23,080 research outputs found
The 2000 Periastron Passage of PSR B1259-63
We report here on a sequence of 28 observations of the binary pulsar system
PSR B1259-63/SS2883 at four radio frequencies made with the Australia Telescope
Compact Array around the time of the 2000 periastron passage. Observations made
on 2000 Sep 1 show that the pulsar's apparent rotation measure (RM) reached a
maximum of rad m, some 700 times the value measured
away from periastron, and is the largest astrophysical RM measured. This value,
combined with the dispersion measure implies a magnetic field in the Be star's
wind of 6 mG. We find that the light curve of the unpulsed emission is similar
to that obtained during the 1997 periastron but that differences in detail
imply that the emission disc of the Be star is thicker and/or of higher
density. The behaviour of the light curve at late times is best modelled by the
adiabatic expansion of a synchrotron bubble formed in the pulsar/disc
interaction. The expansion rate of the bubble km s is
surprisingly low but the derived magnetic field of 1.6 G close to that
expected.Comment: 8 pages, 6 figures, 3 tables, LaTeX (mn.sty). Accepted for
publication in the Monthly Notices of the Royal Astronomical Society. Also
available at http://astronomy.swin.edu.au/staff/tconnors/publications.htm
On the detectability of extragalactic fast radio transients
Recent discoveries of highly dispersed millisecond radio bursts by Thornton
et al. in a survey with the Parkes radio telescope at 1.4 GHz point towards an
emerging population of sources at cosmological distances whose origin is
currently unclear. Here we demonstrate that the scattering effects at lower
radio frequencies are less than previously thought, and that the bursts could
be detectable at redshifts out to about in surveys below 1 GHz. Using a
source model in which the bursts are standard candles with bolometric
luminosities ergs/s uniformly distributed per unit
comoving volume, we derive an expression for the observed peak flux density as
a function of redshift and use this, together with the rate estimates found by
Thornton et al. to find an empirical relationship between event rate and
redshift probed by a given survey. The non-detection of any such events in
Arecibo 1.4 GHz survey data by Deneva et al., and the Allen Telescope Array
survey by Simeon et al. is consistent with our model. Ongoing surveys in the
1--2 GHz band should result in further discoveries. At lower frequencies,
assuming a typical radio spectral index , the predicted peak flux
densities are 10s of Jy. As a result, surveys of such a population with current
facilities would not necessarily be sensitivity limited and could be carried
out with small arrays to maximize the sky coverage. We predict that sources may
already be present in 350-MHz surveys with the Green Bank Telescope. Surveys at
150 MHz with 30 deg fields of view could detect one source per hour above
30 Jy.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS on 2013 July
25. Received 2013 July 24; in original form 2013 May 3
Experimental and theoretical electronic structure of EuRh2As2
The Fermi surfaces (FS's) and band dispersions of EuRh2As2 have been
investigated using angle-resolved photoemission spectroscopy. The results in
the high-temperature paramagnetic state are in good agreement with the full
potential linearized augmented plane wave calculations, especially in the
context of the shape of the two-dimensional FS's and band dispersion around the
Gamma (0,0) and X (pi,pi) points. Interesting changes in band folding are
predicted by the theoretical calculations below the magnetic transition
temperature Tn=47K. However, by comparing the FS's measured at 60K and 40K, we
did not observe any signature of this transition at the Fermi energy indicating
a very weak coupling of the electrons to the ordered magnetic moments or strong
fluctuations. Furthermore, the FS does not change across the temperature (~
25K) where changes are observed in the Hall coefficient. Notably, the Fermi
surface deviates drastically from the usual FS of the superconducting
iron-based AFe2As2 parent compounds, including the absence of nesting between
the Gamma and X FS pockets.Comment: 4 pages, 4 figure
Magnetic, Transport, and Thermal Properties of Single Crystals of the Layered Arsenide BaMn2As2
Growth of BaMn2As2 crystals using both MnAs and Sn fluxes is reported. Room
temperature crystallography, anisotropic isothermal magnetization M versus
field H and magnetic susceptibility chi versus temperature T, electrical
resistivity in the ab plane rho(T), and heat capacity C(T) measurements on the
crystals were carried out. The tetragonal ThCr2Si2-type structure of BaMn2As2
is confirmed. After correction for traces of ferromagnetic MnAs impurity phase
using M(H) isotherms, the inferred intrinsic chi(T) data of the crystals are
anisotropic with chi_{ab}/chi_{c} \approx 7.5 at T = 2 K. The temperature
dependences of the anisotropic chi data suggest that BaMn2As2 is a collinear
antiferromagnet at room temperature with the easy axis along the c axis, and
with an extrapolated Neel temperature T_N \sim 500 K. The rho(T) decreases with
decreasing T below 310 K but then increases below \sim 50 K, suggesting that
BaMn2As2 is a small band-gap semiconductor with an activation energy of order
0.03 eV. The C(T) data from 2 to 5 K are consistent with this insulating ground
state, exhibiting a low temperature Sommerfeld coefficient gamma = 0.0(4)
mJ/mol K^2. The Debye temperature is determined from these data to be theta_D =
246(4) K. BaMn2As2 is a potential parent compound for ThCr2Si2-type
superconductors.Comment: 7 pages, 6 figures; v2: typos corrected, additional data and
discussion, accepted for publication in Phys. Rev.
