Search for Black Holes in the Galactic Halo by Gravitational Microlensing

Abstract: Black hole-like objects with mass greater than $10 M_{\odot}$, as discovered by gravitational antennas, can produce long time-scale (several years) gravitational microlensing effects. Considered separately, previous microlensing surveys were insensitive to such events because of their limited duration of 6-7 years. We combined light curves from the EROS-2 and MACHO surveys to the Large Magellanic Cloud (LMC) to create a joint database for 14.1 million stars, covering a total duration of 10.6 years, with fluxes measured through 4 wide passbands. We searched for multi-year microlensing events in this catalog of extended light curves, complemented by 24.1 million light curves observed by only one of the surveys. Our analysis, combined with previous analysis from EROS, shows that compact objects with mass between $10^{-7}$ and $200 M_{\odot}$ can not constitute more than $\sim 20\%$ of the total mass of a standard halo (at $95\%$ CL). We also exclude that $\sim 50\%$ of the halo is made of Black Holes (BH) lighter than $1000 M_{\odot}$.