The outer space treaty was signed between
the USA and the USSR in 1962.
This was the first nuclear non-proliferation agreement and it treaty forbade
nuclear explosions in the outer space. To monitor it the USA launched a set of military
the Vela satellites, designed to look for flashes of soft γ-rays (radiation that is harder than X-rays) which
would have been the characteristic signature of a nuclear explosion in space.
Figure 1: One of the Vela satellites with Earth in the background.
and Roy Olson2 from the Los Alamos National laboratories reported on
the detection of the cosmic γ-ray
bursts. A confirmation by a Russian spacecraft soon followed.
bursts are short flashes of soft γ-rays.
They last from a few milliseconds (the record holder is a 0.6 msec long) to a
few thausand seconds. Following the work of Chryssa Kouvelioutou3 and coworkers from Huntsville, we classify
today bursts as long or short according to their duration (longer or shorter
than 2 seconds). The bursts arrive from
random directions in the sky and disappear. Present dedicated satellites
searching for GRBs detect bursts at a rate of one per day. This can be
translated to one burst per Galaxy per 10 million years.
Figure 2: The light curve (count rate as a function of time) of the first GRB detected by the Vela Satellites on July 2nd 1967
GRBs puzzled theoreticians ever since
their serendipitous discovery and their strange nature lead to numerous
speculations concerning their origin. Mel Ruderman4, from Columbia University discussed the phenomenon in a
conference in the fall of 1974 and noted “there are more theories than bursts”.
I also had my own theory at the time. As a part of my PhD thesis at the Hebrew University
with Jacob Shaham, I5 examined the possibility that GRBs arise from
instabilities in accretion disks around galactic black holes. As it turned out the real answer was very
far, and yet very close at the same time.
consensus formed at the early eighties that GRBs originate from violent
processes taking place on neutron stars in our own Galaxy, the Milky Way. Several either misleading or totally wrong
observations led to the general acceptance of this idea that even made it way
to classroom textbooks. The BATSE (Bursts And Transient Source Experiment)
detector on board of NASA’s Compton-GRO satellite was designed to prove this
A small but vocal minority did not accept
the standard paradigm. Already in 1975
Vladimir Usov (now at the Weizmann Institute) and Gregory Chibisov6
suggested that GRBs could be extra galactic, making them much more powerful
than if they were produced by relativity “local” Galactic objects. Note that a
source at distant galaxy is million times further than a source in our own
Galaxy. This would make an extragalactic bursts 1012 times more
powerful than a Galactic one.
In the mid eighties the extragalactic
option gained some credibility when Jeremy Goodman7 from Princeton
have suggested the first version of what is known today as the Cosmic Fireball
model and explaining how extra-galactic GRBs might work. At the same time Bohdan Paczynski8,
also from Princeton, have shown that the
statistics of the observed GRBs favor extra-galactic rather than Galactic
sources. Together with David Eichelr from Ben
(then at the Technion) and Dave Schramm from the University of Chicago I9
have suggested in 1989 that extra-galactic merging neutron stars binaries could
produce GRBs. This was the first
credible extra-galactic GRB model.
BATSE’s Surprise and the Great Debate
launched on April 5th 1991 on board of NASA’s Compton-GRO satellite.
It was the largest GRB detector ever flown detecting GRBs at rate of one per
day. BATSE could locate the position of a burst within an accuracy of a few
degrees. With this capability the BATSE team10 led by Gerry Fishman
and Chip Meegan from Huntsville
revealed that the bursts are distributed uniformly over the sky and
revolutionized our understanding of GRBs.
The uniform distribution of GRBs was
inconsistent with the expectations of the Galactic model, that the bursts
should have been concentrated in the Galactic plane. The bursts could be either
local or extremely distant. I11, and independently Shude Mao and
Bohdan Paczynksi12 have immediately shown that the bursts’ peak flux
distribution is incompatible with a local origin, leaving the extra-galactic
possibility as the only viable option.
We were able to set a limit on the distances, showing that a typical GRB
is at a distance of ten billion light years away from us.
