Gamma Ray Burst: Most Mysterious and Energetic Event after Big Bang

gamma-ray burst
Artist’s illustration of a bright gamma-ray burst occurring in a star-forming region. Credit: NASA/Swift/Mary Pat Hrybyk-Keith and John Jones

Gamma Ray Burst (GRB) are the most powerful burst in the entire universe after big bang. GRBs outshine every gamma source in the sky even the sun. They brightness is more than hundred times the brightness of the normal supernovae. Since the gamma rays have the highest energy in all the electromagnetic radiation, they are the most lethal for any life form. The Gamma Ray Bursts are able to devastate the life on Earth if happens to occurs in the neighbourhood of the solar system.

Gamma Ray Burst eject two beams of gamma ray in opposite direction and if any one beam is directed towards the Earth only then we can detect it. Since our eyes cannot detect gamma rays and the atmosphere blocks the gamma ray from cosmic sources. These gamma ray burst were remained undiscovered for very long time even after the discovery of telescopes and beginning of satellite era.

Discovery of Gamma Ray Burst

The Gamma Ray Burst were discovered by Vela satellites of United States in 1960s. Vela satellites were sent in space to detect gamma radiation emitted after nuclear test in space. US suspected that USSR will try to violate “Nuclear Test Ban Treaty” signed in 1963. The Vela satellite detected gamma radiation but the spectrum of these gamma radiation were different from the spectrum of gamma radiation emitted after Nuclear Test.

The position of the Gamma Ray Burst could not be located during that time. More Vela satellites were sent to study these Gamma Ray Burst and rough positions of 16 GRBs were located. These position indicated that the origin of these gamma rays were not from Earth, Moon or Sun. They must be originated outside the solar system. These results were not made public until published in 1973.

Various theories were postulated to explain the origin of these Gamma Ray Bursts. The progress was made only after 1991 with the launch of Compton Gamma Ray Observatory having Burst and Transient Source Explorer (BATSE) instrument which was an extremely sensitive gamma ray detector. Huge amount of data was collected and it was found that the GRBs were not located in any particular direction but they were distributed isotopically. If the GRBs would have been originated in the Milky Way then the distribution would have been concentrated only in the galactic plane or around the center. Thus the isotropic distribution indicated that the GRBs are coming from all the directions and their sources must be extragalactic. The distance of GRBs could not be measured at that time. There were still few theories which can explain the galactic origin even with the isotropic distribution.

Afterglow

The models for Gamma Ray Burst postulated that the initial gamma burst should be followed by the slowly fading emission of radiation at longer wavelength which is created due to collision of burst ejecta and the interstellar gases. This slowly fading emission is called “afterglow”. The burst position at longer wavelength could not be determined at that time, thus all the attempts of finding the afterglow was unsuccessful.

The breakthrough came in February, 1997 when the satellite BeppoSAX detected GRB 970228 and when the X-ray satellite was turned toward the direction of burst it detected the fading X-ray emission after the burst. The William Herschel Telescope identified a fading optical counterpart 20 hours after the gamma ray burst. The deep imaging was able to identify a faint, distant host galaxy at location of burst pinpointed by the optical afterglow. Due to faintness of the galaxy, the distance could not be measured at that time.

Italian–Dutch satellite BeppoSAX
The Italian–Dutch satellite BeppoSAX, launched in April 1996, provided the first accurate positions of gamma-ray bursts.

Another breakthrough occurred when the BeppoSAX registered the event GRB 970508. The host galaxy was localized within four hours of burst and measurement were took much faster than the previous event. The redshift of host galaxy was found to be 0.835 which means the burst occurred at the distance of about 6 billion light years away from the Earth.

This two observations of GRB and determination of their distance from the Earth, proved that the Gamma Ray Burst are extragalactic. But the mystery of their origin was still unsolved. The first clue was found when the even GRB 980425 was registered and a bright supernova (SN 1998bw) occurred just within a day at the same location. This coincidence was giving the first clue about the relation between Gamma Ray Burst and the death of very massive stars.

This video shows the first 500 bursts detected by Swift and identifies some of the more notable bursts.

Types of Gamma Ray Burst

The Gamma Ray Burst show various types of complex light curves. No two Gamma light curve are identical and there exist large variations in almost every property. The duration of Gamma Ray Burst varies from millisecond to tens of minutes. There can be a single peak or various individual subpulses. Based on the duration of burst, there exist following types of Gamma Ray Burst:

Short Gamma Ray Bursts

The duration of short Gamma Ray Bursts is less than 2 seconds and their average duration is found to be around 0.3 second. The Short GRB account for 30 % of the Gamma Ray Burst and no afterglow was detected until 2005, thus very little information was known about their origins. Since then several dozens of short Gamma Ray Burst afterglow has been detected and localized. Several of which are associated with the region of little or no star formation like the elliptical galaxies and the center of large galaxy clusters. This confirms that the Short Gamma Ray Bursts are not associated with the death of massive stars and association with supernova was never observed.

See also:

Stellar Evolution: life of star
What is Supernova Explosion?

The true nature of short GRB is still unknown, the leading hypothesis is that they are originated from the merger of a neutron star into another neutron star or a neutron star into a black hole. The merger of neutron star is expected to produce kilonovae and evidence of association of kilonovae with GRB 130603B has been observed.

