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 towards the Galactic bulge, an FFP in the Galactic halo produces on average longer microlensing events with smaller projected source radii towards 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 per cent and 3-6 per cent through ground-based optical surveys and space-based near-infrared surveys, respectively. We then promote these simulations and assume the Roman telescope continuously observes each MC during one 72-d 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 ~0.01-10⁴M_⊕, Roman discovers their lensing effects with the efficiencies $\sim 10\!-\!80{{\ \rm per\ cent}}$, respectively. By adopting $5{{\ \rm per\ cent}}$ as haloes fraction from FFPs, we estimate the expected number of events. The highest number of detectable FFPs which is ~1700-2200 per season per square degree happens for ones with masses ~0.01M_⊕. Our simulations show that Roman potentially extends the mass range of detectable FFPs in haloes down to 5.9 × 10^-7M_⊕ with continuous observations during one observing season from the Large Magellanic Cloud.