4,297 research outputs found
Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA
Based on mapping observations with ASCA, an unusual hot region with a spatial
extent of 1 square degree was discovered between M87 and M49 at a center
coordinate of R. A. = 12h 27m 36s and Dec. = (J2000). The X-ray
emission from the region has a 2-10 keV flux of ergs
s cm and a temperature of keV, which is
significantly higher than that in the surrounding medium of keV. The
internal thermal energy in the hot region is estimated to be ergs with a gas density of cm. A power-law
spectrum with a photon index is also allowed by the data. The hot
region suggests there is an energy input due to a shock which is probably
caused by the motion of the gas associated with M49, infalling toward the M87
cluster with a velocity km s.Comment: 12 pages, 3 figures, accepted to ApJ
Temperature Variation in the Cluster of Galaxies Abell 115 Studied with ASCA
Abell 115 exhibits two distinct peaks in the surface brightness distribution.
ASCA observation shows a significant temperature variation in this cluster,
confirmed by a hardness ratio analysis and spectral fits. A linking region
between main and sub clusters shows a high temperature compared with other
regions. Two possibilities are examined as the cause of the temperature
variation: cooling flows in the main cluster and a shock heating due to the
collision of the subcluster into the main system. Spectral fits with cooling
flow models to the main-cluster data show a mass-deposition rate less than 419
solar-mass/yr. Temperatures in the main cluster, the linking region, and the
subcluster are estimated by correcting for the effects of X-ray telescope
response as 4.9 (+0.7/-0.6), 11 (+12/-4), and 5.2 (+1.4/-1.0) keV,
respectively. The high temperature in the linking region implies that Abell 115
is indeed a merger system, with possible contribution from cooling flows on the
temperature structure.Comment: 23 pages, including 7 Postscript figures, accepted for publication in
Ap
Formation of magnetic impurities and pair-breaking effect in a superfluid Fermi gas
We theoretically investigate a possible idea to introduce magnetic impurities
to a superfluid Fermi gas. In the presence of population imbalance
(, where is the number of Fermi atoms with
pseudospin ), we show that nonmagnetic potential
scatterers embedded in the system are magnetized in the sense that some of
excess -spin atoms are localized around them. They destroy the
superfluid order parameter around them, as in the case of magnetic impurity
effect discussed in the superconductivity literature. This pair-breaking effect
naturally leads to localized excited states below the superfluid excitation
gap. To confirm our idea in a simply manner, we treat an attractive Fermi
Hubbard model within the mean-field theory at T=0. We self-consistently
determine superfluid properties around a nonmagnetic impurity, such as the
superfluid order parameter, local population imbalance, as well as
single-particle density of states, in the presence of population imbalance.
Since the competition between superconductivity and magnetism is one of the
most fundamental problems in condensed matter physics, our results would be
useful for the study of this important issue in cold Fermi gases.Comment: 27 pages, 14 figure
Two Dimensional Electron and Hole Gases at the Surface of Graphite
We report high-quality two-dimensional (2D) electron and hole gases induced
at the surface of graphite by the electric field effect. The 2D carriers reside
within a few near-surface atomic layers and exhibit mobilities up to 15,000 and
60,000 cm2/Vs at room and liquid-helium temperatures, respectively. The
mobilities imply ballistic transport at micron scale. Pronounced Shubnikov-de
Haas oscillations reveal the existence of two types of carries in both 2D
electron and hole gases.Comment: related to cond-mat/0410631 where preliminary data for this
experimental system were reporte
The Structure of the X-Ray Emitting Gas in the Hydra-A Cluster of Galaxies
The temperature and abundance structure in the intracluster medium (ICM) of
the Hydra-A cluster of galaxies is studied with ASCA and ROSAT. The effect of
the large extended outskirts in the point-spread function of the X-Ray
Telescope on ASCA is included in this analysis. In the X-ray brightness
profile, the strong central excess above a single beta-model, identified in the
Einstein and ROSAT data, is also found in the harder energy band (>4keV). A
simultaneous fit of five annular spectra taken with the GIS instrument shows a
radial distribution of the temperature and metal abundance. A significant
central enhancement in the abundance distribution is found, while the
temperature profile suggests that the ICM is approximately isothermal with the
temperature of ~3.5keV. The ROSAT PSPC spectrum in the central 1'.5 region
