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DOI : 10.22661/AAPPSBL.2013.23.2.23
A Big Challenge on Gamma-Ray Bursts...
SHIGEHIRO NAGATAKI
File 1 : A Big Challenge on Gamma.pdf (445 KB)

A Big Challenge on Gamma-Ray Bursts: the Most Powerful Explosions in the Universe

S. NAGATAKI

YUKAWA INSTITUTE OF THEORETICAL PHYSICS, KYOTO UNIVERSITY

1. We were Born from a Supernovae

According to the state-of-the-art theories and observations, the creation of the universe happened 13.77짹0.0059 billion years ago [1]. Suddenly after the creation, an inflaton field(s) caused inflationary expansion of the universe [2]. It made the universe very homogeneous, but simultaneously quantum fluctuations of the inflaton field(s) provided inhomogeneity in energy density, which was inherited by inhomogeneity of matter density through the decay of the inflaton field(s) into particles. The inhomogeneity is imprinted on the cosmic micro wave background [3], and grew gradually due to gravitational instability, making large scale structures (filamentary structures of dark matter and galaxies that include lots of stars) in the universe [4].

When particles are created by the decay of inflaton field(s), the temperature was too high and all the particles were dissociated into elementary particles such as quarks and gluons. As the universe expanded gradually, its temperature also decreased. During the seventeen minutes, from three minutes to twenty minutes after the creation of the universe, nuclear reactions, called big-bang nucleosynthesis, happened efficiently [5]. After that phase, the temperature of the universe became too low to cause nuclear reactions anymore.

At the big-bang nucleosynthesis phase, only light elements, lighter than carbon: such as deuteron, helium, lithium, beryllium, and boron, were synthesized because called big-bang nucleosynthesis density of the universe was not so high as to cause 3-body reactions of helium to make carbon efficiently. 

On the other hand, our bodies, as well as the earth itself, are mainly composed of heavy elements such as carbon, oxygen, and iron. These elements are produced in stars. In stars, especially at the center of stars, the temperature and density are high enough to cause nuclear reactions. In fact, stars support their self-gravity due to the thermal pressure gradient produced by nuclear reactions. As a result, especially in massive stars, their chemical composition has an onion-like structure where an iron core exists at the center, surrounded by silicon, oxygen, carbon, helium, and hydrogen layers.

There is still an important question left. How were the heavy elements injected into space? The heavy elements are trapped in stars. If nothing happened further, there would be no heavy elements in space. The answer is supernova explosions. Stars, especially massive stars, end their lives as supernova explosions injecting heavy elements into space. The material of supernova ejecta enriched the metallicity of gases in space. From the gases, second generation stars were born. These stars end their lives again as supernova explosions providing gases in space moving heavy elements further. Through the cycles, the abundance of heavy elements in space increased. Finally, the solar-system was born about 4.6 billion years ago containing enough heavy elements to produce planets including the earth [6].  Our bodies, mainly made of heavy elements, come from supernova explosions. In other words, we can say that we were born from supernova explosions.

2. Gamma-Ray Bursts: The Most Powerful Explosion in the Universe

If you can see gamma-rays by your eyes, you will find that Milky Way is very bright. This is because we are in the Milky Way. However, about once a day, you will see a pretty bright burst that is brighter than anything else in the universe. This is called a Gamma-Ray Burst (GRB). It is very bright even though it is an extra-galactic object (i.e. it happens out the side of Milky Way). That means GRBs are very powerful explosions. In fact, they are the most powerful explosions in the universe. They emit energy in about ten seconds that is comparable to the energy released by the sun during its whole life.

What is the origin of GRBs? It has been a mystery for about twenty-five years after their discovery. Since 1998, it has turned out that some GRBs are born together with very energetic supernovae, called hypernovae [7]. Currently, about ten examples have been reported where GRBs and supernovae happened simultaneously at the same positions. From these statistics, it was proven with high significance that at least a class of GRBs is happening at the end of the lives of massive stars. From the state-of-the-art theories and observations, GRBs are considered jet-like phenomena and only special massive stars explode as hypernovae with GRB jets.

3. What is the Central Engine of GRBs?

It is very challenging to try to understand the central engine of a hypernova and GRBs formation of a GRB jet. Theoretically, some promising scenarios have been proposed, but they are not proved and lots of studies are necessary for their proof. At least, we can say that something unbelievable should be happening at the center since GRBs are the most powerful explosion in the universe.

