195 research outputs found
X-ray Pulsations from the Central Source in Puppis A
There are several supernova remnants which contain unresolved X-ray sources
close to their centers, presumably radio-quiet neutron stars. To prove that
these objects are indeed neutron stars, to understand the origin of their X-ray
radiation, and to explain why they are radio-quiet, one should know their
periods and period derivatives. We searched for pulsations of the X-ray flux
from the radio-quiet neutron star candidate RX J0822-4300 near the center of
the Puppis A supernova remnant observed with the ROSAT PSPC and HRI. A standard
timing analysis of the separate PSPC and HRI data sets does not allow one to
detect the periodicity unequivocally. However, a thorough analysis of the two
observations separated by 4.56 yr enabled us to find a statistically
significant period ms and its derivative s s. The corresponding characteristic parameters of
the neutron star, age kyr, magnetic field G, and rotational energy loss erg
s, are typical for young radio pulsars. Since the X-ray radiation has a
thermal-like spectrum, its pulsations may be due to a nonuniform temperature
distribution over the neutron star surface caused by anisotropy of the heat
conduction in the strongly magnetized crust.Comment: 9 pages, 2 postscript figures, to appear in ApJ Letters; an
acknowledgment is adde
Observations of cooling neutron stars
Observations of cooling neutron stars allow to measure photospheric radii and
to constrain the equation of state of nuclear matter at high densities. In this
paper we concentrate on neutron stars, which show thermal (photospheric) X-ray
emission and have measured distances. After a short summary of the radio
pulsars falling into this category we review the observational data of the 7
radio quiet isolated neutron stars discovered by ROSAT which have been studied
in detail by Chandra, XMM-Newton and optical observations. Their spectra show
blackbody temperatures between 0.5 and 1 million Kelvin and an optical excess
of a factor of 5-10 over the extrapolation of the X-ray spectrum. Four of these
sources show periodicities between 3.45 and 11.37 s, indicating slow rotation.
The pulsed fractions are small, between 6 and 18 %. The magnetic fields derived
from spin down and/or possible proton cyclotron lines are of the order
10 G. We then discuss RX J1856.5-3754 in detail and suggest that
the remarkable absence of any line features in its X-ray spectrum is due to
effects of strong magnetic fields ( G). Assuming blackbody
emission to fit the optical and X-ray spectrum we derive a conservative lower
limit of the ``apparent'' neutron star radius of 16.5 km (d/117 pc).
This corresponds to the radius for the ``true'' radius of 14 km for a 1.4
M neutron star, indicating a stiff equation of state at high
densities. A comparison of the result with mass-radius relations shows that in
this case a quark star or a neutron star with a quark matter core can be ruled
out with high confidence.Comment: 15 pages, 5 figures, to appear in ASI proceedings of The
Electromagnetic Spectrum of Neutron Stars, Marmaris 2004, very minor revisio
Observability of atomic line features in strong magnetic fields
The physical properties of atoms in superstrong magnetic fields, characteristic of neutron stars, and the possibility of detecting magnetically strongly shifted atomic lines in the spectra of magnetized X-ray pulsars are discussed. It is suggested that it is recommendable to look for magnetically strongly shifted Fe 26 Lyman lines in rotating neutron stars of not too high luminosity using spectrometers working in the energy range 10 - 20 keV, with sensitivities to minus 4 power photons per sq cm and second, and resolution E/delta E approx. 10-100
Logic Integer Programming Models for Signaling Networks
We propose a static and a dynamic approach to model biological signaling
networks, and show how each can be used to answer relevant biological
questions. For this we use the two different mathematical tools of
Propositional Logic and Integer Programming. The power of discrete mathematics
for handling qualitative as well as quantitative data has so far not been
exploited in Molecular Biology, which is mostly driven by experimental
research, relying on first-order or statistical models. The arising logic
statements and integer programs are analyzed and can be solved with standard
software. For a restricted class of problems the logic models reduce to a
polynomial-time solvable satisfiability algorithm. Additionally, a more dynamic
model enables enumeration of possible time resolutions in poly-logarithmic
time. Computational experiments are included
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