980 research outputs found
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
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
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
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
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
- …