125 research outputs found
The nature of the 2014-2015 dim state of RW Aurigae revealed by X-ray, optical, and near-IR observations
No full textThe binary system RW Aur consists of two classical T Tauri stars (CTTSs). The primary recently underwent its second observed major dimming event (ΔV ∼ 2 mag). We present new, resolved Chandra X-ray and UKIRT near-IR (NIR) data as well as unresolved optical photometry obtained in the dim state to study the gas and dust content of the absorber causing the dimming. The X-ray data show that the absorbing column density increased from NH < 0.1 × 1022 cm-2 during the bright state to ≈2 × 1022cm-2 in the dim state. The brightness ratio between dim and bright state at optical to NIR wavelengths shows only a moderate wavelength dependence and the NIR color-color diagram suggests no substantial reddening. Taken together, this indicates gray absorption by large grains (≳ 1 μm) with a dust mass column density of ≳ 2 × 10-4 g cm-2. Comparison with NH shows that an absorber responsible for the optical/NIR dimming and the X-ray absorption is compatible with the ISM's gas-to-dust ratio, i.e., that grains grow in the disk surface layers without largely altering the gas-to-dust ratio. Lastly, we discuss a scenario in which a common mechanism can explain the long-lasting dimming in RW Aur and recently in AA Tau
Establishing the X-ray source detection strategy for eROSITA with simulations
No full textContext. The eROSITA X-ray telescope on board the Spectrum-Roentgen-Gamma satellite has started to detect new X-ray sources over the full sky at an unprecedented rate. Understanding the performance and selection function of the source detection is important for the subsequent scientific analysis of the eROSITA catalogs. Aims. Through simulations, we test and optimize the eROSITA source detection procedures, and we characterize the detected catalog quantitatively. Methods. Taking the eROSITA Final Equatorial-Depth Survey (eFEDS) as an example, we ran extensive photon-event simulations based on our best knowledge of the instrument characteristics, the background spectrum, and the population of astronomical X-ray sources. We introduce a method of analyzing source detection completeness, purity, and efficiency based on the origin of each photon. Results. According to the source detection efficiency measured in the simulation, we chose a two-pronged strategy to build eROSITA X-ray catalogs, creating a main catalog using only the most sensitive band (0.2-2.3 keV) and an independent hard-band-selected catalog using multiband detection in a range up to 5 keV. Because our mock data are highly representative of the real eFEDS data, we used the mock catalogs to measure the completeness and purity of the eFEDS catalogs as a function of multiple parameters, such as detection likelihood, flux, and luminosity. These measurements provide a basis for choosing the eFEDS catalog selection thresholds. The mock catalogs (available with this paper) can be used to construct the selection function of active galactic nuclei and galaxy clusters. A direct comparison of the output and input mock catalogs also gives rise to a correction curve that converts the raw point-source flux distribution into the intrinsic number counts distribution
A deep XMM-Newton X-ray observation of the Chamaeleon I dark cloud
No full textContext.Low-mass stars are known to exhibit strong X-ray emission during the early stages of evolution.
Nearby star forming regions are ideal targets to study the X-ray
properties of pre-main sequence stars.
Aims.A deep XMM-Newton exposure is used to investigate X-ray properties of the pre-main sequence population of the Chamaeleon I star forming region.
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 CHXR 49 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 ratio. X-ray luminosity correlates with bolometric luminosity,
whereas the 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 to
a high level of magnetic activity as the dominant source of their X-ray emission
Discovery of variable X-ray absorption in the cTTS AA Tauri
No full textWe present XMM-Newton X-ray and UV observations of the classical T Tauri star AA Tau covering almost two rotational periods where 8.5 days. Clear, but uncorrelated variability is found at both wavebands. The variability observed at ~2100 Å follows the previously known optical period. Spectral analysis of the X-ray data results in
significant variability in the X-ray absorption such that the times of maximal X-ray absorption and UV extinction coincide. Placing the coronal emission in regions at low up to moderate magnetic latitudes and attribution of the variable X-ray absorption
to accretion curtains and/or the disk warp provides a consistent physical picture. However, the derived X-ray absorption and optical extinction at times of maximal optical/UV brightness, i.e. outside occultation, are difficult to reconcile, requiring additional absorption in a disk wind or a peculiar dust grain distribution
X-ray emission from the remarkable A-type star HR 8799
No full textWe investigate a Chandra observation of the remarkable planet-bearing A5 V star HR 8799, more precisely classified as a kA5 hF0 mA5 star.
