An Introduction to Cosmic Flares: Gamma-Ray Bursts (I)
During the Cold War, a treaty was formulated by the Russians and Americans that neither nation could test nuclear weaponry whatsoever. This Nuclear Weapons (which could produce gamma rays) couldn’t have been tested deep underground as Seismic Wave detectors would soon pick them up. However, the possibility of testing such weapons high up in the atmosphere remained a daring possibility. To prevent it a satellite named VELA was launched.
The VELA satellite came with high-tech scintillators which could pick up signals of Gamma-ray bursts at fluence as low as 10^-8 Joules per square meter. Although it was launched for military purposes, it left behind a prominent impact in the field of Astrophysics. In July of 1967, it picked up the emission of Gamma Ray. However, soon, it was clear that neither the Americans nor the Russians were testing nuclear arsenals. So, it was fairly a natural question to ask where the Burst came from? Precisely speaking, did it come from Space, or the Earth below?
To clarify the situation, a few more satellites similar to VELA were launched. The primary idea was that, if those bursts came down from Earth, the time that took for the radiation to reach the satellites (let’s say 1,2,3) would be identical. On the other hand, if the timings differed, the radiation must be coming from outer space. The satellites soon picked up another burst and it was clear from the timing that the sources indeed came from outer space.
So, the next question would naturally be: Where from outer space were these gamma-ray signals coming from? Was it coming from the outskirts of the solar system or from somewhere deeper at a distance of millions of light-years? Well, the first clue was that the collision must be extremely strong enough to produce gamma rays. The three most probable candidates for these energetic flares were:
i. Collisions between comets at the outskirts of the Solar System
ii. ‘Earthquake’ like ‘quakes’ on the surfaces of Neutron Star
iii. During the formation of Black Holes
The collision between Comets: Comets generally orbit the sun at a distance of 10¹⁶ meters and have an effective radius of 1 kilometer with a density exceeding slightly that of ice. This leaves us with a mass of about 4x10¹² kilograms for a typical comet. Even if we assume that a head-on head collision between two such comets releases the energy of its collision entirely in gamma rays, gives us a total energy output of 4x10¹² joules (all in gamma rays). However, owing to the staggering distance, this would lead to a fluence of an embarrassing 3x10^-21 J/m², a100 trillion times less than what we expected. So, these gamma-ray flashes are clearly arriving from the outer solar system.
Neutron Star Quakes: Let’s now entertain the possibility of such collisions from Neutron Star Quakes. An upper limit to the Neutron Star mass is roughly 1.4 solar masses and with a radius of 10 kilometers. Quakes on such a star would be within a mass of 1/10000th of the total mass and within a radius of 1 meter. Some basic energy conservation calculations reveal a release of energy which is equal to 6x10³⁸ joules. If we were to receive a fluence of about 10^-7 J/m², such neutron stars would have to be placed at a distance of 2x10²² meters or at the outskirts of the Milky Way Galaxy. This phenomenon seems to be a real possibility that has to be taken into consideration.
Energy Released by Black Hole formation: If we assume that a 10 solar mass star collapses into a black hole and half of the star’s mass gravitationally falls on the other half, the total energy released would amount to a staggering 5x10⁴⁷ Joules! A fluence of 10^-7 joule would place such a huge chunk of energy release at a distance of 2x10⁴ Mega parsecs or typically at the other end of the universe! So, gamma-ray bursts from such explosions are indeed a promising possibility.
To be continued…
