High-energy astrophysical phenomena can be experimental sites for fundamental physics. Deeply understanding these phenomena could be a tool toward high-energy and high-density physics. So far, I have been doing studies about the following topics:

Supernova Simulation / The Central Engine of Gamma-Ray Bursts / Core Collpase of First Stars / Gamma-Ray Bursts from First Stars / Others

 
 
 
 

Supernova Simulation

Core-collapse supernovae are violent explosions of massive stars at the end of their lives. Since the maximum brightness could be comparable to its host galaxy, there has been a lot of observations from ancient era. Nevertheless, the explosion mechanism of supernovae is still under the thick veil. The standard scenario is so-called "delayed explosion scenario", in which the copious neutrinos emitted from neutron star heat up the post-shock materials and produce explosion. Even though we include all known physics into numerical simulations, we could not produce successful explosion in computer using spherical symmetry.

Time evolution of entropy profile in axisymmetric simulations

I developed a new hydrodynamic code which incorporates the neutrino radiative transfer in axial symmetry, performed simulations, and obtained the explosion (Suwa et al. 2010). Although the shock propagates up to outside the iron core, the explosion energy is much smaller than the observational constrains. I proposed a mechanism to amplify the neutrino heating efficiency by the neutrino collective oscillation and performed various simulations in parametric manner. I investigated the reasonable parameter region to produce strong explosion (Suwa et al. 2011).

Entropy profile in three-dimensional simulation

The full three-dimensional (3D) code was also developed by the group including myself and the 3D-neutrino radiation hydrodynamic simulation was performed for the first time in the world (Takiwaki, Kotake, & Suwa 2012).

I studied the impact of the nuclear equation of state, which is one of the important ingredients of core-collapse supernova, on the dynamics using multi-dimensional simulation. I found that the equation of state that leads to faster contraction of protoneutron stars implies better condition to produce the explosion (Suwa et al. 2013).

 

Simulation movie is availabe! This is visualization of our 3D simulation done by 4D2U project at NAOJ.

 
 
 
 

The Central Engine of Gamma-Ray Bursts

Gravitational wave spectrum from GRB

One of the most important missing pieces for gamma-ray bursts is the jet production mechanism from the central engine. I showed that using the gravitational wave we can constrain the jet production mechanism (Suwa & Murase 2009).

Among possible candidates of the jet production mechanism, neutrino-pair annihilation is one of the most well-discussed. In this mechanism, copious amount of neutrinos are emitted from neutrino cooling accretion flow. I found that the infalling material in this flow accelerates neutrinos and the non-thermal components enhance the annihilation rate more than 10 times larger (Suwa 2013).

GRBs with shorter duration than 2 seconds are called as short GRBs. The most promising scenario of short GRBs is the merger of compact objects such as neutron stars and black holes. The group including myself performed fully-general relativistic MHD simulation and found that if a neutron star has magnetic field with stronger than 1012 G, magnetic-field driven jets would be launched (Shibata, Suwa, et al. 2011a), which is observable as a counter part of gravitational wave.

 
 
 
 

Core Collapse of First Stars

Spectrum of gravitational wave background originated from the first stars

The first stars in the universe are thought to be more massive than present stars. I performed numerical simulations for core collapse of these very massive stars (Suwa et al. 2007a) and estimated the amplitude of gravitational waves and neutrinos. Although these signals from a single source are too small to detect, integration over the whole cosmic age would be detectable as background components. I showed that both gravitational waves and neutrinos could be observable by future instruments (Suwa et al. 2007b, Suwa et al. 2009).

 
 
 
 

Gamma-Ray Bursts from First Stars

I developed an analytic model which describes a jet propagation inside a massive stellar envelope, in order to investigate the possibility of GRB production by a variety of stars. Using this method, I showed that first stars, which have massive envelope due to weak mass loss, could produce GRBs (Suwa & Ioka 2011). This is a very important fact because the first stars are one of the very important ingredients to understand the whole history of our universe, but the observational methodology is not constructed so far. If these stars could produce GRBs, our observations can reach them using these significantly bright event.

In order to confirm the validity of our analytic model, the group including myself performed numerical simulations of relativistic hydrodynamics (Nagakura, Suwa, & Ioka 2012). By comparing numerical results and analytic prediction, we gave reliability to our analytic modeling.

Since the prediction of typical mass scale of first stars decreases, the group including myself investigated the jet penetrability for less massive first stars and found that these stars could produce GRBs (Nakauchi, Suwa, et al. 2012). By using empirical relation of local GRBs, we gave a prediction for the spectrum and observability for future facilities. In addition, we estimated the brightness of photospheric emission from the cocoon component (Kashiyama, Nakauchi, Suwa, et al. 2013).

 
 
 
 

Others

I have been doing following studies in addition to above projects.

- Gravitational wave background from the early universe and reheating temperature (Nakayama et al. 2008a, 2008b)

- Particle acceleration at the formation of magnetars and high-energy neutrinos (Horiuchi, Suwa, et al. 2008)

- Formulation of covariant radiation transfer equation (Shibata et al. 2011b)

- An alternative method for pulsar equation (Takamori et al. 2014)