211 research outputs found
Thermal emission and magnetic beaming in the radio and X-ray mode-switching PSR B0943+10
PSR B0943+10 is a mode-switching radio pulsar characterized by two emission
modes with different radio and X-ray properties. Previous studies, based on
simple combinations of blackbody and power law models, showed that its X-ray
flux can be decomposed in a pulsed thermal plus an unpulsed non-thermal
components. However, if PSR B0943+10 is a nearly aligned rotator seen pole-on,
as suggested by the radio data, it is difficult to reproduce the high observed
pulsed fraction unless magnetic beaming is included. In this work we reanalyze
all the available X-ray observations of PSR B0943+10 with simultaneous radio
coverage, modeling its thermal emission with polar caps covered by a magnetized
hydrogen atmosphere or with a condensed iron surface. The condensed surface
model provides good fits to the spectra of both pulsar modes, but, similarly to
the blackbody, it can not reproduce the observed pulse profiles, unless an
additional power law with an ad hoc modulation is added. Instead, the pulse
profiles and phase-resolved spectra are well described using the hydrogen
atmosphere model to describe the polar cap emission, plus an unpulsed power
law. For the X-ray brighter state (Q-mode) we obtain a best fit with a
temperature kT~0.09 keV, an emitting radius R~260 m, a magnetic field
consistent with the value of the dipole field of 4x10^12 G inferred from the
timing parameters, and a small angle between the magnetic and spin axis,
=5. The corresponding parameters for the X-ray fainter state (B-mode) are
kT~0.08 keV and R~170 m.Comment: 16 pages, 10 figures, accepted for publication in Ap
Theoretical And Experimental Studies Of Collision-Induced Electronic Energy Transfer From v=0-3 Of The E(0g+) Ion-Pair State Of Br2: Collisions With He And Ar
Collisions of Br(2), prepared in the E(0(g)(+)) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D(\u27), and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions
Absorption Features in Spectra of Magnetized Neutron Stars
The X-ray spectra of some magnetized isolated neutron stars (NSs) show
absorption features with equivalent widths (EWs) of 50 - 200 eV, whose nature
is not yet well known. To explain the prominent absorption features in the soft
X-ray spectra of the highly magnetized (B ~ 10^{14} G) X-ray dim isolated NSs
(XDINSs), we theoretically investigate different NS local surface models,
including naked condensed iron surfaces and partially ionized hydrogen model
atmospheres, with semi-infinite and thin atmospheres above the condensed
surface. We also developed a code for computing light curves and integral
emergent spectra of magnetized neutron stars with various temperature and
magnetic field distributions over the NS surface. We compare the general
properties of the computed and observed light curves and integral spectra for
XDINS RBS\,1223 and conclude that the observations can be explained by a thin
hydrogen atmosphere above the condensed iron surface, while the presence of a
strong toroidal magnetic field component on the XDINS surface is unlikely. We
suggest that the harmonically spaced absorption features in the soft X-ray
spectrum of the central compact object (CCO) 1E 1207.4-5209 (hereafter 1E 1207)
correspond to peaks in the energy dependence of the free-free opacity in a
quantizing magnetic field, known as quantum oscillations. To explore observable
properties of these quantum oscillations, we calculate models of hydrogen NS
atmospheres with B ~ 10^{10} - 10^{11} G (i.e., electron cyclotron energy
E_{c,e} ~ 0.1 - 1 keV) and T_eff = 1 - 3 MK. Such conditions are thought to be
typical for 1E 1207. We show that observable features at the electron cyclotron
harmonics with EWs \approx 100 - 200 eV can arise due to these quantum
oscillations.Comment: 4 pages, 3 figures, conference "Astrophysics of Neutron Stars - 2010"
in honor of M. Ali Alpar, Izmir, Turke
Vertical Structure of the Outer Accretion Disk in Persistent Low-Mass X-Ray Binaries
We have investigated the influence of X-ray irradiation on the vertical
structure of the outer accretion disk in low-mass X-ray binaries by performing
a self-consistent calculation of the vertical structure and X-ray radiation
transfer in the disk. Penetrating deep into the disk, the field of scattered
X-ray photons with energy \,keV exerts a significant influence on
the vertical structure of the accretion disk at a distance
\,cm from the neutron star. At a distance \,cm,
where the total surface density in the disk reaches
\,g\,cm, X-ray heating affects all layers of an
optically thick disk. The X-ray heating effect is enhanced significantly in the
presence of an extended atmospheric layer with a temperature
\,K above the accretion disk. We have derived
simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This
approximation has a \,% accuracy in the range of X-ray photon
energies \,keV.Comment: 19 pages, 8 figures, published in Astronomy Letter
Theoretical Kinetics Study of the F(2P) + NH3 Hydrogen Abstraction Reaction
