arXiv subjects

S. Nagataki

Publications and source records attributed to S. Nagataki.

At least 91 records · Page 5Linked to original sources

arXiv2010

An analysis of the temperature structure of galaxy clusters by means of the thermal Sunyaev-Zel'dovich effect

Measurements of the Sunyaev-Zel'dovich (hereafter SZ) effect distortion of the cosmic microwave background provide us with an independent method to derive the gas temperature of galaxy clusters. In merging galaxy clusters the gas distribution is inhomogeneous and, therefore, the method of temperature measuring based on the SZ effect should be more relevant than that based on an X-ray emission analysis. Here we study a method for measuring the gas temperature in merging clusters by means of the SZ effect. Our calculations of intensity maps of the SZ effect include relativistic corrections considered within the framework of the Wright formalism and utilize a cosmological numerical simulation of a merging galaxy cluster evolved with its baryon physics. We found that the gas temperature in merging clusters can be measured by means of the ratio of the SZ intensity at a low frequency (128 GHz) to that at a high frequency (369 GHz). This SZ intensity ratio permits us to reveal prominent features of the temperature structure caused by violent merger shock waves. Therefore, measurements of the ratio of the SZ intensities are a promising tool for measuring gas temperature in merging galaxy clusters.

astro-ph.CO↗

arXiv2009

Propagation of ultrahigh energy nuclei in clusters of galaxies: resulting composition and secondary emissions

We study the survival of ultrahigh energy nuclei injected in clusters of galaxies, as well as their secondary neutrino and photon emissions, using a complete numerical propagation method and a realistic modeling of the magnetic, baryonic and photonic backgrounds. It is found that the survival of heavy nuclei highly depends on the injection position and on the profile of the magnetic field. Taking into account the limited lifetime of the central source could also lead in some cases to the detection of a cosmic ray afterglow, temporally decorrelated from neutrino and gamma ray emissions. We calculate that the diffusive neutrino flux around 1 PeV coming from clusters of galaxies may have a chance to be detected by current instruments. The observation of single sources in neutrinos and in gamma rays produced by ultrahigh energy cosmic rays will be more difficult. Signals coming from lower energy cosmic rays (E < 1 PeV), if they exist, might however be detected by Fermi, for reasonable sets of parameters.

astro-ph.HE↗

arXiv2009

Understanding of GRB-SN Connection by General Relativistic MHD Simulations

I have developed two numerical codes to investigate the dynamics of collapsars. One is two-dimensional MHD code that are performed using the Newtonian (ZEUS-2D) code where realistic equation of state, neutrino cooling and heating processes are taken into account. The other one is two-dimensional general relativistic magnetohydrodynamic (GRMHD) code. I have performed numerical simulations of collapsars using these codes and realistic progenitor models. In the Newtonian code, it is found that neutrino heating processes are not efficient enough to launch a jet in this study. It is also found that a jet is launched mainly by toroidal fields that are amplified by the winding-up effect. However, since the ratio of total energy relative to the rest-mass energy in the jet is not as high as several hundred, we conclude that the jets seen in this study are not GRB jets. In the GRMHD simulation, it is shown that a jet is launched from the center of the progenitor. We also find that the mass accretion rate after the launch of the jet shows rapid time variability that resembles to a typical time profile of a GRB. We find that the energy flux per unit rest-mass flux is as high as 10^2 at the bottom of the jet. Thus we conclude that the bulk Lorentz factor of the jet can be potentially high when it propagates outward. We also performed two-dimensional relativistic hydrodynamic simulations of the collapsar model to investigate the explosive nucleosynthesis happened there. It is found that the amount of 56Ni is very sensitive to the energy deposition rate. Thus the synthesized 56Ni can be little even if the total explosion energy is as large as 10^52 erg. We conclude that some GRBs can associate with faint supernovae.

astro-ph.HE↗

arXiv2009

Development of General Relativistic Magnetohydrodynamic Code and its Application to Central Engine of Long Gamma-Ray Bursts

