arXiv subjects

Shuta J. Tanaka

Publications and source records attributed to Shuta J. Tanaka.

28 records · Page 2Linked to original sources

arXiv2016

Wide-Band Spectra of Giant Radio Pulses from the Crab Pulsar

We present the results of the simultaneous observation of the giant radio pulses (GRPs) from the Crab pulsar at 0.3, 1.6, 2.2, 6.7, and 8.4 GHz with four telescopes in Japan. We obtain 3194 and 272 GRPs occurring at the main pulse and the interpulse phases, respectively. A few GRPs detected at both 0.3 and 8.4 GHz are the most wide-band samples ever reported. In the frequency range from 0.3 to 2.2 GHz, we find that about 70\% or more of the GRP spectra are consistent with single power laws and the spectral indices of them are distributed from $-4$ to $-1$. We also find that a significant number of GRPs have such a hard spectral index (approximately $-1$) that the fluence at 0.3 GHz is below the detection limit ("dim-hard" GRPs). Stacking light curves of such dim-hard GRPs at 0.3 GHz, we detect consistent enhancement compared to the off-GRP light curve. Our samples show apparent correlations between the fluences and the spectral hardness, which indicates that more energetic GRPs tend to show softer spectra. Our comprehensive studies on the GRP spectra are useful materials to verify the GRP model of fast radio bursts in future observations.

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arXiv2016

A Broadband Emission Model of Magnetar Wind Nebulae

Angular momentum loss by the plasma wind is considered as a universal feature of isolated neutron stars including magnetars. The wind nebulae powered by magnetars allow us to compare the wind properties and the spin-evolution of magnetars with those of rotation-powered pulsars (RPPs). In this paper, we construct a broadband emission model of magnetar wind nebulae (MWNe). The model is similar to past studies of young pulsar wind nebulae (PWNe) around RPPs, but is modified for the application to MWNe that have far less observational information than the young PWNe. We apply the model to the MWN around the youngest ($\sim$ 1kyr) magnetar 1E 1547.0-5408 that has the largest spin-down power $L_{\rm spin}$ among all the magnetars. However, the MWN is faint because of low $L_{\rm spin}$ of 1E 1547.0-5408 compared with the young RPPs. Since most of parameters are not well constrained only by an X-ray flux upper limit of the MWN, we adopt the model parameters from young PWN Kes 75 around PSR J1846-0258 that is a peculiar RPP showing magnetar-like behaviors. The model predicts $γ$-ray flux that will be detected in a future TeV $γ$-ray observation by {\it CTA}. The MWN spectrum does not allow us to test hypothesis that 1E 1547.0-5408 had milliseconds period at its birth because the particles injected early phase of evolution are suffered from severe adiabatic and synchrotron losses. Further both observational and theoretical studies of the wind nebulae around magnetars are required to constrain the wind and spin-down properties of magnetars.

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arXiv2015

Avalanche Photon Cooling by Induced Compton Scattering: Higher-Order Kompaneets Equation

Induced Compton scattering (ICS) is an interaction between intense electro-magnetic radiations and plasmas, where ICS transfers the energy from photons to plasmas. Although ICS is important for laser plasma interactions in laboratory experiments and for radio emission from pulsars propagating in pulsar wind plasmas, the detail of photon cooling process has not been understood. The problem is that, when ICS dominates, evolution of photon spectra is described as a nonlinear convection equation, which makes photon spectra to be multi-valued. Here, we propose a new approach to treat evolution of photon spectra affected by ICS. Starting from the higher-order Kompaneets equation, we find a new equation that resolves the unphysical behavior of photon spectra. In addition, we find the steady-state analytic solution, which is linearly stable. We also successfully simulate the evolution of photon spectra without artificial viscosity. We find that photons rapidly lose their energy by ICS with continuously forming solitary structures in frequency-space. The solitary structures have the logarithmically same width characterized by an electron temperature. The energy transfer from photons to plasma is more effective for broader spectrum of photons such as expected in astrophysical situations.

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arXiv2015

X-ray Emission from Middle-Aged Gamma-Ray Pulsars

Electrons/positrons produced in a pulsar magnetosphere emit synchrotron radiation, which is widely believed as the origin of the non-thermal X-ray emission detected from pulsars. Particles are produced by curvature photons emitted from accelerated particles in the magnetosphere. These curvature photons are detected as pulsed $γ$-ray emissions from pulsars with age $\lesssim10^6$ yr. Using $γ$-ray observations and analytical model, we impose severe constraints on the synchrotron radiation as a mechanism of the non-thermal X-ray emission. In most middle-aged pulsars ($\sim10^5-10^6$ yr) which photon-photon pair production is less efficient in their magnetosphere, we find that the synchrotron radiation model is difficult to explain the observed non-thermal X-ray emission.

