the apparent discrepancy with the standard picture of magnetic activity that predicts mid A-type stars to be virtually X-ray dark.

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arXiv2010

Coronal properties of planet-bearing stars

Do extrasolar planets affect the activity of their host stars? Indications for chromospheric activity enhancement have been found for a handful of targets, but in the X-ray regime, conclusive observational evidence is still missing. We want to establish a sound observational basis to confirm or reject major effects of Star-Planet Interactions (SPI) in stellar X-ray emissions. We therefore conduct a statistical analysis of stellar X-ray activity of all known planet-bearing stars within 30pc distance for dependencies on planetary parameters such as mass and semimajor axis. We find that in our sample, there are no significant correlations of X-ray luminosity or the activity indicator L_X/L_bol with planetary parameters which cannot be explained by selection effects. Coronal SPI seems to be a phenomenon which might only manifest itself as a strong effect for a few individual targets, but not to have a major effect on planet-bearing stars in general.

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arXiv2010

Quiescent and flaring X-ray emission from the nearby M/T dwarf binary SCR 1845-6357

We investigate an XMM-Newton observation of SCR 1845-6357, a nearby, ultracool M8.5/T5.5 dwarf binary. The binary is unresolved in the XMM detectors, however the X-ray emission is very likely from the M8.5 dwarf. We compare its flaring emission to those of similar very low mass stars and additionally present an XMM observation of the M8 dwarf VB 10. We detect quasi-quiescent X-ray emission from SCR 1845-6357 at soft X-ray energies in the 0.2-2.0 keV band, as well as a strong flare with a count rate increase of a factor of 30 and a duration of only 10 minutes. The quasi-quiescent X-ray luminosity of log L_x = 26.2 erg/s and the corresponding activity level of log L_x/L_bol = -3.8 point to a fairly active star. Coronal temperatures of up to 5 MK and frequent minor variability support this picture. During the flare, that is accompanied by a significant brightening in the near-UV, plasma temperatures of 25-30 MK are observed and an X-ray luminosity of L_x= 8 x 10^27 erg/s is reached. SCR 1845-6357 is a nearby, very low mass star that emits X-rays at detectable levels in quasi-quiescence, implying the existence of a corona. The high activity level, coronal temperatures and the observed large flare point to a rather active star, despite its estimated age of a few Gyr.

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arXiv2009

51 Pegasi - a planet-bearing Maunder minimum candidate

We observed 51 Peg, the first detected planet-bearing star, in a 55 ks XMM-Newton pointing and in 5 ks pointings each with Chandra HRC-I and ACIS-S. The star has a very low count rate in the XMM observation, but is clearly visible in the Chandra images due to the detectors' different sensitivity at low X-ray energies. This allows a temperature estimate for 51 Peg's corona of T<1MK; the detected ACIS-S photons can be plausibly explained by emission lines of a very cool plasma near 200eV. The constantly low X-ray surface flux and the flat-activity profile seen in optical CaII data suggest that 51 Peg is a Maunder minimum star; an activity enhancement due to a Hot Jupiter, as proposed by recent studies, seems to be absent. The star's X-ray fluxes in different instruments are consistent with the exception of the HRC Imager, which might have a larger effective area below 200eV than given in the calibration.

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arXiv2009

Altair - the 'hottest' magnetically active star in X-rays

The A7 star Altair is one of the hottest magnetically active stars. Its proximity to the Sun allows a detailed investigation of a corona in X-rays for a star with a shallow convection zone. We used a deep XMM-Newton observation of Altair and analyzed X-ray light curves, spectra, and emission lines, investigated the temporal behavior and properties of the X-ray emitting plasma and studied the global coronal structure. We find that Altair's corona with an X-ray luminosity of L_X =1.4 x 10^27 erg/s and a very low activity level of log L_X/L_bol = -7.4, is located predominantly at low latitude regions. The X-ray emission is dominated by cool plasma (1-4 MK) at low density, and elemental abundances exhibit a solar-like FIP effect and Ne/O ratio. The X-ray brightness varies by 30 % over the observation, most likely due to rotational modulation and minor activity; in contrast, no strong flares or significant amounts of hot plasma were detected. The X-ray activity level of Altair is apparently close to the saturation level, which is reduced by roughly four orders of magnitude when compared to late-type stars. With its fast rotation, Altair provides an inefficient, but very stable dynamo that mainly operates in convective layers below its 'cooler' surface areas around the equator. This dynamo mechanism results in magnetic activity and leads to X-ray properties that are overall very similar to those of the quiescent Sun, despite very different underlying stars.

