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See detailLe Gravettien de Moldavie (30.000-23.000 BP)
Noiret, Pierre ULg

in Paléo. Revue d'Archéologie préhistorique (2007), 19

In Moldavia, three sequences serve as the basis for the chronological context for Gravettian (and Epigravettian) occupations. Two technological phases have been identified, showing independent development ... [more ▼]

In Moldavia, three sequences serve as the basis for the chronological context for Gravettian (and Epigravettian) occupations. Two technological phases have been identified, showing independent development, homogenous in both technological and typological attributes. The Gravettian is characterized by large retouched blades, followed by shouldered points (29,500-23,000 BP), succeeded by the Early Epigravettian (13,500-11,000 BP). [less ▲]

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See detailLe Gravettien du Nord-ouest de l'Europe
Otte, Marcel ULg; Noiret, Pierre ULg

in Paléo. Revue d'Archéologie préhistorique (2007), 19

Around 30 sites have yielded Gravettian assemblages in Belgium. Present fairly early (28,000 BP), the Gravettian appears in trios distinct facies. The earliest includes large pointed blades and tanged ... [more ▼]

Around 30 sites have yielded Gravettian assemblages in Belgium. Present fairly early (28,000 BP), the Gravettian appears in trios distinct facies. The earliest includes large pointed blades and tanged tools; the most recent is marked by microlithic backed elements (truncated elements, microgravettes). These facies likely correspond to regional groups, for which the first can also be found fairly easily in neighbouring regions (England, Wales and northern France). In Rhenania, some 15 sites correspond to two facies (with Font Robert points and with microgravettes), similar but not identical to the Belgian facies. [less ▲]

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See detailGraviers de la Meuse (alluvions modernes et anciennes) en Wallonie
Dassargues, Alain ULg; Wildemeersch, Samuel ULg; Rentier, Céline

in Dassargues, Alain; Walraevens, Kristine (Eds.) Watervoerende lagen & grondwater in België - Aquifères & eaux souterraines en Belgique (2014)

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See detailGravimetric exploration applied to the detection of faults in the area of Mol - Turnhout (Belgium)
Dassargues, Alain ULg; Halleux, Lucien ULg; Monjoie, Albéric ULg et al

in Annales de la Société Géologique de Belgique (1989), 112(2), 431-441

A gravimetrical exploration has been realized in the area of Mol and Turnhout (province of Antwerp, Belgium) to localize steep dipping faults in the Paleozoic bed rock and the Meso- to Cenozoic overburden ... [more ▼]

A gravimetrical exploration has been realized in the area of Mol and Turnhout (province of Antwerp, Belgium) to localize steep dipping faults in the Paleozoic bed rock and the Meso- to Cenozoic overburden. With a 600 to 1000m thick overburden consisting of sand and clay, 10m high steps in the bed rock result in significant gravimetrical anomalies. The interpretation is based upon comparison with theoretical anomalies computed for various density contrasts and depths. After the classical corrections, the Bouguer anomaly has been smoothed and horizontal derivatives computed to reduce the effect of shallow heterogeneities and regional gradients. The results confirm the presence of several faults already detected by other methods and enable a more accurate positionning. [less ▲]

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See detailGravitational lens simulator : a didactical experiment
Surdej, Jean ULg; Pospieszalska-Surdej, Anna ULg

Learning material (2007)

A large concentration of mass may act as a kind of lens, called a gravitational lens. A simple educational experience makes it possible to simulate such effects, see also (http://www.aeos.ulg.ac.be/GL ... [more ▼]

A large concentration of mass may act as a kind of lens, called a gravitational lens. A simple educational experience makes it possible to simulate such effects, see also (http://www.aeos.ulg.ac.be/GL/didactics.php). [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 detailGravitational lenses
Refsdal, S.; Surdej, Jean ULg

in Reports on Progress in Physics (1994), 57

According to Einstein's general theory of relativity, a gravitational field bends electromagnetic waves in much the same way as low atmospheric air layers curve the trajectory of a propagating light ray ... [more ▼]

