Limits on dark matter compact objects implied by supermagnified stars in lensing clusters

ABSTRACT Supermagnified stars are gravitationally lensed individual stars that are located close to a caustic of a lensing galaxy cluster, and have their flux magnified by a large enough factor (typically ${\sim} 1000$) to make them detectable with present telescopes. The maximum magnification is limited by microlensing caused by intracluster stars or other compact objects, which create a network of corrugated critical lines with an angular width proportional to the surface density of microlenses. We consider a set of nine cases of supermagnified stars reported in the literature, and derive an upper limit on the surface density of compact objects, such as primordial black holes, that might be present as a fraction of the dark matter in addition to known intracluster stars. Any such additional compact objects would widen the corrugated critical line network and therefore the width of the distribution of supermagnified stars around the modelled critical lines of the lens. We find that any compact objects, including primordial black holes, with masses above ${\sim} 10^{-6} \, {\rm M}_\odot$ (below which the microcaustics are closer together than the typical angular size of supermagnified stars) cannot account for more than ${\sim} 2$ per cent of the dark matter.