910 research outputs found
The contribution of Narrow-Line Seyfert 1 galaxies to the soft X-ray background
The ROSAT Ultradeep HRI survey in the Lockman Hole contains a complete sample
of 91 X-ray sources with fluxes in the 0.5-2 keV band larger than 1.2 times
10e-15 erg cm-2 s-1, where over about 75 per cent of the sources are quasars or
Seyfert galaxies. During the course of our optical identification work, we have
obtained optical spectra of 67 narrow emission line galaxies (NELG), which are
physically not associated with the X-ray sources. We have derived the
equivalent width (EW) and the full width at half maximum (FWHM) for the most
prominent emission lines of 41 quasars and Seyfert galaxies taken from the
ROSAT Deep Survey (RDS), which has a flux limit of 5.5 times 10e-15 erg cm-2
s-1 in the 0.5-2.0 keV band. Furthermore we have obtained the EW and FWHM
values of the field NELGs. Here we present the spectroscopic discrimination
between RDS Seyfert galaxies and field galaxies (NELG). The analysis of the
emission lines has revealed that a single object out of 69 spectroscopically
identified AGN fits the optical criteria of Narrow-Line Seyfert 1 galaxies
(NLS1). This may indicate that NLS1 contribute only marginally to the soft
X-ray background, but we can not exclude a possible larger contribution.Comment: Invited talk presented at the Joint MPE,AIP,ESO workshop on NLS1s,
Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also available at
http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Seasonal Antarctic pressure variability during the twentieth century from spatially complete reconstructions and CAM5 simulations
As most permanent observations in Antarctica started in the 1950s, understanding Antarctic climate variations throughout the twentieth century remains a challenge. To address this issue, the non-summer multi-decadal variability in pressure reconstructions poleward of 60°S is evaluated and assessed in conjunction with climate model simulations throughout the twentieth and early twenty-first centuries to understand historical atmospheric circulation variability over Antarctica. Austral autumn and winter seasons show broadly similar patterns, with negative anomalies in the early twentieth century (1905–1934), positive pressure anomalies in the middle twentieth century (1950–1980), and negative pressure anomalies in the most recent period (1984–2013), consistent with concurrent trends in the SAM index. In autumn, the anomalies are significant in the context of estimates of interannual variability and reconstruction uncertainty across most of the Antarctic continent, and the reconstructed patterns agree best with model-generated patterns when the simulation includes the forced response to tropical sea surface temperatures and external radiative forcing. In winter and spring, the reconstructed anomalies are less significant and are consistent with internal atmospheric variability alone. The specific role of tropical SST variability on pressure trends in these seasons is difficult to assess due to low reconstruction skill in the region of strongest tropical teleconnections, the large internal atmospheric variability, and uncertainty in the SST patterns themselves. Indirect estimates of pressure variability, whether through sea ice reconstructions, proxy records, or improved models and data assimilation schemes, will help to further constrain the magnitude of internal variability relative to the forced responses expected from SST trends and external radiative forcing
Hosts of Type II Quasars: an HST Study
Type II quasars are luminous Active Galactic Nuclei whose centers are
obscured by large amounts of gas and dust. In this contribution we present
3-band HST images of nine type II quasars with redshifts 0.25<z<0.4 selected
from the Sloan Digital Sky Survey based on their emission line properties. The
intrinsic luminosities of these quasars are thought to be in the range
-24>M_B>-26, but optical obscuration implies that host galaxies can be studied
unencumbered by bright nuclei. Each object has been imaged in three filters
(`red', `green' and `blue') placed between the strong emission lines. The
spectacular, high quality images reveal a wealth of details about the structure
of the host galaxies and their environments. Most galaxies in the sample are
ellipticals, but strong deviations from de Vaucouleurs profiles are found,
especially in the blue band. We argue that most of these deviations are due to
the light from the nucleus scattered off interstellar material in the host
galaxy. This scattered component can make a significant contribution to the
broad-band flux and complicates the analysis of the colors of the stellar
populations in the host galaxy. This extended component can be difficult to
notice in unobscured luminous quasars and may bias the results of host galaxy
studies.Comment: 6 pages including 2 color figures; proceedings of the 'QSO host
galaxies: evolution and environment' conference, Leiden, August 200
Ground state non-universality in the random field Ising model
Two attractive and often used ideas, namely universality and the concept of a
zero temperature fixed point, are violated in the infinite-range random-field
Ising model. In the ground state we show that the exponents can depend
continuously on the disorder and so are non-universal. However, we also show
that at finite temperature the thermal order parameter exponent one half is
restored so that temperature is a relevant variable. The broader implications
of these results are discussed.Comment: 4 pages 2 figures, corrected prefactors caused by a missing factor of
two in Eq. 2., added a paragraph in conclusions for clarit
Inhibiting decoherence via ancilla processes
