229,074 research outputs found
Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 I. Spectropolarimetric observations in all four Stokes parameters
High-resolution spectropolarimetric observations provide simultaneous
information about stellar magnetic field topologies and three-dimensional
distributions of chemical elements. Here we present analysis of a unique full
Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap
star HD 24712. The goal of our work is to examine circular and linear
polarization signatures inside spectral lines and to study variation of the
stellar spectrum and magnetic observables as a function of rotational phase. HD
24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over
a period of 2010-2011. The resulting spectra have S/N ratio of 300-600 and
resolving power exceeding 100000. The multiline technique of least-squares
deconvolution (LSD) was applied to combine information from the spectral lines
of Fe-peak and rare-earth elements. We used the HARPSPol spectra of HD 24712 to
study the morphology of the Stokes profile shapes in individual spectral lines
and in LSD Stokes profiles corresponding to different line masks. From the LSD
Stokes V profiles we measured the longitudinal component of the magnetic field,
, with an accuracy of 5-10 G. We also determined the net linear
polarization from the LSD Stokes Q and U profiles. We determined an improved
rotational period of the star, P_rot = 12.45812 +/- 0.00019d. We measured
from the cores of Halpha and Hbeta lines. The analysis of measurements
showed no evidence for a significant radial magnetic field gradient in the
atmosphere of HD 24712. We used our and net linear polarization
measurements to determine parameters of the dipolar magnetic field topology. We
found that magnetic observables can be reasonably well reproduced by the
dipolar model. We discovered rotational modulation of the Halpha core and
related it a non-uniform surface distribution of rare-earth elements.Comment: Accepted for publication in A&
Undermining User Privacy on Mobile Devices Using AI
Over the past years, literature has shown that attacks exploiting the
microarchitecture of modern processors pose a serious threat to the privacy of
mobile phone users. This is because applications leave distinct footprints in
the processor, which can be used by malware to infer user activities. In this
work, we show that these inference attacks are considerably more practical when
combined with advanced AI techniques. In particular, we focus on profiling the
activity in the last-level cache (LLC) of ARM processors. We employ a simple
Prime+Probe based monitoring technique to obtain cache traces, which we
classify with Deep Learning methods including Convolutional Neural Networks. We
demonstrate our approach on an off-the-shelf Android phone by launching a
successful attack from an unprivileged, zeropermission App in well under a
minute. The App thereby detects running applications with an accuracy of 98%
and reveals opened websites and streaming videos by monitoring the LLC for at
most 6 seconds. This is possible, since Deep Learning compensates measurement
disturbances stemming from the inherently noisy LLC monitoring and unfavorable
cache characteristics such as random line replacement policies. In summary, our
results show that thanks to advanced AI techniques, inference attacks are
becoming alarmingly easy to implement and execute in practice. This once more
calls for countermeasures that confine microarchitectural leakage and protect
mobile phone applications, especially those valuing the privacy of their users
Detection of ultra-weak magnetic fields in Am stars: beta UMa and theta Leo
An extremely weak circularly polarized signature was recently discovered in
spectral lines of the chemically peculiar Am star Sirius A. A weak surface
magnetic field was proposed to account for the observed polarized signal, but
the shape of the phase-averaged signature, dominated by a prominent positive
lobe, is not expected in the standard theory of the Zeeman effect. We aim at
verifying the presence of weak circularly polarized signatures in two other
bright Am stars, beta UMa and theta Leo, and investigating the physical origin
of Sirius-like polarized signals further. We present here a set of deep
spectropolarimetric observations of beta UMa and theta Leo, observed with the
NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares
Deconvolution multiline procedure. To improve the signal-to-noise ratio and
detect extremely weak signatures in Stokes V profiles, we co-added all
available spectra of each star (around 150 observations each time). Finally, we
ran several tests to evaluate whether the detected signatures are consistent
with the behavior expected from the Zeeman effect. The line profiles of the two
stars display circularly polarized signatures similar in shape and amplitude to
the observations previously gathered for Sirius A. Our series of tests brings
further evidence of a magnetic origin of the recorded signal. These new
detections suggest that very weak magnetic fields may well be present in the
photospheres of a significant fraction of intermediate-mass stars. The strongly
asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant
single-sign lobe) are not expected in the standard theory of the Zeeman effect
and may be linked to sharp vertical gradients in photospheric velocities and
magnetic field strengths
