1,683 research outputs found
Statistical equilibrium and ion cyclotron absorption/emission in strongly magnetized plasmas
We calculate the transition rates between proton Landau levels due to
non-radiative and radiative Coulomb collisions in an electron-proton plasma
with strong magnetic field B. Both electron-proton collisions and proton-proton
collisions are considered. The roles of the first-order cyclotron absorption
and second-order free-free absorption and scattering in determining the line
strength and shape as well as the continuum are analysed in detail. We solve
the statistical balance equation for the populations of proton Landau levels.
For temperatures \sim 10^6-10^7 K, the deviations of the proton populations
from LTE are appreciable at density \rho < 0.1 B_{14}^{3.5} g cm^{-3}, where
B_{14}=B/(10^{14} G). We present general formulae for the plasma emissivity and
absorption coefficents under a wide range of physical conditions. Our results
are useful for studying the possibility and the conditions of proton/ion
cyclotron line formation in the near vicinity of highly magnetized neutron
stars.Comment: 17 pages, 8 figures, MNRAS, accepte
Models of magnetized neutron star atmospheres: thin atmospheres and partially ionized hydrogen atmospheres with vacuum polarization
Observed X-ray spectra of some isolated magnetized neutron stars display
absorption features, sometimes interpreted as ion cyclotron lines. Modeling the
observed spectra is necessary to check this hypothesis and to evaluate neutron
star parameters.We develop a computer code for modeling magnetized neutron star
atmospheres in a wide range of magnetic fields (10^{12} - 10^{15} G) and
effective temperatures (3 \times 10^5 - 10^7 K). Using this code, we study the
possibilities to explain the soft X-ray spectra of isolated neutron stars by
different atmosphere models. The atmosphere is assumed to consist either of
fully ionized electron-ion plasmas or of partially ionized hydrogen. Vacuum
resonance and partial mode conversion are taken into account. Any inclination
of the magnetic field relative to the stellar surface is allowed. We use modern
opacities of fully or partially ionized plasmas in strong magnetic fields and
solve the coupled radiative transfer equations for the normal electromagnetic
modes in the plasma. Spectra of outgoing radiation are calculated for various
atmosphere models: fully ionized semi-infinite atmosphere, thin atmosphere,
partially ionized hydrogen atmosphere, or novel "sandwich" atmosphere (thin
atmosphere with a hydrogen layer above a helium layer. Possibilities of
applications of these results are discussed. In particular, the outgoing
spectrum using the "sandwich" model is constructed. Thin partially ionized
hydrogen atmospheres with vacuum polarization are shown to be able to improve
the fit to the observed spectrum of the nearby isolated neutron star RBS 1223
(RX J1308.8+2127).Comment: Accepted for publications in Astronomy and Astrophysics, 9 pages, 12
figure
Constraining the Geometry of the Neutron Star RX J1856.5-3754
RX J1856.5-3754 is one of the brightest, nearby isolated neutron stars, and
considerable observational resources have been devoted to its study. In
previous work, we found that our latest models of a magnetic, hydrogen
atmosphere matches well the entire spectrum, from X-rays to optical (with
best-fitting neutron star radius R=14 km, gravitational redshift z_g~0.2, and
magnetic field B~4x10^12 G). A remaining puzzle is the non-detection of
rotational modulation of the X-ray emission, despite extensive searches. The
situation changed recently with XMM-Newton observations that uncovered 7 s
pulsations at the 1% level. By comparing the predictions of our model (which
includes simple dipolar-like surface distributions of magnetic field and
temperature) with the observed brightness variations, we are able to constrain
the geometry of RX J1856.5-3754, with one angle < 6 deg and the other angle =
20-45 deg, though the solutions are not definitive given the observational and
model uncertainties. These angles indicate a close alignment between the
rotation and magnetic axes or between the rotation axis and the observer. We
discuss our results in the context of RX J1856.5-3754 being a normal radio
pulsar and a candidate for observation by future X-ray polarization missions
such as Constellation-X or XEUS.Comment: 7 pages, 6 figures; MNRAS, accepte
Bandgap properties of two-dimensional low-index photonic crystals
We study the bandgap properties of two-dimensional photonic crystals created
by a lattice of rods or holes conformed in a symmetric or asymmetric triangular
structure. Using the plane-wave analysis, we calculate a minimum value of the
refractive index contrast for opening both partial and full two-dimensional
spectral gaps for both TM and TE polarized waves. We also analyze the effect of
ellipticity of rods and holes and their orientation on the threshold value and
the relative size of the bandgap.Comment: 5 pages, 6 figures, App. Phys. B. styl
Models of hydrostatic magnetar atmospheres at high luminosities
We investigate the possibility of Photospheric Radius Expansion (PRE) during
magnetar bursts. Identification of PRE would enable a determination of the
magnetic Eddington limit (which depends on field strength and neutron star mass
and radius), and shed light on the burst mechanism. To do this we model
hydrostatic atmospheres in a strong radial magnetic field, determining both
their maximum extent and photospheric temperatures. We find that
spatially-extended atmospheres cannot exist in such a field configuration:
typical maximum extent for magnetar-strength fields is ~10 m (as compared to
200 km in the non-magnetic case). Achieving balance of gravitational and
radiative forces over a large range of radii, which is critical to the
existence of extended atmospheres, is rendered impossible in strong fields due
to the dependence of opacities on temperature and field strength. We conclude
that high luminosity bursts in magnetars do not lead to expansion and cooling
of the photosphere, as in the non-magnetic case. We also find the maximum
luminosity that can propagate through a hydrostatic magnetar atmosphere to be
lower than previous estimates. The proximity and small extent of the
photospheres associated with the two different polarization modes also calls
into question the interpretation of two blackbody fits to magnetar burst
spectra as being due to extended photospheres.Comment: Accepted for publication in MNRAS. 14 pages, 6 figures, 2 table
Cyclotron harmonics in opacities of isolated neutron star atmospheres
Some of X-ray dim isolated neutron stars (XDINS) and central compact objects
in supernova remnants (CCO) show absorption features in their thermal soft
X-ray spectra. It has been hypothesized that these features could be due to the
periodic peaks in free-free absorption opacities, caused by either Landau
quantization of electron motion in magnetic fields B<10^{11} G or analogous
quantization of ion motion in magnetic fields B>10^{13} G. Here, I review the
physics behind cyclotron quantum harmonics in free-free photoabsorption,
discuss different approximations for their calculation, and explain why the ion
cyclotron harmonics (beyond the fundamental) cannot be observed.Comment: 12 pages, 5 figures. In v.5, a typo (missed sign factor) in Eq.(9) is
fixe
Thermal conductivity of ions in a neutron star envelope
We analyze the thermal conductivity of ions (equivalent to the conductivity
of phonons in crystalline matter) in a neutron star envelope.
