Supermassive black holes have a complicated life cycle. Sometimes they are “on” and blow up enormous amounts of energy, and sometimes they are “off” where they sleep like dragons in their caves. By comparing the ratio of high-energy to low-energy waves emitted by quasars, astronomers begin to determine how many black holes are sleeping and when they are likely to wake up again.
This is how it works. As far as we can tell, every galaxy has a supermassive black hole in its center. When material falls on this black hole, it compresses and heats up (because the black hole’s extreme gravity tries to pull a fair amount of material into a relatively small volume). All of this friction drives the release of tons of high-energy radiation, what astronomers refer to as a quasar.
Along with the hard stuff comes radio waves, and we can use radio telescopes like LOFAR (the LOw Frequency ARray) and the WSRT (Westerbork Synthesis Radio Telescope) to detect them.
But the intense radiation blasts material away from the black hole and it can no longer feed itself, so the black hole goes to sleep and the quasar switches off along with the radio emission.
Astronomers try to understand the entire life cycle, which can take hundreds of millions of years to complete. You want to know how often quasars light up, how long they burn, and when they are likely to turn on again.
And with a combination of high frequency and low frequency radio waves, you have a new tool.
“High-frequency radio waves lose their energy quickly … while those with a lower frequency do this much more slowly”, says Prof. Dr. Raffaella Morganti, first author of the paper The best of both worlds: Combination of LOFAR and Apertif to solve radio spectral index images.
By using different radio telescopes to observe different frequencies of radio waves and using the combined data to measure the ratio of high-frequency to low-frequency waves, astronomers can see how recently a quasar was shut down: the less high-frequency material, the more time has passed since the last feeding event .
From there, astronomers can create an overview of active quasars, silent quasars, and everyone else in between.
The technique, while powerful, requires new radio surveys to observe as many galaxies as possible and to produce a proper census of the black holes in our universe.
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