Cosmic twinkling: X-rays and active galactic nuclei

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  • Published: Jul 1, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Cosmic twinkling: X-rays and active galactic nuclei

Delayed illumination

The twinkling of heavenly bodies is well known to stargazers the world over and most are aware that atmospheric disturbances and refractive distortions are the underling of that twinkling. But, there are fluctuations in the brightness of galactic objects that are occur on a much grander and longer scale. Take active galactic nuclei, the brightest objects in the universe. They are not lit up permanently, but rather 'flicker' on a very prolonged timescale, particularly poignantly in the X-ray region of the electromagnetic spectrum. This insight is helping researchers in Switzerland and elsewhere to better understand these objects and the black holes that lie at their core.

Supermassive black holes attract matter with incredible force, attracting vast quantities of gas and other material from their surroundings as this material orbits the black hole it gets hotter and hotter through frictional processes until it begins to radiate energy. This is the heart of an active galactic nuclei (AGN). These cosmic entities shine brighter than the hundreds of billions of stars in the galaxy at the centre of which they lie. Take our own galaxy, the Milky Way, the supermassive black hole that lies at its heart is thought to have been an energetic AGN a mere few millions of years ago.

Now, Kevin Schawinski of the Institute for Astronomy at ETH Zurich, Switzerland and colleagues have revealed that AGN do not shine steadily. Rather they flicker like a cosmic candle flame. Writing in the latest Monthly Notices of the Royal Astronomical Society, the team reports how they have captured the X-ray radiation originating from the immediate vicinity of an AGN and also registered the visible light and on the basis of their experimental evidence suggest that AGNs switch "on and off" every couple of hundred thousand years.

Across the universe

The team points out that the visible light does not come from the AGN, but from the interstellar gas in the galaxy. The atoms of the interstellar gas are stimulated to light up in a very specific way by the radiation from the AGN. There is a delay in the glow, just as there is when one lights a gas lamp, before it achieves full brightness. In the case of the AGN the delay is due to the time required by the light to reach the edge of the galaxy and turn on the ‘galactic gas lamp’. Before this occurs, the AGN exists in what looks like a ‘switched-off’ state, although it is nevertheless emitting X-rays.

The team scoured data in extensive collection of observed AGNs detected using X-ray telescopes and found that one in twenty appear to be in this ‘switched-off’ state. Schawinski and colleagues suggest that if five percent of all observed AGN do not give off visible light, this implies that the apparent switched-off phase lasts approximately 5% of each galaxy's existence. The team knew from earlier theoretical work that the AGN switched-off state lasts approximately 10,000 years, which approximates to the length of time needed for light to cross the galaxy.

Just a phase

“This result is crucial in order to understand how an active galactic nucleus influences the surrounding galaxy,” explains Schawinski. Astrophysicists already knew that AGN accumulate gas over several billion years. However, no one knew whether they amassed enough gas over this period to light up. “Now we know that light emitted by an AGN resembles an energy-saving lamp that flickers on and off every 20 milliseconds,” Schawinski adds.

“The 200,000 years should be viewed as an approximation and it is a statistical average,” says Schawinski. Some galaxies may have an AGN phase that lasts much longer than that in another; but it will be several hundred thousand years for all galaxies, the team says. This limitation may shed light on how AGN intervene in the development of their host galaxy. It might be that radiation from the AGN heats up a collapsing gas cloud in which stars might be formed. The heating delays or even prevents the collapse of the gas cloud and thus the formation of stars. However, the AGN must be lit up long enough for this to occur. “Thanks to the estimate of the length of an AGN phase, we are one step closer to the answer to this question,” says Schawinski.

Related Links

Monthly Notices Royal Astronom Soc, 2015, 451, 2517-2523: "Active galactic nuclei flicker: an observational estimate of the duration of black hole growth phases of ~105 yr"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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