arXiv subjects

E. Watanabe

Publications and source records attributed to E. Watanabe.

7 recordsLinked to original sources

arXiv2023

Magnetic Field Structure of the Crab Pulsar Wind Nebula Revealed with IXPE

We report a detailed study of the magnetic-field structure of the Crab pulsar wind nebula, using the X-ray polarization data in 2--8~keV obtained with the Imaging X-ray Polarimetry Explorer. Contamination of the pulsar emission to the data of the nebula region was removed through application of a stringent pulsation phase-cut, extracting a phase range of 0.7--1.0 only. We found that the electric field vector polarization angle (PA) was about $130^{\circ}$ from north to east with the polarization degree (PD) of about 25\% at the pulsar position, indicating that the direction of the toroidal magnetic field is perpendicular to the pulsar spin axis in the region close to the termination shock. The PA gradually deviated from the angle as an increasing function of the distance from the pulsar. There was a region of a low PD to the west of the X-ray torus. Although such a region is expected to be located at the torus edge, where geometrical depolarization due to a steep spatial variation of the PA is expected, the observed low-PD region positionally deviated from the edge. We found that the region of low PD positionally coincided with a dense filament seen in the optical band, and conjecture that the low-PD region may be produced through deflection of the pulsar wind. By comparing the values of the PD at the pulsar position between the data and a model, in which toroidal and turbulent magnetic fields were considered, we estimated the fractional energy of the turbulent magnetic field to be about $2/3$ of the total. We also evaluated a potential polarization of the northern jet in the nebula and derived the PD and PA to be about $30\%$ and $120^{\circ}$, respectively.

astro-ph.HE↗

arXiv2018

Observation of the quantum valley Hall state in ballistic graphene superlattices

In graphene superlattices, bulk topological currents can lead to long-range charge-neutral flow and non-local resistance near Dirac points. A ballistic version of these phenomena has never been explored. Here, we report transport properties of ballistic graphene superlattices. This allows us to study and exploit giant non-local resistances with a large valley Hall angle without a magnetic field. In the low-temperature regime, a crossover occurs toward a new state of matter, referred to as a quantum valley Hall state (qVHS), which is an analog of the quantum Hall state without a magnetic field. Furthermore, a non-local resistance plateau, implying rigidity of the qVHS, emerges as a function of magnetic field, and the collapse of this plateau is observed, which is considered as a manifestation of valley/pseudospin magnetism.

cond-mat.mes-hall↗

arXiv2016

X-ray and Rotational Luminosity Correlation and Magnetic Heating of the Radio Pulsars

Previous works have suggested a correlation between the X-ray luminosity Lx and the rotational luminosity Lrot of the radio pulsars.However, none of the obtained regression lines are statistically acceptable due to large scatters. We construct a statistical model which has an intrinsic Lx-Lrot relation and reproduces the observed Lx distribution about it by using a Monte Carlo simulator, which takes into account the effects obscuring the intrinsic relation,i.e., the anisotropy of radiation, additional heating, uncertainty in distance and detection limit of the instruments. From the ATNF pulsar catalog we collect 57 `ordinary radio pulsars' with significant detection and 42 with upper limits.The sample does not include the high-magnetic field pulsars (>10^{13} G), which are separately analyzed. We obtain a statistically acceptable relation Lx (0.5 - 10 keV)= 10^{31.69} (Lrot / L_0)^{c_1} with c_1 = 1.03 \pm 0.27 and L_0 =10^{35.38}. The distribution about the obtained Lx-Lrot relation is reproduced well by the simulator. Pulsars with abnormally high Lx fall into two types: one is the soft gamma-ray pulsars, and the other is thermally bright pulsars in comparison with the standard cooling curve. On the other hand, pulsars showing low Lx are found to have dim pulsar wind nebulae. We argue that there is an unknown mechanism that governs both the magnetospheric emission and the pulsar wind nebulae, and it might involve the production rate of electron-positron pairs. The high-field pulsars form a distinctive population other than the ordinary pulsars due to their excess luminosities.

