BVR broadband photometry of comets 1P/Halley and 4P/Faye

We analyzed the visible broadband photometric data of comets 1P/Halley and 4P/Faye, obtained during their perihelion passages of 1986 and 1991, respectively at the Sanglokh Observatory (Tajikistan) and the European Southern Observatory (Chile). Applying the Lomb-Scargle periodogram in the V-band magnitudes and B-V color index, we find that the most probable periodicities are 79 ± 6 and 7.36 ± 0.04 days for 1P, and 6.1 ± 0.3 days for 4P. After comparing results of color and magnitude periodograms, we argue there is a systematic difference in the number of signals identified and the level of confidence of the same periodicity in the periodograms. Our results suggest the quest for periodicities in the color of the coma of active comets should be complementary to ones in magnitudes. We have verified that the distribution of the color B-V of Faye’s coma was invariable during and after the possible occurrence of a post-perihelion outburst. We verify a symmetry in the pre- and post-perihelion H₀ photoelectric absolute magnitude of the comet Halley. The same issues were not observed in the B-V color index. We verify that the absolute magnitude H₀ of the comet Halley differs from each other when calculated from the visual or photoelectric magnitudes, due to the section of the coma used to estimate these magnitudes. We also verified that this difference in the photometric aperture can compromise comparisons of B-V color distributions between active comets.