(((How far really hard gamma can travel)))
From: Larry Klaes <lklaes@bbn.com>
Subject: Outburst from a galaxy leaves scientists in a quandry
(Forwarded)
Date: Fri, 22 Sep 2000 01:03:41 GMT
From: Andrew Yee <ayee@nova.astro.utoronto.ca>
Organization: UTCC Campus Access
To: SEDSNEWS@LISTSERV.TAMU.EDU
------------------
New Scientist
UK Contact:
Claire Bowles
New Scientist Press Office, London
claire.bowles@rbi.co.uk
44-207-331-2751
US Contact:
New Scientist Washington office
newscidc@idt.net
202-452-1178
EMBARGOED FOR RELEASE: September 20, 2000, 14:00 EDT US
Outburst from a galaxy leaves scientists in a quandry
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 outré 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.
###
Author: Hazel Muir, New Scientist
New Scientist issue: 23rd September 2000
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