7,790 research outputs found
On the X-ray number of almost smooth convex bodies and of convex bodies of constant width
The X-ray numbers of some classes of convex bodies are investigated. In
particular, we give a proof of the X-ray Conjecture as well as of the
Illumination Conjecture for almost smooth convex bodies of any dimension and
for convex bodies of constant width of dimensions 3, 4, 5 and 6
CCD photometry and new models of 5 minor planets
We present new R filtered CCD observations of 5 faint and moderately faint
asteroids carried out between October, 1998 and January, 1999. The achieved
accuracy is between 0.01-0.03 mag, depending mainly on the target brightness.
The obtained sinodic periods and amplitudes:
683 Lanzia - 4.6+/-0.2 h, 0.13 mag; 725 Amanda - >3.0 h, >=0.40 mag; 852
Wladilena - 4.62+/-0.01 h, 0.32 mag (December, 1998) and 0.27 mag (January,
1999); 1627 Ivar - 4.80+/-0.01, 0.77 mag (December, 1998) and 0.92 mag
(January, 1999). The Near Earth Object 1998 PG unambiguously showed
doubly-periodic lightcurve, suggesting the possibility of a relatively fast
precession (P_1=1.3 h, P_2=5.3 h).
Collecting all data from the literature, we determined new models for 3 minor
planets. The resulting spin vectors and triaxial ellipsoids have been
calculated by an amplitude-method. Sidereal periods and senses of rotation were
calculated for two asteroids (683 and 1627) by a modified epoch-method. The
results are:
683 - lambda_p=15/195+/-25 deg, beta_p=52+/-15 deg, a/b=1.15+/-0.05,
b/c=1.05+/-0.05, P_sid=0.1964156+/-0.0000001 d, retrograde; 852 -
lambda_p=30/210+/-20 deg, beta_p=30+/-10 deg, a/b=2.3+/-0.3, b/c=1.2+/-0.2;
1627 - lambda_p=145/325+/-8 deg, beta_p=34+/-6 deg, a/b=2.0+/-0.1,
b/c=1.09+/-0.05, P_sid=0.1999154+/-0.0000003 d, retrograde. The obtained shape
of 1627 is in good agreement with radar images by Ostro et al. (1990).Comment: 8 pages, accepted for publication in Astronomy and Astrophysics
Suppl. Serie
Generalized Kac's Lemma for Recurrence Time in Iterated Open Quantum Systems
We consider recurrence to the initial state after repeated actions of a
quantum channel. After each iteration a projective measurement is applied to
check recurrence. The corresponding return time is known to be an integer for
the special case of unital channels, including unitary channels. We prove that
for a more general class of quantum channels the expected return time can be
given as the inverse of the weight of the initial state in the steady state.
This statement is a generalization of the Kac lemma for classical Markov
chains
Pushing the limits: K2 observations of the trans-Neptunian objects 2002 GV31 and (278361) 2007 JJ43
We present the first photometric observations of trans-Neptunian objects
(TNOs) taken with the Kepler space telescope, obtained in the course of the K2
ecliptic survey. Two faint objects have been monitored in specifically designed
pixel masks that were centered on the stationary points of the objects, when
their daily motion was the slowest. In the design of the experiment, only the
apparent path of these objects were retrieved from the detectors, i.e. the
costs in terms of Kepler pixels were minimized. Because of the faintness of the
targets we employ specific reduction techniques and co-added images. We measure
rotational periods and amplitudes in the unfiltered Kepler band as follows: for
(278361) 2007 JJ43 and 2002 GV31 we get P_rot=12.097 h and P_rot=29.2 h while
0.10 and 0.35 mag for the total amplitudes, respectively. Future space
missions, like TESS and PLATO are not well suited to this kind of observations.
