519 research outputs found
SARCS strong lensing galaxy groups: I - optical, weak lensing, and scaling laws
We present the weak lensing and optical analysis of the SL2S-ARCS (SARCS)
sample of strong lens candidates. The sample is based on the Strong Lensing
Legacy Survey (SL2S), a systematic search of strong lensing systems in the
photometric Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The SARCS
sample focuses on arc-like features and is designed to contain mostly galaxy
groups. We briefly present the weak lensing methodology that we use to estimate
the mass of the SARCS objects. Among 126 candidates, we obtain a weak lensing
detection for 89 objects with velocity dispersions of the Singular Isothermal
Sphere mass model ranging from 350 to 1000 km/s with an average value of
600km/s, corresponding to a rich galaxy group (or poor cluster). From the
galaxies belonging to the bright end of the group's red sequence (M_i<-21), we
derive the optical properties of the SARCS candidates. We obtain typical
richnesses of N=5-15 galaxies and optical luminosities of L=0.5-1.5e+12 Lsol
(within a radius of 0.5 Mpc). We use these galaxies to compute luminosity
density maps, from which a morphological classification reveals that a large
fraction of the sample are groups with a complex light distribution, either
elliptical or multimodal, suggesting that these objects are dynamically young
structures. We finally combine the lensing and optical analyses to draw a
sample of 80 most secure group candidates, i.e. weak lensing detection and
over-density at the lens position in the luminosity map, to remove false
detections and galaxy-scale systems from the initial sample. We use this
reduced sample to probe the optical scaling relations in combination with a
sample of massive galaxy clusters. We detect the expected correlations over the
probed range in mass with a typical scatter of 25% in the SIS velocity
dispersion at a given richness or luminosity, making these scaling laws
interesting mass proxie
Applying Magnetized Accretion-Ejection Models to Microquasars: a preliminary step
We present in this proceeding some aspects of a model that should explain the
spectral state changes observed in microquasars. In this model, ejection is
assumed to take place only in the innermost disc region where a large scale
magnetic field is anchored. Then, in opposite to conventional ADAF models, the
accretion energy can be efficiently converted in ejection and not advected
inside the horizon. We propose that changes of the disc physical state (e.g.
transition from optically thick to optically thin states) can strongly modify
the magnetic accretion-ejection structure resulting in the spectral
variability. After a short description of our scenario, we give some details
concerning the dynamically self-consistent magnetized accretion-ejection model
used in our computation. We also present some preliminary results of spectral
energy distribution.Comment: Proceeding of the fith Microquasar Workshop, June 7 - 13, 2004,
Beijing, China. Accepted for publication in the Chinese Journal of Astronomy
and Astrophysic
Characterizing SL2S galaxy groups using the Einstein radius
We analyzed the Einstein radius, , in our sample of SL2S galaxy
groups, and compared it with (the distance from the arcs to the center of
the lens), using three different approaches: 1.- the velocity dispersion
obtained from weak lensing assuming a Singular Isothermal Sphere profile
(), 2.- a strong lensing analytical method ()
combined with a velocity dispersion-concentration relation derived from
numerical simulations designed to mimic our group sample, 3.- strong lensing
modeling () of eleven groups (with four new models presented in
this work) using HST and CFHT images. Finally, was analyzed as a function
of redshift to investigate possible correlations with L, N, and the
richness-to-luminosity ratio (N/L). We found a correlation between
and , but with large scatter. We estimate = (2.2 0.9)
+ (0.7 0.2), = (0.4 1.5) + (1.1
0.4), and = (0.4 1.5) + (0.9 0.3) for
each method respectively. We found a weak evidence of anti-correlation between
and , with Log = (0.580.06) - (0.040.1), suggesting
a possible evolution of the Einstein radius with , as reported previously by
other authors. Our results also show that is correlated with L and N
(more luminous and richer groups have greater ), and a possible
correlation between and the N/L ratio. Our analysis indicates that
is correlated with in our sample, making useful to
characterize properties like L and N (and possible N/L) in galaxy groups.
Additionally, we present evidence suggesting that the Einstein radius evolves
with .Comment: Accepted for publication in Astronomy & Astrophysics. Typos correcte
Studying the X-ray hysteresis in GX 339-4: the disc and iron line over one decade
We report on a comprehensive and consistent investigation into the X-ray
emission from GX 339-4. All public observations in the 11 year RXTE archive
were analysed. Three different types of model - single powerlaw, broken
powerlaw and a disc + powerlaw - were fitted to investigate the evolution of
the disc, along with a fixed gaussian component at 6.4 keV to investigate any
iron line in the spectrum. We show that the relative variation in flux and
X-ray colour between the two best sampled outbursts are very similar. The decay
of the disc temperature during the outburst is clearly seen in the soft state.