Figure 3: Compton-GRO with the BATSE detectors at the eight corners.
Figure 4: The uniform distribution of 2704 GRBs observed by BATSE on the sky.
The extra-galactic origin was not
easily accepted. It required the release of a huge amount of energy in a short
time. An extra-galactic GRB releases in a few seconds the energy released by a
star like our Sun during its whole lifetime, several billion years. It was hard
to accept that such energetic explosions exist. It was also hard to imagine how
so much energy can be channeled out so quickly from a small source[ii].
The debate between the Galactic and the extra-galactic camps was particularly
lively13-15. It lasted for several years with a relatively slow
shift towards the extra-galactic camp.
It did not end until 1997 when GRBs’ afterglow was discovered and with
it a definite proof of their extra-galactic origin.
Even before the debate ended, theorist
perfected the Fireball theory that explains how so much energy can be channeled
out from a very compact source in such a short time. Goodman7 has set in the eighties
the basis for this model with his work on a radiation electron-positron
fireball. However, it was clear that this simple model cannot truly explain GRBs.
It led inevitably to a thermal[iii]
spectrum, whereas GRBs clearly show a non-thermal emission. The next step took place in 1990 when with my
student Amotz Shemi16 (at that time at Tel Aviv University) we modified Goodman’s model
to include protons. We have shown that all the energy of the initial fireball
would be converted to a kinetic energy of the protons. If the baryonic load
were small enough this would result in an ultra-relativistic outflow in which
the matter moves at 0.9999 of the speed of light or faster.
Martin Rees from Cambridge
and Peter Meszaros17 from Penn
State completed the
puzzle when they suggested in 1992 that the kinetic energy of the outflow can
be converted back to radiation via collisionless shocks between the outflow and
the surrounding matter. At roughly the
same time with Bohdan Paczynski, Ramesh Narayan (from Harvard) I18
suggested internal shocks within the outflow as an alternative way to convert
the kinetic energy to radiation. The
basic idea was the collisionless shocks accelerate electrons and produce
magnetic fields. The electrons moving within these magnetic fields produce the
observed -rays via the synchrotron process.
Later on Rees and Meszaros19
have shown that the continuous interaction of the relativistic outflow with the
surrounding matter produce a long lasting emission an afterglow. This afterglow
is weaker and at lower wavelength (X-rays, optical and then radio) but it can
last days, weeks or even months after the burst. With my student Re’em Sari, we20
proceeded to show that the prompt γ-ray
emission couldn’t arise from external shocks between the relativistic outflow
and the surrounding material. Internal shocks within the outflow remained the
only viable way to produce the prompt emission. As internal shocks cannot
exhaust all the energy of the outflow some is inevitably left for later
external shocks, and those produce an afterglow. This concept has led to the
commonly accepted internal-external shocks model according to which the prompt
GRB is produced via internal shocks while external shocks produce later the
Figure 5: The internal-external shocks model. A compact source produces a relativistic outflow. Internal shocks within the outflow produce the prompt g-ray emission while external shocks with the surrounding matter produce the lower energy and longer lasting afterglow.
The Italian-Dutch satellite BeppoSAX
confirmed, in the winter of 1997, the
prediction of a long lasting afterglow. BeppoSAX was launched in the
summer of 1996. It included, in addition to an all sky GRB detector, a wide
field camera that could locate the burst and a narrow field instrument that
could be pointed towards this approximate position and localize X-ray emission
with a high precision. On February 28th
1997 the BeppoSAX team, led by Enrico Costa from Rome localized its first GRB and discovered21
X-ray afterglow. Jan van Paradijs and coworkers form Amsterdam University
pointed an optical telescope to the X-ray position and detected a optical
afterglow as well22.
few months later on May 8th 1997 BeppoSAX localized a second burst.
Mark Metzger23 and coworkers from Caltech measured the optical
spectrum of its afterglow and detected redshifted spectral lines indicating a
source at a cosmological redshift of 0.835. The Galactic – extragalactic debate
was finally over!
