All you need to know about GRB's as a beginner.

G

amma-ray bursts (GRBs) are the most brilliant events in the Universe. Long

regarded as an exotic enigma, they have taken centre stage in high-energy astrophysics by virtue of spectacular discoveries during the past six years. It is now clear that they also have important applications in many other areas of astronomy.

GRBs mark the moment of “first light” in the universe; they are tracers of the star formation,

re-ionization, and metallicity histories of the universe; as well as laboratories

for studying core-collapse supernovae.

The source of most GRBs are billions of light years away from our home planet, Earth, and are believed to be extremely energetic phenomenon, so much so that a typical Gamma Ray Burst releases as much energy in a few seconds as our Sun will emit in its entire 10 billion year life span. The detection of GRBs in the first place, in 1967, was completely accidental. A satellite belonging to US armed forces, called The Druid, designed to detect covert nuclear weapons tests on Earth detected a sudden burst of Gamma-rays in the outer solar system.

Radically different hypotheses were proposed, then, to explain these events. Some even suggested that the Russians were testing their nuclear arsenal somewhere in deep space and the bursts originated from there, but no one knew for sure until 1997 when a study of the data gathered in the last 30 years was made with the inclusion of Optical Spectroscopy and Red-shift calculations.

It is now known that 99.95% of the energy of a typical GRB event is collimated in a highly focused jet which is emitted by the collapsing neutron star along its rotational axis at the final stages of its evolution. So, as the beam is greatly focused, most GRB events in the universe will remain undetected from earth-bound telescopes and satellites.

Since 1997, scientists have gathered quite a bit of knowledge about GRBs; not enough to explain these humongous phenomenon for sure, but enough to create some accepted theories.

It is known that when an extremely massive neutron star with very low metallic property and tremendously high rotational velocity is devoured by a black hole, jets of energy are emitted from the very core of the dying star. All this happens in about 2 seconds. After the star is devoured, its remaining fragments form an accretion disk around the black hole and it is slowly swallowed by a death trap. A relatively low amount of energy is emitted for the next few hours, giving rise to the signature 'afterglow' event.

If a GRB event takes place in our galaxy, and its jet is focused on Earth, it is almost certain that the planet will perish.

It is expected that 98% of the species on our planet will perish instantly even if the event takes place for just 1.5 seconds, and in the days following the GRB, our atmospheric ozone layer (which is already 75% destroyed by the lethal dose of radiation)will slowly evaporate. The remaining species will also start to die out due to complete exposure to harmful cosmic rays.

There is evidence that every 5 million years, such extinctive GRB events occurs and it is said that the Ordovician–Silurian extinction event some 450 million years ago was probably caused by a Gamma Ray burst in our milky way galaxy.

Stars like the Wolf-Rayet stars in our galaxy are leading candidates for an extinctive GRB because it has been observed that most of them have used up their nuclear fuel and are at their final stages of evolution.

A case study:

O

n December 26, 2004 a magnitude 9.3 earthquake occurred in the Indian Ocean off the coast of Sumatra in Malaysia.  It caused a powerful tsunami which devastated coastal regions of many countries leaving over 240,000 people either dead or missing.  It was the worst tsunami to affect this area since the 1883 explosion of Krakatoa. The earthquake that produced it was so strong that it exceeded, by a factor of 10, the most powerful earthquake to occur anywhere in the past 25 years.

• Indonesian 9.3 Richter earthquake:
     December 26, 2004 at 00 hours 58 minutes (Universal Time)

It is then with some alarm that we learn that just 44.6 hours later gamma ray telescopes orbiting the Earth picked up the arrival of the brightest gamma ray burst ever recorded.

• Gamma ray burst arrival:
     December 27, 2004 at 21 hours 36 minutes (Universal Time)

    This gamma ray blast was 100 times more intense than any burst that had been previously recorded, equalling the brightness of the full moon, but radiating most of its energy at gamma ray wavelengths.  Gamma ray counts spiked to a maximum in 1.5 seconds and then declined over a 5 minute period with 7.57 second pulsations.  The blast temporarily changed the shape of the Earth's ionosphere, distorting the transmission of long-wavelength radio signals.

 It was determined that the burst originated from the soft gamma ray repeater star, SGR 1806-20, a neutron star 20 kilometres in diameter which rotates once every 7.5 seconds, matching the GRB pulsation period.  SGR 1806-20 is located about 10 degrees northeast of the Galactic centre and about 20,000 to 32,000 light years from us, or about as far away as the Galactic centre.  (Originally, it had been thought to be 45,000 light years from us. but new results place it closer.) The outburst released more energy in a tenth of a second than the Sun emits in 100,000 years.  Other gamma ray bursts, whose explosions were intrinsically more powerful than this one, have been detected at the source of the explosion, but since those explosions originated in other galaxies tens of thousands of times more distant, the bursts were not nearly as bright when they reached our solar system.  What makes the December 27th gamma ray burst unique is that it was the first time that a burst this bright had been observed, one that also happened to originate from within our own Galaxy.

