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The Question
(Submitted May 16, 1997)
Several individuals have told me that they have read an article(s) in the
newspapers recently (either 15 may or 14 May) regarding the detection of
a gamma-ray that was a mother of all gamma-rays. According to these
individuals the gamma-ray came from a source that that had an energy
content larger than that of the energy contained in the known universe!
I don't doubt the story (unfortunately I did not see the article) up to a
point. I'm sure a powerful gamma-ray was detected but I question the
stated energy content. Would you please clarify the stated energy
content (was this gamma-ray detected by NASA astronomers?) and tell me
where I may obtain up-to-date information regarding this detection of such
a powerful gamma-ray.
The Answer
My colleague David Palmer provided the enclosed answer to
your question.
Jim Lochner
for Ask a High-Energy Astronomer
I believe the newspaper articles were referring to the recent discovery of
a gamma-ray burst counterpart with a high redshift. This source, for a few
seconds, produced over a million times as much power as our entire Galaxy,
although much less than the rest of the Universe as a whole.
Pictures are at
http://antwrp.gsfc.nasa.gov/apod/ap970521.html
http://antwrp.gsfc.nasa.gov/apod/ap970513.html
The details:
About once a day, there is a sudden flash of gamma-rays coming from some
random point in the sky. A Gamma-Ray Burst (GRB) is often the brightest
source in the sky, often brighter than everything else combined. When they
were discovered a quarter century ago, it was immediately 'obvious' that
the sources of these GRBs must be relatively nearby in space, otherwise
they would have to produce ridiculous amounts of energy in order to be so
bright.
As better and better instruments were flown, more and more data accumulated
indicating that these GRBs were distant. Rather than believing that the
sources we see were in our little corner of our Galaxy, the astronomical
community was split into those who believed that GRB sources were out at
the distant fringes of our Galaxy, and those who believed that GRBs were at
cosmological distances, far out in the Universe. The Galactic camp
required that GRBs were merely ridiculously bright, but the cosmological
camp required GRB energies that were completely ludicrous--as much energy
as a supernova, released in seconds, and all of it in gamma-rays. (And
this ignores the fact that it is very hard to make gamma-rays efficiently,
so it may require hundreds to thousands of times more total energy to get
that much gamma-ray energy out.)
One reason why GRBs were so mysterious is that nobody had seen them emit
anything but gamma-rays, and gamma-rays are hard to work with. You can't
focus them with mirrors or lenses, so it's very hard to tell where they are
coming from to better accuracy than a degree or so. If you have a 1 degree
position, you can look at it with modern telescopes (although you will
usually have to look at a small region, move the telescope, look at another
small region, etc. since 1 degree is a huge area of the sky in modern
terms) and you will see many many different objects. The problem is, you
will not be able to tell which of those objects, if any, is the source of
the GRB.
Recently, the BeppoSAX spacecraft was launched by ESA. This spacecraft has
an instrument that can locate GRBs (if they fall in its 40 x 40 degree
field of view) to within about 10 arcminutes. It also has an X-ray
telescope which, if pointed to an accuracy of 10 arcminutes, will see a dim
X-ray source and locate it with an accuracy of an arcminute.
Three times this instrument has seen a GRB with its first instrument and,
within 8 hours, pointed its X-ray telescope and seen a fading source which
it could locate to within an arcminute. On two of these occasions, people
using big optical telescopes have seen points of light flare up and die
down at the right location. These points of light are very faint (20th
magnitude) but with large telescopes such as the Hubble Space Telescope,
Mount Palomar, and Keck Observatory they can be studied.
The latest optical source has lines in its spectrum which imply that it is
at a redshift of z = 0.8, about halfway across the Universe. The BATSE
instrument on The Compton Gamma-Ray Observatory measured this GRB as well,
and reports a flux (at Earth) which reached a maximum of 1.7 x
10-7ergs/cm2/s. This gives a total peak power of a
few x 1050 ergs/second, and over the 35 second duration of the burst,
it produced a few x 1051 ergs.
To get a feeling for this, our entire galaxy produces about 3 x 1043
ergs/second at all wavelengths (mostly optical). Therefore, this gamma-ray
burst object was millions of times as bright as our galaxy. That's pretty
bright, but there are maybe 10 to 100 billion galaxies in the Universe. So
the gamma-ray burst was about a 10,000th as bright as the entire Universe.
So it was millions of times brighter than our Galaxy, the brightest thing
in the entire Universe for a few seconds, and was a significant fraction of
the brightness of the Universe, but the press is once again guilty of
exaggeration.
David Palmer
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