Gamma-Ray Bursts 000131 - 090423

Edo Berger,
CfA





Larger illustration.





GRB 090423 is older than
the earliest detected
galaxy or quasar, as of
April 23, 2009 (more).

GRB 090423 is not only the most distant gamma-ray burst ever detected, but it is also the most distant object ever detected in the universe overall (as of April 28, 2009). The previous record holders for age were GRB 080913 (NASA/GSFC press release and images) and GRB 050904 (NASA news release and images; and Price et al, 2005) both of which took place around 12.8 billion years ago. All three gamma-ray bursts (GRB 090423, 080913, and 050904) occurred during the Epoch of Reionization in the early universe.

GRB 000131 was located between 11 and 13 billion light-years (ly) from Sol, possibly within half a billion years of the Cosmic Dark Age before stars were born. Hence, its massive progenitor star (which probably had at least 20 to 30 Solar-masses) must have been slightly older (Andersen et al, 2000). The gamma-ray burst was created by a relatively large supernova that is sometimes called a "hypernova."

VLT (ANTU + FORS 1), ESO


Larger afterglow image.


Due to its extreme distance
and redshift, however, GRB
000131, at arrow, was a
extremely dim object in
lower energy wavelengths
such as visible and
infrared light (more).

Based on its high cosmological redshift, astronomers estimated that GRB 000131 was emitted when the universe was less than one and a half billion years old -- less than 10 percent of an estimated age of 12 to 14 billion years (ESO press release). Despite travelling at the speed of light, its high-energy gamma rays took more than 11 to 12 and a half billion years to reach the Solar System after they were emitted. Although GRB 000131, like other gamma-ray bursts, appears to have taken place in a remote "early galaxy" (or "sub-galactic clumps" of stars) that is smaller than today's luminous galaxies, astronomers found it difficult to detect that extremely dim, sub-galactic clump of stars even with the Hubble Space Telescope, as the observed fading of the afterglow indicated that the maximum brightness of the gamma-ray emission was explosion was at least 10,000 times brighter than its host galaxy. These early galaxies tend to be optically dim and to lack dense molecular cores (Trentham et al, 2002).

STScI, NASA
(Bloom et al, 2001)



Larger image
(8/17-19/01).



GRB 000131 was at
least 10,000 times
brighter than its
remote host galaxy
(more at GRB 000131
and Bloom et al, 2001).

Astronomers calculate that GRB 000131 had at least one trillion times the luminosity of Sol. Quick follow-up observations undertaken with the 8.2-m Antu instrument at European Southern Observatory's Very Large Telescope in the Paranal and the 1.5-meter Danish telescope at La Silla identified a faint, point-like object in visible light that was fading rapidly, the optical counterpart of the gamma-ray burst called the "afterglow" (Pedersen et al, 2000). By the second night, however, the object had faded in brightness to 30 million times fainter than the limit of visibility with the naked eye in Earth's night sky (Bhargavi et al, 2000).

VLT ANTU + FORS 1, ESO



Larger collage image.




Like all gamma-raybursts,
GRB 000131 faded very
rapidly (more).

Although some gamma-ray bursts last less than a second, GRB 000131 was a relatively bright and long-duration emission that lasted more than 100 seconds (R. Marc Kippen for the BATSE/UAH/MSFC Team, 2000). Its optical afterglow was detected 84 hours after burst detection. The rapid fading of that afterglow suggested that the burst was collimated as if from a directed jet of radiation, like many other gamma-ray bursts (Andersen et al, 2000).

VLT ANTU + FORS 1, ESO


Larger illustration.



The spectroscopic redshift of z=4.50
was calculated from the absorption
of light by intervening hydrogen
clouds at a Lyman-alpha break
wavelength of 670.1 nm (more).

Accurate measurement of GRB 000131's redshift required spectroscopic observations. By the time a spectrum of the gamma-ray burst's afterglow was obtained on February 8, 2000, its brightness had decreased further. Indeed, the object had become so faint (R-magnitude 25.3) that a total of 3 hours of exposure time was necessary with the Very Large Telescopes ANTU + FORS1 instruments at the ESO's Paranal Observatory. Based on the extreme, deduced photometric redshift of GRB 000131 indicating that the gamma rays had travelled an extreme long cosmological distance, astronomers predicted a "break" in the red region of the spectrum around 670 to 700 nm from the strong absorption of light from intervening intergalactic hydrogen clouds along the line of sight between GRB 000131 and the Solar System. Such a break is found in the spectrum of all remote objects (from the crowding of absorption lines creating an effect known as the "Lyman-alpha forest" before the Lyman-alpha spectral line at rest wavelength 121.6 nm). This break was indeed found at a wavelength of 670.1 nm, as was the finding that virtually all light at shorter wavelengths from the optical counterpart of GRB 000131 was absorbed by intervening hydrogen clouds. From the rest wavelength of the Lyman-alpha break (121.6 nm), the spectroscopic redshift of GRB 000131 was then determined to be 4.500 +/- 0.015, corresponding to a travel time of more than 90 percent of the age of the Universe and making GRB 000131 the most ancient and remote gamma-ray burst detected at the time -- for which its age and distance could be calculated (Andersen et al, 2000).

