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TOP > Report & Column > The Forefront of Space Science > 2014 > The Realization of Observing the Gamma-Ray Polarization and Studies on the Radiation Mechanism of Gamma-Ray Bursts

The Forefront of Space Science

The Realization of Observing the Gamma-Ray Polarization and Studies on the Radiation Mechanism of Gamma-Ray Bursts
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The IKAROS, a small solar-powered sailing demonstrator, launched in May 2010, is the first spacecraft to successfully demonstrate solar-sail technology in interplanetary space, taking the first step of the development of the next-generation solar-sail technology. With this success, this technology is being studied to be used as a navigation technology for getting to the Jupiter and its Trojan asteroids belt (Referring from Yuichi Tsuda’s “Solar-sail Technology of the IKAROS - Realization of the Outer Space Sailing With Solar-sail TechnologyE Osamu Mori’s “Returning Samples from Jupiter’s Trojan asteroidEand other articles from “ISAS NewsE.

Two kinds of scientific instruments were installed aboard IKAROS. One is a large-area dust counter (ALDN), which is used to detect the dust in the space. The other one is called the Gamma-Ray Burst Polarimeter (GAP), developed by our group (Referring from to “The Gamma-Ray Burst Polarimeter On IKAROSEby Toshio Murakami and etc. ).

The Biggest Explosions in the Universe: Gamma-ray Bursts

Gamma-Ray Bursts are phenomena that release a large amount of energy in gamma-ray band within short time duration of several tens of seconds, and they occur at cosmological distance over 10 billion light-years away. The total gamma-ray energy is much larger than one of supernova explosion. So we recognize the GRB as the largest explosion in the universe. GRBs and their afterglows are extremely bright even if they show within short time duration, so we can use the bright GRB flashes as a probe of distant universe (early universe). This phenomenon has increasingly become important in space science.

However, their radiation mechanism how to release the large amount of energy as gamma-rays has not been solved yet. According to the past observations, there are some evidences of existence of a narrowly collimated jet including photons and particles with relativistic speed more than 99.99% of the speed of light. However there are many issues about the radiation process how to produce the gamma-rays in it. In theoretical predictions, it is considered that there is a strong magnetic field within the relativistic jet, and electrons interacting with it produced the gamma rays, which is the so-called “synchrotron radiationE Since those electrons are moving in the plane perpendicular to the magnetic field under the influence of the Lorentz force, the gamma rays emitted at this time will be strongly polarized in the direction perpendicular to the magnetic field. Therefore, detecting the polarization of the gamma rays is considered the best way to reveal the radiation mechanism of GRBs.

Importance of Polarization Observation

The electromagnetic wave means that the oscillating electric field and magnetic field propagate as a wave. The direction of the oscillating electric field is defined as the direction of polarization. If the directions are completely random, it is called as non-polarized light, while when the directions are perfectly same, it is completely polarized light (or 100% polarized light). Here we only discuss the linear polarization. For example, in everyday life, polarization is used in polarized sunglasses for fishing or skiing. Since the lights reflected by water surfaces and ski slopes are strongly polarized, we can reduce the glare using a polarizing film which can cut such polarized lights. In space observation, from the lights scattered by gases or plasmas, as well as the radiation from the electrons entangled in a uniform magnetic field (the synchrotron radiation mentioned above), we expect to detect strongly polarized lights.

In the GRB observations so far, we have observed 3 types of physical quantities, i.e. the arrival direction, the time variability and the energy of gamma rays. If we add another independent information “the polarizationEhere, we will enable to find the existence of the magnetic field and the geometrical distribution of plasma, and also to approach the radiation mechanism from a new angle. Especially, there is a great advantage in verifying the existence of the magnetic field, which is predicted in the theoretical models.

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