The nature of the dark matter is still one of the most pressing questions in current astrophysics. Many experiments are running or being built/planned in order to test the existence of various particle physics candidates, like wimps, axions, or neutralinos (for a review on the particle physics aspects of dark matter, see Raffelt 1997). There are also intense searches underway for astrophysical dark matter candidates, like black holes (from stellar to galactic mass scales), brown dwarfs, or other compact objects.
Gott (1981) and Paczynski (1986) put forward the idea to use gravitational lensing for the search of compact dark matter objects in galactic halos. Gott (1981) suggested to look for fluctuations in the lightcurves of multiply imaged quasars in order to detect compact objects along the line of sight in the halo of the lensing galaxy, which had been shown earlier by Chang and Refsdal (1979) to have observable consequences. Paczynski (1986) proposed to probe the content of compact dark objects in the Milky Way halo by monitoring the brightnesses of about 107 individual stars in the Large Magellanic Cloud (LMC). The latter paper led to a whole industry of observing projects, like MACHO (Alcock et al. 2000) and EROS (Lasserre et al. 2000). Gott (1981)'s suggestion to study the halos of lensing galaxies by way of monitoring background quasars - though in principle as powerful a method - was pursued, however, with much less fervor in terms of people involved, total observing nights or CPU-time used.
We followed the latter method and present here results on the possible matter contents of the halo of the lensing galaxy in the lens system 0957+561. They are based on comparing lightcurves of the gravitationally lensed double quasar Q0957+561A,B, obtained by four years of monitoring (1995 - 1998) at the 3.5m-Apache Point Observatory (Colley, Kundic & Turner 2000, hereafter CKT) with extensive numerical simulations.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000