233 research outputs found
Gemini Near-infrared Spectroscopy of Luminous z~6 Quasars: Chemical Abundances, Black Hole Masses, and MgII Absorption
We present Gemini near-infrared spectroscopic observations of six luminous
quasars at z=5.86.3. Five of them were observed using Gemini-South/GNIRS,
which provides a simultaneous wavelength coverage of 0.9--2.5 m in cross
dispersion mode. The other source was observed in K band with
Gemini-North/NIRI. We calculate line strengths for all detected emission lines
and use their ratios to estimate gas metallicity in the broad-line regions of
the quasars. The metallicity is found to be supersolar with a typical value of
4 Z_{\sun}, and a comparison with low-redshift observations shows no
strong evolution in metallicity up to z6. The FeII/MgII ratio of the
quasars is 4.9+/-1.4, consistent with low-redshift measurements. We estimate
central BH masses of 10^9 to 10^{10} M_{\sun} and Eddington luminosity ratios
of order unity. We identify two MgII 2796,2803 absorbers with
rest equivalent width W_0^{\lambda2796}>1 \AA at 2.2<z<3 and three MgII
absorbers with W_0^{\lambda2796}>1.5 \AA at z>3 in the spectra, with the two
most distant absorbers at z=4.8668 and 4.8823, respectively. The redshift
number densities (dN/dz) of MgII absorbers with W_0^{\lambda2796}>1.5 \AA are
consistent with no cosmic evolution up to z>4.Comment: 33 pages (including 7 figures and 6 tables), AJ in pres
Measuring The Equations Of State In A Relaxed Magnetohydrodynamic Plasma
We report measurements of the equations of state of a fully relaxed magnetohydrodynamic (MHD) laboratory plasma. Parcels of magnetized plasma, called Taylor states, are formed in a coaxial magnetized plasma gun, and are allowed to relax and drift into a closed flux conserving volume. Density, ion temperature, and magnetic field are measured as a function of time as the Taylor states compress and heat. The theoretically predicted MHD and double adiabatic equations of state are compared to experimental measurements. We find that the MHD equation of state is inconsistent with our data
Magnetothermodynamics: Measurements Of The Thermodynamic Properties In A Relaxed Magnetohydrodynamic Plasma
We have explored the thermodynamics of compressed magnetized plasmas in laboratory experiments and we call these studies ‘magnetothermodynamics’. The experiments are carried out in the Swarthmore Spheromak eXperiment device. In this device, a magnetized plasma source is located at one end and at the other end, a closed conducting can is installed. We generate parcels of magnetized plasma and observe their compression against the end wall of the conducting cylinder. The plasma parameters such as plasma density, temperature and magnetic field are measured during compression using HeNe laser interferometry, ion Doppler spectroscopy and a linear dot{B} probe array, respectively. To identify the instances of ion heating during compression, a PV diagram is constructed using measured density, temperature and a proxy for the volume of the magnetized plasma. Different equations of state are analysed to evaluate the adiabatic nature of the compressed plasma. A three-dimensional resistive magnetohydrodynamic code (NIMROD) is employed to simulate the twisted Taylor states and shows stagnation against the end wall of the closed conducting can. The simulation results are consistent to what we observe in our experiments
High-order-harmonic generation driven by pulses with angular spatial chirp
We present and analyze a technique to drive high-order harmonics by laser pulses with an angular spatial chirp. Results of our numerical simulations show that each harmonic is emitted with an angular chirp which scales inversely with the harmonic order and leads to additional control of the spatial and temporal resolution of the spectrum. In particular, the use of angular chirp leads to separation of the harmonics in two dimensions where (i) high spectral resolution can be achieved and (ii) the temporal periodicity of the harmonic pulse trains can be controlled. We show that this technique does not require carrier-envelope-phase stabilization when using few-cycle laser pulses.The authors thank Profs. Margaret Murnane and Henry Kapteyn for stimulating discussions and support. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007-2013), under REA Grant Agreement No. 328334, and from Junta de Castilla y León (Project SA116U13, UIC016) and MINECO (FIS2013-44174-P, FIS2015-71933-REDT). A.J.-B., A.B., and C.D. acknowledge support from AFOSR FA9550-10-0561. C.D. acknowledges support from NSF EUV ERC. D.H. gratefully acknowledges supports from the Department of Energy BES Award DE-FG02-99ER14982. This work utilized the Janus supercomputer, which is supported by the U.S. National Science Foundation (Grant No. CNS-0821794) and the University of Colorado Boulder
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Thermal conductivity measurements of Summit polycrystalline silicon.
