Outburst from a galaxy leaves scientists in a quandry
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By Hazel Muir
One week in 1997, a mouse of a galaxy between the shoulders of Hercules turned into a monster. "It was rather inconspicuous before," says Heinz Vslk, who watched the action from the island of La Palma. "But all of a sudden it became the strongest source we’ve ever seen."
This outburst from the galaxy Markarian 501 has left scientists in a quandary. The most energetic photons in the blast had trillions of times the energy of a visible photon, and according to the laws of physics as we understand them, they should never have made the vast journey from that galaxy to Earth. They should have been snuffed out by the sea of infrared radiation that fills space.
So what’s going on? Some physicists think there’s something weird happening inside Markarian 501: bunches of photons are ganging up into exotic globs called Bose-Einstein condensates. Others say there’ll be an everyday explanation once they’ve mulled over the facts and figures a little more. But some scientists think that Markarian 501 is telling us something momentous. It might, they say, go down in history as the key to a 21st-century revolution, a theory that at last marries quantum mechanics with Einstein’s theory of gravity. Then this galaxy would be a gateway to a hidden realm of nature where space and time are radically transformed.
Markarian 501 is no newcomer to astronomers. It appears on photos of the constellation Hercules dating back at least a century, and gets its name from Beniamin Markarian, a Georgian astronomer at the Byurakan Astrophysical Observatory in Armenia who started to compile a catalogue of hundreds of bluish galaxies in the 1960s.
Numbers 501 and 421 in his catalogue are special. At around 300 million light years away, they are the two closest examples to Earth of a rare and strange type of astronomical object known as a blazar. Like other kinds of active galaxy, such as quasars and radio galaxies, blazars are thought to be powered by a central black hole which feeds on the gas, dust and stars that whirl around it in a hot disc. Above and below the hole, two jets of energetic protons and electrons shoot millions of light years into space.
Blazars are capricious, flaring up and dimming again within just a few days. Astronomers think that this is because of their orientation. We see an active galaxy as a blazar if one of its jets is pointing towards us, as though we’re looking down the barrel of a gun. The jet sends out a narrow beam of radiation whose brightness can change rapidly as it shifts slightly or its supply of material from the black hole changes.
This special alignment also means we are assailed with ferociously energetic radiation. In 1992, the orbiting Compton Gamma Ray Observatory picked up high-energy gamma rays from Markarian 421. Astrophysicists believe they are cooked up in the jet by superfast particles. As electrons and protons spiral around the jet’s strong magnetic fields, they emit powerful radiation. They could also be colliding with ordinary photons, boosting them to ultra-high energies.
But it wasn’t until March 1997 that astronomers saw what a blazar can do when it really flexes its muscles. They watched astonished as Markarian 501 flared up from one of the puniest gamma-ray sources in the sky to upstage even the Crab Nebula, the debris of an exploded star in our own galactic backyard, which is the brightest steady gamma source in the sky. The outburst lasted several months, and at its peak Markarian 501 was ten times as bright as the Crab, despite being 50 000 times farther away. "The distance difference is just mind-blowing," says Vslk, a director at the Max Planck Institute for Nuclear Physics in Heidelberg.
Air shower
Vslk is a spokesman for an experiment called HEGRA (High Energy Gamma Ray Astronomy), which kept its eye on Markarian 501’s storm from La Palma, in the Canary Islands. When a high-energy gamma ray hits the upper atmosphere, it sparks an "air shower" — a spreading cascade of superfast subatomic particles. These emit light, and because they move faster than the speed of light in air their emissions pile up into blue flashes known as Cerenkov radiation — just as sound waves from supersonic aircraft pile up into a sonic boom.
During 501’s outburst, HEGRA’s six big mirrors saw astoundingly bright blue flashes. These indicated that some of 501’s gamma rays had energies of up to 22 teraelectronvolts (Astronomy and Astrophysics, vol 349, p 11). This is trillions of times as much as a photon of visible light, which has an energy between 1 and 3 electronvolts.
