arXiv subjects

Shota Kisaka

Publications and source records attributed to Shota Kisaka.

53 records · Page 3Linked 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

Electric Field Screening with Back-Flow at Pulsar Polar Cap

Recent $γ$-ray observations suggest that the particle acceleration occurs at the outer region of the pulsar magnetosphere. The magnetic field lines in the outer acceleration region (OAR) are connected to the neutron star surface (NSS). If copious electron--positron pairs are produced near the NSS, such pairs flow into the OAR and screen the electric field there. To activate the OAR, the electromagnetic cascade due to the electric field near the NSS should be suppressed. However, since a return current is expected along the field lines through the OAR, the outflow extracted from the NSS alone cannot screen the electric field just above the NSS. In this paper, we analytically and numerically study the electric-field screening at the NSS taking into account the effects of the back-flowing particles from the OAR. In certain limited cases, the electric field is screened without significant pair cascade if only ultrarelativistic particles ($γ\gg1$) flow back to the NSS. On the other hand, if electron--positron pairs with a significant number density and mildly relativistic temperature, expected to distribute in a wide region of the magnetosphere, flow back to the NSS, these particles adjust the current and charge densities, so that the electric field can be screened without pair cascade. We obtain the condition for the number density of particles to screen the electric field at the NSS. We also find that in ion-extracted case from the NSS, bunches of particles are ejected to the outer region quasi-periodically, which is a possible mechanism of observed radio emission.

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arXiv2015

X-ray-powered macronovae

A macronova (or kilonova) was observed as an infrared excess several days after short gamma-ray burst, GRB 130603B. Although the $r$-process radioactivity is widely discussed as an energy source, it requires huge mass of ejecta from a neutron star (NS) binary merger. We propose a new model that the X-ray excess gives rise to the simultaneously observed infrared excess via thermal re-emission and explore what constraints this would place on the mass and velocity of the ejecta. This X-ray-powered model explains both the X-ray and infrared excesses with a single energy source by the central engine like a black hole, and allows for broader parameter region, in particular smaller ejecta mass $\sim10^{-3}-10^{-2}M_{\odot}$ with iron mixed as suggested by general relativistic simulations for typical NS-NS mergers, than the previous models. We also discuss the other macronova candidates in GRB 060614 and GRB 080503, and implications for the search of electromagnetic counterparts to gravitational waves.

astro-ph.HE↗

arXiv2015

Magnetic field configurations of a magnetar throughout its interior and exterior -- core, crust and magnetosphere

We obtained the magnetic field configurations, including both poloidal and toroidal components, throughout the interior and exterior of magnetars using a realistic equation of state. We divided the magnetized star into the hydromagnetic equilibrium core, Hall equilibrium crust and twisted force-free magnetosphere. We systematically and simultaneously calculated these regions under various boundary conditions using the Green function relaxation method, and noted the following interesting characteristics of these numerical results. First, the strength and structure of core magnetic fields affect the crustal magnetic fields. Second, the current sheet on the core-crust interface affects both internal and external magnetic field configurations. Third, the twisted magnetosphere makes a cross-point of magnetic field lines, such as X-point geometry, in the magnetosphere. The X-point geometry appears and disappears according to the strength of the twisted field in the magnetosphere or the core-crust boundary conditions. Our results mean that both Hall magnetohydrodynamics secular evolution and magnetospheric dynamical evolution are deeply affected by conditions of another region and the core-crust stress of magnetars.

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arXiv2015

Isotropic Detectable X-ray Counterparts to Gravitational Waves from Neutron Star Binary Mergers

Neutron star binary mergers are strong sources of gravitational waves (GWs). Promising electromagnetic counterparts are short gamma-ray bursts (GRBs) but the emission is highly collimated. We propose that the scattering of the long-lasting plateau emission in short GRBs by the merger ejecta produces nearly isotropic emission for $\sim 10^4$ s with flux $10^{-13}-10^{-10}$ erg cm$^{-2}$ s$^{-1}$ at 100 Mpc in X-ray. This is detectable by Swift XRT and wide field X-ray detectors such as ISS-Lobster, Einstein Probe, eROSITA and WF-MAXI, which are desired by the infrared and optical follow-ups to localize and measure the distance to the host galaxy. The scattered X-rays obtain linear polarization, which correlates with the jet direction, X-ray luminosity and GW polarizations. The activity of plateau emission is also a natural energy source of a macronova (or kilonova) detected in short GRB 130603B without the $r$-process radioactivity.

