1,218 research outputs found
Fermi Surface Measurements on the Low Carrier Density Ferromagnet Ca1-xLaxB6 and SrB6
Recently it has been discovered that weak ferromagnetism of a dilute 3D
electron gas develops on the energy scale of the Fermi temperature in some of
the hexaborides; that is, the Curie temperature approximately equals the Fermi
temperature. We report the results of de Haas-van Alphen experiments on two
concentrations of La-doped CaB6 as well as Ca-deficient Ca1-dB6 and
Sr-deficient Sr1-dB6. The results show that a Fermi surface exists in each case
and that there are significant electron-electron interactions in the low
density electron gas.Comment: 4 pages, 5 figures, submitted to PR
1ES 1927+654: Persistent and rapid X-ray variability in an AGN with low intrinsic neutral X-ray absorption and narrow optical emission lines
We present X-ray and optical observations of the X-ray bright AGN 1ES
1927+654. The X-ray observations obtained with ROSAT and Chandra reveal
persistent, rapid and large scale variations, as well as steep 0.1-2.4 keV
(Gamma = 2.6 +/- 0.3) and 0.3-7.0 keV (Gamma = 2.7 +/- 0.2) spectra. The
measured intrinsic neutral X-ray column density is approximately 7e20cm^-2. The
X-ray timing properties indicate that the strong variations originate from a
region, a few hundred light seconds from the central black hole, typical for
type 1 AGN. High quality optical spectroscopy reveals a typical Seyfert 2
spectrum with some host galaxy contamination and no evidence of Fe II
multiplets or broad hydrogen Balmer wings. The intrinsic optical extinction
derived from the BLR and NLR are A_V >= 3.7 and A_V=1.7, respectively. The
X-ray observations give an A_V value of less than 0.58, in contrast to the
optical extinction values. We discuss several ideas to explain this apparent
difference in classification including partial covering, an underluminous BLR
or a high dust to gas ratio.Comment: 8 pages including 10 figures. Accepted for publication in Astronomy
and Astrophysic
Optical conductivity and superconductivity in LaSb
We have measured the resistivity, optical conductivity, and magnetic
susceptibility of LaSb to search for clues as to the cause of the
extraordinarily large linear magnetoresistance and to explore the properties of
the superconducting state. We find no evidence in the optical conductivity for
the formation of a charge density wave state above 20 K despite the highly
layered crystal structure. In addition, only small changes to the optical
reflectivity with magnetic field are observed indicating that the MR is due to
scattering rate, not charge density, variations with field. Although a
superconducting ground state was previously reported below a critical
temperature of 0.4 K, we observe, at ambient pressure, a fragile
superconducting transition with an onset at 2.5 K. In crystalline samples, we
find a high degree of variability with a minority of samples displaying a full
Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The
application of pressure stabilizes the superconducting transition and reduces
the anisotropy of the superconducting phase.Comment: 4 pages with 4 figure
Interstellar Polarization and the Position Angle Orientations of Seyfert 1 Galaxies
We comment on recent spectropolarimetric studies that compare the observed
polarization position angles (PAs) of Seyfert 1 galaxies near H alpha with the
observed orientations of their radio source axes on the sky. For a Seyfert
galaxy in which scattering occurs mainly in an equatorial scattering region,
one expects the polarization PA to be parallel to the radio axis, while in a
case in which light scatters predominantly in the polar regions, the H alpha
polarization PA should be perpendicular to the radio axis. In practice, these
correlations are difficult to establish because a Galactic interstellar
polarization contribution can introduce a significant uncertainty into the
polarization PA determination, even when the magnitude of interstellar
polarization is small. We show how such uncertainties may affect the analysis
of PA alignments and present spectropolarimetric observations of a probe star
along the line of sight to the Seyfert 1 galaxy Mrk 871 that allow us to assess
the intrinsic H alpha polarization and PA of Mrk 871. These results suggest
that spectropolarimetric observations of such probe stars should form an
integral part of future Seyfert galaxy polarization studies.Comment: 15 pages, 3 figures, accepted by MNRAS; made small corrections to the
derived ISP of Mrk 871 and other minor revisions in response to the referee's
recommendation
The Radio Afterglow and the Host Galaxy of the X-Ray Rich GRB 981226
We report the discovery of a radio transient VLA 232937.2-235553, coincident
with the proposed X-ray afterglow for the gamma-ray burst GRB 981226. This GRB
has the highest ratio of X-ray to gamma-ray fluence of all the GRBs detected by
BeppoSAX so far and yet no corresponding optical transient was detected. The
radio light curve of VLA 232937.2-235553 is qualitatively similar to that of
several other radio afterglows. At the sub-arcsecond position provided by the
radio detection, optical imaging reveals an extended R=24.9 mag object, which
we identify as the host galaxy of GRB 981226. Afterglow models which invoke a
jet-like geometry for the outflow or require an ambient medium with a radial
density dependence, such as that produced by a wind from a massive star, are
both consistent with the radio data. Furthermore, we show that the observed
properties of the radio afterglow can explain the absence of an optical
transient without the need for large extinction local to the GRB.Comment: Accepted for publication in the Astrophysical Journal Letters.
