Kicked Neutron Stars and Microlensing

Due to the large kick velocities with which neutron stars are born in supernovae explosions, their spatial distribution is more extended than that of their progenitor stars. The large scale height of the neutron stars above the disk plane makes them potential candidates for microlensing of stars in the Large Magellanic Cloud. Adopting for the distribution of kicks the measured velocities of young pulsars, we obtain a microlensing optical depth of τ ∼ 2N10 × 10 (where N10 is the total number of neutron stars born in the disk in units of 10 ). The event duration distribution has the interesting property of being peaked at T ∼ 60 – 80 d, but for the rates to be relevant for the present microlensing searches would require N10 ∼> 1, a value larger than the usually adopted ones (N10 ∼ 0.1 – 0.2). The ongoing searches of baryonic dark matter in the Galaxy by means of microlensing of stars in the Large Magellanic Cloud (LMC) have produced very surprising results recently. Indeed, the first five events obtained by the EROS (Aubourg et al. 1993) and MACHO (Alcock et al. 1995) collaborations, with durations T ∼ 10 – 29 d, suggested the presence of compact objects with masses ∼ 0.1M⊙, i.e. near the brown dwarf range. However, the new analysis of the first two years of MACHO observations (Alcock et al. 1996) has now returned events with longer durations (besides eliminating their two shortest duration events). These new results are indicative of larger lens masses, in the range 0.1 – 1M⊙, being responsible for the events. In addition, the large event rates observed require a considerable amount of compact objects to be present in the Galaxy. A first difficulty associated with the new range of inferred masses is that the lensing population can no longer be completely ascribed to the unseen brown dwarf counterpart of one of the Galactic stellar components, such as the thick disk (Gould, Miralda–Escudé and Bahcall 1994) or the spheroid (Giudice, Mollerach and Roulet 1994). Although a population of white dwarf remnants in these components could help to fit the observed