A Gamma Ray Puzzle

by Paul Gilster on December 23, 2006

When you’re calculating the odds on life in any region of the galaxy, the rate of supernova explosions comes into play. As we saw yesterday, one factor Nikos Prantzos examined in his recent work on the galactic habitable zone was the effect that hard radiation could have on exposed land life. But what about gamma ray bursts (GRBs)? They’re more powerful and, although rarer than supernovae, can create beamed energy that makes them lethal from larger distances.

One theory is that because gamma ray bursts are associated with regions of low metallicity outside our galaxy, their frequency in the Milky Way is now close to zero. But a reminder of how little we actually know comes in the December 21 Nature, where four papers discuss GRB activity, and in particular a burst picked up by NASA’s Swift satellite last June 14. It’s a cosmic oddity, a kind of hybrid that probably marks the birth of a black hole. But, as Derek Fox (Penn State) says, “This burst — unlike all other long gamma-ray bursts we have seen at close distance — was not accompanied by a supernova, for reasons we do not yet fully understand.”

The current dichotomy for gamma ray bursts runs something like this: long GRBs last more than two seconds and appear to mark the birth of a black hole resulting from a supernova explosion. Short GRBs may be as brief as several milliseconds, and apparently mark the merger of two neutron stars, or a neutron star and a black hole.

The trick with the recent GRB 060614 burst is that it was a long one — 102 seconds — from a place (some 1.6 billion light years away in the constellation Indus) where star formation rates are low and few stars are massive enough to produce supernovae. Moreover, no trace of a supernova can be found there. “This burst was close enough to detect a supernova if it existed,” said Avishay Gal-Yam (Caltech), lead author on one of the Nature papers, “but even Hubble didn’t see anything.”

Equally interesting is GRB 060505, a burst detected last May. That one, originating in a galaxy 1 billion light years away in the direction of the constellation Piscis Austrinus, also left no supernova remnant.

Maybe the merger model needs an overhaul. Or if supernovae were involved in both events, then they may have formed black holes that allowed absolutely no matter to escape, the usual supernova ejecta being completely consumed. In any case, some new process seems to be at work in the two bursts, one that calls into question our understanding of GRBs and therefore our ability to predict their future frequency with accuracy. And it underscores the difficulty in estimating the long-term effects of GRBs on possible extraterrestrial life.

The paper referenced above is Gal-Yam et al., “A novel explosive process is required for the bold gamma-ray burst GRB 060614,” Nature 444 (21 December 2006), pp. 1053-1055 (abstract here). Three other short papers discuss the recent GRB findings in the same issue.

Ron S December 23, 2006 at 15:17

Being within the lethal zone of a SN could possibly obliterate nearly all life since the radiation lasts many planetary rotations, like a turkey on a spit (to bring in a holiday reference). A GRB is so short it only irradiates half the globe. This would be still be devastating but would allow fairly rapid repopulation of the affected hemisphere from the unaffected hemisphere. There may even be enough upper-atmospheric mixing to make ozone-depletion only partial.

If this is right, one nearby SN could be more lethal than many lethal GRB events. A GRB may be more comparable in effect to a large impactor.

Ciprian December 23, 2006 at 18:41

I’m not a scientist, but I took the time to read this http://en.wikipedia.org/wiki/Gamma_ray and it seems to me that a GBR could penetrate the whole Earth, and the biosphere will be devastated no matter where it is located (on the near or the far side of the Earth, relative to GBR).

Ron S December 26, 2006 at 10:16

Ciprian, I had a look at that wikipedia entry and that is not what I read. I would have been surprised to read it since it isn’t so. As noted in the entry gamma ray photons are very energetic and can do lots of damage, but can penetrate only a very short distance into the earth’s surface. Probably no more than a few centimeters. About the only thing that could penetrate the entire planet would have to have extremely high momentum (like a micro-black hole) or have a very low probability of interaction (like neutrinos).

While I was in wikipedia I clicked through to the GRB entry. It has a bit more on its potential for extinction events.

Ciprian December 27, 2006 at 11:52

Ron, as I said, I’m not a scientist. Following your answer, I tried to find some numbers, and I found this: http://en.wikibooks.org/wiki/Basic_Physics_of_Nuclear_Medicine/Attenuation_of_Gamma-Rays

It seems 0.4cm of lead are necessary to halve the incident radiation intensity for a 500keV gamma ray. It would be nice to have someone to compute the Half Value Layer of lead for gamma rays likely to hit Earth if a GBR should occure 500 light years away.

