19,990 research outputs found
Quasi-Periodic Oscillations in Short Recurring Bursts of the magnetars SGR 1806-20 and SGR 1900+14 Observed With RXTE
Quasi-periodic oscillations (QPOs) observed in the giant flares of magnetars
are of particular interest due to their potential to open up a window into the
neutron star interior via neutron star asteroseismology. However, only three
giant flares have been observed. We therefore make use of the much larger data
set of shorter, less energetic recurrent bursts. Here, we report on a search
for QPOs in a large data set of bursts from the two most burst-active
magnetars, SGR 1806-20 and SGR 1900+14, observed with the Rossi X-ray Timing
Explorer (RXTE). We find a single detection in an averaged periodogram
comprising 30 bursts from SGR 1806-20, with a frequency of 57 Hz and a width of
5 Hz, remarkably similar to a giant flare QPO observed from SGR 1900+14. This
QPO fits naturally within the framework of global magneto-elastic torsional
oscillations employed to explain the giant flare QPOs. Additionally, we uncover
a limit on the applicability of Fourier analysis for light curves with low
background count rates and strong variability on short timescales. In this
regime, standard Fourier methodology and more sophisticated Fourier analyses
fail in equal parts by yielding an unacceptably large number of false positive
detections. This problem is not straightforward to solve in the Fourier domain.
Instead, we show how simulations of light curves can offer a viable solution
for QPO searches in these light curves.Comment: accepted for publication in ApJ; 12 pages, 7 figures; code +
instructions at https://github.com/dhuppenkothen/MagnetarQPOSearchPaper ;
associated data products at
http://figshare.com/articles/SGR_1900_14_RXTE_Data/1184101 (SGR 1900+14) and
http://figshare.com/articles/SGR_1806_20_Bursts_RXTE_data_set/1184427 (SGR
1806-20
The lowest singlet-triplet excitation energy of BN: a converged coupled cluster perspective
The notoriously small excitation energy of the BN
diatomic has been calculated using high-order coupled cluster methods.
Convergence has been established in both the 1-particle basis set and the
coupled cluster expansion. Explicit inclusion of connected quadruple
excitations is required for even semiquantitative agreement with
the limit value, while connected quintuple excitations still have
an effect of about 60 cm. Still higher excitations only account for
about 10 cm. Inclusion of inner-shell correlation further reduces
by about 60 cm at the CCSDT, and 85 cm at the CCSDTQ level. Our
best estimate, =18340 cm, is in excellent agreement with
earlier calculations and experiment, albeit with a smaller (and conservative)
uncertainty. The dissociation energy of BN() is =105.740.16
kcal/mol and =103.570.16 kcal/mol.Comment: J. Chem. Phys., in pres
Epidemics and percolation in small-world networks
We study some simple models of disease transmission on small-world networks,
in which either the probability of infection by a disease or the probability of
its transmission is varied, or both. The resulting models display epidemic
behavior when the infection or transmission probability rises above the
threshold for site or bond percolation on the network, and we give exact
solutions for the position of this threshold in a variety of cases. We confirm
our analytic results by numerical simulation.Comment: 6 pages, including 3 postscript figure
The Number of Incipient Spanning Clusters in Two-Dimensional Percolation
Using methods of conformal field theory, we conjecture an exact form for the
probability that n distinct clusters span a large rectangle or open cylinder of
aspect ratio k, in the limit when k is large.Comment: 9 pages, LaTeX, 1 eps figure. Additional references and comparison
with existing numerical results include
The 2-Dimensional Quantum Euclidean Algebra
The algebra dual to Woronowicz's deformation of the 2-\-di\-men\-sion\-al
Euclidean group is constructed. The same algebra is obtained from
via contraction on both the group and algebra levels.Comment: 8 pages, LBL-31711 and UCB-PTH-92/0
A crossing probability for critical percolation in two dimensions
Langlands et al. considered two crossing probabilities, pi_h and pi_{hv}, in
their extensive numerical investigations of critical percolation in two
dimensions. Cardy was able to find the exact form of pi_h by treating it as a
correlation function of boundary operators in the Q goes to 1 limit of the Q
state Potts model. We extend his results to find an analogous formula for
pi_{hv} which compares very well with the numerical results.Comment: 8 pages, Latex2e, 1 figure, uuencoded compressed tar file, (1 typo
changed
Indications for a slow rotator in the Rapid Burster from its thermonuclear bursting behaviour
We perform time-resolved spectroscopy of all the type I bursts from the Rapid
Burster (MXB 1730-335) detected with the Rossi X-ray Timing Explorer. Type I
bursts are detected at high accretion rates, up to \sim 45% of the Eddington
luminosity. We find evidence that bursts lacking the canonical cooling in their
time-resolved spectra are, none the less, thermonuclear in nature. The type I
bursting rate keeps increasing with the persistent luminosity, well above the
threshold at which it is known to abruptly drop in other bursting low-mass
X-ray binaries. The only other known source in which the bursting rate keeps
increasing over such a large range of mass accretion rates is the 11 Hz pulsar
IGR J174802446. This may indicate a similarly slow spin for the neutron star
in the Rapid Burster
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