References of "Claeskens, Jean-François"
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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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See detailDamped LYalpha Absorbers and Gravitational Lensing
Smette, Alain; Claeskens, Jean-François ULg; Surdej, Jean ULg

in Structure and Evolution of the Intergalactic Medium from QSO Absorption Line System, Proceedings of the 13th IAP Astrophysics Colloquium (1997)

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See detailGravitational lensing by damped Ly-alpha absorbers
Smette, Alain; Claeskens, Jean-François ULg; Surdej, Jean ULg

in New Astronomy (1997), 2

Assuming that (i) damped Ly-alpha absorbers (DLAs) arise in present-day-like spiral galaxies which are immersed in isothermal dark matter halos, (ii) that these galaxies obey the Tully-Fisher sigma/sigma ... [more ▼]

Assuming that (i) damped Ly-alpha absorbers (DLAs) arise in present-day-like spiral galaxies which are immersed in isothermal dark matter halos, (ii) that these galaxies obey the Tully-Fisher sigma/sigma_* = (L/L_*)^1/alpha_TF and the Holmberg R_L/R_* = (L/L_*)^alpha_H relations, and (iii) that they follow the Schechter luminosity distribution, we describe how their observed number density (dN/dz), distribution of column density (f(N)) as well as inferred cosmological density of HI (Omega_HI) derived from DLA surveys are affected by gravitational lensing (GL). The `by-pass' effect causes the lines-of-sight (LOSs) towards background QSOs to avoid the central parts of galaxies and reduces their effective cross-section for absorption; the `amplification bias' leads observers to select QSOs whose LOSs preferentially cross galaxies close to their Einstein radius. As a consequence, the determination of the quantities dN/dz, f(N) and Omega_HI from DLA surveys does not only depend on the redshift z and luminosity L of galaxies responsible for the absorbers but also on the column density profile of HI within the galaxies and on the redshift z_q and magnitude b_q of the background QSOs. For most of the existing surveys using b_q <~ 19 QSOs, the amplification bias dominates the combined effect resulting in a slight overestimate of dN/dz, f(N) and Omega_HI. We mainly find that observational strategies presently used to produce high-z DLA surveys result in avoiding the signature of significant GL effects: following our model, we determine that an overestimate of Omega_HI by more than 10% is unlikely for the z > 1.7 existing surveys, but may reach ~= 35% for the low redshift ones. However, we show that, in the absence of extinction by dust and micro-lensing effects, surveys ideally designed to enhance GL effects, i.e. to search for DLAs at z ~ 0.5 in front of very bright (b_q ~= 16), high-z (z_q > 1) QSOs, may lead 1) to overestimate by up to ~= 90% the number of DLAs per unit redshift; 2) to bias the survey towards high HI column density systems so that it could contain up to 4 times as many such systems, thus 3) to overestimate by up to ~= 170% the cosmological density of gas associated with those DLAs. Identification of the galaxies responsible for the DLAs may be severely biased towards luminous galaxies if 2/alpha_TF - alpha_H>0 this latter effect is greatly increased for log N_HI > 21 DLAs. Hence, GL effects on the quantities derived from surveys for z ~ 0.5 DLAs are of the same order, but of opposite direction, as the effects of extinction by dust (cf. Fall & Pei, 1993). However, the GL and dust extinction effects do not compensate each other: combining them in a consistent way is necessary to interpret existing DLA surveys. Furthermore, the effects due to micro-lensing should be simultaneously taken into account. We intend to report the results on the complex interplay between macro-lensing, micro-lensing and dust in a subsequent paper. We briefly present statistical tests specifically designed to check whether GL affects existing DLA surveys, and assuming that extinction by dust is negligible. We only find indications of GL effects for the z < 1 ones which, if confirmed, might even be stronger than predicted by our model. We show that an independent work on the same subject by Bartelmann & Loeb (1996) incorrectly treats the inclination effects for the intervening galaxies, thus undermining some of their main results and conclusions. [less ▲]

