Gamma Ray Bursts and Colliding Stars Part-1 - Remote Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial
Gamma Ray Bursts and Colliding Stars Part-1

Gamma Ray Bursts

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:

Energy (counts per second)-time diagram of a detected Gamma ray event recorded from a military (Vela program) satellite.

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:

Energy-time plot for a 1991 GRB event.

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:

A GRB afterglow associated with an event in the Milky Way, imaged at X-ray wavelength by 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). 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.

Full Sky distibution of up to 800 GRBs; larger ones shown as blotches

The BATSE (Burst and Transient Source Experiment) instrument on the Compton Gamma Ray Observatory (CGRO; see page 20-4) was particularly suited to detecting GRBs. Here is one image of an event that occurred several billion light years away:

A CGRO BATSE image of GRB980329 that was monitored on April 17, 1997; its peak output lasted 8 seconds.

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:

Duration of detectable radiation from a GRB at different 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):

A hyperNova (= GRB ?) event imaged by x-radiation picked up by the Einstein Observatory.

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 (right). 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.

BeppoSAX image pair of X-ray signals from a GRB.
Images taken a day apart of a GRB, in which x-radiation monitored by BeppoSAX is rendered like a visible image.

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 GRB 000301c.

Optical image of GRB000301c made by the HST.

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)

Photo made through the La Palma telescope of a GRB (arrows) at approximately 1 and 8 days after burst.

Although not used a lot for this purpose, Radio telescopes have detected and imaged GRBs. Here is one made by the VLA group:

VLA Radio wavelength image of GRB980329.

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:

HST�s optical image showing a huge outburst of Gamma rays from a possible hyperNova; over a month�s time the output dropped significantly (left).

The image on the right was taken on January 23, 1999 during its maximum. 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.

The cause(s) of GRBs continues to be uncertain and tantalizing. As an aid to the following discussion, use this Fireball Model to provide a framework for the starting energy, the expansion of the GRB, and the time involved in reaching the afterglow phase:

A model for GRB expansion.

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, should 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 very powerful short-time energy release levels. 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 = mc2 relation. There are indications that this energy release may be directed, something like the beam associated with a Pulsar. This is an artist's conception of the beam that seems to be formed for at least some GRBs:

Jet associated with a GRB.

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 in that timeframe. 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.

W49B, a nearby GRB; credit: J. Keohane, JPL

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:

Schematic showing the development and brief history of a typical GRB.
From Scientific American, December 2002

In their model, similar to some others proposed, GRBs are definitely associated explosive processes that will end up 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, Visible 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.

Energy release/time plot for GRB H2652.

The SWIFT satellite obtained this image of GRB 030329:

Visual image of GRB 030329.