8,252 research outputs found
X-ray interferometry with transmissive beam combiners for ultra-high angular resolution astronomy
Interferometry provides one of the possible routes to ultra-high angular
resolution for X-ray and gamma-ray astronomy. Sub-micro-arc-second angular
resolution, necessary to achieve objectives such as imaging the regions around
the event horizon of a super-massive black hole at the center of an active
galaxy, can be achieved if beams from parts of the incoming wavefront separated
by 100s of meters can be stably and accurately brought together at small
angles. One way of achieving this is by using grazing incidence mirrors. We
here investigate an alternative approach in which the beams are recombined by
optical elements working in transmission. It is shown that the use of
diffractive elements is a particularly attractive option. We report
experimental results from a simple 2-beam interferometer using a low-cost
commercially available profiled film as the diffractive elements. A
rotationally symmetric filled (or mostly filled) aperture variant of such an
interferometer, equivalent to an X-ray axicon, is shown to offer a much wider
bandpass than either a Phase Fresnel Lens (PFL) or a PFL with a refractive lens
in an achromatic pair. Simulations of an example system are presented.Comment: To be published in "Experimental Astronomy
A simple variational method for calculating energy and quantum capacitance of an electron gas with screened interactions
We describe a variational procedure for calculating the energy of an electron
gas in which the long-range Coulomb interaction is truncated by the screening
effect of a nearby metallic gate. We use this procedure to compute the quantum
capacitance of the system as a function of electron density and spin
polarization. The accuracy of the method is verified against published
Monte-Carlo data. The results compare favorably with a recent experiment.Comment: 4 pages, 3 figure
On the high coherence of kilo-Hz Quasi-Periodic Oscillations
We have carried out a systematic study of the properties of the kilo-Hertz
quasi-periodic oscillations (QPO) observed in the X-ray emission of the neutron
star low-mass X-ray binary 4U1608-52, using archival data obtained with the
Rossi X-ray Timing Explorer. We have investigated the quality factor, Q, of the
oscillations (defined as the ratio of the frequency of the QPO peak to its full
width at half maximum). In order to minimise the effect of long-term frequency
drifts, power spectra were computed over the shortest times permitted by the
data statistics. We show that the high Q of ~200 reported by Berger et al.
(1996) for the lower frequency kilo-Hz QPO in one of their observations is by
no means exceptional, as we observe a mean Q value in excess of 150 in 14 out
of the 21 observations analysed and Q can remain above 200 for thousands of
seconds. The frequency of the QPO varies over the wide range 560--890 Hz and we
find a systematic trend for the coherence time of the QPO, estimated as tau=Q
/(pi nu), to increase with the frequency, up to a maximum level at ~ 800 Hz,
beyond which it appears to decrease, at frequencies where the QPO weakens.
There is a more complex relationship between tau and the QPO root mean squared
amplitude (RMS), in which positive and negative correlations can be found. A
higher-frequency QPO, revealed by correcting for the frequency drift of the
560-890 Hz one, has a much lower Q (~10) which does not follow the same
pattern. We discuss these results in the framework of competing QPO models and
show that those involving clumps orbiting within or above the accretion disk
are ruled out.Comment: Accepted for publication in MNRAS, 8 pages, 6 figures, 3 Table
A Persistent High-Energy Flux from the Heart of the Milky Way : Integral's view of the Galactic Center
The Ibis/Isgri imager on Integral detected for the first time a hard X-ray
source, IGR J17456-2901, located within 1' of Sgr A* over the energy range
20-100 keV. Here we present the results of a detailed analysis of ~7 Ms of
Integral observations of the GC. With an effective exposure of 4.7 Ms we have
obtained more stringent positional constraints on this HE source and
constructed its spectrum in the range 20-400 keV. Furthermore, by combining the
Isgri spectrum with the total X-ray spectrum corresponding to the same physical
region around SgrA* from XMM data, and collected during part of the Integral
observations, we constructed and present the first accurate wide band HE
spectrum for the central arcmins of the Galaxy. Our complete analysis of the
emission properties of IGR shows that it is faint but persistent with no
variability above 3 sigma contrary to what was alluded to in our first paper.