One of the most promising scenarios is as follows: progenitor stars of GRBs should be rotating rapidly. As a result, rapidly rotating black holes are formed when their iron cores collapse due to their self-gravity. The rotational energy of the black holes can be very huge, and the explosion-energy of hypernovae and GRBs can be explained if a part of the huge energy of the black holes can be extracted. It means that the energy source of hypernovae and GRBs is the rotation energy of the black holes.

You may think that this scenario contains something strange. How can energy be extracted from a black hole? As you know, black holes absorb everything. Even photons cannot escape from a black hole as long as they pass through the horizon of the black hole. Extracting energy from a black hole sounds very strange and does not make sense. However, this is possible. Especially, it has been proven that huge energy can be extracted from a rapidly rotating black hole [8]. A rotating black hole has an ergo-sphere within which negative energy states are allowed. When a black hole absorbs a particle with negative energy, positive energy is extracted from the black hole. This mechanism can work very efficiently when a rapidly rotating black hole is surrounded by electro-magnetic fields since huge negative energy is absorbed in the form of Poynting flux. This is called the Blandford-Znajek effect [9].

For the investigation of the scenario, numerical simulation is a very promising approach since the dynamics of hypernovae and GRBs is very complicated and non-linear.

Figure 1: 2-D simulation of the formation of a GRB jet (blue region) driven by a rapidly rotating black hole [10]. The jet is launched by the Blandford-Znajek effect. The colors represent the ratio of thermal pressure relative to the magnetic pressure on a logarismic scale. The X and Y axis are scaled by natural units (c=G=M=1: c is the speed of light, G is the gravitational constant, and Mis the mass of the black hole).

Especially, general relativity must be taken into account because black holes are general relativistic objects and extracting energy from a rotating black hole is a general relativistic effect. In order to investigate the Blandford-Znajek effect, electro-magnetic fields have to also be taken into account in the simulations.

Magneto-Hydro-Dynamics (GRMHD) is a code used to investigate the central engine of GRBs in massive stars [10]. They performed 2-Dimensional (2-D) simulations (i.e. axial-symmetry is assumed with respect to the rotation axis of massive stars and black holes) and found that powerful jets are launched from the rapidly rotating black holes by the Blandford-Znajek effect, and they propagate along the rotation axis of the black holes (see Fig.1). These jets are naturally driven as a result of the simulations and can become GRB jets later.    

Recently, our group has succeeded to extend our GRMHD code for 3-Dimensional (3-D) simulations [11]. We found that a similar result can be obtained as long as an initial condition is set to be same as 2-D simulations (i.e. an axially-symmetric initial condition, see Figure 2).

On the other hand, when a small perturbation (1% in density) is introduced in the initial condition, it has been found that the perturbation grows rapidly to form an expanding outflow (see Figure 3). This is very new and cutting edge in this field. The outflow might be related to a hypernova component, but further investigation is necessary for the correct interpretation [11].

Figure 2: 3-D simulation of the formation of a GRB jet (blue region) by a rotating black hole [9]. The colors represent the rest-mass density and the white lines represent magnetic fields. The initial condition is set to be axially-symmetric.

These simulations are numerically very expensive and we could not have done them without super-computers. Our simulations were done by HITACH SR16000 at Yukawa Institute for Theoretical Physics, Kyoto University, Cray XE6 at the Academic Center for Computing and Media Studies, Kyoto University, Cray XT4 at the Center for Computational Astrophysics, the National Astronomical Observatory of Japan, HITACHI HA8000-tc/RS425 at the Information Technology Center, The University of Tokyo, and the K computer at the RIKEN Advanced Institute for Computational Science (see Figure 4). However, we are just at the starting point. More numerically expensive simulations are necessary for more realistic simulations to understand the central engine of hypernovae and GRBs. For example, neutrino transfer and a realistic equation-of-state for dense matter have to be taken into account, which are neglected in the current simulations. Especially, neutrino physics can be important for these dynamics. In fact, it is a key process to drive a normal, core-collapse supernovae [12]. In another promising scenario for the central engine of hypernovae and GRBs, neutrinos play a crucial role to drive the GRB jet: lots of neutrinos and anti-neutrinos are emitted from the accretion disks that are formed around the central black hole, and a thermal pressure-driven jet that will become a GRB may be launched along the rotational axis due to pair annihilation of the neutrinos [13]. This is also an interesting possibility. The true answer may be the combination of the Blandford-Znajek effect and pair-annihilation of neutrinos/anti-neutrinos. Furthermore, other promising scenarios may be found in the future. Lots of studies are necessary for the true understanding of these phenomena.

