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Gamma Ray Burst Images (Section 1)

Gamma Ray Burst Images (Section 2)

GRBs - How, Where, Why ?

Cosmic gamma-ray bursts, the brightest explosions in the known universe, may come from the fiery deaths of very massive stars in supernova explosions. Gamma-ray bursts (GRBs) were discovered by accident over 30 years ago. These bursts have been so transient that no one had been able to pinpoint a location or see one with other instruments. That changed on Feb. 28, 1997, when the Beppo-SAX satellite happened to be looking in the right direction when a burst went off. GRB 970228 was the first of these incredibly energetic events to be seen glowing afterward in X-rays and visible light. Now burst counterparts and the afterglows from them are being discovered at a pace that might seem frantic.

For the first time, the Beppo-Sax observatory was able to pinpoint the location of the bursts with sufficient accuracy to enable their detailed study utilizing ground-based telescopes. In modern astrophysics, it is important to examine objects in all parts of the electromagnetic spectrum - from radio wavelengths all the way up to gamma rays - to fully understand what a particular astronomical object is doing. As of June 1998 there were still only a few coincidental GRB counterparts discovered - five in the optical wavelengths, nine in X-rays, and two in the radio wavelength segment of the spectrum.

One of the leading theories for the cause of gamma ray bursts is the collapsar or failed supernova theory. A super-massive star, after burning all of its nuclear fuel, starts to explode as a supernova, but the outer layers of material are too massive to blow off, at which point the explosion becomes an implosion, which then forms jets of matter that burrow out through the poles and rip the star apart. If true, the observation of this event (in any wavelength) would provide the first direct evidence for what produces gamma-ray bursts. Additionally, some theorists suggest that a burst of gamma rays are seen only when one of the jets from the supernova's central black hole is pointed directly toward Earth.

Apparently, the gamma-ray burst of March 26, 1998 (GRB 980326) is also associated with a supernova explosion. This would indicate that some gamma-ray bursts are associated with the formation of black holes during the fiery deaths of their very massive progenitor stars. In the case of gamma-ray bursts, finding a counterpart in another part of the spectrum was crucial because the bursts were so bright and brief that they created more questions than answers about their morphology.

In addition, there are two other popular models concerning the processes which produce GRBs, both of which suggest that the bursts originate from the processes that also form a black hole.^1b In one model, two massive objects such as neutron stars or black holes (both of which are thought to be the end-products of previous supernova explosions) coalesce, forming a single massive black hole. GRB 971214 (discovered Dec. 14, 1997), at one time thought to be the physical representative of this model of GRB formation, started the theorists really scratching their heads because the source was too bright to match the preferred model of two neutron stars colliding.

In the second model, such a black hole is produced in a catastrophic collapse of the core of a massive star which becomes a supernova or "collapsar" and also explodes and forms a black hole. In this case, one would then expect two sources of light: the "afterglow" emission from the gamma-ray burst itself and light from the exploding star, a supernova. The afterglow rapidly declines whereas the supernova explosion gains in brightness over a period of a few weeks, and then gradually fades away.

A visible light afterglow was found following the burst of GRB 980326, which then rapidly faded away. Then a Caltech-led team discovered something never previously observed -- a dramatic rebrightening of optical emission at the position of the gamma-ray burst. Normally, the optical light of a gamma-ray burst vastly outshines its host galaxy for weeks. When the light from the gamma-ray burst fades, the apparent total brightness remains constant: all that remains is the light from the host galaxy. A month after GRB 980326, it looked as though the host galaxy was dominating the light, but the next time the team observed, some eight months after the burst, the "galaxy" was gone. "Galaxies do not just disappear, so we were astonished," Shri Kulkarni said. "Clearly, what we were seeing is a new source of light brightening one month and then fading away.

The team had also obtained spectra of the object at different times, and that provided additional clues. The spectrum of the source right after the burst was blue, which is common, but after a month it was very red. "That alone suggested that we were looking at some different phenomenon happening at the same location, but with a time delay of a few weeks." Both the rebrightening and the spectrum changes are naturally explained by the presence of a supernova. The intensity of the apparent re-burst matches the peak brightness of a supernova seen in a distant galaxy, and its red spectrum also has the right color.

The unexpected rebrightening is now believed to be due to the underlying supernova created in the explosion of the massive star. In this scenario, a black hole is quickly formed in the center of a massive star whose core is unable to support itself against gravity. When the star explodes, powerful jets from the central black hole emerge along the original axis of rotation, and gamma rays are created by the jets - if the jets are not pointed toward Earth, then we see only a supernova and the effects of the exploding star. Gamma rays as well as the light from the supernova would arrive at Earth if the jets were pointing in our direction.

There appears to be good evidence for an underlying supernova in another well-studied gamma-ray burst. However, it is still possible that there are other causes for gamma-ray bursts such as the coalescence of neutron stars. This represents the most direct evidence to date in favor of the massive supernova model. Gamma-ray bursts have over 150 theoretical models about their possible origins, but only a handful can come close to describing the true magnitude of the bursts.

