136 research outputs found
Heating by transverse waves in simulated coronal loops
K.K. was funded by GOA-2015-014 (KU Leuven). T.V.D was supported by the IAP P7/08 CHARM (Belspo) and the GOA-2015-014 (KU Leuven). P.A. acknowledges funding from the UK Science and Technology Facilities Council and the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214).Context. Recent numerical studies of oscillating flux tubes have established the significance of resonant absorption in the damping of propagating transverse oscillations in coronal loops. The nonlinear nature of the mechanism has been examined alongside the Kelvin-Helmholtz instability,which is expected to manifest in the resonant layers at the edges of the flux tubes. While these two processes have been hypothesized to heat coronal loops through the dissipation of wave energy into smaller scales, the occurring mixing with the hotter surroundings can potentially hide this effect. Aims. We aim to study the effects of wave heating from driven and standing kink waves in a coronal loop. Methods. Using the MPI-AMRVAC code, we perform ideal, three dimensional magnetohydrodynamic (MHD) simulations of both (a) footpoint driven and (b) free standing oscillations in a straight coronal flux tube, in the presence of numerical resistivity. Results. We have observed the development of Kelvin-Helmholtz eddies at the loop boundary layer of all three models considered here, as well as an increase of the volume averaged temperature inside the loop. The main heating mechanism in our setups was Ohmic dissipation, as indicated by the higher values for the temperatures and current densities located near the footpoints. The introduction of a temperature gradient between the inner tube and the surrounding plasma, suggests that the mixing of the two regions, in the case of hotter environment, greatly increases the temperature of the tube at the site of the strongest turbulence, beyond the contribution of the aforementioned wave heating mechanism.PostprintPeer reviewe
SIREN – A network infrastructure for emergencies.
The SIREN project (Secure, Interoperable, UAV-assisted, Rapid Emergency Deployment Communication and sensing Infrastructure) implements a secure, distributed, open, self-configured and emergency-aware network and service platform for automated, secure and dependable support of multiple mission critical applications in highly demanding and dynamic emergency environments
Structure of the SMC - Stellar component distribution from 2MASS data
The spatial distribution of the SMC stellar component is investigated from
2MASS data. The morphology of the different age populations is presented. The
center of the distribution is calculated and compared with previous
estimations. The rotation of the stellar content and possible consequence of
dark matter presence are discussed. The different stellar populations are
identified through a CMD diagram of the 2MASS data. Isopleth contour maps are
produced in every case, to reveal the spatial distribution. The derived density
profiles are discussed. The older stellar population follows an exponential
profile at projected diameters of about 5 kpc (~5 deg) for the major axis and
~4 kpc for the minor axis, centred at RA: 0h:51min, Dec: -73deg 7' (J2000.0).
The centre coordinates are found the same for all the different age population
maps and are in good accordance with the kinematical centre of the SMC. However
they are found considerably different from the coordinates of the centre of the
gas distribution. The fact that the older population found on an exponential
disk, gives evidence that the stellar content is rotating, with a possible
consequence of dark matter presence. The strong interactions between the MCs
and the MilkyWay might explain the difference in the distributions of the
stellar and gas components. The lack in the observed velocity element, that
implies absence of rotation, and contradicts with the consequences of
exponential profile of the stellar component, may also be a result of the
gravitational interactions.Comment: 7 Pages, 6 figures, accepted for publication in A&
Deep XMM-Newton observations of the northern disc of M31. I. Source catalogue
We carried out new observations of two fields in the northern ring of M31
with XMM-Newton with two exposures of 100 ks each and obtained a complete list
of X-ray sources down to a sensitivity limit of ~7 x 10^34 erg s^-1 (0.5 - 2.0
keV). The major objective of the observing programme was the study of the hot
phase of the ISM in M31. The analysis of the diffuse emission and the study of
the ISM is presented in a separate paper. We analysed the spectral properties
of all detected sources using hardness ratios and spectra if the statistics
were high enough. We also checked for variability. We cross-correlated the
source list with the source catalogue of a new survey of the northern disc of
M31 carried out with Chandra and Hubble (Panchromatic Hubble Andromeda
Treasury, PHAT) as well as with other existing catalogues. We detected a total
of 389 sources, including 43 foreground stars and candidates and 50 background
sources. Based on the comparison to the Chandra/PHAT survey, we classify 24