Figure 6: The afterglow of GRB970228 as observed by the Hubble space telescope more than half a year after the burst.
Radio observers from NRAO, headed by
Dale Frail24 discovered radio afterglow from the May 8th
burst. Jeremy Goodman25 was
quick to show that the radio observations of this bursts provided the first
direct confirmations to the idea that was beyond the fireball model: GRBs
involve ultra-relativistic motion at a 0.99 of the speed of light or even
faster. Later on in March 2003 Greg
Taylor from NRAO26 and coworkers gave a stronger and conclusive
proof for this idea using the radio observations of the afterglow of GRB
The propagation of the blast wave into
the surrounding matter was described by Roger Blandford and Chris McKee27 who
calculated already in the seventies the self-similar propagation of a
relativistic blast wave. With Re’em Sari and Ramesh Narayan28
we have combined this solution with synchrotron emission to predict the
afterglow light curves. The solution
describes well the late phases of the afterglow,. Ralph Wijers and Titus Galama29
were the first who compared this solution with the observed afterglow data in
order to determine the physical conditions within the afterglow. This method
was used commonly later.
Figure 7: Radio afterglow of GRB030392 showing an extended figure and revealing relativistic motion.
As it took several hours to turn
BeppoSAX towards the direction of the localized GRB the observations described
only the late stages of the afterglow. Later on when NASA’s satellite Swift observed the very early afterglow
it became clear that the situation is more complicated and there are puzzles
that this simple theory does not explain.
The detection of the afterglow enabled
redshift determinations. With those precise estimates of the distances and of
the corresponding energies followed. As
the energy implied in some cases was unreasonably large it became clear the
emission is not uniformed and it is beamed towards us. James Rhoads30
from the Kitt Peak National Observatory and independently together with Re’em Sari, Jules Halpern
(from Columbia University) we31 have
predicted that jets would have a clear signature on the afterglow, known today
as a “jet break”. Such a signature was indeed observed shortly afterwards by
Shri Kulkarni from Caltech and coworkers32 and enabled us to
estimate the angular width of GRB jets as a few degrees. With beaming taken into account we know today
that a typical GRB emits about 1051 erg during the few seconds that
Figure 8: a schematic description of a jet break that occurs when the jet’s slows down and begin expanding sideways. The insert shows the expected light curve.
The most interesting question is, what
is the origin of GRBs and what are their progenitors. Timing is one argument.
The rapid fluctuations seen in the prompt light curves indicate that the source
must be very small – not more than several hundred kilometers wide. The
enormous energy released in these explosions is another clue. It is only two
orders of magnitude smaller than the binding energy of a compact object – a
neutron star or a black hole. No other known source can release so much energy
so rapidly. This implies that the bursts involve the formation of one of the
two. Already in 1993 I33 have suggested that the second, a black
hole, is strongly favored over the first. Each GRB signals, therefore, the
birth of a new black hole.
As matter collapses to form a black
hole a fraction that has too much angular momentum forms an accretion disk
around the black hole. With one tenth of a solar mass the energy powering the
GRB arises from the gravitational energy released when this material accretes
onto the black hole[iv].
Figure 9: A schematic picture of a GRB, a black hole accretion disk and a jet.
Collapsar and Long GRBs
Stan Woosley34 from Santa Cruz suggested in
1993 that “failed supernovae” – collapsing star that failed to explode like a
regular supernova – produce GRBs. In the late nineties BeppoSAX began
identifying the positions of GRBs and their host galaxies. Bohdan Paczynski35
was quick to notice that GRBs arise preferably in star forming Galaxies.
As massive stars live very short time (on astronomical scale), stellar
death and supernovae are associated with star formation. Making the link
Paczynski suggested that GRBs are related to powerful supernovae. At the same time Andrew MacFadyen and Stan
Woosley36 developed the Collapsar model, showing that a relativistic
jet can punch a hole in the collapsing stellar envelope and produce a GRB.
According to this idea the collapsing star forms in its center a black hole
surrounded by a massive accretion disk.