Figure 3: Time sequence (from top-left to bottom-right) of a 3-D GRMHD simulation with initial perturbation in density [11]. The colors represent the rest-mass density. The simulation itself is 3-D, but this figure shows a slice of the system on the equatorial plane.

Figure 4 (Courtesy of RIKEN): the K computer at the RIKEN Advanced Institute for Computational Science. The K computer has 88,128 nodes and each node has 8 cores. It was top-ranked in June and November 2011 in the TOP500 supercomputer list (10.51PFlops for LINPACK).   

For this purpose, we need to improve the numerical codes and perform very expensive numerical simulations where the simulations will have 6-dimensions (3-dimensions in space and 3-dimensions in the momentum space of neutrinos.) This is because the distribution function of neutrinos is not guaranteed to be thermal and isotropic. Definitely, next generation super-computers are necessary for this purpose.

We would like to find the true answer to these extreme phenomena, from the most powerful explosion in the universe, by taking the state-of-the-art physics into account with the help of super-computers.

4. GRBs, Are you Angels or Fallen Angels?

There are lots of mysteries and unsolved problems in the universe, of course. GRBs are frequently nominated as promising candidates to solve the mysteries and unsolved problems. This is because GRBs are fascinating actors/actresses in the universe. They may be true answers (Angels), but maybe not (Fallen Angels).

Figure 5 (Courtesy of RIKEN): Schematic picture of JEM-EUSO that is planned for launching in 2017. JEM-EUSO will be mounted to the International Space Station (ISS) so that air showers made by UHECRs can be detected from space.    

Figure 6 (Courtesy of RIKEN):

The Superconducting Ring Cyclotron (SRC) of the Radioactive Isotope Beam Factory (RIBF). SRC is the largest (weighing 8,300 tons) and most powerful in the world. RIBF is a facility generating unstable nuclei of all elements up to uranium in order to study their properties.

For example, GRBs may be the longest rulers to measure the size of the universe itself, by which we can estimate the composition of the universe (baryon, dark matter, dark energy), because GRBs are very bright objects. GRBs can also be promising sources of gravitational waves, which should exist as long as general relativity is correct, although they have not been detected yet. GRBs may produce Ultra-High Energy Cosmic Rays (UHECRs) whose energy is as high as 100 EeV, which is about 1000 times greater than the energy the LHC can produce on the earth. In other words, GRBs may be the most powerful accelerators in the universe [14]. The origin of UHECRs may be identified by the on-going projects like the Telescope Array Project and/or future projects like the JEM-EUSO Project, Figure 5. If GRBs produce UHECRs, some of them should interact with the gamma-rays of GRBs, producing Very-High-Energy neutrinos (VHE-neutrinos). The VHE-neutrinos may be detected on the earth with a GRB simultaneously [15]. GRBs may be the sites where very rare and heavy nuclei such as Gold and Uranium, (see also Figure 6) are produced. These nuclei are called rapid-process (r-process) nuclei. For the formation of the nuclei, very high temperatures and entropy, lots of free neutrons (produced by electron captures and/or absorptions of electron-type anti-neutrinos by protons), and rapid expansion are necessary. GRBs may satisfy such a special condition.  Lots of interesting studies and discussions on GRBs are going on.

5. Concluding Remarks

It will be a big challenge to the limits of human capability to try to understand the most powerful explosions in the universe. However, I believe this is possible. This will be achieved together with the progress of human beings' understanding of physics, the development of super-computers, and the state-of-the-art observations.

References

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Dr. Shigehiro Nagataki is an associate chief scientist of The Institute of Physical and Chemical Research (RIKEN). He became an assistant professor of The University of Tokyo in 2002, then he moved to YITP in 2004. Starting in April 2013, he has become an associate chief scientist of RIKEN to establish and manage a new laboratory, the "Astrophysical Big-Bang Laboratory", as the principal investigator. His research field is High-Energy Astrophysics including Gamma-Ray Bursts and Supernova Explosions.

 
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