2,765 research outputs found
The polarization properties of Broad Absorption Line QSOs: observational results
Correlations between BAL QSO intrinsic properties and polarization have been
searched for. Some results are summarized here, providing possible constraints
on BAL outflow models.Comment: 4 pages, 5 figures; to appear in ``Mass Outflow in Active Galactic
Nuclei: New Perspectives'' ASP Conference Series, eds. D.M. Crenshaw, S.B.
Kraemer, and I.M. Georg
The optical polarization of radio-loud and radio-intermediate broad absorption line quasi-stellar objects
On the basis of a sample of approximately 50 broad absorption line quasi-stellar objects (BAL QSOs), we investigate possible correlations between BAL QSO radio properties and other spectral characteristics, including polarization. For this purpose new polarization data have been obtained. The main result of our statistical study is the absence of significant correlations between the radio-to-optical flux ratio R[SUP]star [/SUP] and all other quantities: the polarization p_0 of the continuum, the slope of the continuum, the balnicity and detachment indices which characterize the BAL profiles, and the terminal velocity of the flow v_max. The claimed anticorrelation between R[SUP]star [/SUP] and v_max is therefore not confirmed, as well as the correlation between R[SUP]star [/SUP] and p_0 predicted by some models. Although marginally significant, the only possible correlations occur for the BAL QSOs with low-ionization troughs. Based on observations collected at the European Southern Observatory (ESO, La Silla
Polarimetry and photometry of the peculiar main-belt object 7968 = 133P/Elst-Pizarro
133P/Elst-Pizarro is an object that has been described as either an active
asteroid or a cometary object in the main asteroid belt. Here we present a
photometric and polarimetric study of this object in an attempt to infer
additional information about its origin.
With the FORS1 instrument of the ESO VLT, we have performed during the 2007
apparition of 133P/Elst-Pizarro quasi-simultaneous photometry and polarimetry
of its nucleus at nine epochs in the phase angle range 0 - 20 deg. For each
observing epoch, we also combined all available frames to obtain a deep image
of the object, to seek signatures of weak cometary activity. Polarimetric data
were analysed by means of a novel physical interference modelling.
The object brightness was found to be highly variable over timescales <1h, a
result fully consistent with previous studies. Using the albedo-polarization
relationships for asteroids and our photometric results, we found for our
target an albedo of about 0.06-0.07 and a mean radius of about 1.6 km.
Throughout the observing epochs, our deep imaging of the comet detects a tail
and an anti-tail. Their temporal variations are consistent with an activity
profile starting around mid May 2007 of minimum duration of four months. Our
images show marginal evidence of a coma around the nucleus. The overall light
scattering behaviour (photometry and polarimetry) resembles most closely that
of F-type asteroids.Comment: Accepted by Astronomy and Astrophysic
Multi-point observations of intermittency in the cusp regions
International audienceIn this paper we investigate the statistical properties of magnetic field fluctuations measured by the four Cluster spacecraft in the cusp and close to the interface with the magnetospheric lobes, magnetopause and magnetosheath. At lower altitudes along the outbound orbit of 26 February 2001, the magnetic field fluctuations recorded by all four spacecraft are random and their Probability Distribution Functions (PDFs) are Gaussian at all scales. The flatness parameter, F ? related to the kurtosis of the time series, is equal to 3. At higher altitudes, in the cusp and its vicinity, closer to the interface with the magnetopause and magnetosheath, the PDFs from all Cluster satellites are non-Gaussian and show a clear intermittent behavior at scales smaller than ?G? 61 s (or 170 km). The flatness parameter increases to values greater than 3 for scales smaller than ?G. A Haar wavelet transform enables the identification of the "events" that produce sudden variations of the magnetic field and of the scales that have most of the power. The LIM parameter (i.e. normalized wavelet power) indicates that events for scales below 65 s are non-uniformly distributed throughout the cusp passage. PDFs, flatness and wavelet analysis show that at coarse-grained scales larger than ?G the intermittency is absent in the cusp. Fluctuations of the magnetic energy observed during the same orbit in the magnetosheath show PDFs that tend toward a Gaussian at scales smaller than ?G found in the cusp. The flatness analysis confirms the decreasing of ?G from cusp to magnetosheath. Our analysis reveals the turbulent cusp as a transition region from a non-intermittent turbulent state inside the magnetosphere to an intermittent turbulent state in the magnetosheath that has statistical properties resembling the solar wind turbulence. The observed turbulent fluctuations in the cusp suggests a phenomenon of nonlinear interactions of plasma coherent structures as in contemporary models of space plasma turbulence
Confirmation of the existence of coherent orientations of quasar polarization vectors on cosmological scales
In order to verify the existence of coherent orientations of quasars
polarization vectors on very large scales, we have obtained new polarization
measurements for a sample of quasars located in a given region of the
three-dimensional Universe where the range of polarization position angles was
predicted in advance. For this new sample, the hypothesis of uniform
distribution of polarization position angles may be rejected at the 1.8%
significance level on the basis of a simple binomial test. This result provides
an independent confirmation of the existence of alignments of quasar
polarization vectors on very large scales. In total, out of 29 polarized
quasars located in this region of the sky, 25 have their polarization vectors
coherently oriented. This alignment occurs at redshifts 1-2 suggesting the
presence of correlations in objects or fields on Gpc scales. More global
statistical tests applied to the whole sample of polarized quasars distributed
all over the sky confirm that polarization vectors are coherently oriented in a
few groups of 20-30 quasars. Some constraints on the phenomenon are also
derived. Considering more particularly the quasars in the selected region of
the sky, we found that their polarization vectors are roughly parallel to the
plane of the Local Supercluster. But the polarization vectors of objects along
the same line of sight at lower redshifts are not accordingly aligned. We also
found that the known correlations between quasar intrinsic properties and
polarization are not destroyed by the alignment effect. Several possible
mechanisms are discussed, but the interpretation of this orientation effect
remains puzzling.Comment: Accepted for publication in Astronomy & Astrophysic
Evolution of the Dust Coma in Comet 67P/Churyumov-Gerasimenko Before 2009 Perihelion
Comet 67P/Churyumov-Gerasimenko is the main target of ESA's Rosetta mission
and will be encountered in May 2014. As the spacecraft shall be in orbit the
comet nucleus before and after release of the lander {\it Philae}, it is
necessary necessary to know the conditions in the coma. Study the dust
environment, including the dust production rate and its variations along its
preperihelion orbit. The comet was observed during its approach to the Sun on
four epochs between early-June 2008 and mid-January 2009, over a large range of
heliocentric distances that will be covered by the mission in 2014. An
anomalous enhancement of the coma dust density was measured towards the comet
nucleus. The scalelength of this enhancement increased with decreasing
heliocentric distance of the comet. This is interpreted as a result of an
unusually slow expansion of the dust coma. Assuming a spherical symmetric coma,
the average amount of dust as well as its ejection velocity have been derived.