According to Einstein's general theory of relativity, a gravitational field bends electromagnetic waves in much the same way as low atmospheric air layers curve the trajectory of a propagating light ray. Large mass concentrations in the universe can thereby act as a type of lens, a gravitational lens. After briefly reviewing the history of gravitational lensing since the early thoughts of Newton in 1704 until the serendipitous discovery of the first gravitational lens system in 1979, the authors recall the basic principles of atmospheric and gravitational lensing. They then describe a simple optical gravitational lens experiment which has the merit of accounting for all types of image configuration observed among currently known gravitational lens systems. Various types of gravitational lens models are described in detail as well as the resulting image properties of a distant source. An updated list as well as colour illustrations of the best known examples of multiply imaged quasars, radio rings and giant luminous arcs and arclets are presented. Some of these observations are discussed in detail. Finally, the authors show how it is possible to use gravitational lensing as a cosmological and astrophysical tool, the most interesting applications being the determination of the Hubble parameter H[SUB]0[/SUB], the mass of very distant lensing galaxies as well as the distribution of luminous and dark matter in the universe. They also show how to determine the size and structure of distant quasars from observations of micro-lensing effects. [less ▲]

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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)

Not Available

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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 detail'Gravitational lenses in the Universe', The proceedings of the 31st Liège International Astrophysical Colloquium
Surdej, Jean ULg; Fraipont-Caro, D.; Gosset, Eric ULg et al

Book published by Université de Liège (1993)

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See detailGravitational lenses: observations
Surdej, Jean ULg

in invited talk delivered during the first general meeting of the European Astronomical Society "The impact of space research on astronomy" (1992)

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See detailGravitational lenses: observations, invited talk
Surdej, Jean ULg

in Proceedings of the First general meeting of the European Astronomical Society "The impact of space research on astronomy" (1992)

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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)

Not Available

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See detailGravitational lensing as a tool: future observational prospects
Surdej, Jean ULg

Conference (1993)

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See detailGravitational Lensing as a Tool: Future Observational Prospects
Surdej, Jean ULg; Refsdal, S.

in Wamsteker, W.; Longair, Malcol S.; Kondo, Y. (Eds.) Frontiers of Space and Ground-Based Astronomy (1994)

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See detailGravitational Lensing by a Wine Glass
Surdej, Jean ULg

Article for general public (1999)

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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 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 detailGravitational lensing in quasar samples
Claeskens, Jean-François ULg; Surdej, Jean ULg

in Astronomy and Astrophysics Review (2002), 10(4), 263-311

The first cosmic mirage was discovered approximately 20 years ago as the double optical counterpart of a radio source. This phenomenon had been predicted some 70 years earlier as a consequence of General ... [more ▼]

The first cosmic mirage was discovered approximately 20 years ago as the double optical counterpart of a radio source. This phenomenon had been predicted some 70 years earlier as a consequence of General Relativity. We present here a summary of what we have learnt since. The applications are so numerous that we had to concentrate on a few selected aspects of this new field of research. This review is focused on strong gravitational lensing, i.e. the formation of multiple images, in QSO samples. It is intended to give the reader an up-to-date status of the observations and to present an overview of its most interesting potential applications in cosmology and astrophysics, as well as numerous important results achieved so far. The first section follows an intuitive approach to the basics of gravitational lensing and is developed in view of our interest in multiply imaged quasars. The astrophysical and cosmological applications of gravitational lensing are outlined in Sect. 2 and the most important results are presented in Sect. 5. Sections 3 and 4 are devoted to the observations. Finally, conclusions are summarized in the last section. We have tried to avoid duplication with existing (and excellent) introductions to the field of oravitational lensing. For this reason, we did not concentrate on the individual properties of specific lens models, as these are already well presented in Narayan and Bartelmann (1996) and on a more intuitive ground in Refsdal and Surdej (1994). Wambsganss (1998) proposes a broad view on gravitational lensing, in astronomy; the reviews by Fort and Mellier (1994) and Hattori et al. (1999) deal with lensing by galaxy clusters, microlensing in the Galaxy and the local group is reviewed by Paczynski (1996) and a general panorama on weak lensing is given by Bartelmann and Schneider (1999) and Mellier (1999). The monograph on the theory of gravitational lensing by Schneider, Ehlers and Falco (1992) also remains a reference in the field. [less ▲]

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