10,848 research outputs found
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
HI ``Tails'' from Cometary Globules in IC1396
IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized
by a single O6.5V star and containing bright-rimmed cometary globules. We have
made the first arcmin resolution images of atomic hydrogen toward IC 1396, and
have found remarkable ``tail''-like structures associated with some of the
globules and extending up to 6.5 pc radially away from the central ionizing
star. These HI ``tails'' may be material which has been ablated from the
globule through ionization and/or photodissociation and then accelerated away
from the globule by the stellar wind, but which has since drifted into the
``shadow'' of the globules.
This report presents the first results of the Galactic Plane Survey Project
recently begun by the Dominion Radio Astrophysical Observatory.Comment: 11 pages, 5 postscript figures, uses aaspp4.sty macros, submitted in
uuencoded gzipped tar format, accepted for publication in Astrophysical
Journal Letters, colour figures available at
http://www.drao.nrc.ca/~schieven/news_sep95/ic1396.htm
Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S=1/2 chains in CuCl2*2((CD3)2SO)
Field-dependent specific heat and neutron scattering measurements were used
to explore the antiferromagnetic S=1/2 chain compound CuCl2 * 2((CD3)2SO). At
zero field the system acquires magnetic long-range order below TN=0.93K with an
ordered moment of 0.44muB. An external field along the b-axis strengthens the
zero-field magnetic order, while fields along the a- and c-axes lead to a
collapse of the exchange stabilized order at mu0 Hc=6T and mu0 Hc=3.5T,
respectively (for T=0.65K) and the formation of an energy gap in the excitation
spectrum. We relate the field-induced gap to the presence of a staggered
g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective
staggered fields for magnetic fields applied along the a- and c-axes.
Competition between anisotropy, inter-chain interactions and staggered fields
leads to a succession of three phases as a function of field applied along the
c-axis. For fields greater than mu0 Hc, we find a magnetic structure that
reflects the symmetry of the staggered fields. The critical exponent, beta, of
the temperature driven phase transitions are indistinguishable from those of
the three-dimensional Heisenberg magnet, while measurements for transitions
driven by quantum fluctuations produce larger values of beta.Comment: revtex 12 pages, 11 figure
Analysis of the thin layer of Galactic warm ionized gas in the range 20 < l < 30 deg, -1.5 < b < +1.5 deg
We present an analysis of the thin layer of Galactic warm ionized gas at an
angular resolution ~ 10'. This is carried out using radio continuum data at 1.4
GHz, 2.7 GHz and 5 GHz in the coordinate region 20 < l < 30 deg, -1.5 < b <
+1.5 deg. For this purpose, we evaluate the zero level of the 2.7 and 5 GHz
surveys using auxiliary data at 2.3 GHz and 408 MHz. The derived zero level
corrections are T_{zero}(2.7 GHz)=0.15 +/- 0.06 K and T_{zero}(5 GHz)=0.1 +/-
0.05 K. We separate the thermal (free-free) and non-thermal (synchrotron)
component by means of a spectral analysis performed adopting an antenna
temperature spectral index -2.1 for the free-free emission, a realistic spatial
distribution of indices for the synchrotron radiation and by fitting,
pixel-by-pixel, the Galactic spectral index. We find that at 5 GHz, for |b| = 0
deg, the fraction of thermal emission reaches a maximum value of 82%, while at
1.4 GHz, the corresponding value is 68%. In addition, for the thermal emission,
the analysis indicates a dominant contribution of the diffuse component
relative to the source component associated with discrete HII regions.Comment: 9 pages, 9 figures, accepted to MNRA
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
Gamma-ray Flares and VLBI Outbursts of Blazars
A model is developed for the time dependent electromagnetic - radio to
gamma-ray - emission of active galactic nuclei, specifically, the blazars,
based on the acceleration and creation of leptons at a propagating
discontinuity or {\it front} of a Poynting flux jet. The front corresponds to a
discrete relativistic jet component as observed with
very-long-baseline-interferometry (VLBI). Equations are derived for the number,
momentum, and energy of particles in the front taking into account synchrotron,
synchrotron-self-Compton (SSC), and inverse-Compton processes as well as
photon-photon pair production. The apparent synchrotron, SSC, and
inverse-Compton luminosities as functions of time are determined. Predictions
of the model are compared with observations in the gamma, optical and radio
bands. The delay between the high-energy gamma-ray flare and the onset of the
radio is explained by self-absorption and/or free-free absorption by external
plasma. Two types of gamma-ray flares are predicted depending on pair creation
in the front.Comment: 11 pages, submitted to ApJ. 10 figures can be obtained from R.
Lovelace by sending postal address to [email protected]
The S=1/2 chain in a staggered field: High-energy bound-spinon state and the effects of a discrete lattice
We report an experimental and theoretical study of the antiferromagnetic
S=1/2 chain subject to uniform and staggered fields. Using inelastic neutron
scattering, we observe a novel bound-spinon state at high energies in the
linear chain compound CuCl2 * 2((CD3)2SO). The excitation is explained with a
mean-field theory of interacting S=1/2 fermions and arises from the opening of
a gap at the Fermi surface due to confining spinon interactions. The mean-field
model also describes the wave-vector dependence of the bound-spinon states,
particularly in regions where effects of the discrete lattice are important. We
calculate the dynamic structure factor using exact diagonalization of finite
length chains, obtaining excellent agreement with the experiments.Comment: 16 pages, 7 figures, accepted by Phys. Rev.
Magnetization of small lead particles
The magnetization of an ensemble of isolated lead grains of sizes ranging
from below 6 nm to 1000 nm is measured. A sharp disappearance of Meissner
effect with lowering of the grain size is observed for the smaller grains. This
is a direct observation by magnetization measurement of the occurrence of a
critical particle size for superconductivity, which is consistent with
Anderson's criterion.Comment: 7 pages, 5 figures, Submitted to PR
TRIS III: the diffuse galactic radio emission at
We present values of temperature and spectral index of the galactic diffuse
radiation measured at 600 and 820 MHz along a 24 hours right ascension circle
at declination . They have been obtained from a subset of
absolute measurements of the sky temperature made with TRIS, an experiment
devoted to the measurement of the Cosmic Microwave Background temperature at
decimetric-wavelengths with an angular resolution of about .
Our analysis confirms the preexisting picture of the galactic diffuse
emission at decimetric wavelength and improves the accuracy of the measurable
quantities. In particular, the signal coming from the halo has a spectral index
in the range above 600 MHz, depending on the sky position. In the
disk, at TRIS angular resolution, the free-free emission accounts for the 11%
of the overall signal at 600 MHz and 21% at 1420 MHz. The polarized component
of the galactic emission, evaluated from the survey by Brouw and Spoelstra,
affects the observations at TRIS angular resolution by less than 3% at 820 MHz
and less than 2% at 600 MHz. Within the uncertainties, our determination of the
galactic spectral index is practically unaffected by the correction for
polarization.
Since the overall error budget of the sky temperatures measured by TRIS at
600 MHz, that is 66 mK(systematic)18 mK (statistical), is definitely smaller
than those reported in previous measurements at the same frequency, our data
have been used to discuss the zero levels of the sky maps at 150, 408, 820 and
1420 MHz in literature. Concerning the 408 MHz survey, limiting our attention
to the patch of sky corresponding to the region observed by TRIS, we suggest a
correction of the base-level of K.Comment: Accepted for publication in the Astrophysical Journa
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