 Astronomers have theorized that gamma ray bursts might travel in association with gravity wave bursts.  In the course of their flight through space, gamma rays would be deflected by gravitational fields and would be scattered by dust and cosmic ray particles they encountered, so they would be expected to travel slightly slower than their associated gravity wave burst which would pass through space unimpeded.  

After a 45,000 year light-speed journey, a gamma ray burst arrival delay of 44.6 hours would not be unexpected.  It amounts to a delay of just one part in 9 million.  So if the gravity wave travelled at the speed of light (c), the gamma ray burst would have averaged a speed of 0.99999989 c, just 0.11 millionths slower.  There is also the possibility that at the beginning of its journey the gravity wave may have had a superluminal speed.

 The 9.3 Richter earthquake was ten times stronger than any other earthquake during the past 25 years, and was followed just 44.6 hours later on December 27th by a very intense gamma ray burst, which was 100 fold brighter than any other in the past 25 year history of gamma ray observation. It seems difficult to pass off the temporal proximity of these two Class I events as being just a matter of coincidence.  A time period of 25 years compared to a time separation of 44.6 hours amounts to a time ratio of about 5000:1.  For two such unique events to have such a close time proximity is highly improbable if they are not somehow related.  But, as mentioned above, gravity waves would very likely be associated with gamma ray bursts, and they would be expected to precede them.

 Many have inquired if there might be a connection between these two events. Not thinking of the gravity wave connection, astronomers have been reluctant to admit there might be a connection since they know of no mechanism by which gamma rays by themselves could trigger earthquakes.  They admit that the December 27th gamma ray burst had slightly affected the ionization state of the Earth's atmosphere, but this by itself should not have caused earthquakes.  However, if a longitudinal gravity potential wave pulse were to accompany a gamma ray burst, the mystery becomes resolved.  The connection between earthquakes and gamma ray bursts now becomes plausible.

 In his 1983 Ph.D. dissertation, Paul LaViolette brought attention to the terrestrial dangers of Galactic core explosions, pointing out that the arrival of the cosmic ray superwave they produced would be signalled by a high intensity gamma ray burst which would also generate EMP effects. He also noted that a strong gravity wave might be expected to travel forward at the forefront of this superwave and might be the first indication of a superwave's arrival.  He pointed out that such gravity waves could induce substantial tidal forces on the Earth during their passage which could induce earthquakes and cause polar axis torquing effects.  

[Please note, the gravity potential gradient associated with a stellar explosion or core explosion would drop off in intensity inversely with distance travelled (according to 1/r).  That is, it does not have force-distance dependence similar to the lunar tidal force.  So the impact would be quite significant. ]

LaViolette presents evidence showing that the superwave that passed through the solar system around 14,200 years ago had triggered supernova explosions as it swept through the Galaxy.  Among these were the Vela and Crab supernova explosions whose explosion dates align with this superwave event horizon.  He points out that these explosions could be explained if a gravity wave accompanied this superwave; it could have produced tidal forces which could have triggered unstable stars to explode as it passed through.   

Note that if this gamma ray burst had been as close as 10 light years it would have completely destroyed the ozone layer.  By comparison, the Galactic superwaves LaViolette has postulated to have been generated as a result of an outburst of our Galaxy's core and to have impacted the Solar system during the last ice age, would have impacted the solar system with a cosmic ray electron volley having an energy intensity 100 times greater than this hypothetical 10 light year distant stellar gamma ray burst.  

 In comparison, SGR 1806-20 has been estimated to have a stellar mass of 150 solar masses, whereas our Galactic core has a mass of 2.6 million solar masses.  In its present active phase, SGR 1806-20 is estimated to have luminosity 40 million times that of the Sun, whereas during its active phase the Galactic centre could reach luminosities of 400 trillion times that of the Sun.   So it is understandable that if the Galactic centre were to erupt, it would produce a gamma ray burst and a gravity wave far more intense than the outburst from this star.

 If anything, the December 27, 2004 gamma ray burst shows us that we do not live in a peaceful celestial environment.  And if the December 26th earthquake was in fact part of this same celestial event, we see that this stellar eruption claimed many lives.  For this reason, it is important that we prepare for the possibility of even stronger events in the future, the arrival of superwaves issuing from the core of our Galaxy.  Like the December 26th earthquake and the December 27th gamma ray burst, the next superwave will arrive unexpectedly.  

                                                 

 It would have been possible to determine whether a Galactic gravity wave had indeed immediately preceded the December 26th earthquake by examining data from gravity wave telescopes.  Since seismic waves from the Indonesian earthquake would have taken some time to propagate through the Earth to these gravity wave antenna, their signature could be distinguished from the gravity wave coming from SGR 1806-20.  However, the major gravity wave telescopes were unfortunately not in line at that time.   LIGO (Laser Interferometer Gravity Wave Observatory), which consists of two correlated telescopes, one in Washington state and one in Louisiana, each having a four kilometre long laser interferometer beam path, was in the process of being made operational and unfortunately was not collecting data at that time. An email was sent to the staff of the TAMA gravity wave antenna in Japan.  Dr. Takahashi, who is responsible for the detector, replied that their telescope was unfortunately not operating during that week since they were making modifications at the time.  So, at present, the gravity wave hypothesis remains neither confirmed nor disproven.