8,796 research outputs found

### The Relativistic Precession Model for QPOs in Low Mass X-ray Binaries

The relativistic precession model for quasi periodic oscillations, QPOs, in
low mass X-ray binaries is reviewed. The behaviour of three simultaneous types
of QPOs is well matched in terms of the fundamental frequencies for geodesic
motion in the strong field of the accreting compact object for reasonable star
masses and spin frequencies. The model ascribes the higher frequency kHz QPOs,
the lower frequency kHz QPOs and the horizontal branch oscillations to the
Keplerian, periastron precession and nodal precession frequencies of matter
inhomogeneities orbiting close to the inner edge of the accretion disk. The
remarkable correlation between the centroid frequency of QPOs in both neutron
star and black hole candidate low mass X-ray binaries is very well fit by the
model. QPOs from low mass X-ray binaries might provide an unprecedented
laboratory to test general relativity in the strong field regime.Comment: 12 pages, 2 embedded figures, Proceedings of X-ray Astronomy '999 -
Stellar Endpoints, AGN and the Diffuse Background, 2000. G. Malaguti, G.
Palumbo & N. White (eds), Gordon & Breach (Singapore

### Boundary Spatiotemporal Correlations in a Self-Organized Critical Model of Punctuated Equilibrium

In a semi-infinite geometry, a 1D, M-component model of biological evolution
realizes microscopically an inhomogeneous branching process for $M \to \infty$.
This implies in particular a size distribution exponent $\tau'=7/4$ for
avalanches starting at a free end of the evolutionary chain. A bulk--like
behavior with $\tau'=3/2$ is restored if `conservative' boundary conditions
strictly fix to its critical, bulk value the average number of species directly
involved in an evolutionary avalanche by the mutating species located at the
chain end. A two-site correlation function exponent ${\tau_R}'=4$ is also
calculated exactly in the `dissipative' case, when one of the points is at the
border. These results, together with accurate numerical determinations of the
time recurrence exponent $\tau_{first}'$, show also that, no matter whether
dissipation is present or not, boundary avalanches have the same space and time
fractal dimensions as in the bulk, and their distribution exponents obey the
basic scaling laws holding there.Comment: 5 pages, 3 eps figure

### Branching Interfaces with Infinitely Strong Couplings

A hierarchical froth model of the interface of a random $q$-state Potts
ferromagnet in $2D$ is studied by recursive methods. A fraction $p$ of the
nearest neighbour bonds is made inaccessible to domain walls by infinitely
strong ferromagnetic couplings. Energetic and geometric scaling properties of
the interface are controlled by zero temperature fixed distributions. For
$p<p_c$, the directed percolation threshold, the interface behaves as for
$p=0$, and scaling supports random Ising ($q=2$) critical behavior for all
$q$'s. At $p=p_c$ three regimes are obtained for different ratios of ferro vs.
antiferromagnetic couplings. With rates above a threshold value the interface
is linear ( fractal dimension $d_f=1$) and its energy fluctuations, $\Delta E$
scale with length as $\Delta E\propto L^{\omega}$, with $\omega\simeq 0.48$.
When the threshold is reached the interface branches at all scales and is
fractal ($d_f\simeq 1.046$) with $\omega_c \simeq 0.51$. Thus, at $p_c$,
dilution modifies both low temperature interfacial properties and critical
scaling. Below threshold the interface becomes a probe of the backbone geometry
(\df\simeq{\bar d}\simeq 1.305; $\bar d$ = backbone fractal dimension ),
which even controls energy fluctuations ($\omega\simeq d_f\simeq\bar d$).
Numerical determinations of directed percolation exponents on diamond
hierarchical lattice are also presented.Comment: 16 pages, 3 Postscript figure

### Pseudoknots in a Homopolymer

After a discussion of the definition and number of pseudoknots, we reconsider
the self-attracting homopolymer paying particular attention to the scaling of
the number of pseudoknots at different temperature regimes in two and three
dimensions. Although the total number of pseudoknots is extensive at all
temperatures, we find that the number of pseudoknots forming between the two
halves of the chain diverges logarithmically at (in both dimensions) and below
(in 2d only) the theta-temparature. We later introduce a simple model that is
sensitive to pseudoknot formation during collapse. The resulting phase diagram
involves swollen, branched and collapsed homopolymer phases with transitions
between each pair.Comment: submitted to PR

### Scaling and efficiency determine the irreversible evolution of a market

In setting up a stochastic description of the time evolution of a financial
index, the challenge consists in devising a model compatible with all stylized
facts emerging from the analysis of financial time series and providing a
reliable basis for simulating such series. Based on constraints imposed by
market efficiency and on an inhomogeneous-time generalization of standard
simple scaling, we propose an analytical model which accounts simultaneously
for empirical results like the linear decorrelation of successive returns, the
power law dependence on time of the volatility autocorrelation function, and
the multiscaling associated to this dependence. In addition, our approach gives
a justification and a quantitative assessment of the irreversible character of
the index dynamics. This irreversibility enters as a key ingredient in a novel
simulation strategy of index evolution which demonstrates the predictive
potential of the model.Comment: 5 pages, 4 figure

### The Discovery of 8.7s Pulsations from the Ultrasoft X-Ray Source 4u0142+614

We discovered a periodicity at about 8.7s from the X--ray sources 4U0142+61,
previously considered a possible black hole candidate on the basis of its
ultrasoft spectrum. The pulsations are visible only in the 1--4 keV energy
range, during an observation obtained with the EXOSAT satellite in August 1984.
A search for delays in the pulse arrival times caused by orbital motion gave
negative results. In the same data, periodic oscillations at 25 minutes had
been previously found in an additional hard spectral component dominating above
4 keV which arises from the X-ray transient RX J0146.9+6121, discovered with
ROSAT and identified with a Be star. Though the very high (>10^4) X--ray to
optical flux ratio of 4U0142+61 is compatible with models based on an isolated
neutron star, the simplest explanation involves a low mass X--ray binary with a
very faint companion, similar to 4U1626--67. The discovery of periodic
pulsations from 4U0142+61 weakens the phenomenological criterion that an
ultrasoft spectral component is a signature of accreting black holes.Comment: plain LaTeX v3.1, 14 pages + 2 PostScript figures available upon
request to [email protected] . To appear on The Astrophysical Journal,
Letters. SISSA ref.: 106/94/

### On the anomalous X-ray afterglows of GRB 970508 and GRB 970828

Recently, BeppoSAX and ASCA have reported an unusual resurgence of soft X-ray
emission during the afterglows of GRB 970508 and GRB 970828, together with
marginal evidence for the existence of Fe-lines in both objects. We consider
the implications of the existence of a torus of iron-rich material surrounding
the sites of gamma ray bursts as would be expected in the SupraNova model; in
particular, we show that the fireball will quickly hit this torus, and bring it
to a temperature ~3x10^7 K. Bremsstrahlung emission from the heated up torus
will cause a resurgence of the soft X-ray emission with all expected
characteristics (flux level, duration and spectral hardening with time)
identical to those observed during the reburst. Also, thermal emission from the
torus will account for the observed iron line flux. These events are also
observable, for instance by new missions such as SWIFT, when beaming away from
our line sight makes us miss the main burst, as Fast (soft) X-ray Transients,
with durations ~10^3 s, and fluences ~10^-7-10^-4 erg cm^-2. This model
provides evidence in favor of the SupraNova model for Gamma Ray Bursts.Comment: To appear in MN Pink pages, MN-LateX, no figure

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