In order to investigate formation of relativistic jets at the center of a progenitor of a long gamma-ray burst (GRB), we develop a two-dimensional general relativistic magnetohydrodynamic (GRMHD) code. We show the code passes many, well-known test calculations, by which the reliability of the code is confirmed. Then we perform a numerical simulation of a collapsar using a realistic progenitor model. It is shown that a jet is launched from the center of the progenitor. We also find that the mass accretion rate after the launch of the jet shows rapid time variability that resembles to a typical time profile of a GRB. The structure of the jet is similar to the previous study: a poynting flux jet is surrounded by a funnel-wall jet. Even at the final stage of the simulation, bulk Lorentz factor of the jet is still low, and total energy of the jet is still as small as 10^48 erg. However, we find that the energy flux per unit rest-mass flux is as high as 10^2 at the bottom of the jet. Thus we conclude that the bulk Lorentz factor of the jet can be potentially high when it propagates outward. It is shown that the outgoing poynting flux exists at the horizon around the polar region, which proves that the Blandford-Znajek mechanism is working. However, we conclude that the jet is launched mainly by the magnetic field amplified by the gravitational collapse and differential rotation around the black hole, rather than the Blandford-Znajek mechanism.

astro-ph.HE↗

arXiv2008

High Energy neutrino signals from the Epoch of Reionization

We perform a new estimate of the high energy neutrinos expected from GRBs associated with the first generation of stars in light of new models and constraints on the epoch of reionization and a more detailed evaluation of the neutrino emission yields. We also compare the diffuse high energy neutrino background from Population III stars with the one from "ordinary stars" (Population II), as estimated consistently within the same cosmological and astrophysical assumptions. In disagreement with previous literature, we find that high energy neutrinos from Population III stars will not be observable with current or near future neutrino telescopes, falling below both IceCube sensitivity and atmospheric neutrino background under the most extreme assumptions for the GRB rate. This rules them out as a viable diagnostic tool for these still elusive metal-free stars.

astro-ph↗

arXiv2006

Very High Energy Neutrinos Originating from Kaons in Gamma-Ray Bursts

We simulate neutrino production in a gamma-ray burst (GRB) with the most detailed method to date. We show that the highest energy neutrinos from GRBs mainly come from kaons. Although there is little chance to detect such neutrinos, attempts of detection are very important to prove physical conditions in GRBs.

astro-ph↗

arXiv2003

High-Energy Neutrinos Produced by Interactions of Relativistic Protons in Shocked Pulsar Winds

We have estimated fluxes of neutrinos and gamma-rays that are generated from decays of charged and neutral pions from a pulsar surrounded by supernova ejecta in our galaxy, including an effect that has not been taken into consideration, that is, interactions between high energy cosmic rays themselves in the nebula flow, assuming that hadronic components are the energetically dominant species in the pulsar wind. Bulk flow is assumed to be randomized by passing through the termination shock and energy distribution functions of protons and electrons behind the termination shock are assumed to obey the relativistic Maxwellians. We have found that fluxes of neutrinos and gamma-rays depend very sensitively on the wind luminosity, which is assumed to be comparable to the spin-downluminosity. In the case where B=10^{12}G and P=1ms, neutrinos should be detected by km^3 high-energy neutrino detectors such as AMANDA and IceCube. Also, gamma-rays should be detected by Cherenkov telescopes such as CANGAROO and H.E.S.S. as well as by gamma-ray satellites such as GLAST. On the other hand, in the case where B=10^{12}G and P=5ms, fluxes of neutrinos and gamma-rays will be too low to be detected even by the next-generation detectors. However, even in the case where B=10^{12}G and P=5ms, there is a possibility that very high fluxes of neutrinos may be realized at early stage of a supernova explosion (t \le 1yr), where the location of the termination shock is very near to the pulsar. We also found that there is a possibility that protons with energies \sim 10^5 GeV in the nebula flow may interact with the photon field from surface of the pulsar and produce much pions, which enhances the intensity of resulting neutrinos and gamma-rays.

astro-ph↗

arXiv2003

Small Scale Clustering in the Isotropic Arrival Distribution of Ultra-High Energy Cosmic Rays and Implications for Their Source Candidates