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arXiv2009

X-ray emission from the M9 dwarf 1RXS J115928.5-524717 - Quasi-quiescent coronal activity at the end of the main sequence

Methods: We analyse an XMM-Newton observation of 1RXS J115928.5-524717, an ultracool dwarf with spectral type M9 and compare its X-ray properties to those of other similar very late-type stars. Results: We clearly detected 1RXS J115928.5-524717 at soft X-ray energies in all EPIC detectors. Only minor variability was present during the observation and we attribute the X-ray emission to quasi-quiescent activity. The coronal plasma is described well by a two-temperature model at solar metallicity with temperatures of 2 MK and 6 MK and an X-ray luminosity of about L_x = 1.0 x 10^26 erg/s in the 0.2-2.0 keV band. The corresponding activity level of log L_x/L_bol = -4.1 points to a moderately active star. Altogether, X-ray activity from very low-mass stars shows similar trends to more massive stars, despite their different interior structure. 1RXSJ115928.5-524717 is, after LHS 2065, the second ultracool M9 dwarf that emits X-rays at detectable levels in quasi-quiescence. While faint in absolute numbers,both stars are rather X-ray active, implying the existence of an efficient dynamo mechanisms that is capable of creating magnetic activity and coronal X-ray emission.

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arXiv2008

Further X-ray detections of Herbig stars

The interpretation of X-ray detections from Herbig Ae/Be stars is disputed as it is not clear if these intermediate-mass pre-main sequence stars are able to drive a dynamo and ensuing phenomena of magnetic activity. Alternative X-ray production mechanisms, related to stellar winds, star-disk magnetospheres, or unresolved late-type T Tauri star companions have been proposed. In a series of papers we have been investigating high-resolution X-ray Chandra images of Herbig Ae/Be and main-sequence B-type stars to test the T Tauri hypothesis by spatially resolving known visual companions from the primaries. Here we report on six as yet unpublished Chandra exposures from our X-ray survey of Herbig stars. The target list comprises six Herbig stars with known cool companions, and three further A/B-type stars that are serendipitously in the Chandra field-of-view. In this sample we record a detection rate of 100%, i.e. all A/B-type stars display X-ray emission at levels of log(L_x/L_bol) ~ -5...-7. The analysis of hardness ratios confirms that HAeBe's have hotter and/or more absorbed X-ray emitting plasma than more evolved B-type stars. Radiative winds are ruled out as exclusive emission mechanism on basis of the high X-ray temperatures. Confirming earlier results, the X-ray properties of Herbig Ae/Be stars are not vastly different from those of their late-type companion stars (if such are known). The diagnostics provided by the presently available data leave open if the hard X-ray emission of Herbig stars is due to young age or indicative of further coronally active low-mass companion stars. In the latter case, our detection statistics imply a high fraction of higher-order multiple systems among Herbig stars.

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arXiv2008

Quiescent X-ray emission from the M9 dwarf LHS 2065

We investigate an archival XMM-Newton observation of LHS 2065, an ultracool dwarf with spectral type M9. We clearly detect LHS 2065 at soft X-ray energies in less than 1 h effective exposure time above the 3 sigma level with the PN and MOS1 detector. No flare signatures are present and we attribute the X-ray detection to quasi-quiescent activity. From the PN data we derive an X-ray luminosity of L_x = 2.2 +/- 0.7 x 10^26 erg/s in the 0.3-0.8 keV band, the corresponding activity level of log L_x/L_bol = -3.7 points to a rather active star. Indications for minor variability and possible accompanying spectral changes are present, however the short exposure time and poor data quality prevents a more detailed analysis. LHS 2065 is one of the coolest and least massive stars that emits X-rays at detectable levels in quasi-quiescence, implying the existence of a corona.

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arXiv2008

Neon and oxygen in low activity stars: towards a coronal unification with the Sun

The disagreement between helioseismology and a recent downward revision of solar abundances has resulted in a controversy about the true neon abundance of the Sun and other stars. We investigated XMM-Newton and Chandra high-resolution X-ray spectra of weakly and moderately active stars (log L_x/L_bol = -5...-7) and determined their coronal Ne/O abundance ratio by using two linear combinations of strong emission lines as well as a global-fitting method. The sample stars show a correlation between their Ne/O ratio and stellar activity in the sense that stars with a higher activity level show a higher Ne/O ratio. We find that the Ne/O ratio decreases in our sample from values of Ne/O~0.4 down to Ne/O~0.2, suggesting that ratios similar to 'classical' solar values are rather common for low activity stars. A significantly enhanced neon abundance as the solution to the solar modeling problem seems unlikely. We find no indications of a peculiar position of the Sun among other stars.

