July 2, 1986..... A very powerful gamma ray from the distant universe was suddenly caught on an American satellite monitoring the Soviets. With this coincidental discovery was born a new eye to see our universe - gamma ray astronomy! An unimaginably powerful cosmic ray or cosmic ray factory, which hid pulsars, supernova debris, as well as other extragalactic sources, began to find its trace. Could this powerful gamma ray cause our cancer?
Gamma-ray astronomy is an unprecedentedly beautiful and fascinating field of study in astronomy. Numerous very powerful charged particles are constantly raining down on our heads. We call this charged particle cosmic ray or cosmic ray. About 90% of cosmic rays are protons, 9% helium and the rest are high-mass atoms. The kinetic energy of cosmic rays is huge. So much so, that there is no laboratory or factory in our world that can give so much kinetic energy to any atom. But in the factory of nature, this powerful cosmic ray is constantly being produced.
Now naturally the question comes to our mind that how can the factory of nature be able to create such powerful particles? Gamma-ray astronomy helps us solve this puzzle of the universe. Gamma-ray astronomy is relatively new compared to other branches of astronomy. In the last 10 years, gamma-rays have entered the golden age of astronomy. Today we will learn about this new astronomy.
The measure of strength
First of all, we use the electron volt (eV) as a unit of energy in many departments of physics (1 eV = 1.6 x 10–12 erg). The kinetic energy of this cosmic ray or cosmic ray can be up to 1020 eV (i.e., twenty one zero after one). The magnitude of a tennis ball that travels at a speed of 90 kilometers per hour is equal to the speed of a tennis ball. However, the energy stored in this tennis ball is divided into 1024 atomic particles. Thus, the amount of energy contained in a particle of a tennis ball is less than 1 electron volt.
In the case of cosmic rays, on the other hand, this amount of energy, that is, 1020 eV energy, is stored in just one particle! Through this analysis we can understand what an infinite amount of energy this cosmic ray can contain. To date, we have been able to make a maximum of about 13 TeV of proton particles in the accelerator “Large Hadron Collider (LHC)” that we have developed in the CERN laboratory on the border of France and Switzerland for 20 years. (Teraelectron volts; 1 TeV = 1012 eV = 109 keV (kilo electron volts) = 108 MeV (mega electron volts) = 103 GeV (giga electron volts).) The energy of the cosmos is about a million times greater.
History of the discovery of cosmic rays
Let's turn the pages of history. The cosmic ray was discovered in 1912 by an Austrian researcher, Victor Hess. He used a gas balloon to rise from the surface to measure a type of radiation. We call this radiation ionization radiation and we use electroscope to measure this ionization. It was initially thought that this ionizing radiation was mainly caused by charged particles or radioactive rays coming from our surface. And as a result, its effects will gradually diminish as it moves upwards from the surface. Victor Hess tried to prove this conventional idea by experiment. He observed that the higher the balloon, the higher the effect of ionizing radiation. If this radiation were created for high-powered charged particles or radioactive rays emitted from the earth's surface, the effects of this radiation would gradually decrease with altitude. But Victor Hess saw the opposite.
From this unprecedented observation, Victor Hess concluded that the charge particles that cause this ionizing radiation are not coming from the surface, but from space, and that is why the effects of this radiation continue to increase with altitude. These charged particles are later called cosmic rays. Professor Hess was awarded the Nobel Prize in 1936 for this groundbreaking discovery.
We have countless cosmic factories in this vast expanse of space, where very high-potential positive and negative particles are constantly being accelerated. Now the question is, what are these factories? Where is their origin? How do we find these factories?
We know from a very general knowledge of physics that the motion of a charged particle bends as it passes through a magnetic field. The trajectory of charged cosmic rays also bends as they pass through the inter-galactic and interstellar magnetic fields. So when they reach the earth's surface, there is no way to know exactly where they came from. Therefore, we have no clear idea about the origin of this cosmic ray. This is where another type of wave, the electromagnetic gamma-ray wave, is of immense importance. This gamma ray can make it very easy for us to find those cosmic factories.
But, what is the relation of this cosmic ray with this electromagnetic wave?
The relationship between cosmic rays or cosmic rays and gamma rays is very deep! The cosmic ray radiates high-energy gamma rays after its birth by interacting with the surrounding magnetic field or relatively low-energy electromagnetic waves. The higher the energy of the cosmic ray, the higher the energy of the gamma ray.
