On the detection of free-floating planets through microlensing towards the Magellanic Clouds

In this work, we study detecting free-floating planets (FFPs) by microlensing observations towards the Magellanic Clouds (MCs). In comparison to similar events toward the Galactic bulge, an FFP in the Galactic halo produces on average longer microlensing events with smaller projected source radii toward these clouds. For these microlensing events, the relative lens-source velocities are on average smaller. The MC self-lensing events due to FFPs suffer from severe finite-source effects. We first simulate microlensing events due to FFPs towards MCs and assume a log-uniform step function for their mass. The efficiencies for capturing their lensing signatures (with signal-to-noise greater than $50$) are found to be $0.1$-$0.6\%$ and $3$-$6\%$ through ground-based optical surveys and space-based near-infrared surveys, respectively. We then promote these simulations and assume the \wfirst~telescope continuously observes each MC during one $72$-day season with the $15$min observing cadence. From simulated microlensing events with the resolvable source stars at the baseline due to FFPs with the masses $\sim 0.01$-$10^{4}M_{\oplus}$, \wfirst~discovers their lensing effects with the efficiencies $\sim 10$-$80\%$, respectively. By adopting $5\%$ as halos fraction from FFPs we estimate the expected number of events. The highest number of detectable FFPs which is $\sim1700$-$2200$ per season per square degree happens for ones with masses $\sim 0.01 M_{\oplus}$. Our simulations show that \wfirst~potentially extends the mass range of detectable FFPs in halos down to $5.9\times 10^{-7} M_{\oplus}$ with continuous observations during one observing season from the Large Magellanic Cloud.