28,954 research outputs found
A spatially resolved limb flare on Algol B observed with XMM-Newton
We report XMM-Newton observations of the eclipsing binary Algol A (B8V) and B
(K2III). The XMM-Newton data cover the phase interval 0.35 - 0.58, i.e.,
specifically the time of optical secondary minimum, when the X-ray dark B-type
star occults a major fraction of the X-ray bright K-type star. During the
eclipse a flare was observed with complete light curve coverage. The decay part
of the flare can be well described with an exponential decay law allowing a
rectification of the light curve and a reconstruction of the flaring plasma
region. The flare occurred near the limb of Algol B at a height of about 0.1R
with plasma densities of a few times 10^11 cm^-3 consistent with spectroscopic
density estimates. No eclipse of the quiescent X-ray emission is observed
leading us to the conclusion that the overall coronal filling factor of Algol B
is small.Comment: 8 pages, 7 figures, accepted by A&
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.Comment: 4 pages, 3 figures, accepted by A&
X-ray activity cycle on the active ultra-fast rotator AB Dor A? Implication of correlated coronal and photometric variability
Although chromospheric activity cycles have been studied in a larger number
of late-type stars for quite some time, very little is known about coronal
activity-cycles in other stars and their similarities or dissimilarities with
the solar activity cycle. While it is usually assumed that cyclic activity is
present only in stars of low to moderate activity, we investigate whether the
ultra-fast rotator AB Dor, a K dwarf exhibiting signs of substantial magnetic
activity in essentially all wavelength bands, exhibits a X-ray activity cycle
in analogy to its photospheric activity cycle of about 17 years and possible
correlations between these bands. We analysed the combined optical photometric
data of AB Dor A, which span ~35 years. Additionally, we used ROSAT and
XMM-Newton X-ray observations of AB Dor A to study the long-term evolution of
magnetic activity in this active K dwarf over nearly three decades and searched
for X-ray activity cycles and related photometric brightness changes. AB Dor A
exhibits photometric brightness variations ranging between 6.75 < Vmag < 7.15
while the X-ray luminosities range between 29.8 < log LX [erg/s] < 30.2 in the
0.3-2.5 keV. As a very active star, AB Dor A shows frequent X-ray flaring, but,
in the long XMM-Newton observations a kind of basal state is attained very
often. This basal state probably varies with the photospheric activity-cycle of
AB Dor A which has a period of ~17 years, but, the X-ray variability amounts at
most to a factor of ~2, which is, much lower than the typical cycle amplitudes
found on the Sun.Comment: 10 page
Spatially resolved X-ray emission of EQ Pegasi
We present an analysis of an XMM-Newton observation of the M dwarf binary EQ
Pegasi with a special focus on the the spatial structure of the X-ray emission
and the analysis of light curves. Making use of data obtained with EPIC
(European Photon Imaging Camera) we were for the first time able to spatially
resolve the two components in X-rays and to study the light curves of the
individual components of the EQ Peg system. During the observation a series of
moderate flares was detected, where it was possible to identify the respective
flaring component.Comment: 6 pages, 11 figures, accepted by A&
DN Tauri - coronal activity and accretion in a young low-mass CTTS
We present a deep XMM-Newton observation of DN Tau, a young M0-type accreting
CTTS and investigate its X-ray properties and X-ray generating mechanisms.
Specifically we examine the presence of X-ray emission from magnetic activity
and accretion shocks and compare our new X-ray data with UV data taken
simultaneously and with X-ray/UV observations performed before. We find that
the X-ray emission from DN Tau is dominated by coronal plasma generated via
magnetic activity, but also clearly detect a contribution of the accretion
shocks to the cool plasma component at about 2 MK as consistently inferred from
density and temperature analysis. Typical phenomena of active coronae like
flaring, the presence of very hot plasma at 30 MK and an abundance pattern
showing the inverse FIP effect are seen on DN Tau. Strong variations in the
emission measure of the cooler plasma components between the 2005 and 2010 XMM
data point to accretion related changes. The UV light curve taken
simultaneously is in general not related to the X-ray brightness, but exhibits
clear counterparts during the observed X-ray flares. The X-ray properties of DN
Tau are similar to those of more massive CTTS, but its low mass and large
radius shift the accretion shocks to lower temperatures, reducing their imprint
in the X-ray regime.Comment: 11 pages, 8 figures, accepted for publication in A&
A magnetic cycle of tau Bootis? The coronal and chromospheric view
Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter.
During the last years spectropolarimetric observations led to the hypothesis
that this star may host a global magnetic field that switches its polarity once
per year, indicating a very short activity cycle of only one year duration. In
our ongoing observational campaign, we have collected several X-ray
observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to
characterize tau Boo's corona and chromosphere over the course of the supposed
one-year cycle. Contrary to the spectropolarimetric reconstructions, our
observations do not show indications for a short activity cycle.Comment: 4 pages, 2 figures, appeared in Astronomical Notes 333, 1, 26-29
(2012
Vortex pattern development on the upper surface of a swept wing at high angle of attack
An experimental study, based on a swept wing, was undertaken in the water tunnel and the wind tunnel at low speeds, with a view to improving the understanding of the intervening phenomena and to make easier their modelling. The vortex flow effects on the wing are first illustrated from global effort measurements and static pressure distributions. The domain of existence of this type of flow is deduced as a function of both sweep angle and angle of attack. By a phenomenological study in the water tunnel, an attempt is made to describe the physical pattern of the vortex flow, from its formation near the apex to its breakdown at the trailing edge. Lastly, by means of a clinometric probe, the flow field over the wing is determined
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