1,812 research outputs found
RHESSI Observations of the Solar Flare Iron-line Feature at 6.7 keV
Analysis of RHESSI 3--10 keV spectra for 27 solar flares is reported. This
energy range includes thermal free--free and free--bound continuum and two line
features, at 6.7keV and 8keV, principally due to highly ionized iron (Fe). We
used the continuum and the flux in the so-called Fe-line feature at 6.7keV to
derive the electron temperature T_e, the emission measure, and the Fe-line
equivalent width as functions of time in each flare. The Fe/H abundance ratio
in each flare is derived from the Fe-line equivalent width as a function of
T_e. To minimize instrumental problems with high count rates and effects
associated with multi-temperature and nonthermal spectral components, spectra
are presented mostly during the flare decay phase, when the emission measure
and temperature were smoothly varying. We found flare Fe/H abundance ratios
that are consistent with the coronal abundance of Fe (i.e. 4 times the
photospheric abundance) to within 20% for at least 17 of the 27 flares; for 7
flares, the Fe/H abundance ratio is possibly higher by up to a factor of 2. We
find evidence that the Fe XXV ion fractions are less than the theoretically
predicted values by up to 60% at T_e=25 MK appear to be displaced from the most
recent theoretical values by between 1 and 3 MK.Comment: To be published, Ap
Summary of the 13th IACHEC Meeting
We summarize the outcome of the 13th meeting of the International
Astronomical Consortium for High Energy Calibration (IACHEC), held at Tenuta
dei Ciclamini (Avigliano Umbro, Italy) in April 2018. Fifty-one scientists
directly involved in the calibration of operational and future high-energy
missions gathered during 3.5 days to discuss the current status of the X-ray
payload inter-calibration and possible approaches to improve it. This summary
consists of reports from the various working groups with topics ranging from
the identification and characterization of standard calibration sources,
multi-observatory cross-calibration campaigns, appropriate and new statistical
techniques, calibration of instruments and characterization of background, and
communication and preservation of knowledge and results for the benefit of the
astronomical community.Comment: 12 page
FK Comae Berenices, King of Spin: The COCOA-PUFS Project
COCOA-PUFS is an energy-diverse, time-domain study of the ultra-fast
spinning, heavily spotted, yellow giant FK Com (HD117555; G4 III). This single
star is thought to be a recent binary merger, and is exceptionally active by
measure of its intense ultraviolet and X-ray emissions, and proclivity to
flare. COCOA-PUFS was carried out with Hubble Space Telescope in the UV
(120-300 nm), using mainly its high-performance Cosmic Origins Spectrograph,
but also high-precision Space Telescope Imaging Spectrograph; Chandra X-ray
Observatory in the soft X-rays (0.5-10 keV), utilizing its High-Energy
Transmission Grating Spectrometer; together with supporting photometry and
spectropolarimetry in the visible from the ground. This is an introductory
report on the project.
FK Com displayed variability on a wide range of time scales, over all
wavelengths, during the week-long main campaign, including a large X-ray flare;
"super-rotational broadening" of the far-ultraviolet "hot-lines" (e.g., Si IV
139 nm (T~80,000 K) together with chromospheric Mg II 280 nm and C II 133 nm
(10,000-30,000 K); large Doppler swings suggestive of bright regions
alternately on advancing and retreating limbs of the star; and substantial
redshifts of the epoch-average emission profiles. These behaviors paint a
picture of a highly extended, dynamic, hot (10 MK) coronal magnetosphere around
the star, threaded by cooler structures perhaps analogous to solar prominences,
and replenished continually by surface activity and flares. Suppression of
angular momentum loss by the confining magnetosphere could temporarily postpone
the inevitable stellar spindown, thereby lengthening this highly volatile stage
of coronal evolution.Comment: to be published in ApJ
Interactions of the magnetospheres of stars and close-in giant planets
Since the first discovery of an extrasolar planetary system more than a
decade ago, hundreds more have been discovered. Surprisingly, many of these
systems harbor Jupiter-class gas giants located close to the central star, at
distances of 0.1 AU or less. Observations of chromospheric 'hot spots' that
rotate in phase with the planetary orbit, and elevated stellar X-ray
luminosities,suggest that these close-in planets significantly affect the
structure of the outer atmosphere of the star through interactions between the
