Detailed Photometric Characterization of 'Oumuamua with Gemini North

'Oumuamua is the long-awaited first bridge between extrasolar planetary systems and our own Solar System. The body was discovered with the Pan-STARRS telescope on UT 19 October 2017 and became intensively observed immediately after. Our team was awarded 12 hr of observation time on the Gemini North telescope in Hawaii - the longest run ever allocated to observations of 'Oumuamua on a telescope of this class. On UT 27 and 28 October 2017, we obtained over 400 images suitable for accurate time-resolved photometry, having an effective integration time of 3.58 hr and spanning a total of 8.06 hr (Drahus et al. 2018, NatAs 2, 407). A combined ultra-deep image of 'Oumuamua shows no signs of cometary activity, providing the most compelling evidence that the object is morphologically an asteroid — thus adding to the mystery of the remarkably strong non-gravitational acceleration detected in the object's orbital motion (Micheli et al. 2018, Nature 559, 223). An accurate light curve reveals an enormous range of brightness variation with a full range reaching 2.6±0.2 mag, suggesting a highly elongated shape of the body with the long-to-short axis ratio of >4.9. We also determined the effective rotation period to be 7.56±0.1 hr, estimated the equivalent size to be about 150 m, and we found that - contrary to previous reports by other teams - the requirement of rotational stability allows for the body to be strengthless and have a bulk density of typical Solar System asteroids. Most significantly, our light curve does not repeat exactly from one night to another, implying that 'Oumuamua is in a non-principal-axis rotation state. 'Oumuamua's tumbling is consistent with an ancient collision in the body's home planetary system, but might have also been caused by the mysterious non-gravitational forces during 'Oumuamua's passage through the Solar System.