1,018 research outputs found
A revised radiometric calibration for the Hinode/EIS instrument
A preliminary assessment of the in-flight radiometric calibration of the
Hinode EUV Imaging Spectrometer (EIS) is presented. This is done with the line
ratio technique applied to a wide range of observations of the quiet Sun,
active regions and flares from 2006 until 2012. The best diagnostic lines and
the relevant atomic data are discussed in detail. Radiances over the quiet Sun
are also considered, with comparisons with previous measurements. Some
departures in the shapes of the ground calibration responsivities are found at
the start of the mission. These shapes do not change significantly over time,
with the exception of the shorter wavelengths of the EIS short-wavelength (SW)
channel, which shows some degradation. The sensitivity of the SW channel at
longer wavelengths does not show significant degradation, while that of the
long-wavelength (LW) channel shows a significant degradation with time. By the
beginning of 2010 the responsivity of the LW channel was already a factor of
two or more lower than the values measured on the ground. A first-order
correction is proposed. With this correction, the main ratios of lines in the
two channels become constant to within a relative 20%, and the He II 256 A
radiances over the quiet Sun also become constant over time. This correction
removes long-standing discrepancies for a number of lines and ions, in
particular those involving the strongest Fe X, Fe XIII, Fe XIV, Fe XVII, and Fe
XXIV lines, where discrepancies of factors of more than two were found. These
results have important implications for various EIS science analyses, in
particular for measurements of temperatures, emission measures and elemental
abundances.Comment: Accepted for publication in A&A (under minor revision
The EUV spectrum of the Sun: SOHO CDS NIS radiances during solar cycle 23
For the first time, we present and discuss EUV radiances of the solar
transition region (TR) and corona obtained during a solar cycle. The
measurements were obtained with the SOHO/coronal diagnostic spectrometer (CDS)
during the period from 1996 to 2010. We find that limb-brightening
significantly affects any characterisation of the solar radiances. We present
the limb-brightening function for the main lines and find that it does not
change measurably during the cycle. We confirm earlier findings that the
radiance histogram of the cooler lines have a well defined, log-normal
quiet-Sun component, although our results differ from previous ones. The width
of the lowest-radiance log-normal distribution is constant along the cycle.
Both the analysis of the centre-to-limb variation and of the radiance
statistical distribution point to a constant QS emission along solar cycle 23.
Lines formed above 1 MK are dramatically affected by the presence of active
regions, and indeed, no "quiet Sun" region can be defined during periods of
maximum activity. Much of the irradiance variability in lines formed below 1.5
MK is due to a change in the emitting area. For hotter lines, the emitting area
saturates to almost 100% of full solar disk at the maximum of activity, while
simultaneously the emission due to active regions increases by more than an
order of magnitude. We show that structures around active regions, sometimes
referred to as dark halos or dark canopies, are common and discuss their
similarities and differences with coronal holes. In particular, we show how
they are well visible in TR lines, contrary to coronal holes.Comment: 15 pages, 11 figures, 2 tables; in press in: Astronomy & Astrophysic
GRMHD in axisymmetric dynamical spacetimes: the X-ECHO code
We present a new numerical code, X-ECHO, for general relativistic
magnetohydrodynamics (GRMHD) in dynamical spacetimes. This is aimed at studying
astrophysical situations where strong gravity and magnetic fields are both
supposed to play an important role, such as for the evolution of magnetized
neutron stars or for the gravitational collapse of the magnetized rotating
cores of massive stars, which is the astrophysical scenario believed to
eventually lead to (long) GRB events. The code is based on the extension of the
Eulerian conservative high-order (ECHO) scheme [Del Zanna et al., A&A 473, 11
(2007)] for GRMHD, here coupled to a novel solver for the Einstein equations in
the extended conformally flat condition (XCFC). We fully exploit the 3+1
Eulerian formalism, so that all the equations are written in terms of familiar
3D vectors and tensors alone, we adopt spherical coordinates for the conformal
background metric, and we consider axisymmetric spacetimes and fluid
configurations. The GRMHD conservation laws are solved by means of
shock-capturing methods within a finite-difference discretization, whereas, on
the same numerical grid, the Einstein elliptic equations are treated by
resorting to spherical harmonics decomposition and solved, for each harmonic,
by inverting band diagonal matrices. As a side product, we build and make
available to the community a code to produce GRMHD axisymmetric equilibria for
polytropic relativistic stars in the presence of differential rotation and a
purely toroidal magnetic field. This uses the same XCFC metric solver of the
main code and has been named XNS. Both XNS and the full X-ECHO codes are
validated through several tests of astrophysical interest.Comment: 18 pages, 9 figures, accepted for publication in A&
Benchmarking atomic data for astrophysics: a first look at the soft X-ray lines
A collection of the best solar and laboratory spectra in the soft X-rays is
used here to perform a preliminary benchmark in this wavelength region, by
comparing observed vs. predicted wavelengths and calibrated solar irradiances.
