Sky and Telescope reports the Hubble Space Telescope spotted a mysterious object that brightened by a factor of at least 120 during more than three months and then faded away. Scientists say this object behaved in a way they've never seen before and aren't even sure where this object was located - they only know that it can't be closer than 130 light years but can't be further than 11 billion light years.
What was it?
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
The project used the Hubble Space Telescope to monitor very distant galaxy clusters for supernovae. On February 21, 2006, in the direction of a far-away cluster in Bootes named CL 1432.5+3332.8 (redshift 1.112, light travel time 8.2 billion years), Hubble began seeing something brighten. It continued brightening for about 100 days and peaked at 21st magnitude in two near-infrared colors. It then faded away over a similar timescale, until nothing was left in view down to 26th magnitude. The object brightened and faded by a factor of at least 120, maybe more.
The mystery object did not behave like any known kind of supernova. It is not even in any detectable galaxy. "The shape of the light curve is inconsistent with microlensing," say the researchers. They recorded three spectra of it — and its spectrum, they write, "in addition to being inconsistent with all known supernova types, is not matched to any spectrum in the Sloan Digital Sky Survey database" of vast numbers of objects. "We suggest that the transient may be one of a new class."