stellar magnetic field and the planetary magnetosphere. Here we carry out the
first detailed three-dimensional MagnetoHydroHynamics (MHD) simulation
containing the two magnetic bodies and explore the consequences of such
interactions on the steady-state coronal structure. The simulations reproduce
the observable features of 1) increase in the total X-ray luminosity, 2)
appearance of coronal hot spots, and 3) phase shift of these spots with respect
to the direction of the planet. The proximate cause of these is an increase in
the density of coronal plasma in the direction of the planet, which prevents
the corona from expanding and leaking away this plasma via a stellar wind. The
simulations produce significant low temperature heating. By including dynamical
effects, such as the planetary orbital motion, the simulation should better
reproduce the observed coronal heating
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.Comment: accepted by A&
Chandra High Resolution X-ray Spectroscopy of AM Her
We present the results of high resolution spectroscopy of the prototype polar
AM Herculis observed with Chandra High Energy Transmission Grating. The X-ray
spectrum contains hydrogen-like and helium-like lines of Fe, S, Si, Mg, Ne and
O with several Fe L-shell emission lines. The forbidden lines in the spectrum
are generally weak whereas the hydrogen-like lines are stronger suggesting that
emission from a multi-temperature, collisionally ionized plasma dominates. The
helium-like line flux ratios yield a plasma temperature of 2 MK and a plasma
density 1 - 9 x10^12 cm^-3, whereas the line flux ratio of Fe XXVI to Fe XXV
gives an ionization temperature of 12.4 +1.1 -1.4 keV. We present the
differential emission measure distribution of AM Her whose shape is consistent
with the volume emission measure obtained by multi-temperature APEC model. The
multi-temperature plasma model fit to the average X-ray spectrum indicates the
mass of the white dwarf to be ~1.15 M_sun. From phase resolved spectroscopy, we
find the line centers of Mg XII, S XVI, resonance line of Fe XXV, and Fe XXVI
emission modulated by a few hundred to 1000 km/s from the theoretically
expected values indicating bulk motion of ionized matter in the accretion
column of AM Her. The observed velocities of Fe XXVI ions are close to the
expected shock velocity for a 0.6 M_sun white dwarf. The observed velocity
modulation is consistent with that expected from a single pole accreting binary
system.Comment: 6 figures, AASTEX style, accepted for publication in Ap
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.Comment: accepted by A&
A correlation between host star activity and planet mass for close-in extrasolar planets?
The activity levels of stars are influenced by several stellar properties,
such as stellar rotation, spectral type and the presence of stellar companions.
In analogy to binaries, planetary companions are also thought to be able to
cause higher activity levels in their host stars, although at lower levels.
Especially in X-rays, such influences are hard to detect because coronae of
cool stars exhibit a considerable amount of intrinsic variability. Recently, a
correlation between the mass of close-in exoplanets and their host star's X-ray
luminosity has been detected, based on archival X-ray data from the ROSAT
All-Sky Survey. This finding has been interpreted as evidence for Star-Planet
Interactions. We show in our analysis that this correlation is caused by
selection effects due to the flux limit of the X-ray data used and due to the
intrinsic planet detectability of the radial velocity method, and thus does not
trace possible planet-induced effects. We also show that the correlation is not
present in a corresponding complete sample derived from combined XMM-Newton and
ROSAT data.Comment: accepted by The Astrophysical Journa
Gap solitons in Bragg gratings with a harmonic superlattice
Solitons are studied in a model of a fiber Bragg grating (BG) whose local
reflectivity is subjected to periodic modulation. The superlattice opens an
infinite number of new bandgaps in the model's spectrum. Averaging and
numerical continuation methods show that each gap gives rise to gap solitons
(GSs), including asymmetric and double-humped ones, which are not present
without the superlattice.Computation of stability eigenvalues and direct
simulation reveal the existence of completely stable families of fundamental
GSs filling the new gaps - also at negative frequencies, where the ordinary GSs
are unstable. Moving stable GSs with positive and negative effective mass are
found too.Comment: 7 pages, 3 figures, submitted to EP
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