The hydrogen abstraction reaction of fluorine with ammonia represents a true chemical challenge because it is very fast, is followed by secondary abstraction reactions, which are also extremely fast, and presents an experimental/theoretical controversy about rate coefficients. Using a previously developed full-dimensional analytical potential energy surface, we found that the F + NH3 → HF + NH2 system is a barrierless reaction with intermediate complexes in the entry and exit channels. In order to understand the reactivity of the title reaction, thermal rate coefficidents were calculated using two approaches: ring polymer molecular dynamics and quasi-classical trajectory calculations, and these were compared with available experimental data for the common temperature range 276–327 K. The theoretical results obtained show behavior practically independent of temperature, reproducing Walther–Wagner’s experiment, but in contrast with Persky’s more recent experiment. However, quantitatively, our results are 1 order of magnitude larger than those of Walther–Wagner and reasonably agree with the Persky at the lowest temperature, questioning so Walther−Wagner’s older data. At present, the reason for this discrepancy is not clear, although we point out some possible reasons in the light of current theoretical calculationsThis work was partially supported by Gobierno de Extremadura, Spain, and FEDER (Project No. IB10001). Y.V.S. acknowledges the support of a Combustion Energy Research Fellowship through the Combustion Energy Frontier Research Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001198. A.F-.R. thanks Xunta de
Galicia through Grant No. 2012/314 para a consolidación e a estruturación de unidades de investigación competitivas do Sistema Universitario de Galicia, 2012S
Phase resolved spectroscopic study of the isolated neutron star RBS 1223 (1RXS J130848.6+212708)
The data from all observations of RBS 1223 (1RXS J130848.6+212708) conducted
by XMM-Newton EPIC pn with the same instrumental setup in 2003-2007 were
combined to form spin-phase resolved spectra. A number of complex models of
neutron stars with strongly magnetized ( G) surface,
with temperature and magnetic field distributions around magnetic poles, and
partially ionized hydrogen thin atmosphere above it have been implemented into
the X-ray spectral fitting package XSPEC for simultaneous fitting of
phase-resolved spectra. A Markov-Chain-Monte-Carlo (MCMC) approach is also
applied to verify results of fitting and estimating in multi parameter models.
The spectra in different rotational phase intervals and light curves in
different energy bands with high S/N ratio show a high complexity. The spectra
can be parameterized with a Gaussian absorption line superimposed on a
blackbody spectrum, while the light curves with double-humped shape show strong
dependence of pulsed fraction upon the energy band (13%-42%), which indicates
that radiation emerges from at least two emitting areas. A model with condensed
iron surface and partially ionized hydrogen thin atmosphere above it allows us
to fit simultaneously the observed general spectral shape and the broad
absorption feature observed at 0.3 keV in different spin phases of RBS 1223. It
allowed to constrain some physical properties of X-ray emitting areas, i.e. the
temperatures ( eV, eV), magnetic field strengths
(G) at the poles, and their distributions
parameters (, indicating an absence of strong toroidal
magnetic field component). In addition, it puts some constraints on the
geometry of the emerging X-ray emission and gravitational redshift
() of RBS 1223.Comment: 7 pages, 7 figures, Astronomy and Astrophysics 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
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
Radiative properties of highly magnetized isolated neutron star surfaces and approximate treatment of absorption features in their spectra
In the X-ray spectra of most X-ray dim isolated neutron stars (XDINSs)
absorption features with equivalent widths (EWs) of 50 -- 200 eV are observed.
We theoretically investigate different models to explain absorption features
and compare their properties with the observations. We consider various
theoretical models for the magnetized neutron star surface: naked condensed
iron surfaces and partially ionized hydrogen model atmospheres, including
semi-infinite and thin atmospheres above a condensed surface. The properties of
the absorption features (especially equivalent widths) and the angular
distributions of the emergent radiation are described for all models. A code
for computing light curves and integral emergent spectra of magnetized neutron
stars is developed. We assume a dipole surface magnetic field distribution with
a possible toroidal component and corresponding temperature distribution. A
model with two uniform hot spots at the magnetic poles can also be employed.
Light curves and spectra of highly magnetized neutron stars with parameters
typical for XDINSs are computed using different surface temperature
distributions and various local surface models. Spectra of magnetized model
atmospheres are approximated by diluted blackbody spectra with one or two
Gaussian lines having parameters, which allow us to describe the model
absorption features. To explain the prominent absorption features in the soft
X-ray spectra of XDINSs a thin atmosphere above the condensed surface can be
invoked, whereas a strong toroidal magnetic field component on the XDINS
surfaces can be excluded.Comment: 54 pages, 17 figures, accepted for publication in A&
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