Brief but intense bursts of extemely energetic gamma rays have been detected since July 2, 1967 by orbiting satellites seeking evidence of nuclear bomb tests on Earth. Lasting from less than a second to several minutes, their origins in or outside the Milky Way galaxy were unknown, although the an Earthly origin was ruled out. It was not until the the late 1990s, however, that astronomers found it possible to locate the sites of some of these events (e.g., with the Beppo-Sax satellite). They also found that GRBs were too evenly distributed to be of nearby origin -- i.e., within the Milky Way (see GRB distribution maps from NASA's Goddard Spaceflight Center).

Unknown artist (more images at NASA)


Larger illustration.



When a black hole is created from
a supernova from a massive star or
a collision between neutron stars
(or a neutron star with a black hole),
one of a pair of bi-polar jets of
gamma rays travelling at near
light-speed may be directed at the
Earth (more).

GRBs have been found to be situated at extremely far (i.e., "cosmological") distances, implying that they must be tremendously powerful as the energy released during a burst lasting less than a second to a few minutes is more than that emitted by Sol during its entire lifetime of about 10 billion years. Indeed, GRBs appear to emit produce even more energy than supernovae or even quasars (which are energetically bright accretion disks and bi-polar jets around supermassive black holes that are most commonly found in the active nuclei of some distant galaxies and possibly even in the pre-galaxy period after the Big Bang). Astronomers now believe, however, that GRBs seems so powerful because most of their energy is being beamed out of bi-polar jets in a brief burst, unlike the later stage of a supernova when neutrinos are emitted from all around the exploding star. Indeed, GRBs may precede most, if not all, supernovae, but they would be far less commonly observed since only a few supernovae of hundreds are likely to be beaming one of their bi-polar jets at the Solar System. These jets require an extremely strong magnetic field that appears to be associated with the creation of a black hole with a debris disk.

André Roquette, ESO


Larger illustration.



Gamma-ray bursts are so luminous,
in part, because one jet of very
fast moving particles and gamma
radiation is fortuitously aimed
at the Solar System and eventually
detected, often after crossing
several billion light-years (more).

Thus far, most GRBs have been of the longer duration types that averaging 20 to 30 seconds long (Gehrels et al, 2002). Most of these have been found to precede large Type-II supernovas of massive stars (sometimes called "hypernova") in star-forming regions within distant galaxies, which is logical since massive stars live such short lives that they don't have time to move far from their birthplace. After the gamma-ray signal disappears, these GRBs exhibit "afterglows" of x-rays, visible light, and radio waves. These afterglows may be produced as the gamma-ray beam of photons traveling at near light-speed towards aimed towards Solar observers hits gas and dust thrown off previously by the dying star. Eventually, however, the neutrinos produced by the supernova are seen after the initial burst of gamma rays. Iron has been detected in the x-ray spectra of the afterglow, as would be expected since iron atoms are known to be synthesized and blown into space by supernova explosions. Since 1997, astronomers have identified more than 20 optical sources in the sky that are associated with gamma-ray bursts (GRBs).

© Josh Bloom
(Chart used with permission)




Larger image.




While gamma rays arrive first in the Solar
System and overshine lower energy emissions,
x-rays, visible and infrared light, and radio
waves associated with the supernova are
eventually perceived as well (more images).

Some gamma-ray bursts, on the other hand, are defined by extremely low luminosities and long spectral lags, indicating that high-and low-energy gamma-ray pulses arrived several seconds apart (Gehrels et al, 2002). These strange GRBs appear to occur at the same rate as certain types of supernovae, called Types Ib and Ic, which occurs when the core of a massive star implodes. In contrast to Type-Ia supernovae such as Tycho's Star and Supernova 1997ff, Types Ib and Ic do not exhibit a silicon line and are even less understood than Type Ia. Types Ib and Ic are believed to correspond to stars ending their lives (as Type-II supernovae), but such stars would have lost their hydrogen before, and so hydrogen lines don't appear on their spectra (more discussion). A Type Ib supernova may result from a high-mass star that has blown off much of its outer hydrogen and helium shells and so most closely resembles a Type Ia supernova. It is somewhat dimmer as much of the light is absorbed by the surrounding nebula of material that the star has just recently blown off, and no helium is seen in their spectra. A Type Ic supernova may be produced by a high-mass star that has blown off much of its outer hydrogen layer while still retaining a significant helium layer, and so it is similar to a Type Ib except that helium is seen in its spectrum.

© Werner Benger, Zuse Institute Berlin,
Albert Einstein Institute
(Artwork used with permission)

Larger illustration from a movie.


Some short-duration GRBs may be
the product of mergers between
neutron stars (or neutron stars
and black holes) in close binary
systems (more from Insights
Magazine and the movie).

