  We report the discovery of large-amplitude X-ray variability from the
direction of the previously unknown, optically inactive galaxy pair
RXJ1242.6-1119. The X-ray source shows variability by a factor > 20 between the
ROSAT all-sky survey and a later pointed observation separated by ~1.5 yr. Its
spectrum is extremely soft with photon index Gamma_x ~ -5, among the steepest
ever observed among galaxies. Based on the redshift derived from the optical
spectra, z=0.05, the source's intrinsic luminosity is large, L_x > 9x10^{43}
erg/s. Surprisingly, the optical spectra of both galaxies are characterized by
absorption lines and do not show signs of (Seyfert) activity. This makes
RXJ1242-11 the third candidate for giant-amplitude variability in an otherwise
non-active galaxy, the first two being NGC 4552 (in the UV; Renzini et al.
1995) and NGC 5905 (in X-rays; Bade et al. 1996, Komossa & Bade 1999). Several
mechanisms to explain this unexpected and peculiar behavior are investigated.
The most likely one seems to be an accretion event onto an otherwise dormant
supermassive black hole (SMBH), e.g., by a tidal disruption event.
