Detailed phase function of comet 28P/Neujmin 1

In 2000, comet 28P/Neujmin 1 was an ideal candidate for a phase function study: during its April opposition it reached a phase angle of α = 0.8°, and previous observations at similar heliocentric distances ($r \geq 4$ AU) indicated that the comet was likely to be inactive. We observed this comet using ESO's NTT and 2.2m telescopes at La Silla, on 6 epochs from April to August 2000, covering α = 0.8-8°. These data were combined with a large set of data from 1985 to 1997. In order to disentangle the rotation effects from the phase effects, we obtained complete rotation coverage at opposition, providing us with a lightcurve template. We have obtained an improved rotation period of $12.75 \pm 0.03$ hrs, with a full amplitude of $\Delta m = 0.45 \pm 0.05$ mag, which yields an axis ratio lower limit of 1.51 ± 0.07. For each subsequent epochs, we have obtained enough rotation coverage to re-synchronize the lightcurve fragments with the template, and determine the magnitude change caused by the phase function. The average colours of the nucleus, $V-R = 0.454 \pm 0.050$, and $R-I = 0.413 \pm 0.050$, are similar with those of D-type asteroids and other comet nuclei. The phase function obtained for the nucleus has a linear slope of $0.025 \pm 0.006$ mag deg-1, less steep than that of the mean C-type asteroids