We present numerical simulations on the propagation of UHE protons with energies of $(10^{19.5}-10^{22})$ eV in extragalactic magnetic fields over 1 Gpc. We use the ORS galaxy sample, which allow us to accurately quantify the contribution of nearby sources to the energy spectrum and the arrival distribution, as a source model. We calculate three observable quantities, cosmic ray spectrum, harmonic amplitude, and two point correlation function from our data of numerical simulations. With these quantities, we compare the results of our numerical calculations with the observation. We show that the three observable quantities including the GZK cutoff of the energy spectrum can be reproduced in the case that the number fraction $\sim 10^{-1.7}$ of the ORS galaxies more luminous than -20.5 mag is selected as UHECR sources. In terms of the source number density, this constraint corresponds to $10^{-6}$ Mpc$^{-3}$. However, since mean number of sources within the GZK sphere is only $\sim 0.5$ in this case, the AGASA 8 events above $10^{20.0}$ eV, which do not constitute the clustered events with each other, can not be reproduced. On the other hand, if the cosmic ray flux measured by the HiRes, which is consistent with the GZK cutoff, is correct and observational features about the arrival distribution of UHECRs are same as the AGASA, our source model can explain both the arrival distribution and the flux at the same time. Thus, we conclude that large fraction of the AGASA 8 events above $10^{20}$ eV might originate in the topdown scenarios, or that the cosmic ray flux measured by the HiRes experiment might be better. We also discuss the origin of UHECRs below $10^{20.0}$ eV through comparisons between the number density of astrophysical source candidates and our result ($\sim 10^{-6}$ Mpc$^{-3}$).

astro-ph↗

arXiv2003

A New Method for Calculating Arrival Distribution of Ultra-High Energy Cosmic Rays above 10^19 eV with Modifications by the Galactic Magnetic Field

We present a new method for calculating arrival distribution of UHECRs including modifications by the galactic magnetic field. We perform numerical simulations of UHE anti-protons, which are injected isotropically at the earth, in the Galaxy and record the directions of velocities at the earth and outside the Galaxy for all of the trajectories. We then select some of them so that the resultant mapping of the velocity directions outside the Galaxy of the selected trajectories corresponds to a given source location scenario, applying Liouville's theorem. We also consider energy loss processes of UHE protons in the intergalactic space. Applying this method to our source location scenario which is adopted in our recent study and can explain the AGASA observation above 4 \times 10^{19} eV, we calculate the arrival distribution of UHECRs including lower energy (E>10^19 eV) ones. We find that our source model can reproduce the large-scale isotropy and the small-scale anisotropy on UHECR arrival distribution above 10^19 eV observed by the AGASA. We also demonstrate the UHECR arrival distribution above 10^19 eV with the event number expected by future experiments in the next few years. The interesting feature of the resultant arrival distribution is the arrangement of the clustered events in the order of their energies, reflecting the directions of the galactic magnetic field. This is also pointed out by Alvarez-Muniz et al.(2002). This feature will allow us to obtain some kind of information about the composition of UHECRs and the magnetic field with increasing amount of data.

astro-ph↗

arXiv2003

Explosive Nucleosynthesis Associated with Formation of Jet-induced GRBs in Massive Stars

We perform 2-dimensional relativistic hydrodynamical simulations in the context of collapsar model. Calculations of explosive nucleosynthesis are also accomplished. We investigate the influence of the structure of the progenitor and energy deposition rate on the resulting explosive nucleosynthesis, assuming that 56Ni is mainly synthesized in the jet launched by the neutrino heating. We show the amount of 56Ni is very sensitive to the energy deposition rate. Thus we conclude that it is quite natural not to detect an underlying supernova in some X-ray afterglows as in GRB 010921. We also point out the possibility that the relative abundance of the elements with intermediate mass number such as Si and S in the X-ray afterglow of GRB 011211 may be naturally explained if the energy deposition rate at the central engine is relatively long because little amount of 56Ni should be synthesized under such an environment. If this discussion is true, there should be correlation between the line features in the X-ray afterglow and duration of the GRB. It should be noted that the duration of GRB 011211 is 270 seconds, making it the longest burst ever observed by Beppo-SAX although it suffers from the effect of red-shift (z_host=2.14), which supports our conclusion. Our results also suggest that the type I collapsar model in which the energy deposition rate is relatively low (\dot{E} \sim 10^{51} erg/s) might have difficulty in reproducing the observed amount of 56Ni in a hypernova such as SN 1998bw. This means that the mechanism of the central engine of a hypernova accompanying GRB may be constrained by the discussion of explosive nucleosynthesis.

