4,539 research outputs found
On the Orbital Period of the Intermediate Polar 1WGA J1958.2+3232
Recently, Norton et al. 2002, on the basis of multiwavelength photometry of
1WGA J1958.2+3232, argued that the -1 day alias of the strongest peak in the
power spectrum is the true orbital period of the system, casting doubts on the
period estimated by Zharikov et al. 2001. We re-analyzed this system using our
photometric and spectroscopic data along with the data kindly provided by Andy
Norton and confirm our previous finding. After refining our analysis we find
that the true orbital period of this binary system is 4.35h.Comment: 4 pages, 5 figures, Accepted for publication in A&A Letter
Dual-shaped offset reflector antenna designs from solutions of the geometrical optics first-order partial differential equations
In obtaining solutions to the first-order nonlinear partial differential equations (PDEs) for synthesizing offset dual-shaped reflectors, it is found that previously observed computational problems can be avoided if the integration of the PDEs is started from an inner projected perimeter and integrated outward rather than starting from an outer projected perimeter and integrating inward. This procedure, however, introduces a new parameter, the main reflector inner perimeter radius p(o), when given a subreflector inner angle 0(o). Furthermore, a desired outer projected perimeter (e.g., a circle) is no longer guaranteed. Stability of the integration is maintained if some of the initial parameters are determined first from an approximate solution to the PDEs. A one-, two-, or three-parameter optimization algorithm can then be used to obtain a best set of parameters yielding a close fit to the desired projected outer rim. Good low cross-polarization mapping functions are also obtained. These methods are illustrated by synthesis of a high-gain offset-shaped Cassegrainian antenna and a low-noise offset-shaped Gregorian antenna
Diagnostics of the molecular component of PDRs with mechanical heating. II: line intensities and ratios
CO observations in active galactic nuclei and star-bursts reveal high kinetic
temperatures. Those environments are thought to be very turbulent due to
dynamic phenomena such as outflows and high supernova rates. We investigate the
effect of mechanical heating (MH) on atomic fine-structure and molecular lines,
and their ratios. We use those ratios as a diagnostic to constrain the amount
of MH in an object and also study its significance on estimating the H2 mass.
Equilibrium PDRs models were used to compute the thermal and chemical balance
for the clouds. The equilibria were solved for numerically using the optimized
version of the Leiden PDR-XDR code. Large velocity gradient calculations were
done as post-processing on the output of the PDR models using RADEX. High-J CO
line ratios are very sensitive to MH. Emission becomes at least one order of
magnitude brighter in clouds with n~10^5~cm^-3 and a star formation rate of 1
Solar Mass per year (corresponding to a MH rate of 2 * 10^-19 erg cm^-3 s^-1).
Emission of low-J CO lines is not as sensitive to MH, but they do become
brighter in response to MH. Generally, for all of the lines we considered, MH
increases excitation temperatures and decreases the optical depth at the line
centre. Hence line ratios are also affected, strongly in some cases. Ratios
involving HCN are a good diagnostic for MH, such as HCN(1-0)/CO(1-0) and
HCN(1-0)/HCO^+(1-0). Both ratios increase by a factor 3 or more for a MH
equivalent to > 5 percent of the surface heating, as opposed to pure PDRs. The
first major conclusion is that low-J to high-J intensity ratios will yield a
good estimate of the MH rate (as opposed to only low-J ratios). The second one
is that the MH rate should be taken into account when determining A_V or
equivalently N_H, and consequently the cloud mass. Ignoring MH will also lead
to large errors in density and radiation field estimates.Comment: 38 pages, to appear in A&
VLT observations of the magnetar CXO J164710.2-455216 and the detection of a candidate infrared counterpart
We present deep observations of the field of the magnetar CXOJ164710.2-455216
in the star cluster Westerlund 1, obtained in the near-infrared with the
adaptive optics camera NACO@VLT. We detected a possible candidate counterpart
at the {\em Chandra} position of the magnetar, of magnitudes , , and . The K-band measurements available for two epochs (2006 and
2013) do not show significant signs of variability but only a marginal
indication that the flux varied (at the 2 level), consistent with the
fact that the observations were taken when CXOJ164710.2-455216 was in