indicates a significantly lower temperature than the GIS result. A joint
analysis of the GIS and PSPC data reveals that the spectra can be described by
a two temperature model as well as by a cooling flow model. In both cases, the
hot phase gas with the temperature of ~3.5keV occupies more than 90% of the
total emission measure within 1'.5 from the cluster center. The estimated mass
of the cooler (0.5-0.7keV) component is ~2-6 x 10^9 M_solar, which is
comparable to the mass of hot halos seen in non-cD ellipticals. The cooling
flow model gives the mass deposition rate of 60+-30 M_solar/yr, an order of
magnitude lower than the previous estimation.Comment: 27 pages, 14 figures, AAS LATEX macros v4.0, to appear in The
Astrophysical Journa
Superfluid density and condensate fraction in the BCS-BEC crossover regime at finite temperatures
The superfluid density is a fundamental quantity describing the response to a
rotation as well as in two-fluid collisional hydrodynamics. We present
extensive calculations of the superfluid density \rho_s in the BCS-BEC
crossover regime of a uniform superfluid Fermi gas at finite temperatures. We
include strong-coupling or fluctuation effects on these quantities within a
Gaussian approximation. We also incorporate the same fluctuation effects into
the BCS single-particle excitations described by the superfluid order parameter
\Delta and Fermi chemical potential \mu, using the Nozi\`eres and Schmitt-Rink
(NSR) approximation. This treatment is shown to be necessary for consistent
treatment of \rho_s over the entire BCS-BEC crossover. We also calculate the
condensate fraction N_c as a function of the temperature, a quantity which is
quite different from the superfluid density \rho_s. We show that the mean-field
expression for the condensate fraction N_c is a good approximation even in the
strong-coupling BEC regime. Our numerical results show how \rho_s and N_c
depend on temperature, from the weak-coupling BCS region to the BEC region of
tightly-bound Cooper pair molecules. In a companion paper by the authors
(cond-mat/0609187), we derive an equivalent expression for \rho_s from the
thermodynamic potential, which exhibits the role of the pairing fluctuations in
a more explicit manner.Comment: 32 pages, 12 figure
Quasi-periodic X-ray Flares from the Protostar YLW15
With ASCA, we have detected three X-ray flares from the Class I protostar
YLW15. The flares occurred every ~20 hours and showed an exponential decay with
time constant 30-60 ks. The X-ray spectra are explained by a thin thermal
plasma emission. The plasma temperature shows a fast-rise and slow-decay for
each flare with kT_{peak}~4-6 keV. The emission measure of the plasma shows
this time profile only for the first flare, and remains almost constant during
the second and third flares at the level of the tail of the first flare. The
peak flare luminosities L_{X,peak} were ~5-20 * 10^{31} erg s^{-1}, which are
among the brightest X-ray luminosities observed to date for Class I protostars.
The total energy released in each flare was 3-6*10^{36} ergs. The first flare
is well reproduced by the quasi-static cooling model, which is based on solar
flares, and it suggests that the plasma cools mainly radiatively, confined by a
semi-circular magnetic loop of length ~14 Ro with diameter-to-length ratio
\~0.07. The two subsequent flares were consistent with the reheating of the
same magnetic structure as of the first flare. The large-scale magnetic
structure and the periodicity of the flares imply that the reheating events of
the same magnetic loop originate in an interaction between the star and the
disk due to the differential rotation.Comment: Accepted by ApJ, 9 pages incl. 4 ps figure
On the Fulde-Ferrell State in Spatially Isotropic Superconductors
Effects of superconducting fluctuations on the Fulde-Ferrell (FF) state are
discussed in a spatially isotropic three-dimensional superconductor under a
magnetic field. For this system, Shimahara recently showed that within the
phenomenological Ginzburg-Landau theory, the long-range order of the FF state
is suppressed by the phase fluctuation of the superconducting order parameter.
[H. Shimahara: J. Phys. Soc. Jpn. {\bf 67} (1998) 1872, Physica B {\bf 259-261}
(1999) 492] In this letter, we investigate this instability of the FF state
against superconducting fluctuations from the microscopic viewpoint, employing
the theory developed by Nozi\'eres and Schmitt-Rink in the BCS-BEC crossover
field. Besides the absence of the second-order phase transition associated with
the FF state, we show that even if the pairing interaction is weak, the shift
of the chemical potential from the Fermi energy due to the fluctuations is
crucial near the critical magnetic field of the FF state obtained within the
mean-field theory.Comment: 11 pages, 1 figur
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