hard X-ray sources as new candidates for X-ray binaries (XRBs). In total, we
identified 34 XRBs and candidates and 18 supernova remnants (SNRs) and
candidates. Three of the four brightest SNRs show emission mainly below 2 keV,
consistent with shocked ISM. The spectra of two of them also require an
additional component with a higher temperature. The SNR [SPH11] 1535 has a
harder spectrum and might suggest that there is a pulsar-wind nebula inside the
SNR. We find five new sources showing clear time variability. We also studied
the spectral properties of the transient source SWIFT J004420.1+413702, which
shows significant variation in flux over a period of seven months (June 2015 to
January 2016) and associated change in absorption. Based on the likely optical
counterpart detected in the Chandra/PHAT survey, the source is classified as a
low-mass X-ray binary.Comment: Accepted for publication in A&
Recommended from our members
Web user interface design strategy: Designing for device independence
Until recently, Web services were available only through a desktop web browser. Nowadays, methods of access move beyond the desktop computer towards ubiquitous access through portable devices. As a consequence, users have the chance to interact with a growing diversity of computing devices such as PDAs, smart phones, etc., with diverse characteristics that tend to replace conventional laptop and desktop computers. User interface designers, on the other hand, strive to design usable interfaces to cater for the diverse requirements of these devices. The design strategy proposed in this paper aims at assisting user interface designers in designing for diverse devices recommending a specific line of activities in the process of design. A case study of application of the proposed design strategy is presented, outlining its advantages
Comparative performance of selected variability detection techniques in photometric time series
Photometric measurements are prone to systematic errors presenting a
challenge to low-amplitude variability detection. In search for a
general-purpose variability detection technique able to recover a broad range
of variability types including currently unknown ones, we test 18 statistical
characteristics quantifying scatter and/or correlation between brightness
measurements. We compare their performance in identifying variable objects in
seven time series data sets obtained with telescopes ranging in size from a
telephoto lens to 1m-class and probing variability on time-scales from minutes
to decades. The test data sets together include lightcurves of 127539 objects,
among them 1251 variable stars of various types and represent a range of
observing conditions often found in ground-based variability surveys. The real
data are complemented by simulations. We propose a combination of two indices
that together recover a broad range of variability types from photometric data
characterized by a wide variety of sampling patterns, photometric accuracies,
and percentages of outlier measurements. The first index is the interquartile
range (IQR) of magnitude measurements, sensitive to variability irrespective of
a time-scale and resistant to outliers. It can be complemented by the ratio of
the lightcurve variance to the mean square successive difference, 1/h, which is
efficient in detecting variability on time-scales longer than the typical time
interval between observations. Variable objects have larger 1/h and/or IQR
values than non-variable objects of similar brightness. Another approach to
variability detection is to combine many variability indices using principal
component analysis. We present 124 previously unknown variable stars found in
the test data.Comment: 29 pages, 8 figures, 7 tables; accepted to MNRAS; for additional
plots, see http://scan.sai.msu.ru/~kirx/var_idx_paper
Ipsographing the Dubject; or, The Contradictions of Twitter
Precise control of self-renewal and differentiation of progenitor cells into the cranial neural crest (CNC) pool ensures proper head development, guided by signaling pathways such as BMPs, FGFs, Shh and Notch. Here, we show that murine Sox2 plays an essential role in controlling progenitor cell behavior during craniofacial development. A "Conditional by Inversion" Sox2 allele (Sox2(COIN) ) has been employed to generate an epiblast ablation of Sox2 function (Sox2(EpINV) ). Sox2 (EpINV/+(H)) haploinsufficient and conditional (Sox2(EpINV/mosaic) ) mutant embryos proceed beyond gastrulation and die around E11. These mutant embryos exhibit severe anterior malformations, with hydrocephaly and frontonasal truncations, which could be attributed to the deregulation of CNC progenitor cells during their epithelial to mesenchymal transition. This irregularity results in an exacerbated and aberrant migration of Sox10(+) NCC in the branchial arches and frontonasal process of the Sox2 mutant embryos. These results suggest a novel role for Sox2 as a regulator of the epithelial to mesenchymal transitions (EMT) that are important for the cell flow in the developing head
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