Gamma Ray Bursts

Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are the most violent transient phenomenon occurring in the universe. They are intense, short-lived bursts of extremely energetic gamma rays.  Among the various types of electromagnetic radiation which are radio waves, microwave, visible, ultraviolet, X-rays, and gamma rays, the gamma rays are the most energetic and are 1000-10,000 times more energetic than X-rays. A tremendous amount of energy is released during the GRB event, as a comparison, the energy released during a GRB is 10^27 times more than that released in a nuclear bomb explosion.  Also, the energy released within a few seconds is equivalent to that released by the Sun during its entire lifetime!  Most of the energy produced in a GRB is concentrated in two narrow cones on the opposite sides of the source. The energy released is not spread in all directions but is focused into one precise direction and therefore, GRB is a narrow beam of extremely energetic gamma-rays. Thus, GRBs are the brightest source of cosmic gamma-ray photons in the observable universe and the most powerful explosion in the Universe after the Big Bang. 

Fig 1: Artists conception of a Gamma Ray Burst

GRBs were accidentally discovered in the 1960s during the cold war. In 1963, the US and the Soviet Union signed the Nuclear Test Ban Treaty, but the US suspected that the Soviet Union might secretly conduct nuclear tests hence, to spy on them they launched the Vela satellites equipped with gamma-ray detectors in space to detect gamma radiation pulses. On July 2, 1967, at 14:19 UTC, the Vela 4 and Vela 3 satellites detected a flash of gamma radiation. But these flashes were unlike those coming from the nuclear weapon testing. After detailed analysis, it was found that the radiation did not have a terrestrial or solar origin. At that time, no one had a clue of where this radiation was coming from, most theories suggested that they have their origin in the Milky Way galaxy.

Fig 2: Post-launch separation of Vela 5A and 5B (source: Wikipedia)

With the addition of new space-based instruments such as Compton Gamma Ray Observatory (CGRO) and its Burst and Transient Source Explorer (BATSE) instrument in the 1990s, a large number of GRBs were discovered and it was found that these events were isotropically distributed in space and are not confined to any particular direction. With the addition of more advanced satellites over time, we now discover 1 GRB per day, though astronomers estimate that roughly 500 events are occurring within the same time. To date, we have discovered GRBs from extragalactic sources but they can occur within the Milky way as well.

Fig 3: Green dots from a large area telescope image showing gamma ray burst locations in the sky. (Image credit: NASA/DOE/Fermi LAT Collaboration).

 

Types of GRBs

GRBs are mainly classified into two types based on the duration of the GRB and their progenitors: long duration (Long GRBs) and short duration (Short GRBs). Long GRBs last from 2 seconds to a few minutes and they are thought to be associated with the death of a massive star in supernovas, but not every supernova leads to a gamma-ray burst. About 70% of the GRBs observed are long GRBs. The Short GRBs last for less than 2 seconds and seem to be associated with the merger of binary neutron stars or a neutron star with a black hole. The discovery of short GRB170817, 1.7s after the detection of gravitational wave event GW170817 from the same location in the sky confirmed the association of short GRBs with the merger of compact objects like binary neutron stars. 

Fig 4: An illustration of a gamma-ray burst involving a black hole and a jet of material racing across space. (source :NASA)

GRB emission studies have given us insight into how energy from GRB progenitors is transformed into radiation. There are two types of emissions: Prompt emission and Afterglow emission.  The Prompt emission is the initial high energy burst of gamma rays which is the defining characteristic of the GRB and it also helps in the localization of GRB in the sky. The prompt emission lasts for a few seconds to a few minutes and due to this brevity, prompt emission is not thoroughly studied and understood yet. On the other hand, the GRB afterglow occurs due to the interaction of the radiation with the interstellar medium at a later time. The afterglow is observed at different wavelengths such as X-ray, UV, optical, infrared, and generally remains detectable for days or longer.

Considerable developments in the studies of GRBs have been achieved thanks to the multi-wavelength and multi-messenger approaches. We are now able to simultaneously collect data for a  burst with different instruments working in different wavelength bands, thus providing a holistic understanding of the GRB emission processes.  Gravitational waves provide us information about the formation channel of compact objects and GRB provides us information about what happens after the merger of compact objects thus presenting a complete picture of the formation and evolution of compact objects.  Moreover, all the heavy elements such as gold, silver, platinum are formed during the merger of binary neutron stars hence, a detailed study of the GRBs will give us a better understanding of the formation of heavy elements in the universe. 

Over the decades, astrophysicists across the globe have made significant progress in GRB studies. Nevertheless, there are many challenges and unsolved puzzles in understanding GRBs, so a more detailed analysis is needed. India has many advanced facilities to study GRBs. The first Indian multi-wavelength astronomical satellite AstroSat has detected hundreds of GRBs to date, and its data is used for prompt emission studies. Indian telescopes such as the GROWTH-India telescope (GIT), the first fully robotic Indian telescope, have contributed to detecting GRB afterglow in the optical band. The observations in radio bands are being carried out with the help of the Giant Metrewave Radio Telescope (GMRT). With these facilities, astrophysicists carry out detailed studies of GRBs in multi-wavelength bands to understand the formation and emission mechanisms of these fascinating events occurring in the universe. Also, the upcoming missions will further aid in detecting large numbers of GRBs and their comprehensive analysis.