14,103 research outputs found
The origin of the radio emission from beta Lyrae
In this paper we present new observational evidence that supports the
presence of an extra source of continuum emission in the binary system beta
Lyrae. New VLA and IRAM observations, together with published data from the
literature and ISO archive data, allow us to build the Spectral Energy
Distribution of the binary between 5x10^9 Hz and 5x10^15Hz. The
radio-millimeter part of the spectrum is consistent with free-free emission
from a symbiotic-like wind associated with the primary component and ionized by
the radiation field of the hidden companion. Furthermore, we also consider the
possibility that the observed radio flux originates from collimated radio
structures associated with the mass gaining component and its disk (Conical
thermal jets). An extrapolation of this emission to the far-IR part of the
spectrum indicates that in both cases the free-free contribution at these
frequencies cannot explain the observations and that the observed infrared
excess flux is due principally to the secondary component and its associated
disk.Comment: 8 pages, 3 figures, A&A in pres
R-Parity Violating Supersymmetry Explanation for Large t tbar Forward-Backward Asymmetry
We propose a supersymmetric explanation for the anomalously high forward
backward asymmetry in top pair production measured by CDF and D0. We suppose
that it is due to the t-channel exchange of a right-handed sbottom which
couples to d_R and t_R, as is present in the R-parity violating minimal
supersymmetric standard model. We show that all Tevatron and LHC experiments' t
tbar constraints may be respected for a sbottom mass between 300 and 1200 GeV,
and a large Yukawa coupling >2.2, yielding A_{FB} up to 0.18. The non Standard
Model contribution to the LHC charge asymmetry parameter is Delta
A_C^y=0.017-0.045, small enough to be consistent with current measurements but
non-zero and positive, allowing for LHC confirmation in the future within 20
fb^-1. A small additional contribution to the LHC t tbar production
cross-section is also predicted, allowing a further test. We estimate that 10
fb^-1 of LHC luminosity would be sufficient to rule out the proposal to 95%
confidence level, if the measurements of the t tbar cross-section turn out to
be centred on the Standard Model prediction.Comment: 5 pages, 2 figures, v2 has added comments and references and
increased statistics, leading to more accurate numerical predictions. v3 has
typos in Fig 1 fixed: arrow directions and t and tbar labels. v4 has added
discussion and corrections to Eq 4. v5 has luminosity predictions, additional
checks and small numerical change
The radio lighthouse CU Virginis: the spindown of a single main sequence star
The fast rotating star CU Virginis is a magnetic chemically peculiar star
with an oblique dipolar magnetic field. The continuum radio emission has been
interpreted as gyrosyncrotron emission arising from a thin magnetospheric
layer. Previous radio observations at 1.4 GHz showed that a 100% circular
polarized and highly directive emission component overlaps to the continuum
emission two times per rotation, when the magnetic axis lies in the plane of
the sky. This sort of radio lighthouse has been proposed to be due to cyclotron
maser emission generated above the magnetic pole and propagating
perpendicularly to the magnetic axis. Observations carried out with the
Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this
discovery show that this radio emission is still present, meaning that the
phenomenon responsible for this process is steady on a timescale of years. The
emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On
the light of recent results on the physics of the magnetosphere of this star,
the possibility of plasma radiation is ruled out. The characteristics of this
radio lighthouse provides us a good marker of the rotation period, since the
peaks are visible at particular rotational phases. After one year, they show a
delay of about 15 minutes. This is interpreted as a new abrupt spinning down of
the star. Among several possibilities, a quick emptying of the equatorial
magnetic belt after reaching the maximum density can account for the magnitude
of the breaking. The study of the coherent emission in stars like CU Vir, as
well as in pre main sequence stars, can give important insight into the angular
momentum evolution in young stars. This is a promising field of investigation
that high sensitivity radio interferometers such as SKA can exploit.Comment: Accepted to MNRAS, 8 pages, 7 figures, updated versio
A three-dimensional model for the radio emission of magnetic chemically peculiar stars
In this paper we present a three-dimensional numerical model for the radio
emission of Magnetic Chemically Peculiar stars, on the hypothesis that
energetic electrons emit by the gyrosynchrotron mechanism. For this class of
radio stars, characterized by a mainly dipolar magnetic field whose axis is
tilted with respect to the rotational axis, the geometry of the magnetosphere
and its deformation due to the stellar rotation are determined. The radio
emitting region is determined by the physical conditions of the magnetosphere
and of the stellar wind. Free-free absorption by the thermal plasma trapped in
the inner magnetosphere is also considered. Several free parameters are
involved in the model, such as the size of the emitting region, the energy
spectrum and the number density of the emitting electrons, and the
characteristics of the plasma in the inner magnetosphere. By solving the
equation of radiative transfer, along a path parallel to the line of sight, the
radio brightness distribution and the total flux density as a function of
stellar rotation are computed. As the model is applied to simulate the observed
5 GHz lightcurves of HD37479 and HD37017, several possible magnetosphere
configurations are found. After simulations at other frequencies, in spite of
the large number of parameters involved in the modeling, two solutions in the
case of HD37479 and only one solution in the case of HD37017 match the observed
spectral indices. The results of our simulations agree with the magnetically
confined wind-shock model in a rotating magnetosphere. The X-ray emission from
the inner magnetosphere is also computed, and found to be consistent with the
observations.Comment: 15 pages, 10 figures, A&A in pres
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