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 direction of the previously unknown, optically inactive galaxy pair RXJ1242.6-1119. 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 separated 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 galaxies. 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(43) erg s(-1). 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.