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arXiv2014

Synchrotron X-ray emission from old pulsars

We study the synchrotron radiation as the observed non-thermal X-ray emission from old pulsars ($\gtrsim1-10$Myr) to investigate the particle acceleration in their magnetospheres. We assume that the power-law component of the observed X-ray spectra is caused by the synchrotron radiation from electrons and positrons in the magnetosphere. We consider two pair production mechanisms of X-ray emitting particles, the magnetic and the photon-photon pair productions. High-energy photons, which ignite the pair production, are emitted via the curvature radiation of the accelerated particles. We use the analytical description for the radiative transfer and estimate the luminosity of the synchrotron radiation. We find that for pulsars with the spin-down luminosity $L_{\rm sd}\lesssim10^{33}$ erg s$^{-1}$, the locations of the particle acceleration and the non-thermal X-ray emission are within $\lesssim10^7$cm from the centre of the neutron star, where the magnetic pair production occurs. For pulsars with the spin-down luminosity $L_{\rm sd}\lesssim10^{31}$ erg s$^{-1}$ such as J0108-1431, the synchrotron radiation is difficult to explain the observed non-thermal component even if we consider the existence of the strong and small-scale surface magnetic field structures.

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arXiv2013

Constraint on Pulsar Wind Properties from Induced Compton Scattering off Radio Pulses

Pulsar winds have longstanding problems in energy conversion and pair cascade processes which determine the magnetization $σ$, the pair multiplicity $κ$ and the bulk Lorentz factor $γ$ of the wind. We study induced Compton scattering by a relativistically moving cold plasma to constrain wind properties by imposing that radio pulses from the pulsar itself are not scattered by the wind as was first studied by Wilson & Rees. We find that relativistic effects cause a significant increase or decrease of the scattering coefficient depending on scattering geometry. Applying to the Crab, we consider uncertainties of an inclination angle of the wind velocity with respect to the radio beam $θ_{\rm pl}$ and the emission region size $r_{\rm e}$ which determines an opening angle of the radio beam. We obtain the lower limit $γ\gtrsim10^{1.7}r^{1/2}_{\rm e,3}θ^{-1}_{\rm pl}(1+σ)^{-1/4}$ ($r_{\rm e}=10^3r_{\rm e,3}$ cm) at the light cylinder $r_{\rm LC}$ for an inclined wind $θ_{\rm pl}>10^{-2.7}$. For an aligned wind $θ_{\rm pl}<10^{-2.7}$, we require $γ>10^{2.7}$ at $r_{\rm LC}$ and an additional constraint $γ>10^{3.4}r^{1/5}_{\rm e,3}(1+σ)^{-1/10}$ at the characteristic scattering radius $r_{\rm c}=10^{9.6}r^{2/5}_{\rm e,3}$ cm within which the `lack of time' effect prevents scattering. Considering the lower limit $κ\gtrsim10^{6.6}$ suggested by recent studies of the Crab Nebula, for $r_{\rm e}=10^3$ cm, we obtain the most optimistic constraint $10^{1.7}\lesssimγ\lesssim10^{3.9}$ and $10^{6.6}\lesssimκ\lesssim10^{8.8}$ which are independent of $r$ when $θ_{\rm pl}\sim1$ and $1+σ\sim1$ at $r_{\rm LC}$.

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arXiv2012

Properties of Young Pulsar Wind Nebulae: TeV Detectability and Their Pulsar Properties