Ron S December 27, 2006 at 14:49

Ciprian, have a look at this:
http://xxx.lanl.gov/PS_cache/astro-ph/pdf/0612/0612289.pdf

I, too, am not a scientist nor an expert on this matter, so a quick search seemed in order. This preprint which provides GLAST instrumentation parameters and the types of GRB they’re designed to detect seemed useful. Looks like they are going to detect energies up to 800 GeV, which is over a million times higher than the penetration figure you quoted. I don’t know the answer to your question but it should be possible to get one easily with this number for someone who knows how. Offhand I’d guesstimate that a high proportion of this energy will penetrate the atmosphere and well into the ground. Still not a problem for the far side of the planet, though the earthworms may have a bad day.

My search also pulled up an article on Compton scattering of GRB radiation off the moon and back onto the earth. I didn’t read it closely to see how bright the reflection would be, but it is one way the other hemisphere might be affected if the GRB is strong enough and the moon is opposite the GRB source. For comparison, in visible light the full moon is about 56 db fainter than direct sunlight. So, likely the far hemisphere remains relatively safe.

Of course as Paul points out in the later article, it is very unlikely that Earth has experienced a lethal GRB in its entire history. I would think this should be similarly true for all planets in all galaxies at this present age of the universe.

Paul Dietz December 27, 2006 at 16:00

I thought the real killer from GRBs was the relativistic charged particles in the beamed fireball, not the gamma rays themselves. The muons from high energy particles (produced when the particles strike the atmosphere) can penetrate down the surface, and then well below the surface, if they are sufficiently energetic.

I wonder how coherent the charged particle beam remains. Does its pressure overwhelm the galactic magnetic field?

Ron S December 27, 2006 at 22:52

Paul D, I don’t know much about GRBs, but I would think the existence of these particles and their nature would depend on the mechanism underlying the GRB. I believe there are lingering doubts about the mechanism(s) and beaming. I don’t know, but I’d guess we’ve not detected particles associated with a GRB due to the large distances involved, unless they’re cosmic wave particles.

Assuming for a moment that the GRB is nearby and there is a particle barrage, it would of necessity come after the GRB event and would likely have a duration longer than the GRB due to the range of particle velocities in the barrage, and that duration would increase with distance. That might fry both hemispheres, more like a SN (which is one GRB mechanism).

ljk December 27, 2006 at 23:58

Astrophysics, abstract
astro-ph/0612718

From: Vladimir Vassiliev V [view email]

Date: Sat, 23 Dec 2006 18:05:05 GMT (370kb)
Wide field Ritchey-Chretien telescope for ground-based gamma-ray astronomy

Authors: V. V. Vassiliev, P. F. Brousseau, S. J. Fegan

Comments: 27 pages, 16 figures, submitted to Astropart.Phys

Paper describes a novel wide field Atmospheric Cherenkov Telescope based on modified Ritchey-Chretien design. Its performance characteristics are compared with traditional Davies-Cotton reflector.

http://arxiv.org/abs/astro-ph/0612718

Eric James December 28, 2006 at 1:51

Isn’t water a particularly good insulator against this type of radiation? Would the marine life be safe?

Paul Dietz December 28, 2006 at 11:22

Isn’t water a particularly good insulator against this type of radiation? Would the marine life be safe?

The scenario I vaguely remember seeing had lethal radiation doses down to a depth of 1 km or so in rock. Energetic muons are very penetrating. This would wipe out the entire photosynthetic zone.

Eric James December 28, 2006 at 23:08

At least all the bums in the subway tunnels would be safe! ;)

How deeply would it penetrate water? The photosynthetic zone in water can be a couple of hundred feet or more (depending on clarity). I know that near where I live, the kelp forests easily reach a hundred feet of depth.

Eric James December 28, 2006 at 23:12

Oops, I read meter, you wrote kilometer. The bums wouldn’t be safe after all.

Life’s only hope then would be of the deep thermal vent variety. That’s not good.

ljk May 15, 2007 at 16:42

GRI: The Gamma-Ray Imager mission

Authors: Jürgen KnÖdlseder (CESR)

(Submitted on 12 May 2007)

Abstract: With the INTEGRAL observatory, ESA has provided a unique tool to the astronomical community revealing hundreds of sources, new classes of objects, extraordinary views of antimatter annihilation in our Galaxy, and fingerprints of recent nucleosynthesis processes. While INTEGRAL provides the global overview over the soft gamma-ray sky, there is a growing need to perform deeper, more focused investigations of gamma-ray sources. In soft X-rays a comparable step was taken going from the Einstein and the EXOSAT satellites to the Chandra and XMM/Newton observatories.

