We report the discovery of large-amplitude X-ray variability from the direction of the previously unknown , optically inactive galaxy pair RX J1242.6-1119 .
The X-ray source shows variability by a factor \mathrel { \hbox { \hbox to 0.0 pt { \lower 2.365 pt \hbox { $ \sim$ } } \kern - 3.0 pt \raise 1.72 pt \hbox { $ > $ } } } 20 between the ROSAT all-sky survey and a later pointed observation separated by \sim 1.5 yr. Its spectrum is extremely soft with photon index \Gamma _ { x } \simeq - 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 } \mathrel { \hbox { \hbox to 0.0 pt { \lower 2.365 pt \hbox { $ \sim$ } } \kern - 3.0 pt \raise 1.72 pt \hbox { $ > $ } } } 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 RX J1242-11 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 .