A capability for measuring the thermal conductivity of microelectromechanical systems (MEMS) materials using a steady state resistance technique was developed and used to measure the thermal conductivities of SUMMiT{trademark} V layers. Thermal conductivities were measured over two temperature ranges: 100K to 350K and 293K to 575K in order to generate two data sets. The steady state resistance technique uses surface micromachined bridge structures fabricated using the standard SUMMiT fabrication process. Electrical resistance and resistivity data are reported for poly1-poly2 laminate, poly2, poly3, and poly4 polysilicon structural layers in the SUMMiT process from 83K to 575K. Thermal conductivity measurements for these polysilicon layers demonstrate for the first time that the thermal conductivity is a function of the particular SUMMiT layer. Also, the poly2 layer has a different variation in thermal conductivity as the temperature is decreased than the poly1-poly2 laminate, poly3, and poly4 layers. As the temperature increases above room temperature, the difference in thermal conductivity between the layers decreases
Black hole masses and enrichment of z ~ 6 SDSS quasars
We present sensitive near-infrared spectroscopic observations for a sample of
five z ~ 6 quasars. These are amongst the most distant, currently known quasars
in the universe. The spectra have been obtained using ISAAC at the VLT and
include the CIV, MgII and FeII lines. We measure the FeII/MgII line ratio, as
an observational proxy for the Fe/alpha element ratio. We derive a ratio of
2.7+/-0.8 for our sample, which is similar to that found for lower redshift
quasars, i.e., we provide additional evidence for the lack of evolution in the
FeII/MgII line ratio of quasars up to the highest redshifts. This result
demonstrates that the sample quasars must have undergone a major episode of
iron enrichment in less than one Gyr and star formation must have commenced at
z > 8. The linewidths of the MgII and CIV lines give two estimates for the
black hole masses. A third estimate is given by assuming that the quasars emit
at their Eddington luminosity. The derived masses using these three methods
agree well, implying that the quasars are not likely to be strongly lensed. We
derive central black hole masses of 0.3-5.2 10^9 solar masses. We use the
difference between the redshift of MgII (a proxy for the systemic redshift of
the quasar) and the onset of the Gunn Peterson trough to derive the extent of
the ionized Stromgren spheres around our target quasars. The derived physical
radii are about five Mpc. Using a simple ionization model, the emission of the
central quasars would need of order 10^6-10^8 year to create these cavities in
a surrounding intergalactic medium with a neutral fraction between 0.1 and 1.0.
As the e-folding time scale for the central accreting black hole is on the
order of a few times 10^7 year, it can grow by one e-folding or less within
this time span.Comment: Accepted by ApJ, 15 pages, 8 figure
QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion Quantum Monte Carlo
We review recent advances in the capabilities of the open source ab initio
Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for
greater efficiency and reproducibility. The auxiliary field QMC (AFQMC)
implementation has been greatly expanded to include k-point symmetries,
tensor-hypercontraction, and accelerated graphical processing unit (GPU)
support. These scaling and memory reductions greatly increase the number of
orbitals that can practically be included in AFQMC calculations, increasing
accuracy. Advances in real space methods include techniques for accurate
computation of band gaps and for systematically improving the nodal surface of
ground state wavefunctions. Results of these calculations can be used to
validate application of more approximate electronic structure methods including
GW and density functional based techniques. To provide an improved foundation
for these calculations we utilize a new set of correlation-consistent effective
core potentials (pseudopotentials) that are more accurate than previous sets;
these can also be applied in quantum-chemical and other many-body applications,
not only QMC. These advances increase the efficiency, accuracy, and range of
properties that can be studied in both molecules and materials with QMC and
QMCPACK
Evidence for periodic accretion–ejection in LS I +61°303
The stellar binary system LS I +61°303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to γ-ray energies. The orbital modulation of radio spectral index, X-ray, and GeV γ-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accretion–ejection events for LS I +61°303 along the 26.5 d orbit. However, the existing multiwavelength data are not simultaneous. In this paper, we report the results of a campaign covering radio, X-ray, and γ-ray observations of the system along one single orbit. Our results confirm the two predicted events along the orbit and in addition show that the positions of radio and γ-ray peaks are coincident with X-ray dips as expected for radio and γ-ray emitting ejections depleting the X-ray emitting accretion flow. We discuss future observing strategies for a systematic study of the accretion–ejection physical processes in LS I +61°303
Evidence for periodic accretion–ejection in LS I +61°303
The stellar binary system LS I +61°303, composed of a compact object in an eccentric orbit around a B0 Ve star, emits from radio up to γ-ray energies. The orbital modulation of radio spectral index, X-ray, and GeV γ-ray data suggests the presence of two peaks. This two-peaked profile is in line with the accretion theory predicting two accretion–ejection events for LS I +61°303 along the 26.5 d orbit. However, the existing multiwavelength data are not simultaneous. In this paper, we report the results of a campaign covering radio, X-ray, and γ-ray observations of the system along one single orbit. Our results confirm the two predicted events along the orbit and in addition show that the positions of radio and γ-ray peaks are coincident with X-ray dips as expected for radio and γ-ray emitting ejections depleting the X-ray emitting accretion flow. We discuss future observing strategies for a systematic study of the accretion–ejection physical processes in LS I +61°303
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