Calibration of the NuSTAR High Energy Focusing X-ray Telescope
We present the calibration of the \textit{Nuclear Spectroscopic Telescope
Array} (\nustar) X-ray satellite. We used the Crab as the primary effective
area calibrator and constructed a piece-wise linear spline function to modify
the vignetting response. The achieved residuals for all off-axis angles and
energies, compared to the assumed spectrum, are typically better than \%
up to 40\,keV and 5--10\,\% above due to limited counting statistics. An
empirical adjustment to the theoretical 2D point spread function (PSF) was
found using several strong point sources, and no increase of the PSF half power
diameter (HPD) has been observed since the beginning of the mission. We report
on the detector gain calibration, good to 60\,eV for all grades, and discuss
the timing capabilities of the observatory, which has an absolute timing of
3\,ms. Finally we present cross-calibration results from two campaigns
between all the major concurrent X-ray observatories (\textit{Chandra},
\textit{Swift}, \textit{Suzaku} and \textit{XMM-Newton}), conducted in 2012 and
2013 on the sources 3C\,273 and PKS\,2155-304, and show that the differences in
measured flux is within 10\% for all instruments with respect to \nustar
The Sources of HCN and CH3OH and the Rotational Temperature in Comet 103P/Hartley 2 from Time-Resolved Millimeter Spectroscopy
One of the least understood properties of comets is the compositional
structure of their nuclei, which can either be homogeneous or heterogeneous.
The nucleus structure can be conveniently studied at millimeter wavelengths,
using velocity-resolved spectral time series of the emission lines, obtained
simultaneously for multiple molecules as the body rotates. Using this
technique, we investigated the sources of CH3OH and HCN in comet 103P/Hartley
2, the target of NASA's EPOXI mission, which had an exceptionally favorable
apparition in late 2010. Our monitoring with the IRAM 30 m telescope shows
short-term variability of the spectral lines caused by nucleus rotation. The
varying production rates generate changes in brightness by a factor of 4 for
HCN and by a factor of 2 for CH3OH, and they are remarkably well correlated in
time. With the addition of the velocity information from the line profiles, we
identify the main sources of outgassing: two jets, oppositely directed in a
radial sense, and icy grains, injected into the coma primarily through one of
the jets. The mixing ratio of CH3OH and HCN is dramatically different in the
two jets, which evidently shows large-scale chemical heterogeneity of the
nucleus. We propose a network of identities linking the two jets with
morphological features reported elsewhere, and postulate that the chemical
heterogeneity may result from thermal evolution. The model-dependent average
production rates are 3.5x10**26 molec/s for CH3OH and 1.25x10**25 molec/s for
HCN, and their ratio of 28 is rather high but not abnormal. The rotational
temperature from CH3OH varied strongly, presumably due to nucleus rotation,
with the average value being 47 K.Comment: Published in ApJ 756, 80 (2012). Supplementary materials available at
http://www.its.caltech.edu/~mdrahus/103p_paperII.htm
Recommended from our members
Time Resolved Spectroscopy Of Cataclysmic Variables - SS Cygni
NSF AST76-23882, AST 79-06340McDonald Observator
Diagnostic of stellar magnetic fields with cumulative circular polarisation profiles
Information about stellar magnetic field topologies is obtained primarily
from high-resolution circular polarisation (Stokes ) observations. Due to
their generally complex morphologies, the stellar Stokes profiles are
usually interpreted with elaborate inversion techniques such as Zeeman Doppler
imaging (ZDI). Here we further develop a new method of interpretation of
circular polarisation signatures in spectral lines using cumulative Stokes
profiles (anti-derivative of Stokes ). This method is complimentary to ZDI
and can be applied for validation of the inversion results or when the
available observational data are insufficient for an inversion. Based on the
rigorous treatment of polarised line formation in the weak-field regime, we
show that, for rapidly rotating stars, the cumulative Stokes profiles
contain information about the spatially resolved longitudinal magnetic field
density. Rotational modulation of these profiles can be employed for a simple,
qualitative characterisation of the stellar magnetic field topologies. We apply
this diagnostic method to the archival observations of the weak-line T Tauri
star V410 Tau and Bp He-strong star HD 37776. We show that the magnetic field
in V410 Tau is dominated by an azimuthal component, in agreement with the ZDI
map that we recover from the same data set. For HD 37776 the cumulative Stokes
profile variation indicates the presence of multiple regions of positive
and negative field polarity. This behaviour agrees with the ZDI results but
contradicts the popular hypothesis that the magnetic field of this star is
dominated by an axisymmetric quadrupolar component.Comment: 11 pages, 9 figures; accepted for publication in A&
The Magnetic Topology of the Weak-Lined T Tauri Star V410 - A Simultaneous Temperature and Magnetic Field Inversion
We present a detailed temperature and magnetic investigation of the T Tauri
star V410 Tau by means of a simultaneous Doppler- and Zeeman-Doppler Imaging.
