Physical characterization of the active asteroid (6478) Gault

Abstract

We report dense lightcurve photometry, $BVR_{c}$ colors and phase - mag curve of (6478) Gault, an active asteroid with sporadic comet-like ejection of dust. We collected optical observations along the 2020 Jul-Nov months during which the asteroid appear always star-like, without any form of perceptible activity. We found complex lightcurves, with low amplitude around opposition and a bit higher amplitude far opposition, with a mean best rotation period of $2.46_{\pm 0.02}$ h. Shape changes were observed in the phased lightcurves after opposition, a probable indication of concavities and surface irregularities. We suspect the existence of an Amplitude-Phase Relationship in $C$ band. The mean colors are $B-V = +0.84_{\pm 0.04}$, $V-R_{c} = +0.43_{\pm 0.03}$ and $B-R_{c} = +1.27_{\pm 0.02}$, 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 $p_V=0.13_{\pm 0.04}$, 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 to estimate a diameter of about 3-4 km, so Gault may be smaller than previously thought.