Theoretical UBVRI colors of iron core white dwarfs

We explore photometric properties of hypothetical iron core white dwarfs and compute their expected colors in UBVRI Johnson broadband system. Atmospheres of iron core WDs in this paper consist of pure iron covered by a pure hydrogen layer of an arbitrary column mass. LTE model atmospheres and theoretical spectra are calculated on the basis of Los Alamos TOPS opacities and the equation of state from the OPAL project, suitable for nonideal Fe and H gases. We have also computed UBVRI colors of the models and determined an area on the B-V vs. U-B and U-B vs. V-I planes, occupied by both pure Fe, and pure H model atmospheres of WD stars. Finally, we search for iron core white dwarf candidates in the available literature.Comment: 13 pages, 12 figures, Astronomy & Astrophysics (2003) in prin

Dark-state suppression and optimization of laser cooling and fluorescence in a trapped alkaline-earth-metal single ion

We study the formation and destabilization of dark states in a single trapped 88Sr+ ion caused by the cooling and repumping laser fields required for Doppler cooling and fluorescence detection of the ion. By numerically solving the time-dependent density matrix equations for the eight-level system consisting of the sublevels of the 5s 2S1/2, 5p 2P1/2, and 4d 2D3/2 states, we analyze the different types of dark states and how to prevent them in order to maximize the scattering rate, which is crucial for both the cooling and the detection of the ion. The influence of the laser linewidths and ion motion on the scattering rate and the dark resonances is studied. The calculations are then compared with experimental results obtained with an endcap ion trap system located at the National Research Council of Canada and found to be in good agreement. The results are applicable also to other alkaline earth ions and isotopes without hyperfine structure

X-ray-binary spectra in the lamp post model

[Abridged] Context. The high-energy radiation from black-hole binaries may be due to the reprocessing of a lamp located on the black hole axis, emitting X-rays. The observed spectrum is made of 3 components: the direct spectrum; the thermal bump; and the reflected spectrum made of the Compton hump and the iron-line complex. Aims. We aim at computing accurately the complete reprocessed spectrum (thermal bump + reflected) of black-hole binaries over the entire X-ray band. We also determine the strength of the direct component. Our choice of parameters is adapted to a source showing an important thermal component. Methods. We compute in full GR the illumination of a thin disk by a lamp along the rotation axis. We use the ATM21 radiative transfer code to compute the spectrum emitted along the disk. We ray trace this local spectrum to determine the reprocessed spectrum as observed at infinity. We discuss the dependence of the local and ray-traced spectra on the emission angle and spin. Results. We show the importance of the angle dependence of the total disk specific intensity spectrum emitted by the illuminated atmosphere when the thermal disk emission if fully taken into account. High spin implies high temperature in the inner regions, so the emitted thermal disk spectrum covers the iron-line complex. As a result we locally observe absorption lines produced in the hot disk atmosphere. Absorption lines are narrow and disappear after ray tracing the local spectrum. Conclusions. Our results mainly highlight the importance of considering the angle dependence of the local spectrum when computing reprocessed spectra, as was already found in a recent study. The main new result of our work is to show the importance of computing the thermal bump of the spectrum, as this feature can change considerably the observed iron-line complex.Comment: 12 pages, 10 figures, accepted in A&A; 2 paragraphs added in section 2 wrt version

Disk emission and atmospheric absorption lines in black hole candidate 4U 1630-472

We re-analyzed SUZAKU data of the black hole candidate 4U 1630-472 being in the high/soft state. We show that the continuum X-ray spectrum of 4U 1630-472 with iron absorption lines can be satisfactorily modeled by the spectrum from an accretion disk atmosphere. Absorption lines of highly ionized iron originating in hot accretion disk atmosphere can be an alternative or complementary explanation to the wind model usually favored for these type of sources. We model continuum and line spectra using a single model. Absorption lines of highly ionized iron can origin in upper parts of the disk atmosphere which is intrinsically hot due to high disk temperature. Iron line profiles computed with natural, thermal and pressure broadenings match very well observations. We showed that the accretion disk atmosphere can effectively produce iron absorption lines observed in 4U 1630-472 spectrum. Absorption line arising in accretion disk atmosphere is the important part of the observed line profile, even if there are also other mechanisms responsible for the absorption features. Nevertheless, the wind theory can be an artifact of the fitting procedure, when the continuum and lines are fitted as separate model components.Comment: 10 pages, 4 figures, accepted by Astronomy and Astrophysic

Motional frequency shifts of trapped ions in the Lamb-Dicke regime

First order Doppler effects are usually ignored in laser driven trapped ions when the recoil frequency is much smaller than the trapping frequency (Lamb-Dicke regime). This means that the central, carrier excitation band is supposed to be unaffected by vibronic transitions in which the vibrational number changes. While this is strictly true in the Lamb-Dicke limit (infinitely tight confinement), the vibronic transitions do play a role in the Lamb-Dicke regime. In this paper we quantify the asymptotic behaviour of their effect with respect to the Lamb-Dicke parameter. In particular, we give analytical expressions for the frequency shift, ``pulling'' or ``pushing'', produced in the carrier absorption band by the vibronic transitions both for Rabi and Ramsey schemes. This shift is shown to be independent of the initial vibrational state.Comment: 9 pages, 6 figure

Broadband, unpolarized repumping and clearout light sources for Sr+^+ single-ion clocks

Future transportable optical clocks require compact and reliable light sources. Here, broadband, unpolarized repumper and state clearout sources for Sr+ single-ion optical clocks are reported. These turn-key devices require no frequency stabilization nor external modulators. They are fiber based, inexpensive, and compact. Key characteristics for clock operation are presented, including optical spectra, induced light shifts and required extinction ratios. Tests with an operating single-ion standard show a clearout efficiency of 100%. Compared to a laser-based repumper, the achievable fluorescence rates for ion detection are a few tens of per cent lower. The resulting ion kinetic temperature is 1--1.5 mK, near the Doppler limit of the ion system. Similar repumper light sources could be made for Ca+ (866 nm) and Ba+ (650 nm) using semiconductor gain media.Comment: 4 pages, 6 figure