4,575 research outputs found
Mapping the Milky Way bulge at high resolution: the 3D dust extinction, CO, and X factor maps
Three dimensional interstellar extinction maps provide a powerful tool for
stellar population analysis. We use data from the VISTA Variables in the Via
Lactea survey together with the Besan\c{c}on stellar population synthesis model
of the Galaxy to determine interstellar extinction as a function of distance in
the Galactic bulge covering and . We adopted a
recently developed method to calculate the colour excess. First we constructed
the H-Ks vs. Ks and J-Ks vs. Ks colour-magnitude diagrams based on the VVV
catalogues that matched 2MASS. Then, based on the temperature-colour relation
for M giants and the distance-colour relations, we derived the extinction as a
function of distance. The observed colours were shifted to match the intrinsic
colours in the Besan\c{c}on model as a function of distance iteratively. This
created an extinction map with three dimensions: two spatial and one distance
dimension along each line of sight towards the bulge. We present a 3D
extinction map that covers the whole VVV area with a resolution of 6' x 6',
using distance bins of 0.5 kpc. The high resolution and depth of the photometry
allows us to derive extinction maps for a range of distances up to 10 kpc and
up to 30 magnitudes of extinction in . Integrated maps show the same
dust features and consistent values as other 2D maps. We discuss the spatial
distribution of dust features in the line of sight, which suggests that there
is much material in front of the Galactic bar, specifically between 5-7 kpc. We
compare our dust extinction map with high-resolution maps towards
the Galactic bulge, where we find a good correlation between and
. We determine the X factor by combining the CO map and our dust
extinction map. Our derived average value is consistent with the canonical
value of the Milky Way.Comment: 11 pages, 18 figures, accepted for publication in
Astronomy&Astrophysic
XMM-Newton Spectra of Intermediate-Mass Black Hole Candidates: Application of a Monte-Carlo Simulated Model
We present a systematic spectral analysis of six ultraluminous X-ray sources
(NGC1313 X-1/X-2, IC342 X-1, HoIX X-1, NGC5408 X-1 and NGC3628 X-1) observed
with XMM-Newton Observatory. These extra-nuclear X-ray sources in nearby
late-type galaxies have been considered as intermediate-mass black hole
candidates. We have performed Monte-Carlo simulations of Comptonized
multi-color black-body accretion disks. This unified and self-consistent
spectral model assumes a spherically symmetric, thermal corona around each disk
and accounts for the radiation transfer in the Comptonization. We find that the
model provides satisfactory fits to the XMM-Newton spectra of thesources. The
characteristic temperatures of the accretion disks (T_in), for example, are in
the range of ~ 0.05-0.3 keV, consistent with the intermediate-mass black hole
interpretation. We find that the black hole mass is typically about a few times
10^3 M_\odot and has an accretion rate ~ 10^{-6} - 10^{-5} M_\odot yr^{-1}. For
the spectra considered here, we find that the commonly used multi-color
black-body accretion disk model with an additive power law component, though
not physical, provides a good mathematical approximation to the Monte-Carlo
simulated model. However, the latter model provides additional constraints on
the properties of the accretion systems, such as the disk inclination angles
and corona optical depths.Comment: 23 pages, 4 figures, 5 tables. ApJ accepted, July 2004 issu
Phase diagram of disordered fermion model on two-dimensional square lattice with -flux
A fermion model with random on-site potential defined on a two-dimensional
square lattice with -flux is studied. The continuum limit of the model
near the zero energy yields Dirac fermions with random potentials specified by
four independent coupling constants. The basic symmetry of the model is
time-reversal invariance. Moreover, it turns out that the model has enhanced
(chiral) symmetry on several surfaces in the four-dimensional space of the
coupling constants. It is shown that one of the surfaces with chiral symmetry
has Sp(n)Sp(n) symmety whereas others have U(2n) symmetry, both of
which are broken to Sp(n), and the fluctuation around a saddle point is
described, respectively, by Sp( WZW model and U(2n)/Sp(n) nonlinear sigma
model. Based on these results, we propose a phase diagram of the model.Comment: 13 pages, 2 figure
Multi-frequency Study of the LMC Supernova Remnant (SNR) B0513-692 and New SNR Candidate J051327-6911
We present a new multi-wavelength study of supernova remnant (SNR) B0513-692
in the Large Magellanic Cloud (LMC). The remnant also has a strong, superposed,
essentially unresolved, but unrelated radio source at its north-western edge,
J051324-691049. This is identified as a likely compact HII region based on
related optical imaging and spectroscopy. We use the Australia Telescope
Compact Array (ATCA) at 4790 and 8640 MHz to determine the large scale
morphology, spectral index and polarization characteristics of B0513-692 for
the first time. We detect a strongly polarized region (49%) in the remnant's
southern edge. Interestingly we also detect a small (~40 arcsec) moderately
bright, but distinct optical, circular shell in our Halpha imagery which is
adjacent to the compact HII region and just within the borders of the NE edge
of B0513-692. We suggest this is a separate new SNR candidate based on its
apparently distinct character in terms of optical morphology in 3 imaged
emission lines and indicative SNR optical spectroscopy (including enhanced
optical [SII] emission relative to Halpha).Comment: 12 page