What is hard to explain is why the gamma rays made it to Earth. When a high-energy gamma ray and an infrared photon collide, they have enough energy to mutate into an electron and a positron. So the gamma rays should be gradually mopped up by the sea of far-infrared photons that fills space, emitted by forming stars and hot dust.
How far the gamma rays get depends on how many far-infrared photons are out there. In the past two years, several teams of scientists have taken old images from NASA’s Cosmic Background Explorer (COBE) satellite and the European Space Agency’s Infrared Space Observatory (ISO) and used some novel mathematical tricks to cancel out the infrared from our own Solar System and Galaxy. The results show that the far-infrared background is so bright that gamma rays with energies of more than 10 teraelectronvolts should never reach the Earth from as far away as Markarian 501. So why did we see them?
Perhaps the gamma rays are colluding against us, says Peter Biermann, an astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn. He and his colleagues suggest that several gamma rays from Markarian 501 might merge into a Bose-Einstein condensate — a densely packed globule of lower-energy photons that have exactly the same positions.
This should happen to light from a super-efficient laser — one far more efficient than any yet built on Earth. Nature does build lasers: in many active galaxies, X-rays make clouds of water vapour emit microwave laser light. The Universe is dotted with billions of these microwave lasers, or "masers". But is a natural, super-efficient, ultra-high energy laser plausible? Biermann claims that it could conceivably happen when a group of excited atoms in a blazar’s jet all stimulate each other to emit light at the same time (Astrophysical Journal Letters, vol 524, p 91).
Say the blazar fired out a Bose-Einstein condensate of 20 identical gamma rays with energies of 1 teraelectronvolt each. Because these photons have relatively low energy, they would be unimpeded by the far-infrared background. Arriving together in the Earth’s atmosphere, they would dump 20 teraelectronvolts of energy at the same point in the atmosphere, just like a single 20-teraelectronvolt gamma ray.
Scientists are now taking another look at HEGRA’s observations to test this idea. They’re looking for a subtle difference between the air showers a high-energy photon would trigger in the atmosphere and those produced by a ball of less energetic ones. Though they both have the same total energy, a lone high-energy photon would create a narrower, more chaotic air shower. "It’s like if you have a very heavy truck in a accident in the highway — there’s an incredible scatter." says Biermann. "But 20 teeny trucks might do next to nothing."
Natural gamma-ray lasers may sound like an outlandish explanation, but another possibility would be far more momentous. Giovanni Amelino-Camelia, a physicist at the University of Rome, believes that Markarian 501’s gamma rays might be subject to an entirely new kind of physics that rules the high-energy world. For decades, physicists have been trying to marry quantum theory with general relativity, Einstein’s theory of gravity. Most of their fledgling theories of quantum gravity predict that on tiny scales, approaching 10**-35 metres, our picture of smooth space and time falls apart, giving way to a seething froth of quantum gravity fluctuations dubbed space-time foam.
If so, odd things start to happen. As photon energies get higher, the speed of light might start to drop off by a tiny amount, because the very short wavelength would mean that the light started to "feel" the bumpiness of space-time. "A very rough analogy is that if you roll a soccer ball across a table with lots of tiny ridges, it will travel at roughly the same speed it would have done if there were no ridges," says Amelino-Camelia. "But if you roll a tiny little ball, its path will be strongly altered by all the little valleys in the table."
Feeling the bumps in space would not only slow very high-energy photons, it would help them avoid infrared photons. Raymond Protheroe of the University of Adelaide in South Australia and Hinrich Meyer of Wuppertal University in Germany calculate that provided quantum gravity does indeed kick in at a scale of 10**-35 metres, this could give 20-teraelectronvolt photons just the edge they need to ignore the far-infrared background and make it from Markarian 501 to Earth.
What’s most compelling, Amelino-Camelia says, is that this could also explain another cosmic conundrum. Protons with giant energies of more than 10**20 electronvolts are occasionally detected hitting our atmosphere. For years, astrophysicists have wondered why. The only known sources that could produce such energy are distant active galaxies, which means that these protons should also be eaten up by background radiation — this time the relic microwave radiation of the big bang (New Scientist, 7 December 1996, p 38).