The latter increases exponentially with decreasing heliocentric distance (\rh),
ranging from about 1 m/s at 3 AU to about 25-35 m/s at 1.4 AU. Based on these
results we describe the dust environment at those nucleocentric distances at
which the spacecraft will presumably be in orbit.
Astronomy and Astrophysics, in pressComment: 5 pages, 4 figure
Ensemble Properties of Comets in the Sloan Digital Sky Survey
We present the ensemble properties of 31 comets (27 resolved and 4
unresolved) observed by the Sloan Digital Sky Survey (SDSS). This sample of
comets represents about 1 comet per 10 million SDSS photometric objects.
Five-band (u,g,r,i,z) photometry is used to determine the comets' colors,
sizes, surface brightness profiles, and rates of dust production in terms of
the Af{\rho} formalism. We find that the cumulative luminosity function for the
Jupiter Family Comets in our sample is well fit by a power law of the form N(<
H) \propto 10(0.49\pm0.05)H for H < 18, with evidence of a much shallower fit
N(< H) \propto 10(0.19\pm0.03)H for the faint (14.5 < H < 18) comets. The
resolved comets show an extremely narrow distribution of colors (0.57 \pm 0.05
in g - r for example), which are statistically indistinguishable from that of
the Jupiter Trojans. Further, there is no evidence of correlation between color
and physical, dynamical, or observational parameters for the observed comets.Comment: 19 pages, 8 tables, 11 figures, to appear in Icaru
Albedos of Main-Belt Comets 133P/Elst-Pizarro and 176P/LINEAR
We present the determination of the geometric R-band albedos of two main-belt
comet nuclei based on data from the Spitzer Space Telescope and a number of
ground-based optical facilities. For 133P/Elst-Pizarro, we find an albedo of
p_R=0.05+/-0.02 and an effective radius of r_e=1.9+/-0.3 km (estimated
semi-axes of a~2.3 km and b~1.6 km). For 176P/LINEAR, we find an albedo of
p_R=0.06+/-0.02 and an effective radius of r_e=2.0+/-0.2 km (estimated
semi-axes of a~2.6 km and b~1.5 km). In terms of albedo, 133P and 176P are
similar to each other and are typical of other Themis family asteroids, C-class
asteroids, and other comet nuclei. We find no indication that 133P and 176P are
compositionally unique among other dynamically-similar (but inactive) members
of the Themis family, in agreement with previous assertions that the two
objects most likely formed in-situ. We also note that low albedo (p_R<0.075)
remains a consistent feature of all cometary (i.e., icy) bodies, whether they
originate in the inner solar system (the main-belt comets) or in the outer
solar system (all other comets).Comment: 11 pages, 3 figures, accepted for publication in ApJ
Polarization properties of broad absorption line QSOs : new statistical clues
We report the results of statistical tests performed on a sample of 139 broad
absorption line (BAL) QSOs with good quality optical spectra and/or optical
polarization data. Correlations between optical indices and the polarization
degree p_0 are systematically searched for. In order to identify the most
important correlations, we perform a principal component analysis with a sample
of 30 BAL QSOs and eight quantities (including p_0). Most of the variance in
the data is contained in two principal components (PC): PC1 is mainly dominated
by the correlation between the balnicity index BI and the strength of the feii
emission, and may be driven by the accretion rate of matter onto the central
compact object. The variance in PC2 is essentially due to the anti-correlation
between p_0 and the detachment index DI. PC2 may be related to the orientation
of the BAL QSOs with respect to the line of sight. We also consider a
spectropolarimetric sample of 21 BAL QSOs for which we define four indices
describing the polarization properties of the absorption and emission lines. We
find a possible anti-correlation between the detachment index and a quantity SI
which measures the ratio of the depths of the civ absorption in the polarized
flux and in the total flux. This correlation indicates that in BAL QSOs with P
cygni profiles, the BAL troughs in the polarized flux are nearly as deep as in
the total flux while, in BAL QSOs with detached absorptions, the BAL troughs in
the polarized flux are much weaker than in the total flux. We show that our
main results may be explained in the framework of a 'two-component' wind model.
In this model, the broad absorption occurs in a dense equatorial wind emerging
from the accretion disk, while scattering and polarization mainly take place in
a polar region.Comment: 18 pages, 8 figures, to be published in Astronomy & Astrophysic
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