Many gamma-ray bursts go undetected, MIT
MIT News Office,
August 4, 2003
**Science Contact available at the end of this
Cambridge, Mass. -- An MIT researcher estimates in
the Aug. 10 issue of Astrophysical Journal Letters that there
are roughly 450 gamma-ray bursts or X-ray flashes occurring in
the observable universe for every 1 detectable by orbiting
Shining as brightly as a million trillion suns yet seldom
lasting even one minute, gamma-ray bursts (GRBs) were a great
astronomical mystery only recently solved when they were
conclusively shown to be linked to cataclysmic explosions
called supernovae that mark the deaths of very massive stars.
Gamma-ray bursts come to us -- across billions of light
years of space and hence billions of years of time -- from
wholly random directions of the sky about once a day, but
astronomers have long suspected they see only a small portion
of the total number actually occurring. Until the recent
gamma-ray burst/supernova link was made, proving this or even
deriving a number of "actual-to-observed" bursts based on
observations was exceedingly difficult.
With this link established, two scientists at MIT's Laser
Interferometer Gravitational Wave Observatory (LIGO) have just
derived an "actual-to-observed" ratio of 450-1. That is, there
are roughly 450 GRBs occurring in the observable universe for
every one that's detectable by orbiting satellites designed to
look for them. This is a figure that not only utilizes the
GRB-supernova link but also agrees well with a previous,
independently derived ratio that did not use such a link.
These findings could have important consequences in the
long hunt for elusive gravitational waves--tiny ripples in
space-time predicted by Einstein's theory of general
relativity but as yet never directly observed.
"Our 450-1 figure closely agrees with a 500-1 ratio derived
in 2001 by other scientists, which makes us more confident in
these results," said Maurice van Putten,
assistant professor of applied mathematics. Van Putten
collaborated with post-doctoral researcher Tania Regimbau in
the study. "The earlier figure was based on a method totally
independent of the supernova association involving spectral
characteristics of the gamma-ray emissions themselves."
To derive their figure, van Putten and Regimbau assumed the
now-standard "collapsar" model of GRBs. In that model, the
core of an especially massive star undergoes a gravitational
collapse (likely resulting in a black hole), producing a
massive pressure wave that blasts out of the star in a
particular direction. The blast wave collides with dust and
gas in the surrounding interstellar medium at velocities near
that of light, producing gamma-ray emissions. The type of star
used in the collapsar model is also the type of star that ends
its life in a supernova.
What was missing was observational evidence linking GRBs to
supernovae. That evidence was provided by a burst detected on
March 29, 2003 (and therefore dubbed GRB 030329) by the HETE
satellite, one of the main GRB-seeking satellites. That burst
was so close in astronomical terms--roughly 2 billion light
years away--that astronomers were able to study the
"afterglow" light of progressively less energetic radiation.
What astronomers saw in the spectral analysis of the light
curves was the unmistakable signature of a supernova,
including the presence of oxygen emission lines excited in the
blast. This information provided powerful support to a
previous, even closer blast on April 25, 1998 that had
provided a less conclusive link between supernovae and GRBs.
Once the GRB-supernovae link was established, van Putten
and Regimbau used a "very precious" sample of 33 GRBs whose
distances (unlike most) are well known, to establish a
mathematical relationship between how bright a given burst is
and the rate at which the massive stars form and die.
They could do this because the massive stars involved in
GRBs and supernovae live for only a few tens of millions of
years, as opposed to billions of years. This fact, van Putten
said, means such "massive stars essentially die at their place
One aspect of the collapsar model is that the burst (which
precedes the actual supernova explosion) occurs along a
particular axis in both directions, as opposed to a symmetric,
radial one. Since axes of stars are oriented randomly
throughout the universe, we detect only those bursts along or
near whose axis the Earth happens to lie.
This effect is known as "beaming" and it means the angle
through which the blast of energy is seen is relatively small
for most observed blasts--no more than a few degrees of sky.
Van Putten said this "beaming" effect is factored into their
figure because the relationship is based on peak brightness.
A BOON TO WAVE SEARCH
Van Putten said the confirmation that there are so many
more GRBs than we actually detect is potentially a boon in the
quest to find gravitational waves. These minute waves in
space-time are thought to be produced by massive objects
undergoing extreme events, such as the formation of new black
holes or collision-coalescence of existing black holes or
neutron stars. Since GRBs usually mark the creation of a new
black hole, gravitational waves ought to be emitted. And
unlike the beamed electromagnetic energy from GRBs,
gravitational waves should travel out more or less smoothly in
Van Putten said knowing a ratio of actual to observed GRBs
will help LIGO precisely because most GRBs are so distant that
their gravitational waves won't be detectable by this array.
Instead, the findings will give astronomers a sense of how
often to expect a detectable gravitational wave produced by a
sufficiently close GRB.
"This is an important finding for LIGO because these
findings can give us a good handle on the local
gravitational-wave event rate. It doesn't matter if the burst
is beamed toward us or not because the gravitational wave
energy is not beamed," van Putten said.
"Given what LIGO is capable of seeing, and using our
results, we would expect an event rate of perhaps one per
year, as opposed to one in 450 years, which would be
hopeless," he said.
George R. Ricker, principal investigator for the MIT-run High Energy Transient
Explorer (HETE2) satellite, hailed the findings as just
the kind of success for which he and his colleagues had hoped.
"The HETE mission is rapidly transforming GRBs from vague
cosmic mysteries into incisive cosmological probes. This new
work is exactly the kind of stimulating research which we
dreamed HETE's success would bring about," Ricker said.
Prof. Maurice H.P.M. van Putten
LIGO Group at MIT, Cambridge, Mass.