Title: APO Time Resolved Color Photometry of Highly-Elongated Interstellar Object 1I/'Oumuamua,
Abstract: We report on $g$, $r$ and $i$ band observations of the Interstellar Object
'Oumuamua (1I) taken on 2017 October 29 from 04:28 to 08:40 UTC by the Apache
Point Observatory (APO) 3.5m telescope's ARCTIC camera. We find that 1I's
colors are $g-r=0.41\pm0.24$ and $r-i=0.23\pm0.25$, consistent with the visible
spectra of Masiero (2017), Ye et al. (2017) and Fitzsimmons et al. (2017), 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 towards the end of the run. By
combining our APO photometric time series data with the Discovery Channel
Telescope (DCT) data of Knight et al. (2017), taken 20 h later on 2017 October
30, we construct an almost complete light curve with a most probable lightcurve
period of $P \simeq 4~{\rm h}$. Our results imply a double peaked rotation
period of 8.1 $\pm$ 0.02 h, with a peak-to-peak amplitude of 1.5 - 2.1 mags.
Assuming that 1I's shape can be approximated by an ellipsoid, the amplitude
constraint implies that 1I has an axial ratio of 3.5 to 10.3, which is
strikingly elongated. Assuming that 1I is rotating above its critical break up
limit, our results are compatible with 1I having having modest cohesive
strength and may have obtained its elongated shape during a tidal disruption
event before being ejected from its home system. Astrometry useful for
constraining 1I's orbit was also obtained and published in Weaver et al.
(2017).