This “inner engine” produces the relativistic jet and powers the
Figure 10: A numerical simulation of a jet punching a hole in a stellar envelope. The basic ingredient of the Collapsar model.
not have to wait for a long time. In April 1998 Titus Galama and co-workers
from the University
discovered that GRB980425 (the “phone number” of a GRB is simply its date) is
associated with a distant very powerful supernova (SN98bw). Later, Shri
Kulkarni and his student Josh Bloom38-39, also from Caltech, found
evidence for GRB-SN association in many other afterglows. This association was finally confirmed in
the most spectacular way when in the spring of 2003 Kris Staneck40
from Harvard and coworkers and Jens Hjorth41 from Copenhagen and coworkers observed the
supernova SN2003dh emerged beautifully above the afterglow of GRB030329.
By now it is generally accepted that long
GRBs (whose duration is longer than 2 seconds) are associated with a special
type of supernovae called by the experts type Ic. However, the association is
not one to one. Some type Ic supernovae don’t have any GRBs and some long GRBs
are not associated with supernovae (or at least very strong upper limits on its
power). Clearly a few links are still
missing in this puzzle.
Figure 11: The supernova spectrum of SN2003dh arising above the afterglow light curve of GRB030329 and confirming the SN-GRB connection.
Mergers and Short GRBs
star binaries are pair of neutron stars, circling around each other. Several
such systems are known in our Galaxy.
The most famous of those is the binary pulsar discovered in 1975 by
Russel Hulse and Joseph Taylor (then at the University of Amherst).
Figure 12: A numerical simulation of the last stage of a binary neutron star merger. Shown are density contours.
the neutron stars circle around each other they emit gravitational radiation
and their orbit shrinks. Eventually they coalesce and merge forming a black
hole and an accretion disk of debris. Niel Gehrels42 from Goddard Space Center
and coworker, Derek Fox43 from Caltech and coworkers and Edo Berger44
from Caltech and coworkers using NASA’s Swift satellite discovered that, unlike long GRBs that arise only
in star forming spiral galaxies, short GRBs (GRBs whose duration is less than 2
seconds) appear in both elliptical and spiral ones. At times they are located in the outskirts of
their host galaxies. These observations are consistent with the prediction of
the merger model and suggest that neutron star mergers are the progenitors of
short GRBs. However a clear proof is
Neutron star mergers are the strongest
sources of gravitational radiation. As such they are the prime targets for
gravitational radiation detectors like LIGO and Virgo that have been
constructed during the last decade both in the USA
and in Europe.
Already, in 1993 I suggested with Chris Kochanek45 (former
Harvard) that a combined detection of a GRB and a pulse of gravitational
radiation would be the ultimate proof of this model. With the expected upgrade of
LIGO we are eagerly waiting for this confirmation.
Figure 13: The LIGO detector built to detect gravitational radiation from coalescing neutron star binaries.
Swift and recent developments
GRBs continue to be at the focal point of
the astronomical research. In the fall of 2004 NASA has launched Swift: a dedicated GRB satellite46.
By localizing several short GRBs42-44
Swift has demonstrated that, as would
be expected from the neutron star mergers model, those are not located within
star forming regions. Swift has
numerous discoveries concerning the early afterglow. Some of those are
incompatible with the possibly over simplified fireball model. For example,
some of the findings indicate that the central engines that power GRBs (the
accreting black holes) are active long after the prompt emission ceases47.
Other observations suggest that the outflow may not be composed from protons
and electrons, as initially thought, but might contain a significant fraction
of electromagnetic energy (the so called Poynting flux) as suggested already in
1992 by Vladimir Usov48.
Figure 14: NASA’s Swift satellite, a dedicated GRB satellite with rapid follow up capabilities that enable it to catch the early x-ray and optical afterglow.
