964 research outputs found
X-ray bursters and the X-ray sources of the galactic bulge
Type 1 X-ray bursts, optical, infrared, and radio properties of the galactic bulge sources, are discussed. It was proven that these burst sources are neutron stars in low mass, close binary stellar systems. Several burst sources are found in globular clusters with high central densities. Optical type 1 X-ray bursts were observed from three sources. Type 2 X-ray bursts, observed from the Rapid Burster, are due to an accretion instability which converts gravitational potential energy into heat and radiation, which makes them of a fundamentally different nature from Type 1 bursts
A Search for High-Excitation Redshift Systems in the Absorption Spectra of Five Quasars
We have searched the absorption spectra of five quasars for the presence of redshift system dominated by the highly ionized doublets C iv, N v, and O vi, which could be the strongest lines produced by absorbing clouds with collisional ionization temperatures between 10^5 ° and 10^6 °K. There is at most marginal evidence for one such system apiece in the spectra of PHL 957 and 4C 05.34, which are the two quasars with the largest known emission redshifts. Highly ionized redshift systems of this type are not widespread among the five quasars we investigated; the number of redshifts found in the observed spectra is not significantly larger than the number found in similar random-number spectra. Less than 5 percent of the observed absorption lines are identified in a statistically significant way by redshift systems of this type
Model atmospheres and X-ray spectra of iron-rich bursting neutron stars. II. Iron rich Comptonized Spectra
This paper presents the set of plane-parallel model atmosphere equations for
a very hot neutron star (X-ray burst source). The model equations assume both
hydrostatic and radiative equilibrium, and the equation of state of an ideal
gas in local thermodynamic equilibrium (LTE). The equation of radiative
transfer includes terms describing Compton scattering of photons on free
electrons in fully relativistic thermal motion, for photon energies approaching
m_e *c^2. Model equations take into account many bound-free and free-free
energy-dependent opacities of hydrogen, helium, and the iron ions, and also a
dozen bound-bound opacities for the highest ions of iron. We solve model
equations by partial linearisation and the technique of variable Eddington
factors. Large grid of H-He-Fe model atmospheres of X-ray burst sources has
been computed for 10^7 < T_eff < 3*10^7 K, a wide range of surface gravity, and
various iron abundances. We demonstrate that the spectra of X-ray bursters with
iron present in the accreting matter differ significantly from pure H-He
spectra (published in an earlier paper), and also from blackbody spectra.
Comptonized spectra with significant iron abundance are generally closer to
blackbody spectra than spectra of H-He atmospheres. The ratio of color to
effective temperatures in our grid always remains in the range 1.2 < T_c/T_eff
< 1.85. The present grid of model atmospheres and theoretical X-ray spectra
will be used to determine the effective temperatures, radii and M/R ratios of
bursting neutron stars from observational data.Comment: A&A in prin
Fermi Surface Properties of Low Concentration CeLaB: dHvA
The de Haas-van Alphen effect is used to study angular dependent extremal
areas of the Fermi Surfaces (FS) and effective masses of CeLaB alloys for between 0 and 0.05. The FS of these alloys was previously
observed to be spin polarized at low Ce concentration ( = 0.05). This work
gives the details of the initial development of the topology and spin
polarization of the FS from that of unpolarized metallic LaB to that of
spin polarized heavy Fermion CeB .Comment: 7 pages, 9 figures, submitted to PR
Discovery of a 500 pc shell in the nucleus of Centaurus A
Spitzer Space Telescope mid-infrared images of the radio galaxy Centaurus A
reveal a shell-like, bipolar, structure 500 pc to the north and south of the
nucleus. This shell is seen in 5.8, 8.0 and 24 micron broad-band images. Such a
remarkable shell has not been previously detected in a radio galaxy and is the
first extragalactic nuclear shell detected at mid-infrared wavelengths. We
estimate that the shell is a few million years old and has a mass of order
million solar masses. A conservative estimate for the mechanical energy in the
wind driven bubble is 10^53 erg. The shell could have created by a small few
thousand solar mass nuclear burst of star formation. Alternatively, the
bolometric luminosity of the active nucleus is sufficiently large that it could
power the shell. Constraints on the shell's velocity are lacking. However, if
the shell is moving at 1000 km/s then the required mechanical energy would be
100 times larger.Comment: submitted to ApJ Letter
Smart Cities: Towards a New Citizenship Regime? A Discourse Analysis of the British Smart City Standard
Growing practice interest in smart cities has led to calls for a less technology-oriented and more citizen-centric approach. In response, this articles investigates the citizenship mode promulgated by the smart city standard of the British Standards Institution. The analysis uses the concept of citizenship regime and a mixture of quantitative and qualitative methods to discern key discursive frames defining the smart city and the particular citizenship dimensions brought into play. The results confirm an explicit citizenship rationale guiding the smart city (standard), although this displays some substantive shortcomings and contradictions. The article concludes with recommendations for both further theory and practice development
Multi-phase High-Velocity Clouds toward HE 0226-4110 and PG 0953+414
We study the physical conditions, elemental abundances, and kinematics of the
high-velocity clouds (HVCs) along the sight lines toward active galaxies
HE0226-4110 and PG0953+414 using Hubble Space Telescope Imaging Spectrograph
and Far Ultraviolet Spectroscopic Explorer data. Our observations reveal
multiple components of HVC absorption in lines of HI, CII, CIII, CIV, OVI,
SiII, SiIII, and SiIV in both directions. We investigate whether
photoionization by the extragalactic background radiation or by escaping Milky
Way radiation can explain the observed ionization pattern. We find that
photoionization is a good explanation for the CII, CIII, SiII, and SiIII
features, but not for the OVI or CIV associated with the HVCs, suggesting that
two principal phases exist: a warm (T~10^4K), photoionized phase and a hotter
(T=1-3x10^5K), collisionally-ionized phase. The warm HVCs toward HE0226-4110
have high levels of ionization (97-99%), and metallicities ([Z/H] between -0.9
and -0.4) close to those in the Magellanic Stream, which lies eleven degrees
away on the sky at similar velocities. These HVCs have thermal pressures that
would place them close to equilibrium in a fully ionized 10^6 K Galactic corona
with n_H=4-9x10^{-5}cm^{-3} at 50 kpc. A mini-survey of the hot, collisionally
ionized HVC components seen here and in five other sight lines finds that in
11/12 cases, the high ions have kinematics and ionic ratios that are consistent
with an origin in conductive interfaces. However, the broad absorption wing on
the OVI profile toward PG0953+414 is not completely explained by the interface
scenario, and may be tracing the outflow of hot gas into the Milky Way halo as
part of a Galactic fountain or wind.Comment: 27 pages, 12 figures (9 in color), accepted for publication in Ap
A Search for High-Excitation Redshift Systems in the Absorption Spectra of Five Quasars
We have searched the absorption spectra of five quasars for the presence of redshift system dominated by the highly ionized doublets C iv, N v, and O vi, which could be the strongest lines produced by absorbing clouds with collisional ionization temperatures between 10^5 ° and 10^6 °K. There is at most marginal evidence for one such system apiece in the spectra of PHL 957 and 4C 05.34, which are the two quasars with the largest known emission redshifts. Highly ionized redshift systems of this type are not widespread among the five quasars we investigated; the number of redshifts found in the observed spectra is not significantly larger than the number found in similar random-number spectra. Less than 5 percent of the observed absorption lines are identified in a statistically significant way by redshift systems of this type
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