astro-ph↗

arXiv2002

Revision of the Selection Function of the Optical Redshift Survey using the Sloan Digital Sky Survey Early Data Release: Toward an Accurate Estimate of Source Number Density of Ultra-High Energy Cosmic Rays

If Ultra-High Energy Cosmic Rays (UHECRs) are originated from nearby galaxies, modeling of the distribution of nearby galaxies is important to an accurate estimate the source number density of UHECRs. We investigate uncertainty of the selection function of the Optical Redshift Survey (ORS), which we used to construct a source model of UHECRs. The investigation is based on a comparison of numbe counts of ORS galaxies with those of the spectroscopic sample of the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR). We carefully count galaxies in the same absolute magnitude bin from the two samples. We find a slight systematic overestimate of the ORS counts outside 5000 km s$^{-1}$ by about a factor of 2. We revise the selection function of the ORS assuming that the SDSS counts are correct. Our revision is based on the absorption given in the ORS catalog as well as that computed from Schlegel et al. (1998), which is systematically larger than the former by $A_B \sim 0.1$ mag in the region of low absorption. It is found that introduction of Schlegel et al.'s absorption changes one of the parameters of the ORS selection function by more than 10%. The revision should be taken into account in the future analysis of the source number density of UHECRs based on the ORS. Using the revised selection function, we determine the global structure of the Local Supercluster (LSC) with a source model of UHECRs, that is, a number-density model consisting of a uniform spherical halo and an exponential disk. We find that the revision is insignificant in terms of the structure of the LSC. However, the revised selection function will be useful to other studies such as peculiar velocity and correlation function.

astro-ph↗

arXiv2002

TeV Neutrinos from Companion Stars of Rapid-Rotating Neutron Stars

We have estimated the number flux of of mu-neutrinos which are produced due to the hadronic interactions between the cosmic rays coming from a neutron star and the matter in a companion star. The event rate at 1 km^2 detectors of high-energy neutrinos such as ICECUBE, ANTARES and NESTOR is also estimated to be 2.7 \times 10^4 events yr^-1 when the source is located at 10 kpc away from the Earth. We have estimated the number of such a system and concluded that there will be several candidates in our galaxy. Taking these results into consideration, this scenario is promising and will be confirmed by the future observations.

hep-ph↗

arXiv2002

Gamma-Ray Burst Neutrino Background and Star Formation History in the Universe

We estimate the flux of the GRB neutrino background and compute the event rate at SK and TITAND in the collapsar model, assuming that GRB formation rate is proportional to the star formation rate. We find that the flux and the event rate depend sensitively on the mass-accretion rate. Although the detection of signals from GRBs seems to be difficult by SK, we find that we can detect them by TITAND (optimistically about 20 events per year in the energy range where the energy of anti-electron neutrino is larger than 50 MeV). Therefore, we show that we will obtain the informations on the mass-accretion rate of collapsar by observing the GRB neutrino background, which, in turn, should give us informations on the total explosion energy of GRBs.

astro-ph↗

arXiv2001

Anisotropic $e^+ e^-$ pressure due to the QED effect in strong magnetic fields and the application to the entropy production in neutrino-driven wind

We study the equation of state of electron in strong magnetic fields which are greater than the critical value $B_c \simeq 4.4 \times 10^{13}$ Gauss. We find that such a strong magnetic field induces the anisotropic pressure of electron. We apply the result to the neutrino-driven wind in core-collapse supernovae and find that it can produce large entropy per baryon, $S \sim 400 k_B$. This mechanism might successfully account for the production of the heavy nuclei with mass numbers A = 80 -- 250 through the r-process nucleosynthesis.

hep-ph↗

arXiv2001

Specifying the Environments around GRB, Explaining the Fe line in the X-Ray Afterglow of GRB000214