Therefore, we encourage to include the brightest TNOs around their stationary
points in each observing campaign to exploit this unique capability of the K2
Mission -- and therefore to provide unbiased rotational, shape and albedo
characteristics of many objects.Comment: Accepted for publication in ApJ Letters, 5.2 pages in emulateapj
style, misspelled 2007 JJ43 designation correcte
Evidence for fresh frost layer on the bare nucleus of comet Hale--Bopp at 32 AU distance
Here we report that the activity of comet Hale--Bopp ceased between late 2007
and March, 2009, at about 28 AU distance from the Sun. At that time the comet
resided at a distance from the Sun that exceeded the freeze-out distance of
regular comets by an order of magnitude. A Herschel Space Observatory PACS scan
was taken in mid-2010, in the already inactive state of the nucleus. The albedo
has been found to be surprisingly large (8.10.9%{}), which exceeds the
value known for any other comets. With re-reduction of archive HST images from
1995 and 1996, we confirm that the pre-perihelion albedo resembled that of an
ordinary comet, and was smaller by a factor of two than the post-activity
albedo. Our further observations with the Very Large Telescope (VLT) also
confirmed that the albedo increased significantly by the end of the activity.
We explain these observations by proposing gravitational redeposition of icy
grains towards the end of the activity. This is plausible for such a massive
body in a cold environment, where gas velocity is lowered to the range of the
escape velocity. These observations also show that giant comets are not just
the upscaled versions of the comets we know but can be affected by processes
that are yet to be fully identified.Comment: 7 pages, 6 figures, accepted for publication in Ap
Quantized recurrence time in iterated open quantum dynamics
The expected return time to the original state is a key concept
characterizing systems obeying both classical or quantum dynamics. We consider
iterated open quantum dynamical systems in finite dimensional Hilbert spaces, a
broad class of systems that includes classical Markov chains and unitary
discrete time quantum walks on networks. Starting from a pure state, the time
evolution is induced by repeated applications of a general quantum channel, in
each timestep followed by a measurement to detect whether the system has
returned to the original state. We prove that if the superoperator is unital in
the relevant Hilbert space (the part of the Hilbert space explored by the
system), then the expectation value of the return time is an integer, equal to
the dimension of this relevant Hilbert space. We illustrate our results on
partially coherent quantum walks on finite graphs. Our work connects the
previously known quantization of the expected return time for bistochastic
Markov chains and for unitary quantum walks, and shows that these are special
cases of a more general statement. The expected return time is thus a
quantitative measure of the size of the part of the Hilbert space available to
the system when the dynamics is started from a certain state
THREE-DIMENSIONAL STABLY ADMISSIBLE PREY-PREDATOR MODELS
This paper deals with the most interesting three dimensional Volterra systems, which
have first a sign stable interaction matrix. This matrix is stably admissible too. Then we
consider a balanced interaction matrix, which is not sign stable, because it has a cycle,
but it is stably admissible, and lost we consider an interesting not stably admissible case
Main-Belt Asteroids in the K2 Engineering Field of View
Unlike NASA's original Kepler Discovery Mission, the renewed K2 Mission will
stare at the plane of the Ecliptic, observing each field for approximately 75
days. This will bring new opportunities and challenges, in particular the
presence of a large number of main-belt asteroids that will contaminate the
photometry. The large pixel size makes K2 data susceptible to the effect of
apparent minor planet encounters. Here we investigate the effects of asteroid
encounters on photometric precision using a sub-sample of the K2 Engineering
data taken in February, 2014. We show examples of asteroid contamination to
facilitate their recognition and distinguish these events from other error
sources. We conclude that main-belt asteroids will have considerable effects on
K2 photometry of a large number of photometric targets during the Mission, that
will have to be taken into account. These results will be readily applicable
for future space photometric missions applying large-format CCDs, such as TESS
and PLATO.Comment: accepted for publication in AJ, 6 page
The 106Cd(α, α)106Cd elastic scattering in a wide energy range for γ process studies
Date of Acceptance: 15/04/2015Alpha elastic scattering angular distributions of the 106Cd(α, α)106Cd reaction were measured at three energies around the Coulomb barrier to provide a sensitive test for the α + nucleus optical potential parameter sets. Furthermore, the new high precision angular distributions, together with the data available from the literature were used to study the energy dependence of the locally optimized α + nucleus optical potential in a wide energy region ranging from ELab=27.0MeV down to 16.1 MeV.The potentials under study are a basic prerequisite for the prediction of α-induced reaction cross sections and thus, for the calculation of stellar reaction rates used for the astrophysical γ process. Therefore, statistical model predictions using as input the optical potentials discussed in the present work are compared to the available 106Cd + alpha cross section data.Peer reviewe
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