808 research outputs found
The Kondo crossover in shot noise of a single quantum dot with orbital degeneracy
We investigate out of equilibrium transport through an orbital Kondo system
realized in a single quantum dot, described by the multiorbital impurity
Anderson model. Shot noise and current are calculated up to the third order in
bias voltage in the particle-hole symmetric case, using the renormalized
perturbation theory. The derived expressions are asymptotically exact at low
energies. The resulting Fano factor of the backscattering current is
expressed in terms of the Wilson ratio and the orbital degeneracy as
at zero temperature. Then,
for small Coulomb repulsions , we calculate the Fano factor exactly up to
terms of order , and also carry out the numerical renormalization group
calculation for intermediate in the case of two- and four-fold degeneracy
(). As increases, the charge fluctuation in the dot is suppressed,
and the Fano factor varies rapidly from the noninteracting value to the
value in the Kondo limit , near the crossover region
, with the energy scale of the hybridization .Comment: 10 pages, 4 figure
Sgr A East and its surroundings observed in X-rays
We report the results of an XMM-Newton observation of Sgr A East and its
surroundings. The X-ray spectrum of Sgr A East is well represented with a
two-temperature plasma model with temperatures of ~1 and ~4 keV. Only the iron
abundance shows clear spatial variation; it concentrates in the core of Sgr A
East. The derived plasma parameters suggest that Sgr A East originated in a
single supernova. Around Sgr A East, there is a broad distribution of hard
X-ray emission with a superimposed soft excess component extending away from
the location of Sgr A East both above and below the plane. We discuss the
nature of these structures as well as the close vicinity of Sgr A*.Comment: 7 pages, 3 figures, Accepted for publication in Advances in Space
Research, as a proceeding paper for the 34th COSPAR E1.4 "High Energy Studies
of Supernova Remnants and Neutron stars" held at Houston, Texas, USA during
10-19 Oct 2002; also found in
http://www.star.le.ac.uk/~mas/research/paper/#Sakano2003cos
XMM-Newton observations of Sagittarius A East
We present an analysis of a recent XMM-Newton observation of Sgr A East, a
supernova remnant located close to the Galactic Centre. Very high quality X-ray
spectra reveal many emission lines from highly ionized atoms consistent with a
multi-temperature thin thermal plasma in ionization equilibrium. We use a
two-temperature model to fit the spectra and derive temperatures of 1 keV and 4
keV. There is significant concentration of iron towards the centre of the X-ray
source such that the iron abundance varies from ~4 times solar in the core down
to ~0.5 solar in the outer regions, which contrasts with the rather uniform
distribution of other metals such as sulfur, argon and calcium, which have
abundances in the range 1--3. The derived total energy, mass, and the abundance
pattern are consistent with a single supernova event, either of type-Ia or
type-II origin, involving a relatively low-mass progenitor star. A weak 6.4-keV
neutral iron fluorescence line is also detected, the illumination source most
likely being Sgr A East itself. The morphology and spectral characteristics of
Sgr A East show no clear linkage to putative past activity in Sgr A*.Comment: 12 pages, 8 figures, to appear in MNRAS, figures with full resolution
available at http://www.star.le.ac.uk/~mas/research/paper/#Sakano2003mnra
Kondo Effect in Multiple-Dot Systems
We study the Kondo effect in multiple-dot systems for which the inter- as
well as intra-dot Coulomb repulsions are strong, and the inter-dot tunneling is
small. The application of the Ward-Takahashi identity to the inter-dot
dynamical susceptibility enables us to analytically calculate the conductance
for a double-dot system by using the Bethe-ansatz exact solution of the SU(4)
impurity Anderson model. It is clarified how the inter-dot Kondo effect
enhances or suppresses the conductance under the control of the gate voltage
and the magnetic field. We then extend our analysis to multiple-dot systems
including more than two dots, and discuss their characteristic transport
properties by taking a triple-dot system as an example.Comment: 8 pages, 9 figure
Decaying Sterile Neutrinos as a Heating Source in the Milky Way Center
Recent Chandra and Newton observations indicate that there are
two-temperature components ( 8 keV, 0.8 keV) of the diffuse x-rays
emitted from deep inside the center of Milky Way. We show that this can be
explained by the existence of sterile neutrinos, which decay to emit photons
that can be bound-free absorbed by the isothermal hot gas particles in the
center of Milky Way. This model can account for the two-temperature components
naturally as well as the energy needed to maintain the 8 keV temperature
in the hot gas. The predicted sterile neutrino mass is between 16-18 keV.Comment: Accepted by MNRAS with minor correction
Registration of the First Thermonuclear X-ray Burst from AX J1754.2-2754
During the analysis of the INTEGRAL observatory archival data we found a
powerful X-ray burst, registered by JEM-X and IBIS/ISGRI telescopes on April
16, 2005 from a weak and poorly known source AX J1754.2-2754. Analysis of the
burst profiles and spectrum shows, that it was a type I burst, which result
from thermonuclear explosion on the surface of nutron star. It means that we
can consider AX J1754.2-2754 as an X-ray burster. Certain features of burst
profile at its initial stage witness of a radiation presure driven strong
expansion and a corresponding cooling of the nutron star photosphere. Assuming,
that the luminosity of the source at this phase was close to the Eddington
limit, we estimated the distance to the burst source d=6.6+/-0.3 kpc (for
hidrogen atmosphere of the neutron star) and d=9.2+/-0.4 kpc (for helium
atmosphere).Comment: 12 pages, 6 figure
ASCA Observations of the Jet Source XTE J1748-288
XTE J1748-288 is a new X-ray transient with a one-sided radio jet. It was
observed with ASCA on 1998/09/06 and 1998/09/26, 100 days after the onset of
the radio-X-ray outburst. The spectra were fitted with an attenuated power-law
model, and the 2-6-keV flux was 4.6 * 10^{-11} erg s^{-1} cm^{-2} and 2.2 *
10^{-12} on 09/06 and 09/26, respectively. The light curve showed that the
steady exponential decay with an e-folding time of 14 days lasted over 100 days
and 4 orders of magnitude from the peak of the outburst. The celestial region
including the source had been observed with ASCA on 1993/10/01 and 1994/09/22,
years before the discovery. In those period, the flux was < 10^{-13} erg s^{-1}
cm^{-2}, below ASCA's detection limit. The jet blob colliding to the
environmental matter was supposedly not the X-ray source, although the emission
mechanism has not been determined. A possible detection of a K line from highly
ionized iron is discussed.Comment: 11 pages, 4 figures, submitted to ApJL. Fig2 is replaced with correct
on
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