References of "Claeskens, Jean-François"
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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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See detailSystematic search for gravitational mirages among general quasar samples
Moreau, O.; Claeskens, Jean-François ULg; Gosset, Eric 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 (1993)

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See detailOptical imaging of gravitational lenses B1422+231
Remy, M.; Surdej, Jean ULg; Smette, A. et al

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

We report here on some of the major developments achieved on optical observations of gravitational lenses since the last conference devoted to 'Gravitational Lensing'.

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See detailSearch for gravitational lenses in a large sample of highly luminous quasars and statistical determination of cosmological parameters
Claeskens, Jean-François ULg; Crampton, D.; Filippenko, A. V. et al

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

Results on gravitational lensing statistics applied to a sample of 469 highly luminous quasars are reported.

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See detailOptical imaging of the gravitational lens system B 1422+231
Remy; Surdej, Jean ULg; Smette, A. et al

in Astronomy and Astrophysics (1993), 278

Direct optical images (Bessel V, R and Gunn i; average seeing of 0.88 seconds) of B1422+231, obtained in March 1993 at European Southern Observatory (ESO) (La Silla), have been decomposed by fitting ... [more ▼]

Direct optical images (Bessel V, R and Gunn i; average seeing of 0.88 seconds) of B1422+231, obtained in March 1993 at European Southern Observatory (ESO) (La Silla), have been decomposed by fitting multiple Point Spread Functions (PSFs). We detect the optical counterparts of the four known radio point-like components. No additional optical image is found, but due to the compactness of the configuration, our detection limit for additional point sources is rather low (19.3 in R). The deduced positions and photometry of the four optical components agree pretty well with the radio and IR observations and this study provides further evidence supporting the gravitational lens origin of B1422+231. [less ▲]

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

in Benvenuti, Piero; Schreier, Ethan J (Eds.) Proceedings of a ST-ECF/STScI Workshop "Science with the Hubble Space Telescope" (1992)

We present here some recent results on gravitational lensing statistics as applied to a sample of 470 highly luminous quasars.

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See detailOptical observations of gravitational lenses
Surdej, Jean ULg; Claeskens, Jean-François ULg; Hutsemekers, Damien ULg et al

in Kayser, Rainer; Schramm, Thomas; Nieser, Lars (Eds.) Proceedings of the Hamburg conference on ’Gravitational Lenses’, Lecture Notes in Physics 406 (1992)

We report here on some of the major developments achieved on optical observations of gravitational lenses since the last conference devoted to 'Gravitational Lensing'.

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