References of "Claeskens, Jean-François"
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See detailGravitational lensing studies with the 4-m International Liquid Mirror Telescope (ILMT) (poster contribution)
Jean, C.; Claeskens, Jean-François ULg; Surdej, Jean ULg

in Brainerd, T. G.; Kochanek, C. S. (Eds.) Proceedings of the Conference "Gravitational lensing: recent progress and future goals" (2001, October 01)

A 4-m International Liquid Mirror Telescope (ILMT) is being built in the north of Chile by an international consortium and will become operational in two years from now. We present here a short ... [more ▼]

A 4-m International Liquid Mirror Telescope (ILMT) is being built in the north of Chile by an international consortium and will become operational in two years from now. We present here a short description of the telescope as well as estimates of the microlensing, macrolensing and weak lensing effects expected from a deep, multicolor imaging survey made with such a telescope. [less ▲]

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See detailWeighing nearby stars with GAIA ?
Dib, S.; Surdej, Jean ULg; Claeskens, Jean-François ULg

in Conference proceedings of "European meeting, Census of the Galaxy: Challenges for photometry and spectrometry with GAIA. (2001, August 01)

Microlensing consists in two major effects: (1) variation in the apparent position of the background sources (astrometric component) and (2) flux variations of the background sources (photometric ... [more ▼]

Microlensing consists in two major effects: (1) variation in the apparent position of the background sources (astrometric component) and (2) flux variations of the background sources (photometric component). While the latter has been extensively used in the search for dark objects in the Galactic disk and halo (projects like MACHO (Alcock et al, 1997), EROS (Derue et al, 1999), OGLE (Paczynski et al, 1994)), the first effect has not yet been part of a systematic observational program, simply because the observations of very slight displacements in the positions of background sources requires an astrometric accuracy which current telecopes do not yet provide. We investigate here whether the astrometric accuracy of GAIA could enable such measurements and, as a consequence, enable new, direct and original measurements of the mass of nearby stars. [less ▲]

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See detailThe 4-m International Liquid Mirror Telescope Project (ILMT) (poster)
Poels, Joël ULg; Borra, E.; Claeskens, Jean-François ULg et al

in Harnden, F. R.; Primini, F. A.; Payne, H. E. (Eds.) Astronomical Data Analysis Software & Systems X Conference (2001)

The working principle of liquid mirror telescopes (LMTs) is first reviewed along with their advantages and disadvantages over classical telescopes. For several reasons (access to regions near the south ... [more ▼]

The working principle of liquid mirror telescopes (LMTs) is first reviewed along with their advantages and disadvantages over classical telescopes. For several reasons (access to regions near the south galactic pole, galactic center, good image quality, etc.), an excellent site for such an LMT is the Atacama desert. A deep (B~24 mag) LMT survey at latitudes near -22deg -- -29deg will cover ~90 square degrees at high galactic latitude and be especially useful for gravitational lensing studies, for the identification of various classes of interesting extragalactic objects (cf. clusters, supernovae, etc. at high redshift) and for subsequent follow-up observations with 8 m-class telescopes. A short description of the handling of data products is also presented. [less ▲]

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See detailSurveys with the 4-m International Liquid Mirror Telescope
Poels, Joël ULg; Moreau, O.; Manfroid, Jean ULg et al

in Mining the Sky (2001)

The working principle of liquid mirror telescopes (LMTs) is first reminded as well as their advantages and disadvantages over classical telescopes. For several obvious reasons (access to regions near the ... [more ▼]

The working principle of liquid mirror telescopes (LMTs) is first reminded as well as their advantages and disadvantages over classical telescopes. For several obvious reasons (access to regions near the south galactic pole, galactic center, good image quality, ...), a best site location for such a LMT is somewhere in the Atacama desert. At latitudes near -22 - -29 degree, a deep (B = 24 mag.) LMT survey will approximately cover 90 square degrees at high galactic latitude, specially useful for gravitational lensing studies, for the identification of various classes of interesting extragalactic objects (cf. clusters, supernovae, etc. at high redshift) and subsequent follow-up observations with 8m-class telescopes. A short description of the handling of data products is also presented. [less ▲]