astro-ph.HE↗

arXiv2013

Field-induced Confined States in Graphene

We report an approach to confine the carriers in single-layer graphene, which leads to quantum devices with field-induced quantum confinement. We demonstrated that the Coulomb-blockade effect evolves under a uniform magnetic field perpendicular to the graphene device. Our experimental results show that field-induced quantum dots are realized in graphene, and a quantum confinement-deconfinement transition is switched by the magnetic field.

cond-mat.mes-hall↗

arXiv2004

Spectroscopic Studies of Extremely Metal-Poor Stars with Subaru/HDS:I. Observational Data

We have obtained high-resolution (R $\simeq$ 50,000 or 90,000), high-quality (S/N $\ga$ 100) spectra of 22 very metal-poor stars ([Fe/H] $\la$ -2.5) with the High Dispersion Spectrograph fabricated for the 8.2m Subaru Telescope. The spectra cover the wavelength range from 3500 to 5100 Å; equivalent widths are measured for isolated lines of numerous elemental species, including the $α$ elements, the iron-peak elements, and the light and heavy neutron-capture elements. Errors in the measurements and comparisons with previous studies are discussed. These data will be used to perform detailed abundance analyses in the following papers of this series. Radial velocities are also reported, and are compared with previous studies. At least one moderately r-process-enhanced metal-poor star, HD 186478, exhibits evidence of a small-amplitude radial velocity variation, confirming the binary status noted previously. During the course of this initial program, we have discovered a new moderately r-process-enhanced, very metal-poor star, CS 30306-132 ([Fe/H] $= -2.4$; [Eu/Fe] $= +0.85$), which is discussed in detail in the companion paper.

astro-ph↗

arXiv2002

Chemical composition of carbon-rich, very metal-poor subgiant LP625-44 observed with the Subaru/HDS

We have obtained a high resolution (R~90,000) spectrum of the carbon- and s-process-element-rich, very metal-poor ([Fe/H]=-2.7) subgiant LP625-44, as well as that of HD140283 (a metal-poor subgiant with normal abundance ratio) for comparison, with the High Dispersion Spectrograph (HDS) for the Subaru Telescope for detailed abundance study. The oxygen abundance derived from the O I triplet around 7770A is uncertain, but the excess of oxygen in LP625-44 seems remarkable (perhaps by nearly a factor 10), in comparison with that of HD140283 derived from the same lines. The Na enhancement in LP625-44 is by about a factor 50, suggesting hydrogen burning in the 22Ne-rich layer in an asymptotic giant branch star which produces the abundance pattern of this object. In our new spectrum of LP625-44, the Pb I lambda 3683A line has been detected, as well as the Pb I lambda 4057A line which has already been studied, confirming the Pb abundance (log epsilon(Pb)~1.9) derived by the previous work. The abundance ratio of s-process elements at the second peak (e.g., La, Ce and Nd) to that at the third peak (Pb) in LP 625-44 is significantly higher (by a factor 5) than that in other three s-process element-rich objects recently studied by van Eck et al.. Recent theoretical works have modeled the s-process nucleosynthesis in the radiative layer of asymptotic giant branch stars in the inter-pulse phase, and the above results means that these processes produced a large scatter in the abundance ratios. Another possiblity is that different processes (e.g., s-process nucleosynthesis during thermal pulses) have contributed to heavy elements in the early Galaxy.

astro-ph↗

arXiv2001

Photospheric Abundances of Volatile and Refractory Elements in Planet-Harboring Stars

By using the high-dispersion spectra of 14 bright planet-harboring stars (along with 4 reference stars) observed with the new coude echelle spectrograph at Okayama Astrophysical Observatory, we investigated the abundances of volatile elements (C, N, O, S, Zn; low condensation temperature Tc) in order to examine whether these show any significant difference compared to the abundances of other refractory elements (Si, Ti, V, Fe, Co, Ni; high Tc) which are known to be generally overabundant in those stars with planets, since a Tc-dependence is expected if the cause of such a metal-richness is due to the accretion of solid planetesimals onto the host star. We found, however, that all elements we studied behave themselves quite similarly to Fe (i.e., [X/Fe]~0) even for the case of volatile elements, which may suggest that the enhanced metallicity in those planet-bearing stars is not so much an acquired character (by accretion of rocky material) as rather primordial.

astro-ph↗