General conditions are derived for preventing the decoherence of a single
two-state quantum system (qubit) in a thermal bath. The employed auxiliary
systems required for this purpose are merely assumed to be weak for the general
condition while various examples such as extra qubits and extra classical
fields are studied for applications in quantum information processing. The
general condition is confirmed with well known approaches towards inhibiting
decoherence. A novel approach for decoherence-free quantum memories and quantum
operations is presented by placing the qubit into the center of a sphere with
extra qubits on its surface.Comment: pages 8, Revtex
Accurate Atmospheric Parameters at Moderate Resolution Using Spectral Indices: Preliminary Application to the MARVELS Survey
Studies of Galactic chemical and dynamical evolution in the solar
neighborhood depend on the availability of precise atmospheric parameters
(Teff, [Fe/H] and log g) for solar-type stars. Many large-scale spectroscopic
surveys operate at low to moderate spectral resolution for efficiency in
observing large samples, which makes the stellar characterization difficult due
to the high degree of blending of spectral features. While most surveys use
spectral synthesis, in this work we employ an alternative method based on
spectral indices to determine the atmospheric parameters of a sample of nearby
FGK dwarfs and subgiants observed by the MARVELS survey at moderate resolving
power (R~12,000). We have developed three codes to automatically normalize the
observed spectra, measure the equivalent widths of the indices and, through the
comparison of those with values calculated with pre-determined calibrations,
derive the atmospheric parameters of the stars. The calibrations were built
using a sample of 309 stars with precise stellar parameters obtained from the
analysis of high-resolution FEROS spectra. A validation test of the method was
conducted with a sample of 30 MARVELS targets that also have reliable
atmospheric parameters from high-resolution spectroscopic analysis. Our
approach was able to recover the parameters within 80 K for Teff, 0.05 dex for
[Fe/H] and 0.15 dex for log g, values that are lower or equal to the typical
external uncertainties found between different high-resolution analyzes. An
additional test was performed with a subsample of 138 stars from the ELODIE
stellar library and the literature atmospheric parameters were recovered within
125 K for Teff, 0.10 dex for [Fe/H] and 0.29 dex for log g. These results show
that the spectral indices are a competitive tool to characterize stars with the
intermediate resolution spectra.Comment: Accepted for publication in AJ. Abstract edited to comply with arXiv
standards regarding the number of character
A Low Latitude Halo Stream around the Milky Way
We present evidence for a ring of stars in the plane of the Milky Way, extending at least from l = 180 deg to l = 227 deg; the ring could encircle the Galaxy. The low Galactic latitude structure is at a fairly constant distance of kpc from the Galactic Center above the Galactic plane, and has kpc in the region sampled below the Galactic plane. The evidence includes five hundred SDSS spectroscopic radial velocities of stars within 30 deg of the plane. The velocity dispersion of the stars associated with this structure is found to be 27 km/s at (l,b) = (198,-27), 22 km/s at (l,b) = (225, 28), 30 km/s at (l,b) = (188, 24), and 30 km/s at (l,b) = (182, 27) degrees. The structure co-rotates with the Galactic disk stars at km/s. The narrow measured velocity dispersion is inconsistent with power law spheroid or thick disk populations. We compare the velocity dispersion in this structure with the velocity dispersion of stars in the Sagittarius dwarf galaxy tidal stream, for which we measure a velocity dispersion of 20 km/s at (l,b) = (165, -55) degrees. We interpret our measurements as evidence for a tidally disrupted satellite of to solar masses which rings the Galaxy
Resonant cancellation of off-resonant effects in a multilevel qubit
Off-resonant effects are a significant source of error in quantum
computation. This paper presents a group theoretic proof that off-resonant
transitions to the higher levels of a multilevel qubit can be completely
prevented in principle. This result can be generalized to prevent unwanted
transitions due to qubit-qubit interactions. A simple scheme exploiting dynamic
pulse control techniques is presented that can cancel transitions to higher
states to arbitrary accuracy.Comment: 4 pages, Revtex, submitted for publicatio
A twentieth century perspective on summer Antarctic pressure change and variability and contributions from tropical SSTs and ozone depletion
During the late 20th 33 Century, the Antarctic atmospheric circulation has changed
and significantly influenced the overall Antarctic climate, through processes including a
poleward shift of the circumpolar westerlies. However, little is known about the full
spatial pattern of atmospheric pressure over the Antarctic continent prior to 1979. Here
we investigate surface pressure changes across the entire Antarctic continent back to
1905 by developing a new summer pressure reconstruction poleward of 60°S. We find
that only across East Antarctica are the recent pressures significantly lower than pressures
in the early 20th 40 century; we also discern periods of significant positive pressure trends in
the early 20th 41 century across the coastal South Atlantic sector of Antarctica. Climate
model simulations reveal that both tropical sea surface temperature variability and other
radiative forcing mechanisms, in addition to ozone depletion, have played an important
role in forcing the recent observed negative trends
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