We search for intrinsic X-ray emission, a diagnostic for studying the possible activity of intermediate-mass stars.
In the regime of mid/late A-type stars a strong decline in magnetic activity occurs towards hotter stars because of the vanishing of the outer convection zone.
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 × 1028 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 late A/early F-type stars, in agreement with its
classification on the basis of hydrogen lines and its 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
X-ray emission from the M9 dwarf 1RXS J115928.5-524717
No full textAims. X-ray emission is an important diagnostic for studying magnetic activity in presumably fully convective, very low-mass stars with virtually neutral photospheres.
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 1026 erg/s in the 0.2-2.0 keV band. The corresponding activity level of log LX/ points to a moderately active star. Altogether, X-ray activity from very low-mass stars shows similar trends as more massive stars, despite their different interior structure.
Conclusions. The nearby star 1RXS J115928.5-524717 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 relatively X-ray active, implying an efficient dynamo mechanism that is capable of creating magnetic activity and coronal X-ray emission
The eROSITA Final Equatorial-Depth Survey (eFEDS). The AGN catalog and its X-ray spectral properties
No full textContext. The eROSITA Final Equatorial Depth Survey (eFEDS), observed with eROSITA ahead of its planned 4-yr all-sky survey, is the largest contiguous-field X-ray survey at present. It yielded a large sample of X-ray sources with very rich multiband photometric and spectroscopic coverage. Aims. We present here the eFEDS active galactic nuclei (AGN) catalog and the eROSITA X-ray spectral properties of the eFEDS sources. Methods. Using a Bayesian method, we performed a systematic X-ray spectral analysis for all the eFEDS sources. We adopted multiple spectral models, including single-component power-law or hot-plasma models and double-component models of a power law plus soft excess. We investigated the capacity of eROSITA X-ray spectra for constraining AGN spectral shapes through a detailed analysis of the posterior parameter probability distribution functions. Hierarchical Bayesian modeling was used to recover the spectral parameter distribution of the sample. The source fluxes and luminosities were measured from the posterior of the spectral fitting. Results. The eFEDS AGN catalog (22 079 sources) comprises ~80% of the eFEDS point sources. Despite a large number of faint sources, our spectral fitting provides reasonable measurements of spectral shapes and intrinsic luminosities for a majority of the sources. Because of sample selection bias, this AGN catalog is dominated by X-ray unobscured sources, with an obscured (logNH > 21.5) fraction of 8%; the power-law emission of the hot corona is also relatively soft, with a typical slope of 2.0. For type-I AGN, the X-ray emission is well correlated with the UV emission with the usual anticorrelation between the X-ray to UV spectral slope αOX and the UV luminosity. The X-ray spectral properties measured with various models are presented for all the eFEDS sources
Altair – the “hottest” magnetically active star in X-rays
No full textContext. The onset of stellar magnetic activity is related to the operation of dynamo processes that require the
development of an outer convective layer. This transition of stellar interior structure is expected to occur in late A-type stars.
Aims. 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.
Methods. We used a deep XMM-Newton observation of Altair and analyzed X-ray light curves, spectra, and emission lines. We investigated the temporal behavior and properties of the
X-ray emitting plasma and studied the global coronal structure of Altair.
Results. Altair's corona with an X-ray luminosity of erg/s and an activity level of
is located predominantly at low latitude regions and exhibits X-ray properties that are overall very similar to those of other weakly active stars.
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.
Conclusions. 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 similar to those of the quiescent Sun, despite very different underlying stars
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