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See detailThe relevance of HST observations for studies of quasars, gravitational lenses and intervening gas clouds "
Surdej, Jean ULg; Claeskens, Jean-François ULg; Jean, C. et al

in Space Sciences, Vol. II, Part 1: Space Scientific Research in Belgium (2001)

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See detailQSO colors in the proposed GAIA photometric systems
Claeskens, Jean-François ULg; Royer, P.; Surdej, Jean ULg

in Proceedings of the GAIA Workshop (2001)

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See detailMicrolensing observations with the 4-m International Liquid Mirror Telescope
Claeskens, Jean-François ULg; Jean, Christophe ULg; Surdej, Jean ULg

in Menzies, J. W.; Sackett, P. D. (Eds.) The proceedings of the Conference “Microlensing 2000: A New Era of Microlensing Astrophysics” (2001)

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See detailDetermination of the mass of nearby stars from astrometric microlensing observations
Dib, S.; Claeskens, Jean-François ULg; Surdej, Jean ULg

in Surdej, Jean; Swings, J.-P.; Caro, D. (Eds.) et al Proceedings of the LIAC “From optical to millimetric interferometry, scientific and technological challenges” (2001)

The possibility of determining, with a precision of 10%, the mass of bright, nearby stars by means of astrometric microlensing observations is investigated in the context of the future ground-based and ... [more ▼]

The possibility of determining, with a precision of 10%, the mass of bright, nearby stars by means of astrometric microlensing observations is investigated in the context of the future ground-based and spaceborne high precision astrometric instruments, such as the VLT interferometer, GAIA and SIM. [less ▲]

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See detailHST and ground-based observations of the gravitational lens system Q1009-0252 A & B
Claeskens, Jean-François ULg; Khmil, S. V.; Lee, Dong Wook et al

in Astronomy and Astrophysics (2001), 367

In this paper, we propose a critical study of direct images and spectra obtained for the doubly imaged QSO Q1009-0252 A & B (=LBQS1009-0252 A & B). First, we report new observations with the WFPC2 onboard ... [more ▼]

In this paper, we propose a critical study of direct images and spectra obtained for the doubly imaged QSO Q1009-0252 A & B (=LBQS1009-0252 A & B). First, we report new observations with the WFPC2 onboard the Hubble Space Telescope (HST), in which the lensing galaxy is very clearly detected. We combine these with existing NICMOS observations in order to characterize the nature and redshift of the lens. Our HST observations are also fitted with simple analytical lens models; estimates expected for the time delay and M/L of the lensing galaxy are derived. Secondly, new and existing ground-based observations are used in order to establish the first multicolour lightcurves of the lensed components. Although the time resolution is low, flux variability is detected for both components. Finally, ground-based spectroscopic observations published by Surdej et al. (1993) are re-processed with a new method designed to extract spectra with small angular separations. The final spectra are analysed on the basis of a simple model including macrolensing and microlensing amplifications and dust extinction. Based on observations collected at the European Southern Observatory (La Silla, Chile) and with the Hubble Space Telescope. [less ▲]

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See detailGravitational lensing
Surdej, Jean ULg; Claeskens, Jean-François ULg

in Bleeker, J. A.; Geiss, J.; Huber, M. (Eds.) The Century of Space Science, Volume I (2001)

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See detailQSO mass constraints from gravitational lensing studies of quasar pairs. The cases of Q1548+114 A & B and Q1148+0055 A & B
Claeskens, Jean-François ULg; Lee, D.-W.; Remy, M. et al

in Astronomy and Astrophysics (2000), 356

New constraints on the mass of quasars are derived from gravitational lensing studies of the QSO pairs Q1548+114 A & B and Q1148+0055 A & B, for which new ground-based and Hubble Space Telescope (HST ... [more ▼]

