Spectral evolution of the luminous blue variable NGC 2363-V1. I. Observations and qualitative analysis of the ongoing giant eruption

We describe the spectroscopic evolution of the extragalactic luminous blue variable NGC 2363-V1 from 1997 to 2004. Photometrically, this star has gradually faded by 0.3 mag in the V band over this period, while spectra obtained with the Hubble Space Telescope show dramatic changes: a gradual strengthening of the UV flux (a fourfold increase at 120 nm) is accompanied by a weakening of all Balmer lines (a factor of 3 at H alpha), the disappearance of the absorption component of the P Cygni profile at H alpha, the appearance and strengthening of He i lines in emission, and a shift of the main source of UV line-blanketing from Fe II to Fe IV. These long-term changes show that the star's effective temperature has been constantly rising between 1997 and 2004, while its mass-loss rate has significantly decreased. We also note a possible increase in the bolometric luminosity of V1 during this period.