Among dozens young pulsar wind nebulae, some have been detected in TeV γ-rays (TeV PWNe), while others have not (non-TeV PWNe). The TeV emission detectability is not correlated either with the spin-down power or with the characteristic age of their central pulsars, and it is an open problem what determines the detectability. To study this problem, we investigate spectral evolution of five young non-TeV PWNe, 3C58, G310.6-1.6, G292.0+1.8, G11.2-0.3 and SNR B0540-69.3. We use a spectral evolution model which has been developed to be applied to young TeV PWNe in our previous works. TeV γ-ray flux upper limits of non-TeV PWNe give upper or lower limits on parameters, such as the age of the PWN and the fraction of the spin-down power going to the magnetic energy injection (the fraction parameter). Combined with other independent observational and theoretical studies, we can guess a plausible value of the parameters for each object. For 3C58, we prefer the parameters with an age of 2.5 kyr old and the fraction parameter of 3.0x10^{-3}, although the spectral modeling alone does not rule out a shorter age and a higher fraction parameter. The fraction parameter of 3.0x10^{-3} is also consistent for other non-TeV PWNe and then the value is regarded as common to young PWNe including TeV PWNe. Moreover, we find that the intrinsic properties of the central pulsars are similar, 10^{48-50}erg for the initial rotational energy and 10^{42-44}erg for the magnetic energy (2x10^{12} - 3x10^{13}G for the dipole magnetic field strength at their surfaces). The TeV detectability is correlated with the total injected energy and the energy density of the interstellar radiation field around PWNe. Except for G292.0+1.8, a broken power-law injection of the particles well reproduces the broadband emission from non-TeV PWNe.

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arXiv2011

Study of Four Young TeV Pulsar Wind Nebulae with a Spectral Evolution Model

We study four young Pulsar Wind Nebulae (PWNe) detected in TeV gamma-rays, G21.5-0.9, G54.1+0.3, Kes 75, and G0.9+0.1, using the spectral evolution model developed and applied to the Crab Nebula in our previous work. We model the evolution of magnetic field and particle distribution function inside a uniformly expanding PWN considering a time-dependent injection from the pulsar and radiative and adiabatic losses. Considering uncertainties in the interstellar radiation field (ISRF) and their distance, we study two cases for each PWN. Because TeV PWNe have a large TeV gamma-rays to X-rays flux ratio, the magnetic energy of the PWNe accounts for only a small fraction of the total energy injected (typically a few x 10^{-3}). The gamma-ray emission is dominated by inverse Compton scattering off the infrared photons of the ISRF. A broken power-law distribution function for the injected particles reproduces the observed spectrum well, except for G0.9+0.1. For G0.9+0.1, we do not need a low energy counterpart because adiabatic losses alone are enough to reproduce the radio observations. High energy power-law indices at injection are similar (2.5 -- 2.6), while low energy power-law indices range from 1.0 to 1.6. The lower limit of the particle injection rate indicates that the pair multiplicity is larger than 10^4. The corresponding upper limit of the bulk Lorentz factor of the pulsar winds is close to the break energy of the broken power-law injection, except for Kes 75.

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arXiv2010

A Model of the Spectral Evolution of Pulsar Wind Nebulae

We study the spectral evolution of PWNe taking into account the energy injected when they are young. We model the evolution of the magnetic field inside a uniformly expanding PWN. Considering time dependent injection from the pulsar and coolings by radiative and adiabatic losses, we solve the evolution of the particle distribution function. The model is calibrated by fitting the calculated spectrum to the observations of the Crab Nebula at an age of a thousand years. The spectral evolution of the Crab Nebula in our model shows that the flux ratio of TeV gamma-rays to X-rays increases with time, which implies that old PWNe are faint in X-rays, but not in TeV gamma-rays. The increase of this ratio is because the magnetic field decreases with time and is not because the X-ray emitting particles are cooled more rapidly than the TeV gamma-ray emitting particles. Our spectral evolution model matches the observed rate of the radio flux decrease of the Crab Nebula. This result implies that our magnetic field evolution model is close to the reality. Finally, from the viewpoint of the spectral evolution, only a small fraction of the injected energy from the Crab Pulsar needs to go to the magnetic field, which is consistent with previous studies.

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arXiv2009

Molecular Clouds as a Probe of Cosmic-Ray Acceleration in a Supernova Remnant

We study cosmic-ray acceleration in a supernova remnant (SNR) and the escape from it. We model nonthermal particle and photon spectra for the hidden SNR in the open cluster Westerlund 2, and the old-age mixed-morphology SNR W 28. We assume that the SNR shock propagates in a low-density cavity, which is created and heated through the activities of the progenitor stars and/or previous supernova explosions. We indicate that the diffusion coefficient for cosmic-rays around the SNRs is less than ~1% of that away from them. We compare our predictions with the gamma-ray spectra of molecular clouds illuminated by the cosmic-rays (Fermi and H.E.S.S.). We found that the spectral indices of the particles are ~2.3. This may be because the particles were accelerated at the end of the Sedov phase, and because energy dependent escape and propagation of particles did not much affect the spectrum.

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