484 research outputs found
Sgr A* Polarization: No ADAF, Low Accretion Rate, and Non-Thermal Synchrotron Emission
The recent detection of polarized radiation from Sgr A* requires a
non-thermal electron distribution for the emitting plasma. The Faraday rotation
measure must be small, placing strong limits on the density and magnetic field
strength. We show that these constraints rule out advection-dominated accretion
flow models. We construct a simple two-component model which can reproduce both
the radio to mm spectrum and the polarization. This model predicts that the
polarization should rise to nearly 100% at shorter wavelengths. The first
component, possibly a black-hole powered jet, is compact, low density, and
self-absorbed near 1 mm with ordered magnetic field, relativistic Alfven speed,
and a non-thermal electron distribution. The second component is poorly
constrained, but may be a convection-dominated accretion flow with dM/dt~10^-9
M_Sun/yr, in which feedback from accretion onto the black hole suppresses the
accretion rate at large radii. The black hole shadow should be detectable with
sub-mm VLBI.Comment: 4 pages, 1 figure, accepted by ApJL, several changes from submitted
versio
The Bianchi groups are subgroup separable on geometrically finite subgroups
We show that for certain arithmetic groups, geometrically finite subgroups
are the intersection of finite index subgroups containing them. Examples are
the Bianchi groups and the Seifert-Weber dodecahedral space. In particular, for
manifolds commensurable with these groups, immersed incompressible surfaces
lift to embeddings in a finite sheeted covering.Comment: 19 page
From isolated subgroups to generic permutation representations
Let be a countable group, the (compact, metric)
space of all subgroups of with the Chabauty topology and
the collection of isolated
points. We denote by the (Polish) group of all permutations of a countable
set . Then the following properties are equivalent: (i)
is dense in , (ii) admits a
"generic permutation representation". Namely there exists some such that the collection of permutation
representations is co-meager in
. We call groups satisfying these properties
solitary. Examples of solitary groups include finitely generated LERF groups
and groups with countably many subgroups.Comment: 21 page
Marriage, Intimacy and Risk of HIV Infection in South West Uganda
Long-term, monogamous, relationships are often portrayed as protective in HIV prevention campaigns. Focusing on marriage in a community in south west Uganda, we examine why and how people enter long term relationships, what their expectations are and what factors sustain those relationships. Qualitative data were collected using in-depth interviews with 50 men and women randomly selected from a General Population Cohort. The results showed that managing expectations to sustain marriage is challenging; however the socio-economic and cultural benefits of marriage: having children, property acquisition as well as securing societal status tend to overshadow the costs associated with risks from infidelity such as sexually transmitted infections (including HIV). Recognising the compromises that couples may make to sustain their marriage is an important step towards acknowledging that `being faithful’ may be about staying together and showing commitment, not sexual exclusivity. Keywords: marriage; long-term relationship; HIV epidemic; UgandaLes relations monogames à long terme, sont souvent dépeintes comme une protection dans les campagnes de prévention du VIH. En mettant l'accent sur le mariage dans une communauté dans le sud ouest de l'Ouganda, nous examinons pourquoi et comment les gens entrent dans des relations à long terme, quelles sont leurs attentes et quels facteurs soutiennent ces relations. Les données qualitatives ont été recueillies au moyen d'entrevues en profondeur avec 50 hommes et femmes choisis au hasard à partir d'une population générale cohorte. Les résultats ont montré que la gestion des attentes pour soutenir le mariage est difficile, mais les avantages socio-économiques et culturels du mariage: avoir des enfants, acquis de la propriété ainsi que l’obtention d’un bon état-civil dans la société, ont tendance à éclipser les coûts associés aux risques de l'infidélité telles que les infections sexuellement transmissibles (y compris le VIH). La reconnaissance des compromis que les couples peuvent faire pour soutenir leur mariage, est une étape importante vers la reconnaissance du fait que « être fidèle» peut signifier rester ensemble et de faire preuve de l'engagement, pas l'exclusivité sexuelle. Mots clés: mariage; relation à long terme; Épidémie de VIH; Ougand
On the Origin of Polarization near the Lyman Edge in Quasars
Optical/UV radiation from accretion disks in quasars is likely to be partly
scattered by a hot plasma enveloping the disk. We investigate whether the
