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

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

Simultaneous solution of Kompaneets equation and Radiative Transfer equation in the photon energy range 1 - 125 KeV

Radiative transfer equation in plane parallel geometry and Kompaneets equation is solved simultaneously to obtain theoretical spectrum of 1-125 KeV photon energy range. Diffuse radiation field is calculated using time-independent radiative transfer equation in plane parallel geometry, which is developed using discrete space theory (DST) of radiative transfer in a homogeneous medium for different optical depths. We assumed free-free emission and absorption and emission due to electron gas to be operating in the medium. The three terms $n, n^2$ and $\displaystyle \bigg({\frac {\partial n}{\partial x_k}}\bigg)$ where $n$ is photon phase density and $\displaystyle x_k= \bigg({\frac {h \nu} {k T_e}} \bigg)$, in Kompaneets equation and those due to free-free emission are utilized to calculate the change in the photon phase density in a hot electron gas. Two types of incident radiation are considered: (1) isotropic radiation with the modified black body radiation $I^{MB}$ [1] and (2) anisotropic radiation which is angle dependent. The emergent radiation at $\tau=0$ and reflected radiation $\tau=\tau_{max}$ are calculated by using the diffuse radiation from the medium. The emergent and reflected radiation contain the free-free emission and emission from the hot electron gas. Kompaneets equation gives the changes in photon phase densities in different types of media. Although the initial spectrum is angle dependent, the Kompaneets equation gives a spectrum which is angle independent after several Compton scattering times.Comment: 31 pages, 8 figures, Accepte

Measurement of Lande g factor of 5D5/2 state of BaII with a single trapped ion

We present the first terrestrial measurement of the Lande g factor of the 5D5/2 state of singly ionized barium. Measurements were performed on single Doppler-cooled 138Ba+ ions in a linear Paul trap. A frequency-stabilized fiber laser with nominal wavelength 1.762 um was scanned across the 6S1/25D5/2 transition to spectroscopically resolve transitions between Zeeman sublevels of the ground and excited states. From the relative positions of the four narrow transitions observed at several different values for the applied magnetic field, we find a value of 1.2020+/-0.0005 for g of 5D5/2.Comment: 3 figure

The Nature of the Emission Components in the Quasar/NLS1 PG1211+143

We present the study of the emission properties of the quasar PG1211+143, which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of observational data analyzed by us and collected from the literature, we study the temporal and spectral variability of the source in the optical/UV/X-ray bands and we propose a model that explains the spectrum emitted in this broad energy range. In this model, the intrinsic emission originating in the warm skin of the accretion disk is responsible for the spectral component that is dominant in the softest X-ray range. The shape of reflected spectrum as well as Fe K line detected in hard X-rays require the reflecting medium to be mildly ionized (xi~500). We identify this reflector with the warm skin of the disk and we show that the heating of the skin is consistent with the classical alpha P_{tot} prescription, while alpha P_{gas} option is at least two orders of magnitude too low to provide the required heating. We find that the mass of the central black hole is relatively small (M_BH~10^7- 10^8 Msun, which is consistent with the Broad Line Region mapping results and characteristic for NLS1 class.Comment: 22 pages, 10 figures, accepted to Ap

Atomic Spectral Features During Thermonuclear Flashes on Neutron Stars

The gravitational redshift measured by Cottam, Paerels and Mendez for the neutron star (NS) in the low-mass X-ray binary EXO 0748-676 depends on the identification of an absorption line during a type I burst as the H$\alpha$ line from hydrogenic Fe. We show that Fe is present above the photosphere as long as $\dot M>4\times 10^{-13}M_\odot {\rm yr^{-1}}$ during the burst. In this limit, the total Fe column is $N_{\rm Fe}\approx 3\times 10^{19}{\rm cm^{-2}}$ for incident material of solar abundances and only depends on the nuclear physics of the proton spallation. The Fe destruction creates many heavy elements with $Z<26$ which may imprint photo-ionization edges on the NS spectra during a radius expansion event or in a burst cooling tail. Detecting these features in concert with those from Fe would confirm a redshift measurement. We also begin to address the radiative transfer problem, and find that a concentrated Fe layer with $kT=1.2-1.4 {\rm keV}$ and column $N_{\rm Fe}= 7-20 \times 10^{20} {\rm cm}^{-2}$ (depending on the line depth) above the hotter continuum photosphere is required to create the H$\alpha$ line of the observed strength. This estimate must be refined by considerations of non-LTE effects as well as resonant line transport. Until these are carried out, we cannot say whether the Fe column from accretion and spallation is in conflict with the observations. We also show that hydrogenic Fe might remain in the photosphere due to radiative levitation from the high burst flux.Comment: Substantially revised version, to appear in Ap J Letter

The cooling rate of neutron stars after thermonuclear shell flashes

Thermonuclear shell flashes on neutron stars are detected as bright X-ray bursts. Traditionally, their decay is modeled with an exponential function. However, this is not what theory predicts. The expected functional form for luminosities below the Eddington limit, at times when there is no significant nuclear burning, is a power law. We tested the exponential and power-law functional forms against the best data available: bursts measured with the high-throughput Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer. We selected a sample of 35 'clean' and ordinary (i.e., shorter than a few minutes) bursts from 14 different neutron stars that 1) show a large dynamic range in luminosity, 2) are the least affected by disturbances by the accretion disk and 3) lack prolonged nuclear burning through the rp-process. We find indeed that for every burst a power law is a better description than an exponential function. We also find that the decay index is steep, 1.8 on average, and different for every burst. This may be explained by contributions from degenerate electrons and photons to the specific heat capacity of the ignited layer and by deviations from the Stefan-Boltzmann law due to changes in the opacity with density and temperature. Detailed verification of this explanation yields inconclusive results. While the values for the decay index are consistent, changes of it with the burst time scale, as a proxy of ignition depth, and with time are not supported by model calculations.Comment: 10 pages, 7 figures, recommended for publication in A&

Lifetime measurement of the metastable 3d 2D5/2 state in the 40Ca+ ion using the shelving technique on a few-ion string

We present a measurement of the lifetime of the metastable 3d 2D5/2 state in the 40Ca+ ion, using the so-called shelving technique on a string of five Doppler laser-cooled ions in a linear Paul trap. A detailed account of the data analysis is given, and systematic effects due to unwanted excitation processes and collisions with background gas atoms are discussed and estimated. From a total of 6805 shelving events, we obtain a lifetime tau=1149+/-14(stat.)+/-4(sys.)ms, a result which is in agreement with the most recent measurements.Comment: 10 pages, 7 figures. Submitted for publicatio