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astro-ph/9512103 15 Dec 1995
Monitoring of the Einstein Cross with the Nordic Optical Telescope
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. We report results from five years of monitoring of the Einstein Cross (QSO2237+0305) with the Nordic Optical Telescope. The photometry, mainly in the R and I bands, has been performed by a PSF fitting and 'cleaning' procedure, in which the four image components as well as the host galaxy and its nucleus are iteratively removed. The resulting lightcurves exhibit several microlensing features; one event may have a timescale as short as 14 days. Variations on timescales of several years are found in all four images. This becomes even more convincing when our data are combined with data published for 1986-89. No clear high amplification event was observed during the period. A brightening of all four components during 1994 is interpreted as intrinsic variation. Key words: gravitational lenses -- image processing -- PSF photometry -- QSO 2237+0305 1. Introduction After some preliminary observations in 1990, a program was started in 1991 at the Nordic Optical Telescope (NOT) on the island ...
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Lucky imaging is a proven technique for near diffraction limited imaging in the visible; however, data reduction and analysis techniques are relatively unexplored in the literature. In this paper we use both simulated and real data to test and calibrate improved guide star registration methods and noise reduction techniques. In doing so we have produced a set of "best practice" recommendations. We show a predicted relative increase in Strehl ratio of ~ 50% compared to previous methods when using faint guide stars of ~17th magnitude in I band, and demonstrate an increase of 33% in a real data test case. We also demonstrate excellent signal to noise in real data at flux rates less than 0.01 photons per pixel per frame above the background flux level.
The time delays between brightness variations in different optical bands have been measured for a large number of Seyfert galaxies and low-redshift quasars. These time delays represent the travel time of light between different emission regions of the source and can be used to test variability scenarios. Based on multiwavelength observations of the lensed quasars: Q2237+0305, SBS1520+530, HE2149-2745, HE1104-1805 and UM673, we explore a variability mechanism in the redshift range 1.4 z 2.7 . From cross-correlation analysis of the light curves of the quasars we find that the brightness variations at longer wavelengths may follow the brightness variations at shorter wavelengths by a few days. An independent analysis of the multiband light curves of nonlensed quasars from the same redshift interval confirms the evidence of a time delay. A comparison of the observed time delays with the delays expected from a model of an accretion disc irradiated by the central variable source shows that reprocessing might be the primary mechanism responsible for the observed variability on the considered timescales.
The innermostregion of quasars can be resolved by microlensing on scales down to a few AU. For this purpose, X-rays originating from the innermost regions can be selectively amplified by microlensing resulting from the “caustic crossing” events, and observations at X-ray bands are most preferable. If the observation has been done, lower limits of black hole mass will be determined, because information about gravitational potential of the black hole which is surrounded by accretion disk will be obtained.
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