3,371 research outputs found
Turbulence, Complexity, and Solar Flares
The issue of predicting solar flares is one of the most fundamental in
physics, addressing issues of plasma physics, high-energy physics, and
modelling of complex systems. It also poses societal consequences, with our
ever-increasing need for accurate space weather forecasts. Solar flares arise
naturally as a competition between an input (flux emergence and rearrangement)
in the photosphere and an output (electrical current build up and resistive
dissipation) in the corona. Although initially localised, this redistribution
affects neighbouring regions and an avalanche occurs resulting in large scale
eruptions of plasma, particles, and magnetic field. As flares are powered from
the stressed field rooted in the photosphere, a study of the photospheric
magnetic complexity can be used to both predict activity and understand the
physics of the magnetic field. The magnetic energy spectrum and multifractal
spectrum are highlighted as two possible approaches to this.Comment: 2 figure
On finite-temperature holographic QCD in the Veneziano limit
Holographic models in the T=0 universality class of QCD in the limit of large
number N_c of colors and N_f massless fermion flavors, but constant ratio
x_f=N_f/N_c, are analyzed at finite temperature. The models contain a
5-dimensional metric and two scalars, a dilaton sourcing TrF^2 and a tachyon
dual to \bar qq. The phase structure on the T,x_f plane is computed and various
1st order, 2nd order transitions and crossovers with their chiral symmetry
properties are identified. For each x_f, the temperature dependence of p/T^4
and the quark-antiquark -condensate is computed. In the simplest case, we find
that for x_f up to the critical x_c\sim 4 there is a 1st order transition on
which chiral symmetry is broken and the energy density jumps. In the conformal
window x_c<x_f<11/2, there is only a continuous crossover between two conformal
phases. When approaching x_c from below, x_f\to x_c, temperature scales
approach zero as specified by Miransky scaling.Comment: 66 pages, 29 figure
A search for the presence of magnetic fields in the two Supergiant Fast X-ray Transients IGR J08408-4503 and IGR J11215-5952
A significant fraction of high-mass X-ray binaries are supergiant fast X-ray
transients (SFXTs). The prime model for the physics governing their X-ray
behaviour suggests that the winds of donor OB supergiants are magnetized. To
investigate if magnetic fields are indeed present in the optical counterparts
of such systems, we acquired low-resolution spectropolarimetric observations of
the two optically brightest SFXTs, IGR J08408-4503 and IGR J11215-5952 with the
ESO FORS2 instrument during two different observing runs. No field detection at
a significance level of 3sigma was achieved for IGR J08408-4503. For IGR
J11215-5952, we obtain 3.2sigma and 3.8sigma detections (_hydr =
-978+-308G and _hydr = 416+-110G) on two different nights in 2016. These
results indicate that the model involving the interaction of a magnetized
stellar wind with the neutron star magnetosphere can indeed be considered to
characterize the behaviour of SFXTs. We detected long-term spectral variability
in IGR J11215-5952, while for IGR J08408-4503 we find an indication of the
presence of short-term variability on a time scale of minutes.Comment: 5 pages, 1 table, 7 figures, accepted for publication in MNRA
Stellar activity as noise in exoplanet detection I. Methods and application to solar-like stars and activity cycles
The detection of exoplanets using any method is prone to confusion due to the
intrinsic variability of the host star. We investigate the effect of cool
starspots on the detectability of the exoplanets around solar-like stars using
the radial velocity method. For investigating this activity-caused "jitter" we
calculate synthetic spectra using radiative transfer, known stellar atomic and
molecular lines, different surface spot configurations, and an added planetary
signal. Here, the methods are described in detail, tested and compared to
previously published studies. The methods are also applied to investigate the
activity jitter in old and young solar-like stars, and over a solar-like
activity cycles. We find that the mean full jitter amplitude obtained from the
spot surfaces mimicking the solar activity varies during the cycle
approximately between 1 m/s and 9 m/s. With a realistic observing frequency a
Neptune mass planet on a one year orbit can be reliably recovered. On the other
hand, the recovery of an Earth mass planet on a similar orbit is not feasible
with high significance. The methods developed in this study have a great
potential for doing statistical studies of planet detectability, and also for
investigating the effect of stellar activity on recovered planetary parameters.Comment: Accepted to MNRA
Magnetic field geometry and chemical abundance distribution of the He-strong star CPD -57 3509
The magnetic field of CPD -57 3509 was recently detected in the framework of
the BOB (B fields in OB stars) collaboration. We acquired low-resolution
spectropolarimetric observations of CPD -57 3509 with FORS2 and high-resolution
UVES observations randomly distributed over a few months to search for
periodicity, to study the magnetic field geometry, and to determine the surface
distribution of silicon and helium. We also obtained supplementary photometric
observations at a timeline similar to the spectroscopic and spectropolarimetric
observations. A period of 6.36d was detected in the measurements of the mean
longitudinal magnetic field. A sinusoidal fit to our measurements allowed us to
constrain the magnetic field geometry and estimate the dipole strength in the
range of 3.9-4.5kG. Our application of the Doppler imaging technique revealed
the presence of He I spots located around the magnetic poles, with a strong
concentration at the positive pole and a weaker one around the negative pole.
In contrast, high concentration Si III spots are located close to the magnetic
equator. Further, our analysis of the spectral variability of CPD -57 3509 on
short time scales indicates distinct changes in shape and position of line
profiles possibly caused by the presence of beta Cep-like pulsations. A small
periodic variability in line with the changes of the magnetic field strength is
clearly seen in the photometric data.Comment: 11 pages, 5 tables, 7 figures, accepted for publication in MNRA
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