This result, in conjunction with the spectral characteristics of the X-ray
emission from this region, suggests that the source is most likely not
point-like but, rather, that it is a compact, yet diffuse, non-thermal emission
region. The centroid of IGR is estimated to be R.A.=17h45m42.5,
decl.=-28deg59'28'', offset by 1' from the radio position of Sgr A* and with a
positional uncertainty of 1'. Its 20-400 keV luminosity at 8 kpc is L=5.4x10^35
erg/sec. Very recently, Hess detected of a source of ~TeV g-rays also located
within 1' of Sgr A*. We present arguments in favor of an interpretation
according to which the photons detected by Integral and Hess arise from the
same compact region of diffuse emission near the central BH and that the
supernova remnant Sgr A East could play an important role as a contributor of
very HE g-rays to the overall spectrum from this region.Comment: 14 pages, 11 figures, Accepted for publication in Ap
The problem of shot selection in basketball
In basketball, every time the offense produces a shot opportunity the player
with the ball must decide whether the shot is worth taking. In this paper, I
explore the question of when a team should shoot and when they should pass up
the shot by considering a simple theoretical model of the shot selection
process, in which the quality of shot opportunities generated by the offense is
assumed to fall randomly within a uniform distribution. I derive an answer to
the question "how likely must the shot be to go in before the player should
take it?", and show that this "lower cutoff" for shot quality depends
crucially on the number of shot opportunities remaining (say, before the
shot clock expires), with larger demanding that only higher-quality shots
should be taken. The function is also derived in the presence of a
finite turnover rate and used to predict the shooting rate of an
optimal-shooting team as a function of time. This prediction is compared to
observed shooting rates from the National Basketball Association (NBA), and the
comparison suggests that NBA players tend to wait too long before shooting and
undervalue the probability of committing a turnover.Comment: 7 pages, 2 figures; comparison to NBA data adde
Spiral Waves in Chaotic Systems
Spiral waves are investigated in chemical systems whose underlying
spatially-homogeneous dynamics is governed by a deterministic chaotic
attractor. We show how the local periodic behavior in the vicinity of a spiral
defect is transformed to chaotic dynamics far from the defect. The
transformation occurs by a type of period doubling as the distance from the
defect increases. The change in character of the dynamics is described in terms
of the phase space flow on closed curves surrounding the defect.Comment: latex file with three postscript figures to appear in Physical review
Letter
Troubles with Bayesianism: An introduction to the psychological immune system
A Bayesian mind is, at its core, a rational mind. Bayesianism is thus well-suited to predict and explain mental processes that best exemplify our ability to be rational. However, evidence from belief acquisition and change appears to show that we do not acquire and update information in a Bayesian way. Instead, the principles of belief acquisition and updating seem grounded in maintaining a psychological immune system rather than in approximating
a Bayesian processor
EXIST: The Ultimate Spatial/Temporal Hard X-ray Survey
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed mission to conduct an all-sky imaging hard x-ray (HX) survey (~5â600 keV) with ~0.05mCrab sensitivity (5Ï; 6mo.; ~5â100keV) comparable to the ROSAT soft x-ray survey, and to provide the maximum sensitivity and resolution (spatial and temporal) HX imager as the Next Generation GRB mission. Its primary science goals are to i) identify and measure obscured AGN and constrain the accretion luminosity of the universe as well as the cosmic IR background from Blazar spectra coincident with GeV-TeV observations, ii) measure spectra, variability and locations for the faintest GRBs to study the most energetic events in the universe and the earliest epoch of star formation, and iii) study black holes on all scales, from x-ray transients to luminous AGN. EXIST would incorporate a very large area (~8m^2) imaging Cd-Zn-Te detector and coded aperture telescope array with nearly half-sky instantaneous view which images the full sky each orbit. With fixed zenith pointing, it could be mounted on the ISS or a free flyer and would complement both GLAST and Constellation-X science if launched before 2010, as recommended by the Astronomy and Astrophysics Decadal Survey
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