| Summary: | We report on g-, r-, and i-band observations of the Interstellar Object 1I/'Oumuamua (1I) taken on 2017 October 29 from 04:28 to 08:40 UTC by the Apache Point Observatory (APO) 3.5 m telescope's ARCTIC camera. We find that 1I's colors are and g - r = 0.41 ± 0.24 and r - i = 0.23 ± 0.25, consistent with visible spectra and most comparable to the population of solar system C/D asteroids, Trojans, or comets. We find no evidence of any cometary activity at a heliocentric distance of 1.46 au, approximately 1.5 months after 1I's closest approach distance to the Sun. Significant brightness variability was seen in the r observations, with the object becoming notably brighter toward the end of the run. By combining our APO photometric time series data with the Discovery Channel Telescope data of Knight et al., taken 20 hr later on 2017 October 30, we construct an almost complete lightcurve with a most probable single-peaked lightcurve period of P ≃ 4 hr. Our results imply a double-peaked rotation period of 8.1 ± 0.02 hr, with a peak-to-trough amplitude of 1.5-2.1 mag. Assuming that 1I's shape can be approximated by an ellipsoid, the amplitude constraint implies that 1I has an axial ratio of 3.5-10.3, which is strikingly elongated. Assuming that 1I is rotating above its critical break up limit, our results are compatible with 1I having modest cohesive strength and may have obtained its elongated shape during a tidal distortion event before being ejected from its home system.
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