The microlensing interpretation of the optical afterglow of GRB 000301C seems naively surprising , since a simple estimate of the stellar microlensing rate gives less than one in four hundred for a flat \Omega _ { \Lambda } = 0.7 cosmology , whereas one event was seen in about thirty afterglows .
Considering baryonic MACHOs making up half of the baryons in the universe , the microlensing probability per burst can be roughly 5 \% for a GRB at redshift z = 2 .
We explore two effects that may enhance the probability of observing microlensed gamma-ray burst afterglows : binary lenses and double magnification bias .
We find that the consideration of binary lenses can increase the rate only at the \sim 15 \% level .
On the other hand , because gamma-ray bursts for which afterglow observations exist are typically selected based on fluxes at widely separated wavebands which are not necessarily well correlated ( e.g .
localization in X-ray , afterglow in optical/infrared ) , magnification bias can operate at an enhanced level compared to the usual single–bias case .
Using a simple model for the selection process in two bands , we compute the enhancement to microlensing rate due to magnification bias in two cases : perfect correlation and complete independence of the flux in the two bands .
We find that existing estimates of the slope of the luminosity function of gamma-ray bursts , while as yet quite uncertain , point to enhancement factors of more than three above the simple estimates of the microlensing rate .
We find that the probability to observe at least one microlensing event in the sample of 27 measured afterglows can be 3 - 4 \% for stellar lenses , or as much as 25 \Omega _ { lens } for baryonic MACHOs .
We note that the probability to observe at least one event over the available sample of afterglows is significant only if a large fraction of the baryons in the universe are condensed in stellar–mass objects .