eRASSt J234402.9-352640 optical photometry

The object brightened by a factor of at least 150 in 0.2-2.0keV flux compared to an upper limit found six months previous, reaching an observed peak of 1.76^+0.03^_-0.24_x10^-11^erg/cm^2^/s. The X-ray ignition is associated with a galaxy at z=0.10, making the peak luminosity log10(L0.2-2keV/[erg/s])=44.7+/-0.1.

Around the time of the rise in X-ray flux, the nucleus of the galaxy brightened by approximately 3mag. in optical photometry, after correcting for the host contribution. We present X-ray follow-up data from Swift, XMM-Newton, and NICER, which reveal a very soft spectrum as well as strong 0.2-2.0keV flux variability on multiple timescales. Optical spectra taken in the weeks after the ignition event show a blue continuum with broad, asymmetric Balmer emission lines, and high-ionisation ([OIII]4959,5007) and low-ionisation ([NII]6585, [SII]6716,6731) narrow emission lines.