Thirteen pages. Three Postscript figure
Geotail and LFM comparisons of plasma sheet climatology: 2. Flow variability
[1] We characterize the variability of central plasma sheet bulk flows with a 6-year Geotail data set and a 2-month Lyon-Fedder-Mobarry (LFM) global MHD simulation at two spatial resolutions. Comparing long databases of observed and simulated parameters enable rigorous statistical tests of the model\u27s ability to predict plasma sheet properties during routine driving conditions and represent a new method of global MHD validation. In this study, we use probability density functions (PDFs) to compare the statistics of plasma sheet velocities in the Geotail observations with those in the LFM simulations. We find that the low-resolution model grossly underestimates the occurrence of fast earthward and tailward flows. Increasing the simulation resolution inherently changes plasma sheet mass transport in the model, allowing the development of fast, bursty flows. These flows fill out the wings of the velocity distribution and bring the PDF into closer agreement with observations
Geotail and LFM comparisons of plasma sheet climatology: 1. Average values
[1] We compare the statistics of central plasma sheet properties from 6 years of Geotail observations with 2 months of Lyon-Fedder-Mobarry (LFM) global MHD simulations. This statistical validation effort represents an inherently new method of systematically characterizing and quantifying global MHD model performance. For our comparison, we identify the central plasma sheet in the observations and simulation by identical criteria and select the simulation interval to ensure statistically similar distributions of solar wind conditions in both studies. After verifying our plasma sheet selection by inspecting the magnetic signatures of both studies, we compare the resultant number densities, thermal pressures, thermal energies, and bulk flows as functions of position across the equatorial plane. We find that the LFM model successfully reproduces the gross features of the global plasma sheet in a statistical sense. However, our comparison also reveals certain systematic discrepancies between the model and the observations. The LFM predicts a plasma sheet which is too dense, too cool, and exhibits faster globally averaged bulk flows than the observed plasma sheet. By quantifying the LFM overestimate of ionospheric transpolar potential and showing that ΦPC correlates with plasma sheet flow speed, we demonstrate that 15% of the plasma sheet velocity discrepancy is reflected in a ΦPC overestimate. This statistical validation effort represents an essential first step toward the rigorous, quantitative evaluation of a global MHD model in the plasma sheet
Echelle long-slit optical spectroscopy of evolved stars
We present echelle long-slit optical spectra of a sample of objects evolving
off the AGB, most of them in the pre-planetary nebula (pPN) phase, obtained
with the ESI and MIKE spectrographs at Keck-II and Magellan-I, respectively.
The total wavelength range covered with ESI (MIKE) is ~3900 to 10900 A (~3600
to 7200A). In this paper, we focus our analysis mainly on the Halpha profiles.
Prominent Halpha emission is detected in half of the objects, most of which
show broad Halpha wings (up to ~4000 km/s). In the majority of the
Halpha-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles.
In ~37% of the objects Halpha is observed in absorption. In almost all cases,
the absorption profile is partially filled with emission, leading to complex,
structured profiles that are interpreted as an indication of incipient post-AGB
mass-loss. All sources in which Halpha is seen mainly in absorption have F-G
type central stars, whereas sources with intense Halpha emission span a larger
range of spectral types from O to G. Shocks may be an important excitation
agent of the close stellar surroundings for objects with late type central
stars. Sources with pure emission or P Cygni Halpha profiles have larger J-K
color excess than objects with Halpha mainly in absorption, which suggests the
presence of warm dust near the star in the former. The two classes of profile
sources also segregate in the IRAS color-color diagram in a way that intense
Halpha-emitters have dust grains with a larger range of temperatures.
(abridged)Comment: 68 pages, 14 figures, accepted for publication in ApJS (abstract
abridged
First XMM-Newton study of two Narrow-Line Seyfert 1 galaxies discovered in the Sloan Digital Sky Survey
The Early Data Release of the Sloan Digital Sky Survey (SDSS) contains 150
Narrow-Line Seyfert 1 (NLS1) galaxies, most of them previously unknown. We
present here the study of the X-ray emission from two of these active galaxies
(SDSS J030639.57+000343.2 and SDSS J141519.50-003021.6), based upon XMM-Newton
observations. The spectral and timing characteristics of the two sources are
presented and compared against the typical properties of known NLS1 galaxies.
We found that these two NLS1 are within the dispersion range of the typical
values of this class of AGN, although with some interesting features that
deserve further studies.Comment: 5 pages, 8 figures. Accepted for publication on A&A Main Journa
Self-similar solution of a nonsteady problem of nonisothermal vapour condensation on a droplet growing in diffusion regime
This paper presents a mathematically exact self-similar solution to the joint
nonsteady problems of vapour diffusion towards a droplet growing in a
vapour-gas medium and of removal of heat released by a droplet into a
vapour-gas medium during vapour condensation. An equation for the temperature
of the droplet is obtained; and it is only at that temperature that the
self-similar solution exists. This equation requires the constancy of the
droplet temperature and even defines it unambiguously throughout the whole
period of the droplet growth. In the case of strong display of heat effects,
when the droplet growth rate decreases significantly, the equation for the
temperature of the droplet is solved analytically. It is shown that the
obtained temperature fully coincides with the one that settles in the droplet
simultaneously with the settlement of its diffusion regime of growth. At the
obtained temperature of the droplet the interrelated nonsteady vapour
concentration and temperature profiles of the vapour-gas medium around the
droplet are expressed in terms of initial (prior to the nucleation of the
droplet) parameters of the vapour-gas medium. The same parameters are used to
formulate the law in accordance with which the droplet is growing in diffusion
regime, and also to define the time that passes after the nucleation of the
droplet till the settlement of diffusion regime of droplet growth, when the
squared radius of the droplet becomes proportionate to time. For the sake of
completeness the case of weak display of heat effects is been studied.Comment: 12 pages, 4 figure
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