New constraints on the mass of quasars are derived from gravitational lensing studies of the QSO pairs Q1548+114 A & B and Q1148+0055 A & B, for which new ground-based and Hubble Space Telescope (HST) direct imagery have been obtained. In the case of Q1548+114 A & B, QSO A has been resolved into its host galaxy and a close companion. The non-detection with HST of a secondary lensed image of the background QSO in the close vicinity of the foreground one and the modeling of the host of QSO A, of the companion and of field galaxies with Singular Isothermal Spheres (SIS) yield a robust upper limit on the central compact mass of 4.5 10[SUP]11[/SUP] M[SUB]sun[/SUB]. On the other hand, the combined mass of Q1148+0055 B plus host must be smaller than 6.5 10[SUP]11[/SUP] M[SUB]sun[/SUB] since no secondary lensed image has been detected with HST. Photometry and relative astrometry of all the detected objects are reported. Based on data collected with the Hubble Space Telescope and at the European Southern Observatory, La Silla, Chile [less ▲]

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See detailLensing properties of 7 damped Lyalpha absorbing galaxy-QSO pairs
Le Brun, V.; Smette, A.; Surdej, Jean ULg et al

in Astronomy and Astrophysics (2000), 363

Le Brun et al. (1997) presented the first identifications of the galaxies giving rise to 7 intermediate redshift damped Lyalpha (DLA) absorption systems. Here, we study the gravitational lensing ... [more ▼]

Le Brun et al. (1997) presented the first identifications of the galaxies giving rise to 7 intermediate redshift damped Lyalpha (DLA) absorption systems. Here, we study the gravitational lensing properties of these foreground galaxies based on their observed optical appearance and on the absence of any secondary lensed quasar image. We consider the possibility that any secondary image be hidden due to extinction by dust, but find it unlikely. We derive upper limits on the amplification factor affecting the luminosity of the background quasars; in each case, this factor is found to be less than 0.3 mag. We also obtain upper limits on the total mass of the damped Ly-alpha galaxies, within radii equal to the quasar impact parameters. Mass-to-light ratios are found to be consistent with existing estimates based on X-ray emission or on motion of dwarf satellites. Although we show that lensing is not important in this sample, we note that existing DLA surveys used to determine the cosmological density of gas at z < 1 are based on samples of quasars brighter than the ones considered here and for which the amplification bias is likely to be stronger. [less ▲]

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See detailGravitational Lensing by Damped LY alpha Absorbers
Smette, A.; Claeskens, Jean-François ULg; Surdej, Jean ULg

in Carilli, C. L.; Radford, S. J. E.; Menten, K. M. (Eds.) et al Highly Redshifted Radio Lines, ASP Conf. Series Vol. 156 (1999)

Spiral galaxies are thought to be the main responsible for the damped Ly-alpha (DLA) systems seen in QSO spectra. They can also act as gravitational lenses, affecting quantities derived in DLA surveys ... [more ▼]

Spiral galaxies are thought to be the main responsible for the damped Ly-alpha (DLA) systems seen in QSO spectra. They can also act as gravitational lenses, affecting quantities derived in DLA surveys. Assuming that z > 0 spiral galaxies are similar to local ones, we find that, at z 0.5, the number density of DLA systems may be over-estimated by up to 90% and the HI cosmological density (Omega[SUB]HI[/SUB]) by up to 170% in a survey using bright b[SUB]q[/SUB] = 16, z[SUB]q[/SUB] ga 2 QSOs and in the absence of important extinction by dust. Applying our model to existing surveys, we find that Omega[SUB]HI[/SUB] is significantly over-estimated only in the z < 1.7 ones (by 34%). Furthermore, statistical tests indicate that these surveys are indeed affected by gravitational lensing at a 2.3% confidence level. If luminosity functions for flat-spectrum radio-sources and optically selected QSOs are similar in shape and slopes, similar Gling effects should affect surveys for 21cm absorbers using bright, high-z flat-spectrum radio-sources. [less ▲]

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See detailSurveys with a 4 m Liquid Mirror Telescope
Jean, C.; Claeskens, Jean-François ULg; Surdej, Jean ULg

in Morgant, Faffaella; Couch, Warrick J. (Eds.) Looking Deep in the Southern Sky (1999)