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See detailExistence of a short period (3.5-4 hours) in the photometric variability of WR 66.
Rauw, Grégor ULg; Gosset, Eric ULg; Manfroid, Jean ULg et al

in Astronomy and Astrophysics (1996), 306

We report here on an independent detection of a short period (~4hours) in the photometric variations of WR66, thus confirming in broad terms the discovery by Antokhin et al.. In addition, we present the ... [more ▼]

We report here on an independent detection of a short period (~4hours) in the photometric variations of WR66, thus confirming in broad terms the discovery by Antokhin et al.. In addition, we present the first spectroscopic variability analysis for this star. A few peculiarities of the spectrum of WR66 are also discussed. Finally, we perform a brief examination of different possible origins of the phenomenon. [less ▲]

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See detailJ03.13 A and B: a new multiply imaged QSO candidate.
Claeskens, Jean-François ULg; Surdej, Jean ULg; Remy, M.

in Astronomy and Astrophysics (1996), 305

Careful analysis of direct R CCD frames obtained for selected Highly Luminous Quasars in March 1993 with the MPI/ESO 2.2m telescope has revealed that the quasar J03.13 consists of at least two point-like ... [more ▼]

Careful analysis of direct R CCD frames obtained for selected Highly Luminous Quasars in March 1993 with the MPI/ESO 2.2m telescope has revealed that the quasar J03.13 consists of at least two point-like components, constituting therefore a new gravitational lens candidate. Follow-up direct imagery of this interesting object with the New Technology Telescope (NTT) + SUSI in February 1994, using the Bessel B, R and Gunn-i filters, tends to confirm the gravitational lens hypothesis. The mean angular separation and magnitude difference between the A & B images are found to be 0.84"+/-0.03" and 2.1+/-0.1mag, respectively. A spatially unresolved, medium resolution spectrum of J03.13, obtained with the NTT + EMMI in February 1994, shows that the QSO has a redshift z=2.55, with Lyalpha and CIV absorptions at z=2.34 and MgII, MgI and FeII absorptions at z=1.085. The latter absorptions might be associated with a sigma=206km/s lens galaxy. B, R and Gunn-i photometric observations of J03.13 during 1994 and 1995 do not show evidence for flux variations exceeding 0.03mag. HST direct imagery with WFPC2 and FOS spectroscopy of J03.13 A & B are planned during cycle 5. [less ▲]

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See detailGround-based and HST Direct Imaging of HLQs
Surdej, Jean ULg; Jaunsen, A. O.; Claeskens, Jean-François ULg et al

in Kochanek, C.; Hewitt, J. (Eds.) Astrophysical Applications of Gravitational Lensing (1996)

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See detailGravitational Lenses and Damped Ly-alpha Systems
Smette, A.; Claeskens, Jean-François ULg; Surdej, Jean ULg

in Kochanek, C. S.; Hewitt, Jacqueline N (Eds.) Astrophysical applications of gravitational lensing: proceedings of the 173rd Symposium of the International Astronomical Union (1996)

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See detailJ03.13 AB: A New Multiply Imaged QSO Candidate
Claeskens, Jean-François ULg; Surdej, Jean ULg; Remy, M.

in Kochanek, C. S.; Hewitt, Jacqueline N (Eds.) Astrophysical applications of gravitational lensing: proceedings of the 173rd Symposium of the International Astronomical Union (1996)

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See detailGravitational Lenses Among Highly Luminous Quasars: Large Optical Surveys
Claeskens, Jean-François ULg; Jaunsen, A. O.; Surdej, Jean ULg

in Kochanek, C. S.; Hewitt, Jacqueline N (Eds.) Astrophysical applications of gravitational lensing: proceedings of the 173rd Symposium of the International Astronomical Union (1996)

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See detailDetermination of the Cosmological Density of Compact Objects using Gravitational Lensing and the HST
Claeskens, Jean-François ULg; Surdej, Jean ULg

in Benvenuti, Piero; Macchetto, F. D.; Schreier, E. J. (Eds.) Science with the Hubble Space Telescope - II (1996)