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arXiv2015

Long-Lasting Black-Hole Jets in Short Gamma-Ray Bursts

Whether a short gamma-ray burst (GRB) is caused by a black hole (BH) or a neutron star (NS) after the merger of a NS binary is a crucial problem. We propose a BH model that explains short GRBs with long-lasting activities such as extended emission and plateau emission up to $\sim10000$ s. To extract the BH rotational energy, the topological evolution of the magnetic field should accompany the mass ejection, mass fallback, and magnetic field reconnection. The observations suggest the magnetic field decay from $\sim10^{14}$ G to $\sim10^{13} - 10^{11}$ G at the BH, bounded below by the pre-merger strength and kept constant while the luminosity is constant, and the fallback mass of $\sim10^{-4} - 10^{-2} M_{\odot}$, comparable to the ejecta mass implied by the macronova (or kilonova) in GRB 130603B. The BH model has implications for gravitational waves and the equation of state of NS matter.

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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.

astro-ph.HE↗

arXiv2015

Energy Sources and Light Curves of Macronovae

A macronova (kilonova) was discovered with a short gamma-ray burst, GRB 130603B, which is widely believed to be powered by the radioactivity of $r$-process elements synthesized in the ejecta of a neutron star binary merger. As an alternative, we propose that macronovae are energized by the central engine, i.e., a black hole or neutron star, and the injected energy is emitted after the adiabatic expansion of ejecta. This engine model is motivated by extended emission of short GRBs. In order to compare the theoretical models with observations, we develop analytical formulae for the light curves of macronovae. The engine model allows a wider parameter range, especially smaller ejecta mass, and better fit to observations than the $r$-process model. Future observations of electromagnetic counterparts of gravitational waves should distinguish energy sources and constrain the activity of central engine and the $r$-process nucleosynthesis.

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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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arXiv2014

The Evolution of High Temperature Plasma in Magnetar Magnetospheres and its Implications for Giant Flares

In this paper we propose a new mechanism describing the initial spike of giant flares in the framework of the starquake model. We investigate the evolution of a plasma on a closed magnetic flux tube in the magnetosphere of a magnetar in the case of a sudden energy release and discuss the relationship with observations of giant flares. We perform one-dimensional numerical simulations of the relativistic magnetohydrodynamics in Schwarzschild geometry. We assume energy is injected at the footpoints of the loop by a hot star surface containing random perturbations of the transverse velocity. Alfvén waves are generated and propagate upward, accompanying very hot plasma that is also continuously heated by nonlinearly generated compressive waves. We find that the front edges of the fireball regions collide at the top of the tube with their symmetrically launched counterparts. This collision results in an energy release which can describe the light curve of initial spikes of giant flares.

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arXiv2013

Search for a correlation between giant radio pulses and hard X-ray emissions in the Crab pulsar

We present the results of the search for a correlation between giant radio pulses (GRPs) at 1.4 GHz and hard X-rays at 15-75 keV from the Crab pulsar. We made simultaneous ground and satellite observations of the Crab pulsar over 12 hours in three occasions in April 2010, March and September 2011, and got a sample of 1.3*10^4 main-pulse phase GRPs. From these samples we have found statistically marginal enhancement (21.5%, 2.70 sigma) of hard X-ray flux within +/- 1.5 degree phase angle of the synchronous peak of main-pulse phase GRPs. This enhancement, if confirmed, implicates that GRPs may accompany plasma density increases in the pulsar magnetosphere.

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arXiv2012

Magnetic field decay with Hall drift in neutron star crusts

The dynamics of magnetic field decay with Hall drift is investigated. Assuming that axisymmetric magnetic fields are located in a spherical crust with uniform conductivity and electron number density, long-term evolution is calculated up to Ohmic dissipation. The nonlinear coupling between poloidal and toroidal components is explored in terms of their energies and helicity. Nonlinear oscillation by the drift in strongly magnetized regimes is clear only around the equipartition between two components. Significant energy is transferred to the poloidal component when the toroidal component initially dominates. However, the reverse is not true. Once the toroidal field is less dominant, it quickly decouples due to a larger damping rate. The polar field at the surface is highly distorted from the initial dipole during the Hall drift timescale, but returns to the initial dipole in a longer dissipation timescale, since it is the least damped one.

