The visual lightcurve of comet C/1995 O1 (Hale–Bopp) from 1995 to 1999

The great comet C/1995 O1 (Hale–Bopp) presented a remarkable opportunity to study its long-term brightness over four years. We used 2240 observations published in the International Comet Quarterly from 17 observers during 1995 July to 1999 September to create a secular lightcurve. In order to account for observer differences, we present a novel algorithm to reduce scatter and increase precision in a lightcurve compiled from many sources. It is implemented in a publicly available code, ICQSPLITTER, which uses a self-consistent statistical approach. To first order, the comet’s lightcurve approximates an r response for both pre- and postperihelion distances. The preperihelion data are better fit with a fifth-order polynomial with inflection points at 4.0, 2.6, 2.1, and 1.1 au, some of which are associated with physical changes in the coma. Outbursts may have occurred a few days before perihelion and at ∼2.2 and 7.4 au postperihelion. The Afρ values derived from the final magnitudes are consistent with an r dependence on heliocentric distance and are within a factor of 2–4 of those derived from spectroscopy and narrowband photometry. We present correlation equations for visual magnitudes and CO and HO production rates that are consistent with the preperihelion brightness increasing due to CO outgassing until about 2.6–3.0 au from the Sun and then are strongly correlated with HO production rates. We also present two generalized correlation equations that may be useful for observation planning and data analysis with the James Webb Space Telescope and other observatories.