Technological advances in the past years in the domain of gamma-ray focusing using Laue diffraction have paved the way towards a new gamma-ray mission, providing major improvements regarding sensitivity and angular resolution. Such a future Gamma-Ray Imager will allow studies of particle acceleration processes and explosion physics in unprecedented detail, providing essential clues on the innermost nature of the most violent and most energetic processes in the Universe.

Comments:

8 pages

Subjects:

Astrophysics (astro-ph)

Journal reference:

Proceedings of the 6th INTEGRAL Workshop: The Obscured Universe (01/01/2007) 8 pages

Report number:

GRI-2007

Cite as:

arXiv:0705.1754v1 [astro-ph]

Submission history

From: Jurgen Knodlseder [view email]

[v1] Sat, 12 May 2007 09:42:06 GMT (733kb)

http://arxiv.org/abs/0705.1754

ljk December 18, 2007 at 10:23

INTERGALACTIC “SHOT IN THE DARK” SHOCKS ASTRONOMERS

GREENBELT, Md. – A team of astronomers has discovered a cosmic explosion that
seems to have come from the middle of nowhere — thousands of light-years from
the nearest galaxy-sized collection of stars, gas, and dust. This “shot in the
dark” is surprising because the type of explosion, a long-duration gamma-ray
burst (GRB), is thought to be powered by the death of a massive star.

“Here we have this very bright burst, yet it’s surrounded by darkness on all
sides,” says Brad Cenko of the California Institute of Technology, Pasadena,
Calif., lead author of the team’s paper, which has been accepted for
publication in The Astrophysical Journal. “The nearest galaxy is more than
88,000 light-years away, and there’s almost no gas lying between the burst and
Earth.”

The blast was detected on January 25, 2007, by several spacecraft of the
Inter-Planetary Network. Observations by NASA’s Swift satellite pinpointed the
explosion, named GRB 070125 for its detection date, to a region of sky in the
constellation Gemini. It was one of the brightest bursts of the year, and the
Caltech/Penn State team moved quickly to observe the burst’s location with
ground-based telescopes.

Using the team’s robotic 60-inch telescope at Palomar Observatory in Calif., the
astronomers discovered that the burst had a bright and fast-fading afterglow in
visible light. This prompted them to observe the afterglow in detail with two of
the world’s largest telescopes, the 8-meter Gemini North telescope and 10-meter
Keck I telescope, both near the summit of Hawaii’s Mauna Kea.

What came next was a total surprise. Contrary to experience with more than a
hundred previous GRBs, Gemini spectra revealed no signs of dense gas and dust
absorbing the light of the afterglow. A trace of magnesium revealed that the
burst took place more than 9.4 billion years ago, as deduced by the shift in
wavelength of the afterglow’s light, and that the surrounding gas and dust was
more tenuous than the environment around any previous burst.

To further pin down the environment that could produce such an unusual
explosion, the group obtained Keck images of the location of GRB 070125 long
after its afterglow light had faded away. Surprisingly, the resulting images
showed no galaxy at this location. “A Keck image could have revealed a very
small, faint galaxy at that distance,” says coauthor Derek Fox of Penn State.

Astronomers have amassed a great deal of evidence that GRBs are triggered by the
explosive deaths of massive stars, which live very short lives. Because of their
short lifespans, massive stars don’t have time to wander far from their
birthplaces, usually dense clouds of gas and dust inside respectable-size
galaxies. So GRB 070125 raises the perplexing question of how a massive star
could be found so far away from any galaxy.

“Big stars live fast and die young, without much time to move around,” says Fox.
“So if this massive star died far away from any galaxy, the key question is, how
did it manage to be born there?” The formation of massive stars requires
similarly massive aggregations of gas and dust, which are usually found in
bright galaxies.

One possibility is that the star formed in the outskirts of an interacting
galaxy, as seen in the famous Hubble Space Telescope picture of the “Tadpole”
galaxy, UGC 10214. “In the local universe, about one percent of star formation
happens in tidal tails, on the outskirts of two interacting galaxies,” says
Cenko. “So it might even make sense to find one in 100 gamma-ray bursts in such
an environment.”

If this idea is correct, it should be possible to detect the tidal tail hosting
GRB 070125 by taking a long exposure with the Hubble Space Telescope. “That’s
definitely our next stop,” says Cenko.

“Many Swift discoveries have left astronomers scratching their heads in
befuddlement,” adds Swift lead scientist Neil Gehrels of NASA Goddard Space
Flight Center in Greenbelt, Md. “But this discovery of a long GRB with no host
galaxy is one of the most perplexing of all.”

For related images to this story, please visit on the Web:

http://www.nasa.gov/centers/goddard/news/topstory/2007/intergalatic_shot.html

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