http://www.nature.com/news/clues-emerge-in-mystery-of-flickering-quasars-1.22376
Some of the Universe's most luminous objects have disappeared much faster than expected
Some of the brightest objects in the Universe — quasars — are vanishing rapidly. Astronomers now think that they understand
this mysterious behaviour, and the answer could help them to explain how galaxies such as the Milky Way evolve.
Quasars are supermassive black holes at the centres of galaxies fed by huge quantities of gas that shine across the visible Universe.
Astronomers have long thought that quasars persist for millions of years before dimming slowly over tens of thousands of years. But
in 2014, Stephanie LaMassa, an astronomer now at the Space Telescope Science Institute in Baltimore, Maryland, discovered a quasar
that seemed to disappear in less than ten years. That’s a blink of an eye, astronomically speaking.
Researchers struggled to explain the oddity. Perhaps a massive dust cloud passed in front of the quasar’s bright beacon and momentarily
blocked its light. Or maybe a star passed too close to the black hole and was rapidly torn apart, causing a bright flare that scientists
mistook for a quasar. It seemed physically impossible that such a bright object could fade in such a short time.
The discovery set in motion the hunt for more of these ‘changing-look’ quasars. The search has identified dozens of these mysterious
beasts, some of which have dimmed more dramatically than the first. The two studies published this month on the preprint server arXiv
suggest that these quasars blaze out of existence because the amount of gas and dust flowing through their accretion disks - the swirl
of hot matter that encircles a black hole - drops dramatically. In effect, the black hole starves.