Gamma Ray Bursts: The most energetic explosions of gamma rays in the universe

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Image: ESO/L. Cal├žada.

Gamma Ray Bursts (GRBs) are extremely energetic explosions of gamma rays and the brightest electromagnetic events known in the universe. A remarkable progress has been achieved in understanding this astonishing astrophysical phenomenon in recent years. This progress has been reached thanks to the wealth of data acquired from X-rays, gamma rays and high-energy observations, as well optical and radio data from telescopes around the planet. Our knowledge has evolved from an elementary paradigm to an astrophysical laboratory involving interactions among many stars.

Recently, astronomers led by Professor R. Ruffini, published an article presenting the various types of Gamma Ray Bursts and their properties. GRBs are broadly classified based on the duration of the burst. Long bursts (L-GRBs) last for more than 2 seconds and up to several minutes, while short bursts (S-GRBs) last for less than 2 seconds.

For long bursts the most popular, perhaps, scenario that tries to explain this astrophysical phenomenon is called induced gravitational collapse. In this paradigm, a CO, He or Wolf-Rayet Star undergoes a Supernova explosion in the presence of a binary Neutron Star companion. Depending on the distance between the Supernova progenitor star and the Neutron Star, different sub-classes of long bursts exist. When the distance is large, the accretion of material from the Supernova onto the Neutron Star is not sufficient for the Neutron Star to reach a large enough mass (critical mass) to gravitationally collapse to a Black Hole. In this case, Gamma Ray Bursts are named X-ray flashes (XRFs). When the progenitor star and the Neutron Star are more tightly bound, then a Black Hole forms and in this case the Gamma Ray Bursts are called binary-driven hypernovae (BdHNe) or authentic L-GRBs.

For short bursts current paradigms indicate mergers of Neutron Star-Neutron Star or Neutron Star-Black Holes binaries. Sub-classes occur in this case, depending on the initial mass of the two mergers. If the initial mass of the two Neutron Stars is large enough to lead to a merged core with mass larger than the critical value required for a Black Hole to be formed, then the Gamma Ray Burst is called authentic S-GRB. When no Black Hole is formed then Short Gamma-Ray Flashes (S-GRFs) occur. Hybrid sub-classes also exist and show features between long and short Gamma Ray Bursts (e.g. Ultrashort Gamma Ray Bursts; U-GRBs).

As the available data increase, our theoretical understanding on the physical mechanisms that generate Gamma Ray Bursts will also evolve and our knowledge of this amazing phenomenon will go deeper, revealing the mysteries that govern it.

 

Publication: Ruffini et al. 2016