Berry's phase in the multimode Peierls states
It is shown that Berry's phase associated with the adiabatic change of local
variables in the Hamiltonian can be used to characterize the multimode Peierls
state, which has been proposed as a new type of the ground state of the
two-dimensional(2D) systems with the electron-lattice interaction.Comment: 2 pages, 2 figure
Local Isoelectronic Reactivity of Solid Surfaces
The quantity w^N(r) = ( 1/ k^2 T_el)[partial n(r, T_el) / partial
T_el]_(v(r),N) is introduced as a convenient measure of the local isoelectronic
reactivity of surfaces. It characterizes the local polarizability of the
surface and it can be calculated easily. The quantity w^N(r) supplements the
charge transfer reactivity measured e.g. by the local softness to which it is
closely related. We demonstrate the applicability and virtues of the function
w^N(r) for the example of hydrogen dissociation and adsorption on Pd(100).Comment: RevTeX, 13 pages, 3 figures, to appear in Phys. Rev. Let
XMM-Newton observations of HESS J1813-178 reveal a composite Supernova remnant
We present X-ray and 12CO(J=1-0) observations of the very-high-energy (VHE)
gamma-ray source HESS J1813-178 with the aim of understanding the origin of the
gamma-ray emission. Using this dataset we are able to undertake spectral and
morphological studies of the X-ray emission from this object with greater
precision than previous studies. NANTEN 12CO(J=1-0) data are used to search for
correlations of the gamma-ray emission with molecular clouds which could act as
target material for gamma-ray production in a hadronic scenario. The NANTEN
12CO(J=1-0) observations show a giant molecular cloud of mass 2.5 10^5
M_{\sun} at a distance of 4 kpc in the vicinity of HESS J1813-178. Even
though there is no direct positional coincidence, this giant cloud might have
influenced the evolution of the gamma-ray source and its surroundings. The
X-ray data show a highly absorbed non-thermal X-ray emitting object coincident
with the previously known ASCA source AX J1813-178 showing a compact core and
an extended tail towards the north-east, located in the centre of the radio
shell-type Supernova remnant (SNR) G12.82-0.2. This central object shows
morphological and spectral resemblance to a Pulsar Wind Nebula (PWN) and we
therefore consider that the object is very likely to be a composite SNR. We
discuss the scenario in which the gamma-rays originate in the shell of the SNR
and the one in which they originate in the central object. We demonstrate, that
in order to connect the core X-ray emission to the VHE gamma-ray emission
electrons have to be accelerated to energies of at least 1 PeV.Comment: Submitted to A&
Giant Molecular Clouds in M33 - I. BIMA All Disk Survey
We present the first interferometric CO(J=1->0) map of the entire H-alpha
disk of M33. The 13" diameter synthesized beam corresponds to a linear
resolution of 50 pc, sufficient to distinguish individual giant molecular
clouds (GMCs). From these data we generated a catalog of 148 GMCs with an
expectation that no more than 15 of the sources are spurious. The catalog is
complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of
2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that
our survey detects ~50% of the CO flux, hence that the total molecular mass of
M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our
catalog are confined largely to the central region (R < 4 kpc). They show a
remarkable spatial and kinematic correlation with overdense HI filaments; the
geometry suggests that the formation of GMCs follows that of the filaments. The
GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper
than that found in the Milky Way and in the LMC. Combined with the total mass,
this steep function implies that the GMCs in M33 form with a characteristic
mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions,
implying that the GMCs have a short quiescent period. Our results suggest the
rapid assembly of molecular clouds from atomic gas, with prompt onset of
massive star formation.Comment: 19 pages, Accepted for Publication in the Astrophysical Journal
Supplemen
Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?
We present observations of two LMC supernova remnants (SNRs), DEM L238 and
DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central
emission, surrounded by a faint shell, is present in both remnants. The central
emission has an entirely thermal spectrum dominated by strong Fe L-shell lines,
with the deduced Fe abundance in excess of solar and not consistent with the
LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and
DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission
originates in gas swept up and heated by the blast wave. A standard Sedov
analysis implies about 50 solar masses in both swept-up shells, SNR ages
between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and
subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich
supernova ejecta are close to collisional ionization equilibrium. Their
presence is unexpected, because standard Type Ia SNR models predict faint
ejecta emission with short ionization ages. Both SNRs belong to a previously
unrecognized class of Type Ia SNRs characterized by bright interior emission.
Denser than expected ejecta and/or a dense circumstellar medium around the
progenitors are required to explain the presence of Fe-rich ejecta in these
SNRs. Substantial amounts of circumstellar gas are more likely to be present in
explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar
SNRs could be remnants of ``prompt'' Type Ia explosions with young (~100 Myr
old) progenitors.Comment: 24 pages, 8 figures, ApJ, in pres
- âŠ