We describe an international project of construction and operation of a 4 m Liquid Mirror Telescope (LMT) led by E. Borra. A LMT, whose main advantage is its very low cost, is particularly well suited for ... [more ▼]

We describe an international project of construction and operation of a 4 m Liquid Mirror Telescope (LMT) led by E. Borra. A LMT, whose main advantage is its very low cost, is particularly well suited for the search and study of gravitational lenses, type Ia supernovae, faint nearby red, brown and white dwarfs, halo stars with high proper motions and, more generally, all variable phenomena like quasars, variable stars, micro-lensing effects, etc. [less ▲]

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See detailGravitational lens studies with a LMT
Surdej, Jean ULg; Claeskens, Jean-François ULg

in Ferrari, M. (Ed.) Proceedings of the International workshop “Science with Liquid Mirror Telescopes” (1998)

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See detailDetection of the lensing galaxy for the double QSO HE 1104-1805
Remy, M.; Claeskens, Jean-François ULg; Surdej, Jean ULg et al

in New Astronomy (1998), 3

High angular resolution ground-based direct imaging (V, R, I_c) of the double QSO HE 1104-1805 has been obtained with the NOT and NTT telescopes. Analysis of these data led to the first detection of the ... [more ▼]

High angular resolution ground-based direct imaging (V, R, I_c) of the double QSO HE 1104-1805 has been obtained with the NOT and NTT telescopes. Analysis of these data led to the first detection of the lensing galaxy. Direct imaging of HE 1104-1805 has subsequently been carried out with the Planetary Camera (WFPC2) through the F555W (nearly Johnson V) and F814W (nearly Kron-Cousins I_c) filters onboard HST. These images confirm the presence of the deflector between the two lensed quasar components (A & B). Direct imaging of this system in the near infrared (IR) at 2.2 mu-m (K'), with IRAC-2b at the Cassegrain focus of the ESO/MPI 2.2 m telescope, indicates that the additional component is indeed a very red extended object with magnitude K ~= 16.5. We present deconvolutions of the HST and IR images using the 2-channel PLUCY method. From the HST observations, we also report very accurate relative positions and brightnesses of the QSO components. By comparison between the ground-based and the HST observations, from February-March 1994 to November 1995, we possibly detect the fading of component A by ~ 0.3 magnitudes in the optical and a corresponding fading of B by about half this value. The observed monotonic decrease of the magnitude difference between B and A as a function of wavelength is consistent with a partial amplification of the A component by microlensing. Evolutionary models for galaxies show that, in order to produce the observed colour indices V - I_c > 2 and 4 < I_c - K < 4.5, the galaxy is likely to be of elliptical type with a redshift 0.95 < z < 1.4. This range is consistent with the redshift z=1.32 measured from the absorption lines of metallic species in the spectra of the quasar components. However, other values for the redshift of the lens inside this range cannot be excluded. This galaxy would presently be among the most distant known gravitational lenses. Several theoretical models are found to fit the observations. The simplest one is a singular isothermal sphere (SIS) lens immersed in an external shear with gamma=0.123. The derived mass-to-light ratio (11 h_50 M_Sun/L_Sun) for the galaxy is large but does not imply an excess of dark matter with respect to other well known lensing galaxies. [less ▲]

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See detailClose projected QSO-Galaxy associations: are they real?
Claeskens, Jean-François ULg; Surdej, Jean ULg

in Astronomy and Astrophysics (1998), 335

The present status of close projected associations between high redshift quasars and foreground galaxies is reviewed in the framework of gravitational lensing, adopting both an observational and a ... [more ▼]