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arXiv2007

Doppler imaging an X-ray flare on the ultrafast rotator BO Mic - A contemporaneous multiwavelength study using XMM-Newton and VLT

We present an analysis of contemporaneous photospheric, chromospheric and coronal structures on the highly active K-dwarf star BO Mic (Speedy Mic). We localize a moderate flare in the stellar atmosphere and study its energetics, size and thermal behaviour. The analysis is based on strictly simultaneous X-ray, UV- and optical observations carried out by XMM-Newton and the VLT. We use Doppler imaging and related methods for the localization of features. Based on X-ray spectroscopy we study the the coronal plasma in and outside the flare. The flare emits in X-rays and UV, but is not detected in white light; it is located at intermediate latitude between an extended spot group and the weakly spotted pole. We estimate its height below 0.4 stellar radii making it clearly distinct in longitude and height from the prominences found about two stellar radii above the surface. While BO Mic's photospheric brightness is modulated due to extended starspots, neither the chromospheric nor the X-ray emission show a pronounced rotational modulation.

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arXiv2007

X-rays from RU Lupi - Accretion and winds in CTTS

Using three XMM-Newton observations we study the X-ray emission of RU Lup, a accreting and wind-driving CTTS. In comparison with other bright CTTS we study possible signatures of accretion and winds in their X-ray emission. Results: We find three levels of activity among the observations of RU Lup, clearly of magnetic origin due to the corresponding high plasma temperatures. Density analysis with the OVII triplet indicates high densities in the cool plasma, suggesting accretion shocks to be a significant contributor to the soft X-ray emission. No strong overall metal depletion is observed. Excess emission at 6.4 keV during the more active phase suggest the presence of iron fluorescence. In accreting stars we find an excess of cool plasma as evidenced by lower OVIII/OVII line ratios compared to main-sequence stars. High density plasma appears to be only present in low-mass CTTS, while accreting stars with intermediate masses (> 2 M_Sun) have lower densities. Conclusions: In all investigated CTTS the characteristics of the cooler X-ray emitting plasma are influenced by the accretion process. We suspect different accretion rates and amount of funneling, possibly linked to stellar mass and radius, to be mainly responsible for the different properties of their cool plasma component. The exceptional strong X-ray absorption in RU Lup and other CTTS is probably related to accretion flows and an optically transparent wind emanating from the star or the disk.

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arXiv2007

X-ray emission from classical T Tauri stars: Accretion shocks and coronae?

Classical T Tauri stars (CTTS) are surrounded by actively accreting disks. According to current models material falls along the magnetic field lines from the disk with more or less free-fall velocity onto the star, where the plasma heats up and generates X-rays. We want to quantitatively explain the observed high energy emission and measure the infall parameters from the data. Absolute flux measurements allow to calculate the filling factor and the mass accretion rate.We use a numerical model of the hot accretion spot and solve the conservation equations. A comparison to data from XMM-Newton and Chandra shows that our model reproduces the main features very well. It yields for TW Hya a filling factor of 0.3% and a mass accretion rate 2e-10 M_sun/yr.

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arXiv2007

X-ray activity cycles in stellar coronae

We present updated results from the ongoing XMM-Newton monitoring program of moderately active, solar-like stars to investigate stellar X-ray activity cycles; here we report on the binary systems Alpha Cen A/B and 61 Cyg A/B. For 61 Cyg A we find a coronal X-ray cycle which clearly reflects the chromospheric activity cycle and is in phase with a ROSAT campaign performed in the 1990s. 61 Cyg A is the first example of a persistent coronal cycle observed on a star other than the Sun. The changes of its coronal properties during the cycle resemble the solar behaviour. The coronal activity of 61 Cyg B is more irregular, but also follows the chromospheric activity. Long-term variability is also present on Alpha Cen A and B. We find that Alpha Cen A, a G2 star very similar to our Sun, fainted in X-rays by at least an order of magnitude during the observation program. This behaviour has never been observed before on Alpha Cen A, but is rather similar to the X-ray behaviour of the Sun. The X-ray emission of the Alpha Cen system is dominated in our observations by Alpha Cen B, which might also have an activity cycle indicated by a significant fainting since 2005.