quiescence. At the same time, we also present colour--magnitude and
colour--colour diagrams in the J, H, and K bands from the 2006 epoch
only, the only one with observations in all three bands, showing that the
candidate counterpart lies in the main bulk of objects describing a relatively
well--defined sequence. Therefore, based on its colours and lack of
variability, we cannot yet associate the candidate counterpart to
CXOJ164710.2-455216. Future near-infrared observations of the field,
following-up a source outburst, would be crucial to confirm the association
from the detection of near-infrared variability and colour evolution.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Exactly solvable model of wormhole supported by phantom energy
We have found a simple exact solution of spherically-symmetrical Einstein
equations describing a wormhole for an inhomogeneous distribution of the
phantom energy. The equation of state is linear but highly anisotropic: while
the radial pressure is negative, the transversal one is positive. At infinity
the spacetime is not asymptotically flat and possesses on each side of the
bridge a regular cosmological Killing horizon with an infinite area,
impenetrable for any particles. This horizon does not arise if the wormhole
region is glued to the Schwarzschild region. In doing so, the wormhole can
enclose an arbitrary amount of the phantom energy. The configuration under
discussion has a limit in which the phantom energy turns into the string dust,
the areal radius tends to the constant. In this limit, the strong gravitational
mass defect is realized in that the gravitational active mass is finite and
constant while the proper mass integrated over the total manifold is infinite.Comment: 6 pages. Two references added, typos corrected. Accepted for
publication in Phys. Rev. D as Rapid Communicatio
Comunidades virtuales en tiempos de flujos: el caso de Second Life
This article studies the new forms of emerging sociability in virtual communities that develop thei activity in the context of what is called “society of flows”. Online communities break the traditional sense of community by establishing a new regime based most of the times on the ephemeral and transitory interests of their users. Likewise, the characteristics that define a virtual 3D world as a community and the types of participation and social organisation that happen within will be explored here using the case of “Second life” as an example.Este artículo estudia las nuevas formas de sociabilidad emergentes en las comunidades virtuales que desarrollan su actividad en el contexto de la llamada “sociedad de flujos”. Las comunidades on line rompen con el sentido tradicional de comunidad instaurando un nuevo régimen basado la mayoría de las veces en los intereses efímeros y transitorios de sus usuarios. Asimismo, tomando como ejemplo el caso de Second Life, se exploran las características que definen a un mundo virtual 3D como comunidad y los tipos de participación y organización social que tienen lugar en su interior.Cet article étudie les nouvelles formes de sociabilité émergeantes dans les communautés virtuelles que développent leur activité dans le contexte ainsi nommé comme la “société des flux”. Les communautés virtuelles cassent le sens traditionnel de communauté en établissant un nouvel régime fondé la plupart de fois sur des intérêts éphémères et transitoires de leurs clients. De la même manière les caractéristiques que définissent un monde virtuel 3D en tant que communauté et les types de participation et de organisation social que incluent sont ici explorés en utilisant le cas de “Second Life” comme exemple
HCO+ and HCN J=3-2 absorption toward the center of Centaurus A
We have investigated the presence of dense gas toward the radio source Cen A
by looking at the absorption of the HCO+ and HCN (3-2) lines in front of the
bright continuum source with the Submillimeter Array. We detect narrow HCO+
(3-2) absorption, and tentatively HCN (3-2), close to the systemic velocity.
For both molecules, the J=3-2 absorption is much weaker than for the J=1-0
line. From simple excitation analysis, we conclude that the gas density is on
the order of a few 10^4 cm^-3 for a column density N(HCO+)/dV of 3x10^12 cm^-2
km^-1 s and a kinetic temperature of 10 K. In particular, we find no evidence
for molecular gas density higher than a few 10^4 cm^-3 on the line of sight to
the continuum source. We discuss the implications of our finding on the nature
of the molecular gas responsible for the absorption toward Cen A.Comment: Accepted for publication in Ap
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