7,265 research outputs found
The Fragmenting Superbubble Associated with the HII Region W4
New observations at high latitudes above the HII region W4 show that the
structure formerly identified as a chimney candidate, an opening to the
Galactic halo, is instead a superbubble in the process of fragmenting and
possibly evolving into a chimney. Data at high Galactic latitudes (b > 5
degrees) above the W3/W4 star forming region at 1420 and 408 MHz Stokes I
(total power) and 1420 MHz Stokes Q and U (linear polarization) reveal an
egg-shaped structure with morphological correlations between our data and the
H-alpha data of Dennison, Topasna, & Simonetti. Polarized intensity images show
depolarization extending from W4 up the walls of the superbubble, providing
strong evidence that the radio continuum is generated by thermal emission
coincident with the H-alpha emission regions. We conclude that the parts of the
HII region hitherto known as W4 and the newly revealed thermal emission are all
ionized by the open cluster OCl 352. Assuming a distance of 2.35 kpc, the ovoid
structure is 164 pc wide and extends 246 pc above the mid-plane of the Galaxy.
The shell's emission decreases in total-intensity and polarized intensity in
various locations, appearing to have a break at its top and another on one
side. Using a geometric analysis of the depolarization in the shell's walls, we
estimate that a magnetic field line-of-sight component of 3 to 5 uG exists in
the shell. We explore the connection between W4 and the Galactic halo,
considering whether sufficient radiation can escape from the fragmenting
superbubble to ionize the kpc-scale H-alpha loop discovered by Reynolds,
Sterling & Haffner.Comment: 42 pages, 14 figures; Accepted for publication in Ap
G55.0+0.3: A Highly Evolved Supernova Remnant
Multi-frequency analysis has revealed the presence of a new supernova
remnant, G55.0+0.3, in the Galactic plane. A kinematic distance of 14 kpc has
been measured from HI spectral line data. The faint, clumpy half-shell is
non-thermal and has a physical radius of 70 pc. Using an evolutionary model,
the age of the remnant is estimated to be on the order of one million years,
which exceeds conventional limits by a factor of five. The remnant may be
associated with the nearby pulsar J1932+2020, which has a spin-down age of 1.1
million years. This work implies that the radiative lifetimes of remnants could
be much longer than previously suggested.Comment: 27 pages, 7 figures in 9 files (figures 1 and 2 require 2 files
each), Accepted for publication in The Astrophysical Journal (Jan. 20, 1998
volume
A radio continuum survey of the southern sky at 1420 MHz. Observations and data reduction
We describe the equipment, observational method and reduction procedure of an
absolutely calibrated radio continuum survey of the South Celestial Hemisphere
at a frequency of 1420 MHz. These observations cover the area 0h < R.A. < 24h
for declinations less than -10 degree. The sensitivity is about 50 mK T_B (full
beam brightness) and the angular resolution (HPBW) is 35.4', which matches the
existing northern sky survey at the same frequency.Comment: 9 pages with 9 figures, A&A, in pres
An angular power spectrum analysis of the DRAO 1.4 GHz polarization survey: implications for CMB observations
The aim of the present analysis is to improve the knowledge of the
statistical properties of the Galactic diffuse synchrotron emission, which
constrains sensitive CMB anisotropy measurements. We have analysed the new DRAO
1.4 GHz polarization survey together with the Stockert 1.4 GHz total intensity
survey and derived the angular power spectra (APSs) of the total intensity, the
polarized emission, and their cross-correlation for the entire surveys and for
three low-intensity regions. The APSs of the diffuse synchrotron emission are
modelled by power laws. For the and modes, a slope of for the multipole range is found. By the
extrapolation of these results to 70 GHz, we can estimate the Galactic
synchrotron contamination of CMB anisotropies, and we find results that are
compatible with the ones coming from WMAP 3-yr data. In the low-intensity
regions, the cosmological primordial B~mode peak at should be
clearly observable for a tensor-to-scalar ratio T/S \gsim 0.5 and a
synchrotron temperature spectral index . Its detection is also
possible for T/S \gsim 0.005 and , in case a separation of the
foreground from the CMB signal could be achieved with an accuracy of . For the TE mode, a mask excluding (for
) or (for ) from the
surveys is sufficient to render the foreground contamination negligible, thus
confirming the ability of WMAP to have a clear view of the
temperature-polarization correlation peak and antipeak series.Comment: 4 pages, 5 figures, will appear on A&A Let
Radio and gamma-ray constraints on dark matter annihilation in the Galactic center
We determine upper limits on the dark matter (DM) self-annihilation cross
section for scenarios in which annihilation leads to the production of
electron--positron pairs. In the Galactic centre (GC), relativistic electrons
and positrons produce a radio flux via synchroton emission, and a gamma ray
flux via bremsstrahlung and inverse Compton scattering. On the basis of
archival, interferometric and single-dish radio data, we have determined the
radio spectrum of an elliptical region around the Galactic centre of extent 3
degrees semi-major axis (along the Galactic plane) and 1 degree semi-minor axis
and a second, rectangular region, also centered on the GC, of extent 1.6
degrees x 0.6 degrees. The radio spectra of both regions are non-thermal over
the range of frequencies for which we have data: 74 MHz -- 10 GHz. We also
consider gamma-ray data covering the same region from the EGRET instrument
(about GeV) and from HESS (around TeV). We show how the combination of these
data can be used to place robust constraints on DM annihilation scenarios, in a
way which is relatively insensitive to assumptions about the magnetic field
amplitude in this region. Our results are approximately an order of magnitude
more constraining than existing Galactic centre radio and gamma ray limits. For
a DM mass of m_\chi =10 GeV, and an NFW profile, we find that the
velocity-averaged cross-section must be less than a few times 10^-25 cm^3 s^-1.Comment: 14 pages, 9 figures. Version accepted for publication in PRD.