The average duration of 0.2 seconds indicate that the size of the source must be less than 0.2 light seconds (about 37000 miles). This suggest very compact source like neutron star. Afterglows of minutes to hours in X-rays are consistent with fragments of neutron star material which is tidally-disrupted (r-process radiation). Thus origin of short Gamma Ray Burst is related to the neutron star merger.

Long Gamma Ray Bursts

The long Gamma Ray Bursts have duration more than 2 seconds and they constitute more than 70 % of all the Gamma Ray Burst. The usually have brightest afterglow and thus have been studied in details as compared to short Gamma Ray Bursts. The long GRB are found to be associated with regions of galaxy with rapid star formation. The long GRB are believed to be originated from the death of massive star undergoing core collapse supernovae.

Gamma Ray Burst
Graph of the time versus number of bursts for the gamma-ray bursts observed by the BATSE instrument on the Compton Gamma-ray Telescope.

Ultra-long Gamma Ray Bursts

Ultra-long Gamma Ray Bursts have duration longer than 10000 seconds. They are believed to be associated with the collapse of blue supergiant stars. Very few ultra-long GRB have been identified till now.

Energetics and Beaming of Gamma Ray Bursts

The Gamma Ray Bursts are the brightest objects despite of happening at such a large distances. Assuming that the Gamma Ray Burst is spherical, the energy output of GRB 080319B turnout to be greater than 1054 erg. This energy is comparable to the order of solar mass, no known process can produce such a large energy in such a short time scale. The total energy output of supernovae are of the order of one thousandth of solar mass. Thus, the radiation in Gamma Ray Burst must be highly beamed which results in very high brightness along the beam direction.

Watch a supercomputer simulation of two neutron stars colliding, merging and forming a black hole. Events such as this are believed to be the source of short gamma-ray bursts. (Credit: NASA/AEI/ZIB/M. Koppitz and L. Rezzolla)
 

The Gamma Ray Burst results in a Black hole at center of the source. It is rapidly rotating and almost certainly has a large magnetic field. It creates a fireball of relativistic electrons, positrons and photons which expands and collide with stellar material and creates gamma ray which emerges from the star in the beams ahead of the blast wave with the speed of light.

Effect of Gamma Ray Bursts and their Rate of occurrence

All the Gamma Ray Bursts observed till today have been occurred in the other galaxies and thus remained harmless to the Earth. But even if single Gamma Ray Burst occurred in the Milky Way pointing towards the Earth, the results would be devastating.

The orbiting satellites have detected about one GRB per day. Since the GRBs are highly beamed so we mostly miss the GRBs when these are not pointed in our direction. So the actual rate of occurrence of GRB should be much larger about 1 per minute. Within our galaxy, the rate is estimated to be lie in between 1 per 100000 and 1000000 years. And less than 10 % of these would be beamed in our direction.

See also:

Why do planets spin?
What is Dark Matter? In Simple Terms Here

If the Earth happens to be lie in the direction of the beam, the Gamma Ray will not be able penetrate the atmosphere but they will damage the ozone layer drastically which will expose the organisms to the deadly radiation. The situation would be like a nuclear bombing on the side of Earth facing the GRB and the other side also will not be remained safe. For example if the GRB occurs in the WR 104 at the distance of 8000 light years and that hits the Earth for duration of 10 seconds, its gamma radiation will deplete 25 % of the ozone layer. This would lead to mass extinction, food chain depletion and starvation.

The GRB which is close enough to destroy the life completely occurs in about 5 billion years. It has been suggested that the Ordovician-Silurian extinction, 450 million year age was caused by a GRB. The species living on the surface and in the upper layer of ocean water like trilobites were hard hit and the species living in the deep water remained unharmed. The high amount of Carbon-14 in the trees during 774-775 is also believed to be caused by a GRB.

Conclusion:

The Gamma Ray Bursts are highly beamed and energetic. Their brightness is more than 100 times the brightness of Supernova. Long RGB are believed to be originated from the collapse of very massive stars and the short RGB are believed to be originated from the merger of a neutron star into other neutron star or a neutron star into a black hole. If the GRB beam hits Earth then the results would be devastating.

The rate of GRB was high in the past because galaxies were small and very big stars can be easily found in them. Since the rate of occurrence of GRB is very low, thus there are chances that the earth will be hit by a GRB in 5 billion years which is enough time for any civilization to grow and die themselves due to some other causes. Thus, this time is perfect for any civilization to grow and become advanced. If you have any doubt or idea, you can comment in the comment box. If you enjoyed this article, please like and share (don’t enjoy alone). If you want to know more about astronomy, you may subscribe us form email notification. You can also follow us on Twitter, Facebook, Google Plus, Tumblr and Pinterest.

References:

http://www.astro.sunysb.edu/lattimer/AST301/lecture_grb.pdf
http://iopscience.iop.org/article/10.1088/0034-4885/69/8/R01/meta
https://en.wikipedia.org/wiki/Gamma-ray_burst
http://imagine.gsfc.nasa.gov/science/objects/bursts1.html
http://earthsky.org/space/gamma-ray-bursts-are-the-most-powerful-explosions-in-the-universe
http://www.outerspacecentral.com/gamma_ray_page.html
http://cerncourier.com/cws/article/cern/59937
http://w.astro.berkeley.edu/research/grbs/grbinfo.html

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