Afterglow

While BATSE and a few other instruments can record the gamma-ray flash of a burst, the rest of the astronomy community have to work with the afterglow - if a counterpart is found in other parts of the spectrum - which can last hours or a year. ^1b Radio emissions from the afterglow provide unique information on the burst environment and the burst progenitor itself. Using radio telescopes in Socorro and other institutions, radio counterparts have been discovered in bursts where optical or X-ray counterparts were also found. In some cases, they were seen in radio and X-rays, but not visible light. Perhaps we are seeing a class of events that are optically dark - being obscured by dust in the areas where the gamma-ray bursts explode.

Afterglows are also seen in visible and near-visible wavelengths and continue to be among the most valued because they help scientists in trying to locate the hosts of bursts. A total of 14 have been observed with the Hubble Space Telescope, and in every case, the gamma-ray burst is right on top of the stellar field.

Many of the bursts are associated with blue galaxies - observed to have high rates of star birth. In many cases, the optical afterglow components are barely visible to Hubble, despite its incredible light gathering power. GRB 970228 (the numbers are the date of the burst) is the landmark sighting because it was the first optical component to be captured in visible light. When Hubble was able to look at it again some weeks later, scientists saw "a small smudge in the sky," a dwarf galaxy with the fading embers of the burst. In several cases, Hubble's resolution and its ability to distinguish colors have allowed scientists to pick out burst afterglows and barely observable host galaxies. Some of these irregular galaxies have the "morphology of a train wreck."

A different spectra was recorded for the famous burst of Jan. 23, 1999 (GRB 990123). The Robotic Optical Transient Search Experiment (ROTSE) caught this burst in optical wavelengths within seconds of its detection by BATSE. It has been suggested that BATSE sees gamma rays produced by internal shock waves as the exploding gas interacts with itself. The optical or visible part is caused by the external shock wave blazing forward through space and ramming into whatever dust and gas are there. That material can vary: some regions like the Coal Sack Nebula are so dense that the absorb light from stars and galaxies behind them, others are empty "superbubbles" swept clean by previous star explosions.Additionally, a late afterglow appears in gamma rays as the external shock wave causes reverse shocks within the expanding explosion. This appears as a smooth tail whose high spot early in the blast is masked by the blast itself. So it is that there is excellent agreement with the fireball model, but there are other oddities to investigate.

For twenty years acientists have been trying to understand what produced the X rays seen in the Centaurus A jet. The length - comparable to the diameter of the Milky Way Galaxy¹ - and shape of the X-ray jet pinned down the source of the radiation. Now it is know that the X-ray emissions are produced by extremely high energy electrons spiraling around a magnetic field. Besides the jets, there is a new population of sources in the center of the galaxy. They are grouped in a sphere around the nucleus, which tells us something very fundamental about how the galaxy, and the supermassive black hole in the center, were formed.

Astronomers have accumulated evidence with optical and infrared telescopes that Centaurus A collided with a small spiral galaxy several hundred million years ago. This collision is believed to have triggered a burst of star formation and supplied gas to fuel the activity of the central black hole. With Chandra, this type of detailed investigation is possible. Now we can see the main jet, the counter jet, and the extension of the jets beyond the galaxy. At a distance of eleven million light years from Earth, Centaurus A has long been a favorite target of astronomers because it is the nearest example of a class of galaxies called active galaxies. Active galaxies are noted for their explosive activity, which is presumed to be due to a supermassive black hole in their center.

The energy output due to the huge central black hole can in many cases affect the appearance of the entire galaxy. The Chandra X-ray image of Centaurus A, made with the High Resolution Camera, shows a bright source in the nucleus of the galaxy at the location of the suspected supermassive black hole. The bright jet extending out from the nucleus to the upper left is due to explosive activity around the black hole that ejects matter at high speeds from the vicinity of the black hole. A counter-jet extending to the lower right can also be seen - this jet is probably pointing away from us, which accounts for its faint appearance.

One of the most intriguing features of supermassive black holes is that they do not suck up all the matter that falls within their sphere of influence. Some of the matter falls inexorably toward the black hole, and some explodes away from the black hole in high-energy jets that move at near the speed of light. The presence of bright X-ray jets in the Chandra image means that electric fields are continually accelerating electrons to extremely high energies over enormous distances.

	Gamma Ray Burst Images (Section 1)
	Gamma Ray Burst Images (Section 2)
	The Most RECENT Gamma Ray Bursts Detected by BATSE
	Click HERE for a CHART OF MASS RATIOS for various STELLAR AND QUASI-STELLAR objects
	Observation Platforms
	Resources

The MAXBC Notices have 16 numbers at the bottom of the Notice. These are the peak countrates in the 8 LADs in the 64-msec sampling rate encountered over the 0 to 10 minutes after the initial trigger in two energy bands: the C1 band (20-50 keV) and the so called "burst channels" band, which is most of the time set to be the sum of the C2 band (50-100) and the C3 band (100-300).  The "burst channels" are those used in the 3 trigger time scales (64, 256, & 1024 msec). The "burst channels" can be set to any combination of the four channels: C1, C2, C3, and C4 (note C4 is E > 300 keV).

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