The benchmark focuses on the Fe IX - Fe XIV ions,for which we have recently
calculated the relevant atomic data, however a few other ions have also been
benchmarked. The iron ions are dominating the soft X-rays, however a large
fraction of the strongest soft X-ray lines due to n=4 -> n=3 transitions were
previously unidentified. The strongest transitions are all identified here, in
particular the decays from the core-excited levels (3s 3p^l 4s, l=$ 5,4,3,2,1
for Fe X, Fe XI, Fe XII, Fe XIII, and Fe XIV respectively) which are the
strongest soft X-ray transitions from these ions. Many new identifications are
proposed, some only tentatively. Good agreement in terms of solar irradiances
between the soft X-ray and EUV (n=3 -> n=3) transitions is found, confirming
the reliability of the new large-scale calculations. Some of the new atomic
data and identifications are particularly important for the Solar Dynamic
Observatory (SDO) Atmospheric Imaging Assembly (AIA) 94 A band.Comment: Accepted for publication in A&A (under revision
Modelling ion populations in astrophysical plasmas: carbon in the solar transition region
The aim of this work is to improve the modelling of ion populations in higher
density, lower temperature astrophysical plasmas, of the type commonly found in
lower solar and stellar atmospheres. Ion population models for these regions
frequently employ the coronal approximation, which assumes conditions more
suitable to the upper solar atmosphere, where high temperatures and lower
densities prevail. Using the coronal approximation for modelling the solar
transition region gives theoretical lines intensities for the Li-like and
Na-like isoelectronic sequences which can be factors of 2-5 times lower than
observed. The works of Burgess & Summers (1969) and Nussbaumer & Storey (1975)
showed the important part ions in excited levels play when included in the
modelling. Their models, however, used approximations for the atomic rates to
determine the ion balance. Presented here is the first stage in updating these
earlier models of carbon by using rates from up-to-date atomic calculations and
more recent photo-ionising radiances for the quiet Sun. Where such atomic rates
are not readily available, in the case of electron-impact direct ionisation and
excitation--auto-ionisation, new calculations have been made and compared to
theoretical and experimental studies. The effects each atomic process has on
the ion populations as density changes is demonstrated, and final results from
the modelling are compared to the earlier works. Lastly, the new results for
ion populations are used to predict line intensities for the solar transition
region in the quiet Sun, and these are compared with predictions from
coronal-approximation modelling and with observations. Significant improvements
in the predicted line intensities are seen in comparison to those obtained from
zero-density modelling of carbon.Comment: Draft accepted by A&A, 13 pages, 15 figure
Solar microflares: a case study on temperatures and the Fe XVIII emission
In this paper, we discuss the temperature distribution and evolution of a
microflare, simultaneously observed by Hinode XRT, EIS, and SDO AIA. We find
using EIS lines that during peak emission the distribution is nearly isothermal
and peaked around 4.5 MK. This temperature is in good agreement with that
obtained from the XRT filter ratio, validating the use of XRT to study these
small events, invisible by full-Sun X-ray monitors such as GOES. The increase
in the estimated Fe XVIII emission in the AIA 94 {\AA} band can mostly be
explained with the small temperature increase from the background temperatures.
The presence of Fe XVIII emission does not guarantee that temperatures of 7 MK
are reached, as is often assumed. We also revisit with new atomic data the
temperatures measured by a SoHO SUMER observation of an active region which
produced microflares, also finding low temperatures (3 - 4 MK) from an Fe XVIII
/ Ca XIV ratio.Comment: 12-13 pages, 17 figures (22 eps-files), 4 tables, accepted by
Astronomy and Astrophysic
Dynamo action in thick disks around Kerr black holes: high-order resistive GRMHD simulations
We present the first kinematic study of an -dynamo in the
General Relativistic Magneto-HydroDynamics (GRMHD) regime, applied to thick
disks orbiting around Kerr black holes and using a fully covariant mean field
dynamo closure for the Ohm law. We show that the -dynamo
mechanism leads to a continuous exponential growth of the magnetic field within
the disk and to the formation of dynamo waves drifting away or toward the
equatorial plane. Since the evolution of the magnetic field occurs
qualitatively in the same fashion as in the Sun, we present also butterfly
diagrams that characterize our models and show the establishment of an
additional timescale, which depends on the microscopic properties of the
turbulent motions, possibly providing an alternative explanation to
periodicities observed in many high-energy astrophysical sources where
accretion onto a rotating black hole is believed to operate.Comment: 5 pages, 4 figures. Accepted for publication in MNRA
Axisymmetric equilibrium models for magnetised neutron stars in Scalar-Tensor Theories
Among the possible extensions of General Relativity that have been put
forward in order to address some long standing issues in our understanding of
the Universe, Scalar-Tensor Theories have received a lot of attention for their
simplicity. Interestingly, some of these predict a potentially observable
non-linear phenomenon, known as \textit{spontaneous scalarisation}, in the
presence of highly compact matter distributions, like the case of neutron
stars. Neutron stars are ideal laboratories to investigate the properties of
matter under extreme conditions, and in particular they are known to harbour
the strongest magnetic fields in the Universe. Here, for the first time, we
present a detailed study of magnetised neutron stars in Scalar-Tensor Theories.
First, we show that the formalism developed for the study of magnetised neutron
stars in General Relativity, based on the \textit{eXtended Conformally Flat
Condition}, can easily be extended in the presence of a non-minimally coupled
scalar field, retaining many of its numerical advantages. We then carry out a
study of the parameter space considering the two extreme geometries of purely
toroidal and purely poloidal magnetic fields, varying both the strength of the
magnetic field and the intensity of scalarisation. We compare our results with
magnetised general-relativistic solutions and un-magnetised scalarised
solutions, showing how the mutual interplay between magnetic and scalar fields
affect the magnetic and the scalarisation properties of neutron stars. In
particular, we focus our discussion on magnetic deformability, maximum mass and
range of scalarisation.Comment: accepted for publication by A&A; minor language corrections; minor
typos correctio
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