We present a model explaining the Fe K alpha line and the continuum in the afterglow of GRB000214. We pose the importance to seek the physically natural environment around GRB000214. For the reproduction of the observation, we need the ring-like remnant around the progenitor like that of SN 1987A produced by the mass-loss of the progenitor and the fireball spread over in every directions. The observation of GRB000214, in which the continuum power law spectrum decreased faster than the line, motivated us to consider the two independent systems for the line emission and the continuum spectrum. At first, the continuum spectrum can be fitted by the afterglow emission of the fireball pointing toward the observer which does not collide with the ring because the emission of GRB and the afterglow are highly collimated to the observer by the relativistic beaming effect. Secondly, the line can be fitted by the fluorescence of the Fe atoms in the ring illuminated by the X-ray afterglow. The significance of this study is that our model may constrain strongly the GRB model. Although the Supranova model assumes the extreme-ring-like remnant produced by the usual supernova explosion, this may not be probable. It is because the supernova remnants are known to be shell-like. The model also assumes two steps of explosions, on the other hand, we need only one explosion of the progenitor. In this sense, our scenario is more natural. Moreover, in the numerical simulations of Hypernova, the jet of the opening angle of only 1 degree is generated. In our model, the fireball which spreads over in every directions reconciles with the observation of 1 percent of the polarization in the observation of SN1998bw which showed the explosion might not be so collimated.

astro-ph↗

arXiv2001

Propagation of UHECRs from the Sources in the Super-Galactic Plane

We have performed the detailed numerical simulations on the propagation of the UHE protons in the energy range $E=(10^{19.5} - 10^{22.0}$) eV in the relatively strong extra-galactic magnetic field with strength $B= (10, 100)$ nG within about 40 Mpc. In this case, the deflection angles of UHECRs become so large that the no counterparts problem is simply solved. As for the source distribution, we assumed that it is proportional to the number distribution of galaxies within the GZK sphere. We have found many clusters, which mean the small-scale anisotropy, in our simulations. It has been also shown that the observed energy spectrum is well reproduced in our models without any fine-tuned parameter. We have used the correlation value in order to investigate statistically the similarity between the distribution of arrival directions of UHECRs and that of galaxies. We have found that each correlation value for each parameter set begins to converge when the number of the detected events becomes ${\large O}(10^3)$. Since the expected number counts by the experiment of the next generation such as TA, HiRes, Auger, and EUSO are thought to be the order of $10^3$, we will be able to determine the source distribution and values of the parameters in this study in the very near future. Compared with the AGASA data, the significant anisotropy on the arrival directions of UHECRs are found in the analysis of first and second harmonics. This may originate from the incompleteness of the ORS database. This problem may be also solved if the source distribution is slightly changed. For example, this problem may be solved if we assume that UHECRs come from some of the galaxies such as AGNs and radio galaxies.

astro-ph↗

arXiv2001

Implications of the Discovery of Millisecond Pulsar in SN 1987A

From the observation of a millisecond pulsar in SN 1987A, the following implications are obtained. 1) The pulsar spindown in SN 1987A is caused by radiating gravitational waves rather than by magnetic dipole radiation and/or relativistic pulsar winds. 2) A mildly deformed shock wave would be formed at the core-collapse and explosion in SN 1987A, which is consistent with the conclusion given in Nagataki (2000). 3) The gravitational waves from the pulsar should be detected in several years using a Fabry-Perot-Michelson interferometer as the gravitational detector, such as LIGO and TAMA. 4) The neutrino oscillation model is not a promising model for the explanation of the kick velocity of the pulsar in SN 1987A. The hydrodynamical instability model is more favored.

astro-ph↗

arXiv2001

Rapid-Process Nucleosynthesis in Neutrino-Magneto-Centrifugally Driven Winds

We have studied whether the rotation and magnetic fields in neutrino-driven winds can be key processes for the rapid-process (r-process) nucleosynthesis. We have examined the features of a steady and subsonic wind solutions which extend the model of Weber and Davis (1967), which is a representative solar wind model. As a result, we found that the entropy per baryon becomes lower and the dynamical timescale becomes longer as the angular velocity becomes higher. These results are inappropriate for the production of the r-process nuclei. As for the effects of magnetic fields, we found that a solution as a steady wind from the surface of the proto-neutron star can not be obtained when the strength of the magnetic field becomes $\ge$ $10^{11}$ G. Since the magnetic field in normal pulsars is of order $10^{12}$ G, a steady wind solution might not be realized there, which means that the models in this study may not be adopted for normal proto-neutron stars. In this situation, we have little choice but to conclude that it is difficult to realize a successful r-process nucleosynthesis in the wind models in this framework.

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