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Weak gravitational lensing by large-scale structure affects the determination of the cosmological deceleration parameter q0. We find that the lensing induced dispersions on truly standard candles are 0.04 and 0.02 mag at redshift z = 1 and z = 0.5, respectively, in a COBE-normalized cold dark matter universe with Omega 0 = 0.40, Lamda 0 = 0.6, H = 65 km s-1 Mpc-1, and sigma 8 = 0.79. It is shown that one would observe q0 = -0.395^{+0.125}_{-0.095} and q0 = - 0.398^{+0.048}_{-0.077} (the error bars are 2 sigma limits) with standard candles with zero intrinsic dispersion at redshift z = 1 and z = 0.5, respectively, compared to the truth of q0 = -0.400. A standard COBE normalized Omega 0 = 1 CDM model would produce three times as much variance and a mixed (hot and cold) dark matter model would lead to an intermediate result. One unique signature of this dispersion effect is its non-Gaussianity. Although the lensing induced dispersion at lower redshift is still significantly smaller than the currently best observed (total) dispersion of 0.12 mag in a sample of type Ia supernovae, selected with the multicolor light curve shape method, it becomes significant at higher redshift. We show that there is an optimal redshift, in the range z ~ 0.5--2.0 depending on the amplitude of the intrinsic dispersion of the standard candles, at which q0 can be most accurately determined.
The gravitationally lensed quasar Q2237+0305 in X-rays: ROSAT/HRI detection of the "Einstein Cross"
(1999)
We report the first detection of the gravitationally lensed quasar Q2237+0305 in X-rays. With a ROSAT/HRI exposure of 53 ksec taken in Nov./Dec. 1997, we found a count rate of 0.006 counts per second for the combined four images. This corresponds to an X-ray flux of 2.2*E(-13) erg/cm(2) /sec and an X-ray luminosity of 4.2*E(45) erg/sec (in the ROSAT energy window 0.1-2.4 keV). The ROSAT/HRI detector is not able to resolve spatially the four quasar images (maximum separation 1.8 arcsec). The analysis is based on about 330 source photons. The signal is consistent with no variability, but with low significance. This detection is promising in view of the upcoming X-ray missions with higher spatial/spectral resolution and/or collecting power (Chandra X-ray Observatory, XMM and ASTRO-E).
Microlensing results from APO monitoring of the double quasar Q0957+561A,B between 1995 and 1998
(2000)
If the halo of the lensing galaxy 0957+561 is made of massive compact objects (MACHOs), they must affect the lightcurves of the quasar images Q0957+561 A and B differently. We search for this microlensing effect in the double quasar by comparing monitoring data for the two images A and B - obtained with the 3.5m Apache Point Observatory from 1995 to 1998 - with intensive numerical simulations. This way we test whether the halo of the lensing galaxy can be made of MACHOs of various masses. We can exclude a halo entirely made out of MACHOs with masses between 10-6 Msun and 10-2 Msun for quasar sizes of less than 3x 1014 h60-1/2 cm, hereby extending previous limits upwards by one order of magnitude.
Planeten um andere Sterne
(2000)
In the roughly 20 years of its existence as an observational science, gravitational lensing has established itself as a valuable tool in many astrophysical fields. In the introduction of this review we briefly present the basics of lensing. Then it is shown that the two propagation effects, lensing and scintillation, have a number of properties in common. In the main part various lensing phenomena are discussed with emphasis on recent observations.
The declining light curve of the optical afterglow of gamma-ray burst (GRB) GRB000301C showed rapid variability with one particularly bright feature at about t-t0=3.8d. This event was interpreted as gravitational microlensing by Garnavich, Loeb & Stanek and subsequently used to derive constraints on the structure of the GRB optical afterglow. In this paper, we use these structural parameters to calculate the probability of such a microlensing event in a realistic scenario, where all compact objects in the universe are associated with observable galaxies. For GRB000301C at a redshift of z=2.04, the a posteriori probability for a microlensing event with an amplitude of m>=0.95mag (as observed) is 0.7 per cent (2.7 per cent) for the most plausible scenario of a flat -dominated Friedmann- Robertson-Walker (FRW) universe with m=0.3 and a fraction f*=0.2 (1.0) of dark matter in the form of compact objects. If we lower the magnification threshold to m>=0.10mag, the probabilities for microlensing events of GRB afterglows increase to 17 per cent (57 per cent). We emphasize that this low probability for a microlensing signature of almost 1mag does not exclude that the observed event in the afterglow light curve of GRB000301C was caused by microlensing, especially in light of the fact that a galaxy was found within 2arcsec from the GRB. In that case, however, a more robust upper limit on the a posteriori probability of ~5 per cent is found. It does show, however, that it will not be easy to create a large sample of strong GRB afterglow microlensing events for statistical studies of their physical conditions on microarcsec scales.
Microlensing of Quasars
(2001)
Microlens-induced variability in multiple quasars can be used to study two cosmological issues of great interest, the size and brightness profile of quasars on one hand, and the distribution of compact (dark) matter along the line of sight on the other. Here a summary is given of recent theoretical progress as well as observational evidence for quasar microlensing, plus a discussion of desired observations and required theoretical studies.
We present CHANDRA observations of the X-ray luminous, distant galaxy cluster RBS797 at z=0.35. In the central region the X-ray emission shows two pronounced X-ray minima, which are located opposite to each other with respect to the cluster centre. These depressions suggest an interaction between the central radio galaxy and the intra-cluster medium, which would be the first detection in such a distant cluster. The minima are symmetric relative to the cluster centre and very deep compared to similar features found in a few other nearby clusters. A spectral and morphological analysis of the overall cluster emission shows that RBS797 is a hot cluster (T=7.7+1.2-1.0 keV) with a total mass of Mtot(r500)= 6.5+1.6-1.2 *E14Msun.
Quasar Microlensing
(2001)
Cosmological microlensing
(2001)