Microlensing constraints on MACHO masses from the double quasar Q0957+561

Abstract

Gravitational lensing can be used to determine whether the dark matter in galactic halos is in the form of objects of stellar or sub-stellar mass. Searches for these MACHOs (MAssive Compact Halo Objects) have become popular in our own Milky Way from studies of the rare events of a MACHO magnifying a star in the Large Magellanic Cloud. Here we show that we can also address this question by investigating well sampled light curves of multiply imaged quasars. The light bundles pass through (halos of) foreground galaxies and are subject to microlensing by MACHOs there. The recently obtained well-sampled light curves for the two images of the double quasar Q0957+561A and B are very similar to each other, except for an overall time delay and scaling factor. This allows us to put limits on the amount of microlensing that took place in this system. By comparison with intensive numerical simulations we can exclude a halo consisting of objects in the mass range 10 −7 to 10 −3 M ⊙ for a quasar size of 10 14 cm. For quasar sizes as large as 3 × 10 15 cm we can exclude only the mass interval 10 −4 to 10 −3 M ⊙. A halo consisting of objects with 10 −2 or 10 −1 M ⊙ cannot be ruled out yet, but it should produce MACHO induced fluctuations in future observations.