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See detailPhotometric monitoring (1987 to 1994) of the gravitational lens candidate UM 425.
Courbin, F.; Magain, Pierre ULg; Remy, M. et al

in Astronomy and Astrophysics (1995), 303

We present the results of a 7 year long photometric monitoring of two components (A and B) of UM 425, thought to be images, separated by 6.5", of the same z=1.47 quasar. These components have been imaged ... [more ▼]

We present the results of a 7 year long photometric monitoring of two components (A and B) of UM 425, thought to be images, separated by 6.5", of the same z=1.47 quasar. These components have been imaged through an R filter in order to obtain their light curves. The photometry was obtained by simultaneously fitting a stellar two-dimensional profile on each component. The brightest image (component A, m_R_=15.7) shows a slow and smooth increase in brightness of 0.2 magnitude in seven years, while the faintest one (component B, m_R_=20.1) displays an outburst of 0.4 magnitude which lasts approximately two years. The variation of component B may be interpreted in two ways, assuming UM 425 is gravitationally lensed. If it is due to an intrinsic variation of the quasar, we derive a lower limit of 3 years on the time delay from the fact that it is not observed in component A. On the other hand, if it is a microlensing "High Amplification Event", we estimate the size of the source to be ~10^-3^pc, in agreement with standard models of AGNs. These observations are consistent with the gravitational lens interpretation of the object. Furthermore, all the CCD frames obtained under the best seeing conditions have been co-added, in an attempt to detect the deflector. The final R image reveals a rich field of faint galaxies in the magnitude range m_R_~22-24. No obvious deflector, nor any system of arcs or arclets is detected, down to a limiting magnitude of m_R_~24. [less ▲]

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See detailIntegral Field Spectroscopy of Selected Extragalactic Objects with FUEGOS
Surdej, Jean ULg; Vanderriest, C.; Angonin-Willaime, M.-C. et al

in Walsh, J.; Danziger, J. (Eds.) Science with the VLT (1995)

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See detailGravitational lensing statistics based on a large sample of highly luminous quasars
Surdej, Jean ULg; Claeskens, Jean-François ULg; Crampton, D. et al

in Astronomical Journal (The) (1993), 105

Results on gravitational lensing statistics applied to a sample of 469 highly luminous quasars are reported. The objects were directly imaged, either from the ground under optimal seeing conditions, or ... [more ▼]

Results on gravitational lensing statistics applied to a sample of 469 highly luminous quasars are reported. The objects were directly imaged, either from the ground under optimal seeing conditions, or using the HST. We have derived values for the effectiveness parameter F of galaxies, modeled by means of singular isothermal spheres, to produce macrolensed images of distant quasars, and upper limits on the density parameter Omega(L) of compact objects with masses between 10 exp 10 and 10 exp 12 solar masses. Adopting H(0)= 50 km/s/Mpc, Omega(0)= 1, and Lambda = 0, we find that at the 99.7 percent confidence level, F is between 0.005 and 0.478 and that Omega(L) is less than 0.02. A critical discussion of these results is presented. Finally, comparing the efficiencies of ground-based and space instruments used to search for gravitational lens systems among highly luminous quasars, we conclude that for the near future, ground-based direct imaging characterized by a good dynamical range still constitutes the best observational strategy. [less ▲]

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See detailTwo new very close pairs of quasars with discordant redshifts and a gravitational lens candidate.
Surdej, Jean ULg; Remy, M.; Smette, A. et al

in Surdej, Jean; Fraipont-Caro, D.; Gosset, Eric (Eds.) et al Gravitational Lenses in the Universe (1993)

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See detailGalaxies statistics around highly luminous quasars: the ESO Sample
van Drom, E.; Surdej, Jean ULg; Magain, Pierre ULg et al

in Surdej, Jean; Fraipont-Caro, Denise; Gosset, Eric (Eds.) et al The proceedings of the 31st Liège International Astrophysical Colloquium "Gravitational lenses in the Universe" (1993)

We present observational evidence on the correlation between the positions of Highly Luminous Quasars and those of galaxies for a sample of 136 objects, observed under good seeing conditions.

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