Yasufumi Kojima

Stable Operation of a 300-m Laser Interferometer with Sufficient Sensitivity to Detect Gravitational-Wave Events within Our Galaxy

TAMA300, an interferometric gravitational-wave detector with 300-m baseline length, has been developed and operated with sufficient sensitivity to detect gravitational-wave events within our galaxy and sufficient stability for observations; the interferometer was operated for over 10 hours stably and continuously. With a strain-equivalent noise level of h approximately 5x10(-21)/sqrt[Hz], a signal-to-noise ratio of 30 is expected for gravitational waves generated by a coalescence of 1.4M-1.4M binary neutron stars at 10 kpc distance. We evaluated the stability of the detector sensitivity with a 2-week data-taking run, collecting 160 hours of data to be analyzed in the search for gravitational waves.

The bottom magnetic field and magnetosphere evolution of neutron star in low-mass X-ray binary

The accretion-induced neutron star (NS) magnetic field evolution is studied through considering the accretion flow to drag the field lines aside and dilute the polar-field strength, and as a result the equatorial field strength increases, which is buried inside the crust on account of the accretion-induced global compression of star crust. The main conclusions of model are as follows: (i) the polar field decays with increase in the accreted mass; (ii) the bottom magnetic field strength of about 10 8 G can occur when the NS magnetosphere radius approaches the star radius, and it depends on the accretion rate as ˙ M 1/2 ; and (iii) the NS magnetosphere radius decreases with accretion until it reaches the star radius, and its evolution is little influenced by the initial field and the accretion rate after accreting ∼0.01 M� , which implies that the magnetosphere radii of NSs in low-mass X-ray binaries would be homogeneous if they accreted the comparable masses. As an extension, the physical effects of the possible strong magnetic zone in the X-ray NSs and recycled pulsars are discussed. Moreover, the strong magnetic fields

Quasi-toroidal oscillations in rotating relativistic stars

Quasi-toroidal oscillations in slowly rotating stars are examined within the framework of general relativity. Unlike the Newtonian case, the oscillation frequency to first order of the rotation rate is not a single value, even for uniform rotation. All the oscillation frequencies of the r-modes are purely neutral and form a continuous spectrum limited to a certain range. The allowed frequencies are determined by the resonance condition between the perturbation and the background mean flow. The resonant frequency varies with the radius according to the general relativistic dragging effect.

Spin period evolution of a recycled pulsar in an accreting binary

We investigate the spin period evolutions of recycled pulsars in binary accreting systems. Taking both the accretion induced field decay and spin-up into consideration, we calculate their spin period evolutions influenced by the initial magnetic field strengths, initial spin periods, and accretion rates. The results indicate that the minimum spin period (or maximum spin frequency) of a millisecond pulsar (MSP) is independent of the initial conditions and accretion rate when the neutron star (NS) accretes the mass of similar to 0.2 M-circle dot. The accretion torque with the fastness parameter and gravitational wave (GW) radiation torque may be responsible for the formation of the minimum spin period (maximum spin frequency). The fastest spin frequency (716 Hz) of MSP can be inferred to associate with a critical fastness parameter of about omega(c) = 0.55. Furthermore, comparisons with the observational data are presented in the field period (B - P) diagram.

First search for gravitational waves from inspiraling compact binaries using TAMA300 data

We analyzed 6 hours of data from the TAMA300 detector by matched filtering, searching for gravitational waves from inspiraling compact binaries. We incorporated a two-step hierarchical search strategy in matched filtering. We obtained an upper limit of 0.59/hour (C.L.=90%) on the event rate of inspirals of compact binaries with mass between 0.3M_solar and 10M_solar and with signal-to-noise ratio greater than 7.2. The distance of 1.4M_solar (0.5M_solar) binaries which produce the signal-to-noise ratio 7.2 was estimated to be 6.2kpc (2.9kpc) when the position of the source on the sky and the inclination angle of the binaries were optimal.

Accuracy of the relativistic Cowling approximation in slowly rotating stars

We have calculated the non-radial oscillation in slowly rotating relativistic stars with the Cowling approximation. The frequencies are compared with those based on the complete linearized equations of general relativity. It is found that the results with the approximation differ by less than about

Magnetic field decay with Hall drift in neutron star crusts

20 %

On the Oscillation Spectra of Ultracompact Stars: an Extensive Survey of Gravitational-Wave Modes

for typical relativistic stellar models. The approximation is more accurate for higher-order modes as in the Newtonian case.

Measuring neutron star mass and radius with three mass-radius relations

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, the long-term evolution is calculated up to Ohmic dissipation. The non-linear coupling between poloidal and toroidal components is explored in terms of their energies and helicity. Non-linear oscillation of the drift in strongly magnetized regimes is clear only around equipartition between the two components. Significant energy is transferred to the poloidal component initially when the toroidal component 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 time-scale, but returns to the initial dipole over a longer dissipation time-scale, since it is the least damped form.

The r-Mode Oscillations in Relativistic Rotating Stars

An extensive survey of gravitational-wave modes for uniform density stars is presented. The study covers stars ranging in compactness from R/M=100 to the limit of stability in general relativity: R/M = 9/4. We establish that polar and axial gravitational-wave modes exist for all these stellar models. Moreover, there are two distinct families of both axial and polar modes. We discuss the physics of these modes and argue that one family is primarily associated with the interior of the star, while the second family is mainly associated with the stellar surface. We also show that the problem of axial perturbations has all the essential features of the polar problem as far as gravitational waves are concerned. This means that the axial problem is much more important than has previously been assumed. We also find some surprising features, such as avoided crossings between the polar gravitational-wave modes and the Kelvin f-mode as the star becomes very compact. This seems to suggest that the f-mode should be considered on equal footing with the polar w-modes for ultracompact stars. All modes may have the main character of trapped modes inside the curvature potential barrier for R/M < 3.