.
Black
holes almost certainly play a role in what are called Gamma Ray Bursts (GRB).
These are the most intense and copious releases of energy observed in the
Universe - less than that of the Big Bang itself but much more than given out
by supernovae or quasars. GRBs can at their outset release enough energy to
give them a luminosity calculated to be 1019 greater than that of
the Sun. They are characterized by extreme outputs over very brief periods,
measured in seconds to minutes at their peak. At least one GRB is observed each
day somewhere in the Universe, so they are rather common events, albeit less
frequent than supernovae.
Despite
being the largest rapid release high energy events in the Cosmos, GRBs were
unknown (sometimes mistaken for ordinary supernovae) before 1967. The manner in
which they were discovered is an interesting example of serendipity: Nuclear
explosions on Earth release large quantities of gamma ray energy. In the 1960s,
the U.S. was seeking ways to detect Soviet nuclear tests, so it built and
orbited gamma ray, x-ray, and neutron detectors on military satellites. In the
U.S. Air Force Vela program, the Vela-4 satellite detected many gamma ray
events, all at times that failed to correlate with any known nuclear blasts on
Earth. These gamma ray events were all proved (eventually) to emanate from well
beyond Earth. Here is a plot of one of the first records:
GRBs give
off tremendous amounts of energy extending through all wavelengths of the EM
spectrum. The diagnostic signature of the GRB that separates it from supernovae
is the predominance of high energy gamma rays over very short time periods. GBRs
can be subdivided into two types: short burst (around 2 seconds) and long burst
(more 2 seconds; initial emissions on the order of 20-30 seconds, with a few
extending up to an hour). This time spike has been observed in GRBs detected by
more sophisticated sensors that monitored such events. Thus, this example:
These
GRBs puzzled astrophysicists. They were first thought to be in the Milky Way. And
in fact some were actually located in our galaxy, where they occur on average
about once in 10000 years. Afterglow radiation from one such event was observed
on February 28, 1997 in the M.W. itself by an Italian X-ray satellite called
BeppoSAX:
But, the
frequency of occurrence, which as more observations were confirmed indicates at
least one GRB every day, suggested that the vast majority of GRBs were located
in galaxies well beyond the Milky Way. As more records of these events accumulated,
it became evident that GRBs are not concentrated in specific regions of the sky
but are distributed at random (isotropic) over the entire sky. GRB's are also
randomly distributed in time - occurring anywhere in the Universe (thus over
the full extent of time since the first galaxies;(from thousands of light years
to 12+ billion l.y.). A large number seem to be distant, near the outer part of
the observed Universe, and hence were most common in the early history of the
Universe. Here is a map of the sky showing many of the larger GRBs, as detected
by the Compton Gamma Ray Observer and BeppoSAX.
The BATSE
(Burst and Transient Source Experiment) instrument on the Compton Gamma Ray
Observatory (CGRO; see page 20-3) was particularly suited to detecting GRBs. Here
is one image of an event that occurred several billion light years away:
These GRB
events should generate radiation at wavelengths longer than those of gamma
rays. As studies of them expanded, traces of individual events were sought by
other satellites that monitor at different wavelengths. The problem is that
evidence of a GRB diminishes rapidly at shorter wavelengths. However, in time
such events were picked up at various wavelengths when alerts were given and
the sky locations established. Now, with experience this is the time frame for
durations of GRBs over a range of wavelengths:
These
signs of lower levels of energy at longer wavelengths persisting around a GRB
are grouped under the general term "afterglow". X-rays proved useful
as GRB signatures provided the searching satellite(s) could check out the source
region within a few days. The x-ray emissions persist over periods of hours to
days. This is one X-ray image of a presumptive GRB that was located in a galaxy
nearby (some has classified this as a hypernova):
Images
acquired by BeppoSAX were especially helpful in the sky survey for GRBs. The
top illustration consists of two intensity contoured images typical of X-ray
renditions; note the reduction in intensity in just four days between February
28 and March 3. Below it is a pair of BeppoSAX images taken first on December
15, showing the GRB as a bright dot and then on December 16 as the afterglow
had faded away.
Special
attention was given to finding GRBs at visible (optical) wavelengths, since
these are especially capable of measuring red shifts by which approximate
distance to the source can be estimated. About half the GRBs give off light in
the visible for durations of a week or more. The HST and the Keck Observatory
in Hawaii were pointed at targets reported by other observing satellites. Here
is the HST image of event GRB000301c.
A ground
telescope imaging of another GRB shows the burst as seen in visible light (here
the print is a negative) at 21 hours (left) and 8 days (right) after first
detection. The rapid fading of the galaxy-sized feature is evident (note
arrows)
Although
not used a lot for this purpose, radio telescopes have detected and imaged
GRBs. Here is one made by the VLA group:
One very
important GRB event led to some intriguing information that indicates that this
phenomenon occurred much more often early in cosmic time (but continues til the
present) and helps to confirm the huge amounts of energy involved. Its
magnitude is equivalent to 100 million billion solar radiances. On December 14,
1997 the CGRO registered this event. Word was sent to BeppoSAX operators and to
the HST and Keck telescopes to look for it as rapidly as possible. All
succeeded. This is how the event was imaged by the HST:
The image
on the right was taken on January 23, 1999 during its maximum. By February 8
(left) this burst (shown at a different scale, in the box) had faded to 1/4
millionth of the visible output.
