8,449 research outputs found
A reanalysis of the Gaia Data Release 2 photometric sensitivity curves using HST/STIS spectrophotometry
CONTEXT: The second Gaia data release (DR2) took place on April 2018. DR2
included photometry for more than 1.3 10^9 sources in G, BP, and RP. Even
though Gaia DR2 photometry is very precise, there are currently three
alternative definitions of the sensitivity curves that show significative
differences. AIMS: The aim of this paper is to improve the quality of the input
calibration data to produce new compatible definitions of the three bands and
to identify the reasons for the discrepancies between previous definitions.
METHODS: We have searched the HST archive for STIS spectra with G430L+G750L
data obtained with wide apertures and combined them with the CALSPEC library to
produce a high quality SED library of 122 stars with a broad range of colors,
including three very red stars. We have used it to compute new sensitivity
curves for G, BP, and RP using a functional analytical formalism. RESULTS: The
new curves are significantly better than the two previous attempts, REV and
WEI. For G we confirm the existence of a systematic bias in magnitude and
correct a color term present in REV. For BP we confirm the need to define two
magnitude ranges with different sensitivity curves and measure the cut between
them at G = 10.87 with a significant increase in precision. The new curves also
fit the data better than either REV or WEI. For RP we obtain a sensitivity
curve that better fits the STIS spectra and we find that the differences with
previous attempts reside in a systematic effect between ground-based and HST
spectral libraries. Additional evidence from color-color diagrams indicate that
the new sensitivity curve is more accurate. Nevertheless, there is still room
for improvement in the accuracy of the sensitivity curves because of the
current dearth of good-quality red calibrators: adding more to the sample
should be a priority before Gaia data release 3 takes place.Comment: Accepted for publication in A&A, revised version with minor change
The Radio Properties of Type Ibc Supernovae
Over the past few years, long-duration gamma-ray bursts (GRBs), including the
subclass of X-ray flashes (XRFs), have been revealed to be a rare variety of
Type Ibc supernova (SN Ibc). While all these events result from the death of
massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those
of ordinary Type Ibc SNe by many orders of magnitude. The observed diversity of
stellar death corresponds to large variations in the energy, velocity, and
geometry of the explosion ejecta. Using multi-wavelength (radio, optical,
X-ray) observations of the nearest GRBs, XRFs, and SNe Ibc, I show that while
GRBs and XRFs couple at least 10^48 erg to relativistic material, SNe Ibc
typically couple less than 10^48 erg to their fastest (albeit non-relativistic)
outflows. Specifically, I find that less than 3% of local SNe Ibc show any
evidence for association with a GRB or XRF. Recently, a new class of GRBs and
XRFs has been revealed which are under-luminous in comparison with the
statistical sample of GRBs. Owing to their faint high-energy emission, these
sub-energetic bursts are only detectable nearby (z < 0.1) and are likely 10
times more common than cosmological GRBs. In comparison with local SNe Ibc and
typical GRBs/XRFs, these explosions are intermediate in terms of both
volumetric rate and energetics. Yet the essential physical process that causes
a dying star to produce a GRB, XRF, or sub-energetic burst, and not just a SN,
remains a crucial open question. Progress requires a detailed understanding of
ordinary SNe Ibc which will be facilitated with the launch of wide-field
optical surveys in the near future.Comment: 8 pages, Proceedings for "Supernova 1987A: 20 Years After: Supernovae
and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W. Weiler, and R.
McCra
Physical activity education in the undergraduate curricula of all UK medical schools: are tomorrow's doctors equipped to follow clinical guidelines?
Physical activity (PA) is a cornerstone of disease prevention and treatment. There is, however, a considerable disparity between public health policy, clinical guidelines and the delivery of physical activity promotion within the National Health Service in the UK. If this is to be addressed in the battle against non-communicable diseases, it is vital that tomorrow's doctors understand the basic science and health benefits of physical activity. The aim of this study was to assess the provision of physical activity teaching content in the curricula of all medical schools in the UK. Our results, with responses from all UK medical schools, uncovered some alarming findings, showing that there is widespread omission of basic teaching elements, such as the Chief Medical Officer recommendations and guidance on physical activity. There is an urgent need for physical activity teaching to have dedicated time at medical schools, to equip tomorrow's doctors with the basic knowledge, confidence and skills to promote physical activity and follow numerous clinical guidelines that support physical activity promotion
Recent Type II Radio Supernovae
We present the results of radio observations, taken primarily with the Very
Large Array, of Supernovae 1993J, 2001gd, 2001em, 2002hh, 2004dj, and 2004et.
We have fit a parameterized model to the multi-frequency observations of each
supernova. We compare the observed and derived radio properties of these
supernovae by optical classification and discuss the implications.Comment: 5 pages, 2 figures, 1 table Conference Proceedings: "Supernova 1987A:
20 Years After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S.
Immler, K.W. Weiler, and R. McCra
Magneto-optical imaging of voltage-controlled magnetization reorientation
We study the validity and limitations of a macrospin model to describe the
voltage-controlled manipulation of ferromagnetic magnetization in nickel thin
film/piezoelectric actuator hybrid structures. To this end, we correlate
simultaneously measured spatially resolved magneto-optical Kerr effect imaging
and integral magnetotransport measurements at room temperature. Our results
show that a macrospin approach is adequate to model the magnetoresistance as a
function of the voltage applied to the hybrid, except for a narrow region
around the coercive field - where the magnetization reorientation evolves via
domain effects. Thus, on length scales much larger than the typical magnetic
domain size, the voltage control of magnetization is well reproduced by a
simple Stoner-Wohlfarth type macrospin model
- …