The expected decay is S_Disc \propto T^4; we measure T^4.75\pm0.23. This
implies that the inner disc radius is approximately constant in the soft state.
We also show a significant anti-correlation between the iron line significant
width and the X-ray flux in the soft state while in the hard state the EW is
independent of the flux. This results in hysteresis in the relation between
X-ray flux and both line flux and EW. To compare the X-ray binary outburst to
the behaviour seen in AGN, we construct a Disc Fraction Luminosity Diagram for
GX 339-4, the first for an X-ray binary. The shape qualitatively matches that
produced for AGN. Linking this with the radio emission from GX 339-4 the change
in radio spectrum between the disc and power-law dominated states is clearly
visible.Comment: Accepted for publication in MNRAS, 20 pages, 17 figures. For high-res
version see http://www.astro.soton.ac.uk/~r.j.dunn/publications.htm
The XMM/BeppoSAX observation of Mkn 841
Mkn 841 has been observed simultaneously by XMM and BeppoSAX in January 2001.
Due to operational contingency, the 30ks XMM observation was split into two
parts, separated by about 15 hours. We first report the presence of a narrow
iron line which appears to be rapidly variable between the two pointings,
requiring a non-standard interpretation. We then focus on the analysis of the
broad band (0.3-200 keV) continuum using the XMM/EPIC, RGS and SAX/PDS data.
The Mkn 841 spectrum is well fitted by a comptonization model in a geometry
more photon-fed than a simple slab geometry above a passive disk. It presents a
relatively large reflection (R>2) which does not agree with an apparently weak
iron line. It also show the presence of a strong soft excess wellfitted by a
comptonized spectrum in a cool plasma, suggesting the presence of a
multi-temperature corona.Comment: 4 pages, 5 figures. Proc. of the meeting: "The Restless High-Energy
Universe" (Amsterdam, The Netherlands), E.P.J. van den Heuvel, J.J.M. in 't
Zand, and R.A.M.J. Wijers Ed
Probing the Slope of Cluster Mass Profile with Gravitational Einstein Rings: Application to Abell 1689
The strong lensing modelling of gravitational ``rings'' formed around massive
galaxies is sensitive to the amplitude of the external shear and convergence
produced by nearby mass condensations. In current wide field surveys, it is now
possible to find out a large number of rings, typically 10 gravitational rings
per square degree. We propose here, to systematically study gravitational rings
around galaxy clusters to probe the cluster mass profile beyond the cluster
strong lensing regions. For cluster of galaxies with multiple arc systems, we
show that rings found at various distances from the cluster centre can improve
the modelling by constraining the slope of the cluster mass profile. We outline
the principle of the method with simple numerical simulations and we apply it
to 3 rings discovered recently in Abell~1689. In particular, the lens modelling
of the 3 rings confirms that the cluster is bimodal, and favours a slope of the
mass profile steeper than isothermal at a cluster radius \sim 300 \kpc. These
results are compared with previous lens modelling of Abell~1689 including weak
lensing analysis. Because of the difficulty arising from the complex mass
distribution in Abell~1689, we argue that the ring method will be better
implemented on simpler and relaxed clusters.Comment: Accepted for publication in MNRAS. Substantial modification after
referee's repor
Discovery of a new INTEGRAL source: IGR J19140+0951
IGR J19140+0951 (formerly known as IGR J19140+098) was discovered with the
INTEGRAL satellite in March 2003. We report the details of the discovery, using
an improved position for the analysis. We have performed a simultaneous study
of the 5-100 keV JEM-X and ISGRI spectra from which we can distinguish two
different states. From the results of our analysis we propose that IGR
J19140+0951 is a persistent Galactic X-ray binary, probably hosting a neutron
star although a black hole cannot be completely ruled out.Comment: 4 pages, 4 figures. Accepted for publication in A&A
A community based algorithm for deriving users' profiles from egocentrics networks: experiment on Facebook and DBLP
International audienceNowadays, social networks are more and more widely used as a solution for enriching users’ profiles in systems such as recommender systems or personalized systems. For an unknown user’s interest, the user’s social network can be a meaningful data source for deriving that interest. However, in the literature very few techniques are designed to meet this solution. Existing techniques usually focus on people individually selected in the user’s social network and strongly depend on each author’s objective. To improve these techniques, we propose using a community-based algorithm that is applied to a part of the user’s social network (egocentric network) and that derives a user social profile that can be reused for any purpose (e.g., personalization, recommendation). We compute weighted user’s interests from these communities by considering their semantics (interests related to communities) and their structural measures (e.g., centrality measures) in the egocentric network graph. A first experiment conducted in Facebook demonstrates the usefulness of this technique compared to individual-based techniques and the influence of structural measures (related to communities) on the quality of derived profiles. A second experiment on DBLP and the author’s social network Mendeley confirms the results obtained on Facebook and shows the influence of the density of egocentrics network on the quality of results
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