When a
redshift distance measurement was made on the GRB, it was found to be some 12
billion light years from Earth, proving the surmise that GRBs have probably
been part of the Universe's history since soon after the Big Bang. It was also
the brightest object yet found at that far distance from Earth.
Thus, the
pattern found for most GRB events is rapid emission of gamma rays followed, as
they fade, by the dominant radiation passing through x-ray, visible, and radio
wavelengths, with the whole sequence being over in less than a few months.
GRBs are
of such high interest that another dedicated satellite has been placed in orbit
to look for these and similar events. This is HETE-2, the High Energy Transient
Explorer, launched October 9, 2000 (the first HETE failed to separate from its
third stage rocket). It is described at this MIT site. One of its most important discoveries is
described four paragraphs down.
The
cause(s) of GRBs continues to be uncertain and tantalizing. The early idea of
the explosion of material sucked in and around a neutron star (see top
illustration on this page for a similar example) has been challenged. But, a
variant postulates a role for a binary pair of neutron stars which, if they
should collide, produce a huge release of energy. Still others attribute the
GRBs to some involvement with quasars. One school holds them to be the outcome
of giant supernovae (hypernovae) which generate the most powerful short-time
energy release levels known in the Universe. A recent hypothesis takes still a
new tack - the GRBs are associated with large clusters of galaxies which
together have such a strong gravitational pull that they accelerate matter both
within and around the galaxies to high speeds that, upon colliding with
intergalactic matter, release energy at the gamma-ray level.
Another
hypothesis, known as the Paczynski Model (also known as a collapsar event) and
now the most favored explanation, starts with a supermassive (type O) rotating
star that collapses to form a black hole that continues to draw more material
around it until a critical state is reached that requires an intense
supernova-like explosion producing the GRB fireball. Essentially, all the mass
involved is suddenly converted to energy in obeyance to the Einstein E = mc2relation.
There are indications that this energy release may be directed, something like
the beam associated with a pulsar. Calculations show that if such a beam
generated from a GRB destruction of a neighboring massive star in the Milky Way
were to strike the Earth, the intensity would destroy everything at and above
the surface - oceans, vegetation, atmosphere, life (fortunately, the
probability of this happening, both in terms of star sizes and of
directionality of the beam, is considered quite low).
In fact,
until the release of information in June 2004 about a GRB only 35000 l.y. away
- either in or just outside the M.W. - no nearby events had ever been
confirmed. The image below shows W49B as a color composite made from Chandra
X-ray data (blue), and Palomar telescope images taken in the IR (green and
red). The estimated initial release of energy over a 1 minute time span is 1013
greater than that of the Sun. The image represent the GRB status soon after the
burst; the colors indicate enrichment in iron. The postulated beam associated
with collapsar events was not oriented straight at the Earth and hence is not
visible here.
Some of
the above information has been extracted from an article in the December 2002
edition of Scientific American, entitled "The Brightest Explosions in the
Universe", by N. Gehrels, L. Piro, and P. Leonard. The article contains
this illustration that summarizes the authors' ideas on the formation of GRBs:
From Scientific American, December 2002
In their
model, similar to some others proposed, GRBs are definitely associated
explosive processes that will end forming black holes. In one common mechanism,
a massive star collapses and explodes as a hypernova, leading to a disk of
matter/energy surrounding a black hole; this is a fast process in the sense
that at a critical time, the hypernova ensues without anything discernible
obviously leading up to it. Alternatively, over a long span of time (millions
of years, the same end result could occur as two neutron stars mutually
orbiting finally crash into each other. The wedge to the right of the 'Central
Engine' conforms to a jet that carries the photons released in the GRB outward
at near light-speed. This material moves outward as "blobs" that
catch-up and coalesce forming internal shock waves that generate the gamma bursts.
With expansion over time, the high energy photons are replaced by those of
progressively lower energies represented by x-rays, light, and radio waves as
the emissions encounter the galactic/intergalactic medium. The final result is
an afterglow that fades over time.
On March
29, 2003, HETE-2 captured a GRB (HETE Burst H2652 is also listed as GRB 030329
and SN2003dh) in a galaxy 2.6 billion light years distant and sent the
occurrence of this event back to Earth so quickly that many observatories were alerted
quick enough to train their telescopes on it within minutes. Thus, for the
first time the earliest stages of a GRB could be monitored. This event proved
one of the brightest ever observed. This plot of HETE data shows how brief was
the main phase of the event.
A radio
telescope image of this GRB taken on April 22 shows a progressive distribution
of decreasing energy moving outward. This seems to confirm the
"fireball" model for ejection of matter (an alternate explanation,
that material is ejected in huge blobs [the "cannonball" model, is
apparently not valid for this observation). The expelled material moves at
nearly the speed of light.
Spectrographic
data showed that the initial burst of H2652 was rich in excited Silicon and
Iron. These elements would be produced in a star whose mass is at least 30x
that of the Sun, which would give rise to temperatures and pressures that
generate nuclear reactions that fuse nuclei into Si and Fe. These are the
conditions that favor a "super-supernova", another way of referring
to hypernovae. Astronomers believe this is convincing evidence for that mode of
generation of most (perhaps all) GRBs.
Needless
to say, GRBs continue to fascinate cosmologists since they represent the
largest and fastest explosive events beyond that of the Big Bang itself. As
they are better understood, they may reveal the action of physical processes
only now being speculated upon, and suggested by particle physics experiments.