Clues emerge in mystery of flickering quasars
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 behaviour¹, ² 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 years³. 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.
1. Sheng, Z. et al. Preprint at https://arxiv.org/abs/1707.02686 (2017).
2. Hutsemékers, D. et al. Preprint at https://arxiv.org/abs/1707.05540 (2017).
3.THE DISCOVERY OF THE FIRST "CHANGING LOOK" QUASAR: NEW INSIGHTS INTO THE PHYSICS AND PHENOMENOLOGY OF ACTIVE GALACTIC NUCLEI
Stephanie M. LaMassa, Sabrina Cales et al
Published 2015 February 20
The Astrophysical Journal, Volume 800, Number 2