We calculate the ion/phonon thermal conductivity in a crystal of atomic
nuclei using variational formalism and performing momentum-space integration by
Monte Carlo method. We take into account phonon-phonon and phonon-electron
scattering mechanisms and show that phonon-electron scattering dominates at not
too low densities. We extract the ion thermal conductivity in ion liquid or gas
from literature.
Numerical values of the ion/phonon conductivity are approximated by
analytical expressions, valid for T>10^5 K and 10^5 g cm^-3 < \rho < 10^14 g
cm^-3. Typical magnetic fields B~10^12 G in neutron star envelopes do not
affect this conductivity although they strongly reduce the electron thermal
conductivity across the magnetic field. The ion thermal conductivity remains
much smaller than the electron conductivity along the magnetic field. However,
in the outer neutron star envelope it can be larger than the electron
conductivity across the field, that is important for heat transport across
magnetic field lines in cooling neutron stars. The ion conductivity can greatly
reduce the anisotropy of heat conduction in outer envelopes of magnetized
neutron stars.Comment: 12 pages, 5 figures; to appear in MNRA
Retention and Activation of Blood-Borne Proteases in the Arterial Wall Implications for Atherothrombosis
All forms of atheroma are characterized by a risk of arterial wall rupture leading to clinical complications. This involves medial and adventitial ruptures in abdominal aortic aneurysm (AAA) and intimal cap rupture in vulnerable atherothrombotic plaques. Extracellular proteases, including metalloproteinases, locally generated plasmin, and leukocyte elastase, are important molecular mediators of atheroma progression via their matrix degradation properties. The pathological evolution of AAA is linked to the biology of its associated mural thrombus. Indeed, in aneurysmal segments lined by a thrombus, the wall is thinner, the extracellular matrix more degraded, and the adventitial inflammatory response greater than in segments that are not. Several lines of evidence highlight the role of the thrombus, in AAA, as a reservoir of blood-borne proteases that conveys them from the lumen to the diseased wall. In stenosing atheroma, both previous and recent studies provide evidence that recurrent intraplaque hemorrhages play a dominant role in the evolution of the lesion toward vulnerability. In this review, we draw a parallel between the role of protease conveyance and activation of the mural thrombus in AAA and of intraplaque hemorrhages in stenosing atheroma. We hypothesize that intraplaque hemorrhages convey blood-borne proteases into lesions, where they are retained and activated upon thrombus/hematoma formation, thus contributing significantly to their deleterious action
Three XMM-Newton observations of the Anomalous X-ray Pulsar 1E 1048.1-5937: long term variations in spectrum and pulsed fraction
We report the results of a recent (July 2004) XMM-Newton Target of
Opportunity observation of the Anomalous X-ray pulsar 1E 1048.1-5937, together
with a detailed re-analysis of previous observations carried out in 2000 and
2003. In July 2004 the source had a 2-10 keV flux of 6.2 erg
cm s and a pulsed fraction P=0.68. The comparison of the
three data sets shows the presence of an anti-correlation between flux and
pulsed fraction, implying that previous estimates of the source energetics
based on the assumption of a large and constant pulsed fraction might be
significantly underestimated. The source spectrum is well described by a power
law plus blackbody model (kT~0.63 keV, photon index ~2.7-3.5) or,
alternatively, by the sum of two blackbodies of which the hotter is Comptonized
by relativistic electrons. In this case the temperatures are kT~0.2-0.3
keV and kT~0.4-0.5 keV and the emitting area of the cooler component is
consistent with the whole neutron star surface. The long term luminosity
variation of a factor >~2 is accompanied by relatively small variations in the
spectral shape. Phase resolved spectroscopy indicates a harder spectrum in
correspondence of the pulse maximum. No spectral features have been detected
with 4 limits on the equivalent width in the range ~10-220 eV,
depending on line energy and width.Comment: 10 pages, 6 figures; accepted for publication in A&
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