Compact Stars as Sources of Gravitational Waves Y. Kojima (Hiroshima Univ.) 小嶌康史 ( 広島大学理学研究科 ) 第 3 回 TAMA シンポジュウム（柏） 2003 年 2 月 6 － 7 日

Available Energy Sources Rotational Energy +( Instabilities) -> Continuous GW Binding Energy +(Externally driven deformation) +(Formation /Phase transition) -> Burst-like GW (cf. Elastic Energy in Crust)

Brief Review of R-Mode Instability Unstable for all rotating stars Importance of coupling evolution of amplitude unstable growth mode coupling Non-linear Saturation Magnetic field: Rezzolla et al (1999) Hydrodynamical Cascade: Gressman et al(2002),Arras et al (2002)

Gressman et al(2002) : Cascade decay of unstable mode Decaying R-mode Scenario?

Various (unknown) instabilities on rotating stars -> Non-axially symmetric deformation -> Gravitational radiation Weakly growing mode has a small amplitude -> Less important for first detection Sources for 2 nd generation detector Implications

A New Type of Compact Stars? RX J :nearby NS? D~100pc 1996 Discovered at X-ray/Optical 2002 Drake et al(ApJ.572, 996) radiation radius~ km -> Quark star? Not yet confirmed 2002 Walter & Lattimer (ApJ.576,L145) 15 (D/117pc)km: Two-components blackbody 2002 Burwitz et al (astro/ph ) 12(D/120pc)km: Neutron star

What’s a quark star? Bare quark star Bodmer-Witten’s Conjecture “Strange matter (u,d, s-quarks) is the ground state” simple description (MIT bag model B is bag constant) (cf.) Hybrid quark star (quark core)

Neutron Stars vs. Quark Stars Static properties (mass M, radius R) ->Compact! -> Strange star as rapidly rotating pulsar with P~0.5ms

Mass and Radius of Compact Star gravitationally bound star self-bound star Lattimer & Prakash(2001)

Dynamical Property of a Star - Characteristic Frequency - Wave form of damped sine curve (frequency and damping time) e.g. Black hole ringing ( Chandrasekhar & Detweiler ) Neutron stars ( Lindblom & Detweiler ) However, excitation is unclear.

Complex Frequency Kojima & Sakata(2002)

Frequency and Damping Time average density relativistic factor Kojima & Sakata(2002)

Results Two parameters from frequency(~ average density) and damping time (~relativistic factor) -> M, R are inferred. Important information for EOS Discriminating quark star from neutron star

Optimistic Estimate Marranghello et al (2002) ‘Phase transitions in neutron stars and GW emission’ Typical binding energy Matched Filter S/N =2 Fraction of energy to non-radial oscillation is not clear.

Summary Simple system (BH/NS binary) for 1 st detection Complicated system as future targets should be studied theoretically (or experimentally).