Birth of gamma-ray astronomy
Gamma-ray astronomy began in a very unexpected way. In 1973, the United States and the Soviet Union signed an agreement banning the testing of nuclear weapons. Under the terms of this agreement, the testing of nuclear weapons in the atmosphere or in reservoirs is strictly prohibited. The United States suspected that the Soviet Union could secretly test nuclear weapons. So America was secretly keeping an eye on the Soviet Union. As part of this surveillance, the United States sent several satellites into space, known as "Vela". Two of these satellites were equipped with gamma-ray detectors. The purpose was that the detection of any nuclear weapon on Earth would easily detect gamma-rays emitted from the radioactive material used in nuclear weapons. On July 2, 1967, a very strong gamma signal was intercepted on the satellite. Surprisingly, scientists found that the gamma radiation did not come from the earth's surface, but from a cosmic source.
The discovery of this cosmic gamma-ray gave birth to gamma-ray astronomy.
Gamma-ray diagnostic method
The power of gamma rays can vary greatly. So no detector is enough to test on gamma rays. It takes different types of detectors. A detector means that a signal is emitted when gamma rays fall on the device. We use satellites to detect less powerful gamma-rays (20 mega electron volts to 100 giga electron volts). And we use surface detectors to detect high-powered gamma-rays. Now naturally we can ask why low energy gamma-rays cannot be detected from the surface?
The gamma-ray satellite detector interacts with electrons and positrons. Part of this electron-positron manic pair detector, called a particle tracker, leaves traces of their own trajectory as they pass through it (Figure 2). From that pattern we can understand from which direction the gamma-ray is coming. Notable among the current satellite-based gamma-ray detectors is NASA's "Fermi Large Area Telescope (Fermi-LAT)".
This detector was launched in 2006. For 12 years, this satellite detector has been searching for the source of numerous gamma rays.
Gamma-rays, on the other hand, are indirectly detected in a very attractive way with a surface detector. Before saying this method, let me tell you a small incident.
We know that gamma-rays emitted from radioactive substances are very harmful. They are capable of creating cancer in our body. Again using them in a controlled manner, we also destroy cancer cells. The energy of this gamma ray (from kiloelectron volts to mega electron volts) is many times less than that of cosmic gamma rays. So now the question is, can this cosmic ray cause cancer in our body?
The answer is - "No". Our atmosphere continues to protect us from such a powerful gamma-ray that causes cancer. When gamma-rays enter our atmosphere, they interact with molecules / atoms in the atmosphere to form electrons and positrons. These high-energy electrons and positrons emit gamma-rays again, which again form pairs of electrons and positrons. This process continues until the energy of the gamma-rays created in the atmosphere is less than the minimum energy required to produce one pair of electrons and positrons. In this way a fountain of charged particles and light particles is created in the atmosphere, which we call an electromagnetic shower.
As these showers descend to the bottom of the atmosphere, these charged particles stimulate the atmosphere to form a large angular field of a kind of blue light that extends 150-200 meters above the surface, which we call the Cherenkov light pool. In this light pool we place our telescopes and detect that blue light, through which we get an image of the electromagnetic shower, which also indirectly helps to indicate the direction of gamma rays. Let me say that the characteristics of this blue light are different from the characteristics of other blue lights. And that's why by detecting it, we can easily pinpoint the direction of gamma rays. The amount of Cherenkov light produced in our atmosphere from low-energy gamma-rays is very small and is easily absorbed into the atmosphere, making them difficult to detect on the surface.
We have many surface-based gamma-ray telescopes in the world. One of them is the Magic (MAGIC: Major Atmospheric Gamma Imaging Cherenkov) telescope. The telescope is located on the island of La-Palma in the Canary Islands of Spain.
Different forms of gamma source
Over the last 10-15 years, with this gamma-ray telescope, we have discovered numerous different sources of gamma-rays. We have found it in both galactic and extra galactic. The galaxy that our solar system belongs to is called the Milky Way, and the sources contained in this Milky Way are called the Galactic Source. And the sources outside our Milky Way are extragalactic. Notable among galactic sources is the supernova Remnant. There are also Pulsar, Pulsar Wind Nebula, Star Cluster, Binary System. Notable outside our Milky Way, the extragalactic source, is the active galactic nucleus, the nucleus of which is a black hole. There are also star burst galaxies where new stars are being born. They are all more or less cosmic ray factories. However, the characteristics and capabilities of each source are different. Through the identification of gamma rays from all these sources and their proper analysis, we can learn in detail about these cosmic ray factories. Through these discoveries we not only learn about these sources, but also through many basic researches in physics.
