MemoryStone

Now you don’t see it, now you do. Something in Bootes truly in the middle of nowhere — apparently not even in a galaxy — brightened by at least 120 times during more than three months and then faded away. Its spectrum was like nothing ever seen, write the discoverers, with “five broad absorption bands between 4100 and 6500 Angstroms and a mostly featureless continuum longward of 6500 Angstroms.” Even the cause of the spectral features is unknown.
K. Barbary and others (via SkyandTelescope.com - Hubble Finds a Mystery Object)