206,334 research outputs found
News on the X-ray emission from hot subdwarf stars
In latest years, the high sensitivity of the instruments on-board the
XMM-Newton and Chandra satellites allowed us to explore the properties of the
X-ray emission from hot subdwarf stars. The small but growing sample of X-ray
detected hot subdwarfs includes binary systems, in which the X-ray emission is
due to wind accretion onto a compact companion (white dwarf or neutron star),
as well as isolated sdO stars, in which X-rays are probably due to shock
instabilities in the wind. X-ray observations of these low-mass stars provide
information which can be useful for our understanding of the weak winds of this
type of stars and can lead to the discovery of particularly interesting binary
systems. Here we report the most recent results we have recently obtained in
this research area.Comment: 8 pages, 3 figures. To appear in the Proceedings of the 8th Meeting
on Hot Subdwarf Stars and Related Objects, 9-15 July 2017, Cracow, Poland.
Eds. A. Baran, A. E. Lynas-Gray, Open Astronomy, in pres
Swift monitoring of the massive X-ray binary SAX J0635.2+0533
SAX J0635.2+0533 is a binary pulsar with a very short pulsation period ( =
33.8 ms) and a high long-term spin down ( 3.8 s
s), which suggests a rotation-powered (instead of an accretion-powered)
nature for this source. While it was discovered at a flux level around
10 erg cm s, between 2003 and 2004 this source was
detected with XMM-Newton at an average flux of about 10 erg cm
s; moreover, the flux varied of over one order of magnitude on time
scales of a few days, sometimes decreasing below erg
cm s. Since both the rotation-powered and the accretion-powered
scenarios have difficulties to explain these properties, the nature of SAX
J0635.2+0533 is still unclear. Here we report on our recent long-term
monitoring campaign on SAX J0635.2+0533 carried out with Swift and on a
systematic reanalysis of all the RXTE observations performed between 1999 and
2001. We found that during this time interval the source remained almost always
active at a flux level above 10 erg cm s.Comment: 8 pages, 6 figures, 2 tables. Accepted for publication in Astronomy &
Astrophysic
IFSM representation of Brownian motion with applications to simulation
Several methods are currently available to simulate paths of the Brownian
motion. In particular, paths of the BM can be simulated using the properties of
the increments of the process like in the Euler scheme, or as the limit of a
random walk or via L2 decomposition like the Kac-Siegert/Karnounen-Loeve
series.
In this paper we first propose a IFSM (Iterated Function Systems with Maps)
operator whose fixed point is the trajectory of the BM. We then use this
representation of the process to simulate its trajectories. The resulting
simulated trajectories are self-affine, continuous and fractal by construction.
This fact produces more realistic trajectories than other schemes in the sense
that their geometry is closer to the one of the true BM's trajectories. The
IFSM trajectory of the BM can then be used to generate more realistic solutions
of stochastic differential equations
Optimality and strong stability of control systems
Optimality and strong stability of control syste
Inflation from superstrings
We investigate the possibility of obtaining inflationary solutions of the
slow roll type from a low energy Lagrangian coming from superstrings. The
advantage of such an approach is that in these theories the scalar potential
has only one free parameter (the Planck scale) and therefore no unnatural fine
tuning may be accommodated. We find that in any viable scheme the dilaton and
the moduli fields have to be stabilized and that before this happens, no other
field may be used as the inflaton. Then inflation may occur due to chiral
matter fields. Demanding that the potential terms associated with the chiral
fields do not spoil the dilaton and moduli minimization leads to severe
constraints on the magnitude of the density fluctuations.Comment: 22 pages, no figures, latex file We have corrected the magnitude of
the density fluctuations, which become smaller than the COBE ones. Some
references have also been added, and a few misprints correcte
A short survey on biharmonic maps between Riemannian manifolds
In this short survey we report on the theory of biharmonic maps between
Riemannian manifolds.Comment: 20 page
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