The present status of close projected associations between high redshift quasars and foreground galaxies is reviewed in the framework of gravitational lensing, adopting both an observational and a theoretical approach. The Non Singular Isothermal Spherical lens model (Hinshaw & Krauss 1987) is used. We confirm that a stronger overdensity q of bright galaxies is expected at small projected angular separations (<= 3arcsec ) from high redshift and bright (thus highly luminous) quasars (HLQs). The effects of a non singular galactic core radius and/or microlensing only slightly enhance this expected overdensity. The predictions reproduce pretty well the scarce observations which are presently available, with the exception of two rather high overdensities previously claimed in the literature (e.g. q=2.9 for theta <= 6arcsec , Webster & Hewett 1990). Uncontrolled morphological selections may affect these controversial results so that additional unbiased observations are badly needed before rejecting gravitational lensing as the mechanism to produce the observed close angular QSO-galaxy associations. We describe three new selected and bias-free samples consisting of a total of 219 different HLQs. Although an apparent overdensity is detected in two of them, these galaxy excesses are found to be statistically not significant. Firm confirmation of the reality of close projected QSO-Galaxy associations is hampered by small number statistics: the expected and observed absolute numbers of such QSO-galaxy associations are still presently found to be very small. Therefore, the corresponding error bars are quite large. We conclude that the best observational strategy to significantly detect (at 3sigma ) an overdensity of galaxies near HLQs is to observe about 1500 HLQs (M_V ~ -29) down to a limiting magnitude Rlim ~ 23. Future automated surveys may provide us with such a large and unbiased database. In conclusions, we find that the lensing-induced correlation between galaxies and HLQs at small angular separations consists of a very interesting but weak effect, much less sensitive to cosmological or astrophysical parameters than the number of multiply imaged sources expected within a sample of HLQs. Based on data collected at the European Southern Observatory, La Silla, Chile [less ▲]

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See detailHST confirmation of the lensed quasar J03.13.
Surdej, Jean ULg; Claeskens, Jean-François ULg; Remy, M. et al

in Astronomy and Astrophysics (1997), 327

In order to definitely prove the claim by Claeskens et al. (1996A&A...305L...9C) that J03.13 is a doubly imaged quasar, WFPC2 direct imaging and FOS spectroscopy of this compact system have been obtained ... [more ▼]

In order to definitely prove the claim by Claeskens et al. (1996A&A...305L...9C) that J03.13 is a doubly imaged quasar, WFPC2 direct imaging and FOS spectroscopy of this compact system have been obtained with HST. These textbook case observations clearly show that J03.13 consists of two point-like images separated by 0.849"+/-0.001" with a magnitude difference of 2.14+/-0.03 in V and I. We see no trace of a lensing galaxy. From the FOS observations, we find that J03.13 A and B have identical spectra within the measurement uncertainties. The data also show that spectroscopic contamination of B by A is negligible. We do confirm the redshift z=2.545 for J03.13 A and B (emission-lines due to Ly-beta lambda1025 and O VI lambdalambda1031, 1037 are also detected), first derived from an unresolved ground-based spectrum of the two components by Claeskens et al. We also find that the absorption line system at z=2.344 (CIV and Ly-alpha) is present in the spectra of A and B. On the contrary, the absorption line system at z=1.085 (MgII) is only present in the spectrum of J03.13 A. If this latter system is associated with the lens, we may expect that image A consists of two radially merging images with a very small angular separation. From the WFPC2 imagery, we can rule out the presence of a third component fainter than A by up to 5.2mag. with an angular separation >=0.13". [less ▲]

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See detailHST observations of the gravitational lens systems HE 1104-1805 and J03.13
Remy, M.; Claeskens, Jean-François ULg; Surdej, Jean ULg

in Casertano, S. (Ed.) The 1997 HST Calibration Workshop with a New Generation of Instruments (1997, January 01)

High angular resolution Principle Component Image (PC1) images of the gravitational lens systems HE 1104-1805 and J03.13 are presented. Using a method described in Remy et al. (1997a), optimal TinyTim ... [more ▼]

High angular resolution Principle Component Image (PC1) images of the gravitational lens systems HE 1104-1805 and J03.13 are presented. Using a method described in Remy et al. (1997a), optimal TinyTim Point Spread Function (PSFs) are constructed to fit at best the lensed point-like components. The derived photometry of the GL components and the detection of the lens galaxy for HE 1104-1805 are discussed. Textbook case Faint Object Spectrograph (FOS) spectra of J03.13 A and B clearly show that this double QSO is a cosmic mirage. [less ▲]

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