Another small proportion of GRBs exhibit comparatively short-duration bursts that average only 0.3 seconds and very little x-ray and optical afterglow (Gehrels et al, 2002). Astronomers believe that these GRBs may be the product of collisions between neutron stars or with black holes in binary systems, when two such objects spiral toward each other and merge into one. These compact-object mergers, however, take billions of years to develop and so are believed to be relative young at less than five billion years old. Hence, such binary systems have time to drift away from star-forming regions, and so short-burst GRBs are found in more dispersed locations, where there is less gas and dust for the relativistic shock wave of an explosion to crash into and light up as a bright and long-lasting afterglow. Just as in the large supernovae (hypernovae) cases, however, the end result is the formation of a single black hole surrounded by a disk. In 2005, astronomers announced that GRB 050709 and GRB 050509B may be have created by collisions involving two neutron stars (more from Chandra X-Ray Observatory) and ESO), but that the presence of a second flare by GRB 050724 was more likely to have been produced by a neutron star's merger with a black hole (ESO).

Aurore Simonnet,
SSU, NASA





Larger illustration.





Many optically dark GRBs
are located in the local
universe but obscured by
surrounding dust (more).

Roughly half of the GRBs detected by NASA's Swift satellite since its 2004 launch are "dark" gamma-ray bursts that are not seen in visible light (and so are virtually invisible to optical telescopes). On May 1, 2009, a team of astronomers submitted a paper which concluded that some dark GRBs actually exploded relatively close to Earth Perley et al, 2009. In results presented on June 8, 2009 at the 214th Meeting of the American Astronomical Society, they reported that many dark GRBs may be obscured by surrounding dust, which may hide entire stellar nurseries (NASA news release; UC Berkeley press release; Rachel Courtland, New Scientist, June 8, 2009; and Keith Cooper, Astronomy Now, June 8, 2009). Using the 60-inch telescope at the Palomar Observatory, they found that found that 14 of 29 GRBs discovered by NASA's Swift satellite were dark, with no visible flash. Follow-up observations with the twin 10-meter telescopes at the Keck Observatory, however, indicated suggested that 3 of the 14 optically dark GRBs had have dimly visible afterglows and that the remaining 11 exploded in known visible galaxies, whose light had not been shifted to longer wavelengths by extreme distance. Although they did not find any dark GRBs that originated in the early universe (such as the subsequently discovered GRB 090423), the astronomers were able to estimate that ancient GRBs probably account for only around 0.2 to 0.7 per cent of all gamma-ray bursts detected since their initial discovery.

Using the 60-inch telescope at the Palomar Observatory, they found that found that 14 of 29 GRBs discovered by NASA's Swift satellite were dark, with no visible flash. Follow-up observations with the twin 10-meter telescopes at the Keck Observatory, however, indicated suggested that 3 of the 14 optically dark GRBs had have dimly visible afterglows and that the remaining 11 exploded in known visible galaxies, whose light had not been shifted to longer wavelengths by extreme distance. Although they did not find any dark GRBs that originated in the early universe (such as the subsequently discovered GRB 090423), the astronomers were able to estimate that ancient GRBs probably account for only around 0.2 to 0.7 per cent of all gamma-ray bursts detected since their initial discovery.

Unknown artist,
SkyWorks Digital, NASA



Larger illustration.



Gamma-ray bursts are
created by the destruction
of stars that focus energy
released in two bi-polar
beams of unusually great
intensity (more).

All GRBs are now thought to be created by explosions that create black holes with large disks of material around them. It now seems likely that an extremely magnetic field builds up during the formation of the disk. The field heats the disk material to such high temperatures that it creates a fireball of gamma rays and plasma and squirts out bi-polar jets of material near the speed of light along the rotational axis. As these blobs of high-speed matter ram into slower blobs of material emitted previously in the exploding fireball, shockwaves are created that generate the observed gamma-ray radiation.

Unknown artist (more images at NASA) -- larger illustration


After the first ("pre-burst") burst of gamma rays, shockwaves
in the fireball create the the main burst that quickly fades
to reveal the more typical "afterglow" of supernova-type
emissions at x-ray, visible light, and other lower energy
wavelengths (more images from NASA).

Some long-duration GRBs are called dark or "ghost" GRBs because they have been found and studied at lower energy wavelengths (mostly x-rays) instead of gamma rays (Gehrels et al, 2002). Moreover, many of these GRBs fail to shine in visible light. Since most of these GRBs lie in regions of star formation which tend to have abundant interstellar dust, visible light may be blocked by dust although x-rays pass through to the Solar System for observation. Some ghost GRBs also may be so far away that many wavelengths of light emitted by them may become absorbed by intergalactic gas. Finally, some ghosts may be intrinsically faint.

Lastly, some long-duration GRBs are x-ray-rich, giving off more x-ray than gamma-ray radiation, or even no detectable gamma radiation at all (Gehrels et al, 2002). Some of these x-ray flashes may come from explosions with a relatively large amount of baryonic matter such as protons that produce a "dirty fireball" with higher inertia from their substance, so that the fireball expands slower and is less energetically able to boost photons into the gamma-ray range. An even more interesting possibility, however, is that x-ray flashers come from explosions in even more distant regions of the universe, where cosmic expansion since the Big Bang would have shifted emitted gamma rays into the x-ray range and intergalactic gas blocks visible afterglow, as none of these x-ray flashes have been observed to have a detectable, visible-light afterglow.