According to calculations announced last month by Amelino-Camelia and Tsvi Piran of the Hebrew University in Jerusalem, the same roughness of space-time needed to explain the gamma rays from Markarian 501 also solves the cosmic ray problem. "The remarkable point is that you have these two problems at very different energy scales and contexts," says Amelino-Camelia. "Yet with the same equations, we can explain both."
Amelino-Camelia admits there’s a lot of guesswork going on. But for the first time, he says, nature might be throwing us some solid clues to quantum gravity. "And even if the correct explanation is different, we are finally obtaining data that are relevant to our understanding of the small-scale structure of space-time," says Amelino-Camelia. "This really is a turning point."
To find out if space-time is truly fuzzy in this way, Amelino-Camelia thinks astronomers should look to gamma-ray bursts. These are bright bursts of gamma rays that appear unpredictably anywhere in the sky and come from distant, mysterious sources. If astronomers could catch a very distant and bright burst, the highest-energy photons may lag slightly behind (Nature, vol 393, p 763). He says present-day detectors aren’t sharp enough to pick up the tiny timing differences necessary. "But on the space station and other orbiting observatories, we’ll acquire this level of sophistication over the next few years."
If Amelino-Camelia’s speculations hold true, they could lead to a change in our concept of time as radical as that brought about by relativity at the beginning of the 20th century. "Special relativity said that time is not absolute. That was the breakthrough for mankind at that time," he says.
Quantum gravity implies that time comes in discrete pieces. What’s more, like Schrsdinger’s proverbial cat, which is neither dead nor alive until we choose to look at it, time would exist as a jumble of different possible values. "The concept of ‘now’ becomes just a rough approximation," says Amelino-Camelia.
Not everyone agrees that Markarian 501’s message is this radical. "That’s a rather outrÈ possibility," says Sheldon Glashow of Boston University. He believes that other experiments have made such large departures from smooth space-time look extremely unlikely, and thinks there’s probably a simpler answer to the puzzle — perhaps that we’ve overestimated the distance to the blazar. If it is closer than we think, energetic gamma rays could make the journey to Earth despite the infrared background.
Erasing the Milky Way
Biermann agrees that something mundane could turn out to be the key. "My gut feeling is that the solution is something simple that we’re just not seeing," he says. Along with Vslk, he’d put his money on the latest far-infrared measurements being wrong. "The measurement of the far-infrared background is notoriously difficult," says Biermann. He thinks that improved tricks for erasing the Milky Way from COBE and ISO images to work out the strength of the far-infrared background might show it to be weaker than we now think.
One way to resolve this would be to look at more distant blazars. A whole new generation of gamma-ray telescopes is due to get under way. Scientists from Germany, France and Italy are building a telescope array in Namibia called HESS (High Energy Stereoscopic System). The array, with up to 16 telescopes, will be 10 times as sensitive as HEGRA, and could pick up blazars 30 times as distant as Markarian 501.
The first four HESS telescopes should start operating next year, along with a German telescope called MAGIC (Major Atmospheric Gamma Imaging Cerenkov Telescope). MAGIC, at the HEGRA site on La Palma, will gather Cerenkov light using a mirror 17 metres across. Further down the line, there are plans for an American telescope called VERITAS (Very Energetic Radiation Imaging Telescope Array System). Sited at the foot of Mount Hopkins in Arizona, this array will have seven mirrors, each 10.4 metres across, and start operating sometime in 2004 or later.
If the new telescopes find that Markarian 501’s even more distant cousins are relentlessly pelting us with 20-teraelectronvolt gamma rays, ordinary physics will be hard pushed to explain why. "This would deepen the suspicion that something dramatically new is happening," says Meyer. Whatever happens, the performance of the Universe’s most histrionic galaxies will be under the spotlight for years to come.
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Author: Hazel Muir, New Scientist
New Scientist issue: 23rd September 2000
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