Moreover we introduce a new line profile reconstruction method based on a
singular value decomposition (SVD) to extract the weak polarized line profiles.
One of the key features of the line profile reconstruction is that the SVD line
profiles are amenable to radiative transfer modeling within our Zeeman-Doppler
Imaging code iMap. The code also utilizes a new iterative regularization scheme
which is independent of any additional surface constraints. To provide more
stability a vital part of our inversion strategy is the inversion of both
Stokes I and Stokes V profiles to simultaneously reconstruct the temperature
and magnetic field surface distribution of V410 Tau. A new image-shear analysis
is also implemented to allow the search for image and line profile distortions
induced by a differential rotation of the star. The magnetic field structure we
obtain for V410 Tau shows a good spatial correlation with the surface
temperature and is dominated by a strong field within the cool polar spot. The
Zeeman-Doppler maps exhibit a large-scale organization of both polarities
around the polar cap in the form of a twisted bipolar structure. The magnetic
field reaches a value of almost 2 kG within the polar region but smaller fields
are also present down to lower latitudes. The pronounced non-axisymmetric field
structure and the non-detection of a differential rotation for V410 Tau
supports the idea of an underlying -type dynamo, which is predicted
for weak-lined T Tauri stars.Comment: Accepted for A&A, 18 pages, 10 figure
Analysis of antenna position measurements and weather station network data during the ALMA Long Baseline Campaign of 2015
In a radio interferometer, the determination of geometrical antenna positions
relies on accurate calibration of the dry and wet delay of the atmosphere above
each antenna. For the Atacama Large Millimeter/Submillimeter Array (ALMA),
which has baseline lengths up to 16 kilometers, the geography of the site
forces the height above mean sea level of the more distant antenna pads to be
significantly lower than the central array. Thus, both the ground level
meteorological values and the total water column can be quite different between
antennas in the extended configurations. During 2015, a network of six
additional weather stations was installed to monitor pressure, temperature,
relative humidity and wind velocity, in order to test whether inclusion of
these parameters could improve the repeatability of antenna position
determinations in these configurations. We present an analysis of the data
obtained during the ALMA Long Baseline Campaign of Oct. through Nov. 2015. The
repeatability of antenna position measurements typically degrades as a function
of antenna distance. Also, the scatter is more than three times worse in the
vertical direction than in the local tangent plane, suggesting that a
systematic effect is limiting the measurements. So far we have explored
correcting the delay model for deviations from hydrostatic equilibrium in the
measured air pressure and separating the partial pressure of water from the
total pressure using water vapor radiometer (WVR) data. Correcting for these
combined effects still does not provide a good match to the residual position
errors in the vertical direction. One hypothesis is that the current model of
water vapor may be too simple to fully remove the day-to-day variations in the
wet delay. We describe possible avenues of improvement, including measuring and
applying more accurate values of the sky coupling efficiency of the WVRs.Comment: 19 pages, 14 figures, 1 table; presented at SPIE Astronomical
Telescopes + Instrumentation 2016, held in Edinburgh, UK on 26 June - 1 July
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