scattering may produce the steep rises in polarization observed blueward of the
Lyman limit in some quasars. We suggest and assess two models. In the first
model, primary disk radiation with a Lyman edge in absorption passes through a
static ionized "skin" covering the disk, which has a temperature about 3 keV
and a Thomson optical depth about unity. Electron scattering in the skin smears
out the edge and produces a steep rise in polarization at lambda < 912 A. In
the second model, the scattering occurs in a hot coronal plasma outflowing from
the disk with a mildly relativistic velocity. We find that the second model
better explains the data. The ability of the models to fit the observed rises
in polarization is illustrated with the quasar PG 1630+377.Comment: submitted to ApJ Letter
Transits and secondary eclipses of HD 189733 with Spitzer
We present limits on transit timing variations and secondary eclipse depth
variations at 8 microns with the Spitzer Space Telescope IRAC camera. Due to
the weak limb darkening in the infrared and uninterrupted observing, Spitzer
provides the highest accuracy transit times for this bright system, in
principle providing sensitivity to secondary planets of Mars mass in resonant
orbits. Finally, the transit data provides tighter constraints on the
wavelength- dependent atmospheric absorption by the planet.Comment: 7 pages, 7 figures, submitted to proceedings of IAU Symposium No. 253
"Transiting Planets
Microlensing variability in the gravitationally lensed quasar Q2237+0305 = the Einstein Cross, I. Spectrophotometric monitoring with the VLT
We present the results of the first long-term (2.2 years) spectroscopic
monitoring of a gravitationally lensed quasar, namely the Einstein Cross
Q2237+0305. The goal of this paper is to present the observational facts to be
compared in follow-up papers with theoretical models to constrain the inner
structure of the source quasar. We spatially deconvolve deep VLT/FORS1 spectra
to accurately separate the spectrum of the lensing galaxy from the spectra of
the quasar images. Accurate cross-calibration of the 58 observations at
31-epoch from October 2004 to December 2006 is carried out with non-variable
foreground stars observed simultaneously with the quasar. The quasar spectra
are further decomposed into a continuum component and several broad emission
lines to infer the variations of these spectral components. We find prominent
microlensing events in the quasar images A and B, while images C and D are
almost quiescent on a timescale of a few months. The strongest variations are
observed in the continuum of image A. Their amplitude is larger in the blue
(0.7 mag) than in the red (0.5 mag), consistent with microlensing of an
accretion disk. Variations in the intensity and profile of the broad emission
lines are also reported, most prominently in the wings of the CIII] and center
of the CIV emission lines. During a strong microlensing episode observed in
June 2006 in quasar image A, the broad component of the CIII] is more highly
magnified than the narrow component. In addition, the emission lines with
higher ionization potentials are more magnified than the lines with lower
ionization potentials, consistent with the results obtained with reverberation
mapping. Finally, we find that the V-band differential extinction by the lens,
between the quasar images, is in the range 0.1-0.3 mag.Comment: 16 pages, 16 figures, A&A accepted, corrected Fig. 1
SPH simulations of Shakura-Sunyaev instability at intermediate accretion rates
We show that a standard Shakura-Sunyaev accretion disc around a black hole
with an accretion rate lower than the critical Eddington limit does show the
instability in the radiation pressure dominated zone. We obtain this result
performing time-dependent simulations of accretion disks for a set of values of
the viscosity parameter and accretion rate. In particular we always find the
occurrence of the collapse of the disc: the instability develops always towards
a collapsed gas pressure dominated disc and not towards the expansion. This
result is valid for all initial configurations we tested. We find significant
convective heat flux that increases the instability development time, but is
not strong enough to inhibit the disc collapse. A physical explanation of the
lack of the expansion phase is proposed considering the role of the radial heat
advection. Our finding is relevant since it excludes the formation of the hot
comptonizing corona -often suggested to be present- around the central object
by the mechanism of the Shakura-Sunyaev instability. We also show that, in the
parameters range we simulated, accretion disks are crossed by significant
amplitude acoustic waves.Comment: 8 pages, 12 Postscript figures, uses natbib.sty, accepted for
publication in MNRA
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