Physical Characterization Of The Active Asteroid (6478) Gault

We report dense light-curve photometry, BVRc colours and phase-mag curve of (6478) Gault, an active asteroid with sporadic comet-like ejection of dust. We collected optical observations along the 2020 July-November months during which the asteroid appear always star-like, without any form of perceptible activity. We found complex light curves, with low amplitude around opposition and a bit higher amplitude far opposition, with a mean best rotation period of 2.46(+/- 0.02) h. Shape changes were observed in the phased light curves after opposition, a probable indication of concavities and surface irregularities. We suspect the existence of an Amplitude-Phase Relationship in C band. The mean colours are B - V = +0.84(+/- 0.04), V - R-c = +0.43(+/- 0.03), and B - R-c = +1.27(+/- 0.02), compatible with an S-type asteroid, but variables with the rotational phase index of a non-homogeneous surface composition. From our phase-mag curve and Shevchenko's empirical photometric system, the geometric albedo result p(V) = 0.13(+/- 0.04), lower than the average value of the S-class. We estimate an absolute mag in V band of about + 14.9 and this, together with the albedo value, allows us to estimate a diameter of about 3-4 km, so Gault may be smaller than previously thought.