Shock absorbing support and restraint means Patent
Shock absorbing couch for body support under high acceleration or deceleration force
Crystallography, magnetic susceptibility, heat capacity, and electrical resistivity of heavy fermion LiVO single crystals grown using a self-flux technique
Magnetically pure spinel compound is a rare -electron
heavy fermion. Measurements on single crystals are needed to clarify the
mechanism for the heavy fermion behavior in the pure material. In addition, it
is known that small concentrations ( mol%) of magnetic defects in the
structure strongly affect the properties, and measurements on single crystals
containing magnetic defects would help to understand the latter behaviors.
Herein, we report flux growth of and preliminary measurements
to help resolve these questions. The magnetic susceptibility of some as-grown
crystals show a Curie-like upturn at low temperatures, showing the presence of
magnetic defects within the spinel structure. The magnetic defects could be
removed in some of the crystals by annealing them at 700 C\@. A very
high specific heat coefficient = 450 mJ/(mol K\@) was obtained
at a temperature of 1.8 K for a crystal containing a magnetic defect
concentration = 0.5 mol%. A crystal with = 0.01 mol% showed a residual resistivity ratio of 50.Comment: 6 pages, 7 figures, Title modifie
Phase Relations in the Li2O-V2O3-V2O5 System at 700 C: Correlations with Magnetic Defect Concentration in Heavy Fermion LiV2O4
The phase relations in the Li2O-V2O3-V2O5 ternary system at 700 C for
compositions in equilibrium with LiV2O4 are reported. This study clarified the
synthesis conditions under which low and high magnetic defect concentrations
can be obtained within the spinel structure of LiV2O4. We confirmed that the
LiV2O4 phase can be obtained containing low (0.006 mol%) to high (0.83 mol%)
magnetic defect concentrations n{defect} and with consistently high magnetic
defect spin S values between 3 and 6.5. The high n{defect} values were obtained
in the LiV2O4 phase in equilibrium with V2O3, Li3VO4, or LiVO2 and the low
values in the LiV2O4 phase in equilibrium with V3O5. A model is suggested to
explain this correlation.Comment: 6 pages, 7 figures; Phys. Rev. B (accepted
Large Miscibility Gap in the Ba(Mn_xFe_{1-x})2As2 System
The compounds BaMn2As2 and BaFe2As2 both crystallize in the
body-centered-tetragonal ThCr2Si2-type (122-type) structure at room temperature
but exhibit quite different unit cell volumes and very different magnetic and
electronic transport properties. Evidently reflecting these disparities, we
have discovered a large miscibility gap in the system Ba(Mn_xFe_{1-x})2As2.
Rietveld refinements of powder x-ray diffraction (XRD) measurements on samples
slow-cooled from 1000 C to room temperature (RT) reveal a two-phase mixture of
BaMn2As2 and Ba(Mn_{0.12}Fe_{0.88})2As2 phases together with impurity phases
for x = 0.2, 0.4, 0.5, 0.6 and 0.8. We infer that there exists a miscibility
gap in this system at 300 K with composition limits 0.12 < x < 1. For samples
quenched from 1000 C to 77 K, the refinements of RT XRD data indicate that the
miscibility gap at RT narrows at 1000 C to 0.2 < x < 0.8. Samples with x=0.4,
0.5 and 0.6 quenched from 1100-1400 C to 77 K contain a single 122-type phase
together with significant amounts of Fe_{1-x}Mn_xAs and FeAs2 impurity phases.
These results indicate that the system is not a pseudo-binary system over the
whole composition range and that the 122-type phase has a significant
homogeneity range at these temperatures. Magnetic susceptibility, electrical
resistivity and heat capacity measurements versus temperature of the
single-phase quenched polycrystalline samples with x = 0.2 and 0.8 and for
lightly doped BaMn2As2 crystals are reported.Comment: 14 pages, 16 figures, 3 tables; published versio
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