A Revised Historical Light Curve of Eta Carinae and the Timing of Close Periastron Encounters

The historical light curve of the 19th century "Great Eruption" of etaCar provides a striking record of violent instabilies encountered by the most massive stars. We report and analyze newly uncovered historical estimates of the visual brightness of etaCar during its eruption, and we correct some mistakes in the original record. The revised light curve looks substantially different from previous accounts: it shows two brief eruptions in 1838 and 1843 that resemble modern supernova impostors, while the final brightening in December 1844 marks the time when etaCar reached its peak brightness. We consider the timing of brightening events as they pertain to the putative binary system in etaCar: (1) The brief 1838 and 1843 events peaked within weeks of periastron if the pre-1845 orbital period is shorter than at present due to the mass loss of the eruption. Each event lasted only 100 days. (2) The main brightening at the end of 1844 has no conceivable association with periastron, beginning more than 1.5yr afterward. It lasted 10yr, with no obvious influence of periastron encounters during that time. (3) The 1890 eruption began to brighten at periastron, but took over 1yr to reach maximum and remained there for almost 10yr. A second periastron passage midway through the 1890 eruption had no effect. While evidence for a link between periastron encounters and the two brief precursor events is compelling, the differences between the three cases above make it difficult to explain all three phenomena with the same mechanism.