Reference section updated/extended
Extragalactic Radio Sources and the WMAP Cold Spot
We detect a dip of 20-45% in the surface brightness and number counts of NVSS
sources smoothed to a few degrees at the location of the WMAP cold spot. The
dip has structure on scales of approximately 1-10 degrees. Together with
independent all-sky wavelet analyses, our results suggest that the dip in
extragalactic brightness and number counts and the WMAP cold spot are
physically related, i.e., that the coincidence is neither a statistical anomaly
nor a WMAP foreground correction problem. If the cold spot does originate from
structures at modest redshifts, as we suggest, then there is no remaining need
for non-Gaussian processes at the last scattering surface of the CMB to explain
the cold spot. The late integrated Sachs-Wolfe effect, already seen
statistically for NVSS source counts, can now be seen to operate on a single
region. To create the magnitude and angular size of the WMAP cold spot requires
a ~140 Mpc radius completely empty void at z<=1 along this line of sight. This
is far outside the current expectations of the concordance cosmology, and adds
to the anomalies seen in the CMB.Comment: revised version, ApJ, in pres
DA495 - an aging pulsar wind nebula
We present a radio continuum study of the pulsar wind nebula (PWN) DA 495
(G65.7+1.2), including images of total intensity and linear polarization from
408 to 10550 MHz based on the Canadian Galactic Plane Survey and observations
with the Effelsberg 100-m Radio Telescope. Removal of flux density
contributions from a superimposed \ion{H}{2} region and from compact
extragalactic sources reveals a break in the spectrum of DA 495 at 1.3 GHz,
with a spectral index below the break and
above it (). The
spectral break is more than three times lower in frequency than the lowest
break detected in any other PWN. The break in the spectrum is likely the result
of synchrotron cooling, and DA 495, at an age of 20,000 yr, may have
evolved from an object similar to the Vela X nebula, with a similarly energetic
pulsar. We find a magnetic field of 1.3 mG inside the nebula. After
correcting for the resulting high internal rotation measure, the magnetic field
structure is quite simple, resembling the inner part of a dipole field
projected onto the plane of the sky, although a toroidal component is likely
also present. The dipole field axis, which should be parallel to the spin axis
of the putative pulsar, lies at an angle of {\sim}50\degr east of the North
Celestial Pole and is pointing away from us towards the south-west. The upper
limit for the radio surface brightness of any shell-type supernova remnant
emission around DA 495 is OAWatt
m Hz sr (assuming a radio spectral index of ), lower than the faintest shell-type remnant known to date.Comment: 25 pages, accepted by Ap
The phonon dispersion of graphite by inelastic x-ray scattering
We present the full in-plane phonon dispersion of graphite obtained from
inelastic x-ray scattering, including the optical and acoustic branches, as
well as the mid-frequency range between the and points in the Brillouin
zone, where experimental data have been unavailable so far. The existence of a
Kohn anomaly at the point is further supported. We fit a fifth-nearest
neighbour force-constants model to the experimental data, making improved
force-constants calculations of the phonon dispersion in both graphite and
carbon nanotubes available.Comment: 7 pages; submitted to Phys. Rev.
Tuning the electron-phonon coupling in multilayer graphene with magnetic fields
Magneto Raman scattering study of the E optical phonons in multi-layer
epitaxial graphene grown on a carbon face of SiC are presented. At 4.2K in
magnetic field up to 33 T, we observe a series of well pronounced avoided
crossings each time the optically active inter Landau level transition is tuned
in resonance with the E phonon excitation (at 196 meV). The width of the
phonon Raman scattering response also shows pronounced variations and is
enhanced in conditions of resonance. The experimental results are well
reproduced by a model that gives directly the strength of the electron-phonon
interaction.Comment: 4 pages, 3 figure
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