Discovery of a giant and luminous X-ray outburst from the optically inactive galaxy pair RX J1242.6-1119

We report the discovery of large-amplitude X-ray variability from the direc tion of the previously unknown, optically inactive galaxy pair RXJ1242.6-11 19. The X-ray source shows variability by a factor greater than or equal to 20 between the ROSAT all-sky survey and a later pointed observation separa ted by similar to 1.5 yr. Its spectrum is extremely soft with photon index Gamma(x) similar or equal to -5, among the steepest ever observed among gal axies. Based on the redshift derived from the optical spectra, z=0.05, the source's intrinsic luminosity is large, L-x greater than or equal to 9 10(4 3) erg s(-1). Surprisingly, the optical spectra of both galaxies are charac terized by absorption lines and do not show signs of (Seyfert) activity. Th is 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; Re nzini et al. 1995) and NGC 5905 (in X-rays; Bade et al. 1996, Komossa & Bad e 1999). Several mechanisms to explain this unexpected and peculiar behavio r 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 disr uption event.