1,944 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
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
Are magnetic fields universal in O-type multiple systems?
Although significant progress has been achieved in recent surveys of the
magnetism in massive stars, the origin of the detected magnetic fields remains
to be the least understood topic in their studies. We present an analysis of 61
high-resolution spectropolarimetric observations of 36 systems with O-type
primaries, among them ten known particle-accelerating colliding-wind binaries
exhibiting synchrotron radio emission. Our sample consists of multiple systems
with components at different evolutionary stages with wide and tight orbits and
different types of interactions. For the treatment of the complex composite
spectra of the multiple systems, we used a special procedure involving
different line masks populated for each element separately. Out of the 36
systems, 22 exhibit in their LSD Stokes V profiles definitely detected Zeeman
features, among them seven systems with colliding winds. For fourteen systems
the detected Zeeman features are most likely associated with O-type components
whereas for three systems we suggest an association with an early B-type
component. For the remaining five systems the source of the field is unclear.
Marginal evidence for the detection of a Zeeman feature is reported for eleven
systems and non-detection for three systems. The large number of systems with
definitely detected Zeeman features presents a mystery, but probably indicates
that multiplicity plays a definite role in the generation of magnetic fields in
massive stars. The newly found magnetic systems are supreme candidates for
spectropolarimetric monitoring over their orbital and rotation periods to
obtain trustworthy statistics on the magnetic field geometry and the
distribution of field strength.Comment: 21 pages, 2 tables, 9 figures, accepted for publication in MNRA
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