astro-ph.HE↗

arXiv2011

TeV cosmic ray electrons from millisecond pulsars

Recent γ-ray observations suggest that the γ-ray millisecond pulsar (MSP) population is separated into two sub-classes with respect to the pair multiplicity. Here, we calculate the cosmic ray electron/positron spectra from MSPs. Based on the assumption of the equipartition in the pulsar wind region the typical energy of electrons/positrons ejected by a MSP with the pair multiplicity of order unity is \sim 50 TeV. In this case, we find that a large peak at 10 - 50 TeV energy range would be observed in the cosmic ray electron/positron spectrum. Even if the fraction of pair starved MSPs is 10%, the large peak would be detectable in the future observations. We also calculate the contribution from MSPs with high pair multiplicity to the electron/positron spectrum. We suggest that if the multiplicity of dominant MSP population is \sim 10^3, electrons/positrons from them may contribute to the observed excess from the background electron/positron flux and positron fraction.

astro-ph.HE↗

arXiv2011

Multi-wavelength emission region of gamma-ray pulsars

Recent obserbations by Fermi Gamma-Ray Space Telescope of gamma-ray pulsars have revealed further details of the structure of the emission region. We investigate the emission region for the multi-wavelength light curve using outer gap model. We assume that gamma-ray and non-thermal X-ray photons are emitted from a particle acceleration region in the outer magnetosphere, and UV/optical photons originate above that region. We also assume that gamma-rays are radiated only by outwardly moving particles, whereas the other photons are produced by particles moving inward and outward. We parametrize the altitude of the emission region. We find that the outer gap model can explain the multi-wavelength pulse behavior. From observational fitting, we also find a general tendency for the altitude of the gamma-ray emission region to depend on the inclination angle. In particular, the emission region for low inclination angle is required to be located in very low altitude, which corresponds to the inner region within the last-open field line of rotating dipole in vacuum. This model suggests a modification of statistics about observed gamma-ray pulsars.

astro-ph.HE↗

arXiv2011

Multi-wavelength emission region of gamma-ray emitting pulsars

Using the outer gap model, we investigate the emission region for the multi-wavelength light curve from energetic pulsars. We assume that gamma-ray and non-thermal X-ray photons are emitted from a particle acceleration region in the outer magnetosphere, and UV/optical photons originate above that region. We assume that gamma-rays are radiated only by outwardly moving particles, whereas the other photons are produced by particles moving inward and outward. We parameterize the altitude of the emission region as the deviation from the rotating dipole in vacuum and determine it from the observed multi-wavelength pulse profile using the observationally constrained magnetic dipole inclination angle and viewing angle of the pulsars. We find that the outer gap model can explain the multi-wavelength pulse behavior by a simple distribution of emissivity, and discuss the possibility of further improvement. From observational fitting, we also find a general tendency for the altitude of the gamma-ray emission region to depend on the inclination angle. In particular, the emission region for low inclination angle is required to be located in very low altitude, which corresponds to the inner region within the last-open field line of rotating dipole in vacuum. This model suggests a modification of statistics about observed gamma-ray pulsars. Number of the sources with low inclination and viewing angles increases compared with previous estimate.

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arXiv2010

Mass-dependent evolution of the relation between supermassive black hole mass and host spheroid mass since z ~ 1

We investigate the evolution of supermassive black hole mass (M_BH) and the host spheroid mass (M_sph) in order to track the history of the M_BH-M_sph relationship. The typical mass increase of M_BH is calculated by a continuity equation and accretion history, which is estimated from the active galactic nucleus (AGN) luminosity function. The increase in M_sph is also calculated by using a continuity equation and a star formation model, which uses observational data for the formation rate and stellar mass function. We find that the black hole to spheroid mass ratio is expected to be substantially unchanged since z~1.2 for high mass objects (M_BH>10^8.5M_SUN and M_sph>10^11.3M_SUN). In the same redshift range, the spheroid mass is found to increase more rapidly than the black hole mass if M_sph>10^11M_SUN. The proposed mass-dependent model is consistent with the current available observational data in the M_BH-M_sph diagram.

astro-ph.CO↗

arXiv2008

The correlation of black hole mass with metallicity index of host spheroid

We investigate the correlation between the mass of the supermassive black holes (SMBHs) and metal abundance, using existing data sets. The SMBH mass $M_{bh}$ is well correlated with integrated stellar feature of Mgb. For 28 galaxies, the best-fit $M_{bh}$-Mgb relation has a small scatter, which is an equivalent level with other well-known relation, such as a correlation between the stellar velocity dispersion and the mass. An averaged iron index $<$Fe$>$ also positively correlates with $M_{bh}$, but the best-fit $M_{bh}$-$<$Fe$>$ relation has a larger scatter. The difference comes from the synthesis and evolution mechanisms, and may be important for the SMBH and star formation history in the host spheroid.

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