Among Swift’s most amazing discoveries was the detection by Judith
Racusin from Penn State and coworkers49 of GRB080319b, a very
powerful burst accompanied by a 5th magnitude optical flash. Optical
flashes accompanying GRBs were not new. With Re’em Sari, I50 have
predicted this phenomenon in the fall of 1998. Luckily enough the first flash
was detected shortly afterwards by Carl Akerlof51 and coworkers from
the University of
Michigan in the winter of
1999. However, the burst of GRB 080319b was the brightest flash ever. It was so
powerful that it could have been seen by a naked eye even thought it came from
7 billion light years away. Had the burst been in our Galaxy this optical flash
would have out-shined the Sun!
Figure 15: The optical flash of the naked eye burst, GRB080319b. A 5th magnitude optical signal, 7 billion light years away.
summer NANA has launched Fermi: a very high-energy -ray satellite with GRB
detector on board. As far as GRBs are concerned Fermi’s major goal is to search
for very high-energy emission (in the range of GeV) from GRBs. Such emission
would give new clues on the nature of the emission processes both during the
prompt phase and during the afterglow.
Figure 16: Fermi, NASA’s latest high energy satellite with GeV detection capabilities and a dedicated GRB detector on board.
So far Fermi has detected GeV emission
from several GRBs. This is less than what was previously expected and somewhat
puzzling. Still the observations have some intriguing achievements. This April,
Fermi as well as Swift has detected a
burst, GRB 090423, at a redshift of 8.1. This is the most distant and hence
also the earliest object seen so far. The age of the Universe at the time that
this burst exploded was only 570 million years old. This implies that some
stars formed very early and that GRB progenitors are among the earliest objects
that have formed in our Universe.
observations of other GRBs have set strong limits on possible quantum gravity
effects that would make the speed of photons energy depend. In other
observations Fermi has shown that the velocity of the outflow in a particular
GRB exceeded 0.999999 of the speed of light52.
Neutrinos and Cosmic Rays and Cosmology
involve particles moving at ultra-relativisitic velocities. Collisions between
these particles can produce very high-energy neutrinos. Such neutrinos are
prime candidates for detection by the km3 that is being built now in
Antarctica. It is amazing that this detector
is so quite and free of background that a detection of even a single neutrino
in coincidence with a GRB would be significant.
Eli Waxman53 and independently
Mordechai Milgtrom and Vladimir Usov54 (all from the Weizmann
Institute) pointed out in 1995 the exciting possibility that GRBs are the
sources of ultra high energy Cosmic Rays. This is intriguing as the origin of
those cosmic rays that are observed at energies above 1019.5eV that
is above the GZK cutoff is still mysterious.
1995 I15 have suggested that as powerful beacons that can be seen
from the most distant parts of the Universe GRBs can replace supernovae as the
classical tool for cosmological tests. However, in order to do so one has to
find a way to estimate with high precision GRBs intrinsic luminosity so that
they can become “standard candles”.
While numerous suggestions have been put forwards in this direction none
has been successful so far.
of these ideas was confirmed yet but they serve to indicate the vast potential
of GRBs and of the extreme conditions that take place in them.
Even though GRBs are at cosmological
distances their -ray flux is at times so large that it is capable of shaking
the Earth ionosphere and creating noticeable disturbances. It is amazing that,
apart from the Sun and a few solar system objects - meteorites and asteroids,
GRBs are the only astronomical objects that have a direct influence of any kind
Imagine now a GRB taking place in our
Galaxy. Such event will be rare as GRB take place only once every 100,000 years
in our Galaxy and only once per 10 million year they will point towards us. But
this could happen. If the burst is near enough its enormous radiation field
could ionize and partially evaporate the Earth atmosphere. While a complete
theory for the possible damage is not in place yet and the essential comparison
with the effects of less powerful but thousand times more frequent supernovae
should be done, it is clear that a GRB in our Galaxy can have devastating
effects for life on Earth. A GRB could
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The Vela satellites were disguised as scientific ones and to justify this
Sterling Colgate1 from the Los Alamos National Laboratory was asked
to make an invited prediction of astronomical bursts of soft g-rays.
rapid variability implied a small size of a few km for the origin.
A thermal spectrum is a black body spectrum and it is described by the Planck
As suggested in my early PhD work GRBs arise in accretion disks around black
holes, but unlike this work these black holes are not Galactic but very distant