2,519 research outputs found
The absence of rapid X-ray variability in active galaxies
Variations on time scales ranging from minutes to several hours in the X-ray flux from 54 observations of 38 active galaxies are identified. The sample is composed mostly of Seyfert I galaxies but also includes radio galaxies, NELG's BL Lacs and 3C 273. Only NGC 6814 varied on time scales as short as 100 sec. No other source was observed to vary with a time scale of less than 12 hours. Large amplitude short term variations are not a characteristic of the X-ray emission from active galaxies. Upper limits on sigma sub I/I ranged from 2% for Cen A, 5% for NGC 4151, to approximately 20% for sources giving 1 ct/sec in the detector. Three objects NGC 3227, NGC 4151 and MCG 5-23-16 show variability consistent with a time scale of approximately 1 day. Ways to reconcile the rapid variability seen for NGC 6814 (and NGC 4051) with the general stability observed for the other objects are considered
Measurement of pilot describing functions in single-controller multiloop tasks
Measurement of pilot describing functions in single controller multiloop task
The Quasi-1D S=1/2 Antiferromagnet Cs2CuCl4 in a Magnetic Field
Magnetic excitations of the quasi-1D S=1/2 Heisenberg antiferromagnet (HAF)
Cs2CuCl4 have been measured as a function of magnetic field using neutron
scattering. For T<0.62 K and B=0 T the weak inter-chain coupling produces 3D
incommensurate ordering. Fields greater than Bc =1.66 T, but less than the
field (~8 T) required to fully align the spins, are observed to decouple the
chains, and the system enters a disordered intermediate-field phase (IFP). The
IFP excitations are in agreement with the predictions of Muller et al. for the
1D S=1/2 HAF, and Talstra and Haldane for the related 1/r^2 chain (the
Haldane-Shastry model). This behaviour is inconsistent with linear spin-wave
theory.Comment: 10 pages, 4 encapsulated postscript figures, LaTeX, to be published
in PRL, e-mail comments to [email protected]
Analytical theory for proton correlations in common water ice
We provide a fully analytical microscopic theory for the proton correlations
in water ice . We compute the full diffuse elastic neutron scattering
structure factor, which we find to be in excellent quantitative agreement with
Monte Carlo simulations. It is also in remarkable qualitative agreement with
experiment, in the absence of any fitting parameters. Our theory thus provides
a tractable analytical starting point to account for more delicate features of
the proton correlations in water ice. In addition, it directly determines an
effective field theory of water ice as a topological phase.Comment: 5 pages, 3 figure
Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1
We show that dramatic spectral evolution associated with dips occurring near
phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times
heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying
faint component which is not attenuated by the variable column and whose flux
is ~10% of that of the unabsorbed bright component. A prominent Fe emission
line at ~6.5 keV is evident during the dips. The absolute line flux outside the
dips is similar to that during the dips, indicating that the line is associated
with the faint component. These results are consistent with a model in which
the bright component is radiation received directly from a compact source while
the faint component may be attributed to scattered radiation. Our results are
also generally consistent with those of Brandt et al., who found that a
partial- covering model could explain ASCA spectra of a low-to-high transition
in Cir X-1. The relative brightness of the two components in our model requires
a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson
scattering in material that mostly surrounds the source. We find that
illumination of such a scattering cloud by the observed direct component would
produce an Fe K-alpha fluorescence flux that is in rough agreement with the
flux of the observed emission line. We also conclude that if the scattering
medium is not highly ionized, our line of sight to the compact source does not
pass through it. Finally, we discuss simple pictures of the absorbers
responsible for the dips themselves.Comment: Accepted for publication in The Astrophysical Journal (23 pages,
including 11 figures
Spectral Evolution of Circinus X-1 Along its Orbit
We report on the spectral analysis of Circinus X-1 observed by the ASCA
satellite in March 1998 along one orbital period. The luminosity of the source
(in the 0.1-100 keV band) ranges from erg s at the
periastron (orbital phase 0.01) to erg s at orbital
phase 0.3. From the spectral analysis and the lightcurve we argue that Cir X-1
shows three states along the orbital evolution. The first state is at the
orbital phase interval 0.97-0.3: the luminosity becames super-Eddington and a
strong flaring activity is present. In this state a shock could form in the
inner region of the system due to the super-Eddington accretion rate, producing
an outflow of ionized matter whose observational signature could be the
prominent absorption edge at keV observed in the energy spectrum at
these phases. In the second state, corresponding to the orbital phase interval
between 0.3 and 0.7, the accretion rate is sub-Eddington and we observe a
weaker outflow, with smaller hydrogen column: the absorption edge is now at
keV with an optical depth a factor of 2.5 to 6 smaller. The third
state corresponds to the orbital phase interval 0.78-0.97. In this state the
best fit model to the spectrum requires the presence of a partial covering
component, indicating that the emission from the compact object is partially
absorbed by neutral matter, probably the atmosphere of the companion star
and/or the accreting matter from the companion.Comment: 18 pages, 3 figures. Accepted by Ap
Rapid X-ray variability in the Seyfert galaxy NGC 6814
The HEAO-1 A-2 high time resolution X-ray observations of the X-ray emitting Seyfert I Galaxy NGC 6814 are reported. In sharp distinction with a sample of over 30 active galactic nuclei this object showed strong X-ray variability on timescales less than 3 hours. The mean flux on a timescale of 90 minutes varied by a factor of approximately 2.5 corresponding to Delta L sub x being approximately 1 x 10 to the 43rd power ergs/sec. An autocorrelation analysis shows a characteristic time for variability of 100 (+60 or -25) seconds. There is no indication of spectral variability with an upper limit on a change in the power law spectral index of the absolute value of Delta gamma .37, for a factor two change in intensity. The constraints of such rapid variability on a wide variety of X-ray source mechanisms are considered
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Stochastic parameterization: uncertainties from convection
In 2005, the ECMWF held a workshop on stochastic parameterisation, at which the convection was seen as being
a key issue. That much is clear from the working group reports and particularly the statement from working group
1 that “it is clear that a stochastic convection scheme is desirable”. The present note aims to consider our current
status in comparison with some of the issues raised and hopes expressed in that working group report
Measurements with the Chandra X-Ray Observatory's flight contamination monitor
NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7
Exact two-spinon dynamical correlation function of the Heisenberg model
We derive the exact contribution of two spinons to the dynamical correlation
function of the spin-1/2 Heisenberg model. For this, we use the isotropic
limits of the exact form factors that have been recently computed through the
quantum affine symmetry of the anisotropic Heisenberg model Comment: 9 pages, Latex, 2 corrections of coefficient
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