The relative lens-source proper motion in MACHO 98-SMC-1

We present photometric and spectroscopic data for the second microlensing event seen toward the Small Magellanic Cloud (SMC), MACHO 98-SMC-1. The lens is a binary. We resolve the caustic crossing and find that the source took 2 δt = 8.5 hr to transit the caustic. We measure the source temperature T_eff = 8000 K both spectroscopically and from the color, (V - I)₀ ∼0.22. We find two acceptable binarylens models. In the first, the source crosses the caustic at φ = 43°.2 and the unmagnified source magnitude is I_s = 22.15. The angle implies that the lens crosses the source radius in time t_* = δt sin φ = 2.92 hr. The magnitude (together with the temperature) implies that the angular radius of the source is θ_* = 0.089 μas. Hence, the proper motion is μ = θ_*/t_* = 1.26 km s^-1 kpc^-1. For the second solution, the corresponding parameters are φ = 30°.6, I_s = 21.81, t_* = 2.15 hr, θ_* = 0.104 μas, and μ = θ_*/t_* = 2.00 km s^-1 kpc^-1. Both proper-motion estimates are slower than 99.5% of the proper motions expected for halo lenses. Both are consistent with an ordinary binary lens moving at ∼75-120 km s^-1 within the SMC itself. We conclude that the lens is most likely in the SMC proper.