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arXiv2006

A deep XMM-Newton X-ray observation of the Chamaeleon I dark cloud

Methods. The northern-eastern fringe of the Chameleon I dark cloud was observed with XMM-Newton, revisiting a region observed with ROSAT 15 years ago. Centered on the extended X-ray source CHXR49 we are able to resolve it into three major contributing components and to analyse their spectral properties. Furthermore, the deep exposure allows not only the detection of numerous, previously unknown X-ray sources, but also the investigation of variability and the study of the X-ray properties for the brighter targets in the field. We use EPIC spectra, to determine X-ray brightness, coronal temperatures and emission measures for these sources, compare the properties of classical and weak-line T Tauri stars and make a comparison with results from the ROSAT observation. Results. X-ray properties of T Tauri stars in Cha I are presented. The XMM-Newton images resolve some previously blended X-ray sources, confirm several possible ones and detect many new X-ray targets, resulting in the most comprehensive list with 71 X-ray sources in the northern Cha I dark cloud. The analysis of medium resolution spectra shows an overlapping distribution of spectral properties for classical and weak-line T Tauri stars, with the X-ray brighter stars having hotter coronae and a higher L_X/L_bol ratio. X-ray luminosity correlates with bolometric luminosity, whereas the L_X/L_bol ratio is slightly lower for the classical T Tauri stars. Large flares as well as a low iron and a high neon abundance are found in both types of T Tauri stars. Abundance pattern, plasma temperatures and emission measure distributions during quiescent phases are attributed toa high level of magnetic activity as the dominant source of their X-ray emission.

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arXiv2006

X-ray accretion signatures in the close CTTS binary V4046 Sgr

We present Chandra HETGS observations of the classical T Tauri star (CTTS) V4046 Sgr. The He-like triplets of O VII, Ne IX, and Si XIII are clearly detected. Similar to the CTTS TW Hya and BP Tau, the forbidden lines of O VII and Ne IX are weak compared to the intercombination line, indicating high plasma densities in the X-ray emitting regions. The Si XIII triplet, however, is within the low-density limit, in agreement with the predictions of the accretion funnel infall model with an additional stellar corona. V4046 Sgr is the first close binary exhibiting these features. Together with previous high-resolution X-ray data on TW Hya and BP Tau, and in contrast to T Tau, now three out of four CTTS show evidence of accretion funnels.

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arXiv2006

XMM-Newton X-ray spectroscopy of classical T Tauri stars

We present results from a comparative study of XMM-Newton observations of four classical T Tauri stars (CTTS), namely BP Tau, CR Cha, SU Aur and TW Hya. In these objects coronal, i.e. magnetic, activity and as recently shown, magnetically funneled accretion are the processes likely to be responsible for the generation of X-ray emission. Variable X-ray emission with luminosities in the order of $10^{30}$ erg/s is observed for all targets. We investigate light curves as well as medium and high-resolution X-ray spectra to determine the plasma properties of the sample CTTS and to study the origin of their X-ray emission and its variability. The emission measure distributions and observed temperatures differ significantly and the targets are dominated either by plasma at high densities as produced by accretion shocks or by predominantly hotter plasma of coronal origin. Likewise the variability of the X-ray luminosity is found to be generated by both mechanisms. Cool plasma at high densities is found in all stars with detected \ion{O}{vii} triplet emission, prevented only for SU~Aur due to strong absorption. A general trend is present in the abundance pattern, with neon being at solar value or enhanced while oxygen, iron and most other metals are depleted, pointing to the presence of the inverse FIP effect in active coronae and possibly grain formation in evolved disks. We find that both accretion shocks and coronal activity contribute to the observed X-ray emission of the targets. While coronal activity is the dominant source of X-ray activity in the majority of the CTTS, the fraction for each process differs significantly between the individual objects.

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arXiv2005

X-rays from Alpha Centauri - The darkening of the solar twin

We present first results from five XMM-Newton observations of the binary system Alpha Centauri, which has been observed in snapshot like exposures of roughly two hours each during the last two years. In all our observations the X-ray emission of the system is dominated by Alpha Cen B, a K1 star. The derived light curves of the individual components reveal variability on short timescales and a flare was discovered on Alpha Cen B during one observation. A PSF fitting algorithm is applied to the event distribution to determine the brightness of each component during the observations. We perform a spectral analysis with multi-temperature models to calculate the X-ray luminosities.We investigate long term variability and possible activity cycles of both stars and find the optically brighter component Alpha Cen A, a G2 star very similar to our Sun, to have fainted in X-rays by at least an order of magnitude during the observation program, a behaviour never observed before on Alpha Cen A, but rather similar to the X-ray behaviour observed with XMM-Newton on HD 81809. We also compare our data with earlier spatially resolved observations performed over the last 25 years.

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