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The r-modes look good again in accreting neutron stars

The r-modes look good again in accreting neutron stars. Ben Owen with Mohit Nayyar. What is an r-mode?. Dominated by Coriolis force Mainly velocity perturbation, little density perturbation Pushes buoys horizontally Extends down into core Has frequency comparable to spin frequency of star

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The r-modes look good again in accreting neutron stars

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  1. The r-modes look good again in accreting neutron stars Ben Owen with Mohit Nayyar LIGO Scientific Collaboration

  2. What is an r-mode? • Dominated by Coriolis force • Mainly velocity perturbation, little density perturbation • Pushes buoys horizontally • Extends down into core • Has frequency comparable to spin frequency of star • Has pattern speed prograde in inertial frame, retrograde in co-rotating frame • Subject to gravitational radiation (CFS) instability LIGO Scientific Collaboration

  3. Why do we care? Instability of r-modes could • survive in realistic neutron stars (with viscosity), • explain why young neutron stars spin so slowly, • explain why rapidly accreting neutron stars (LMXB) spin slowly and within a narrow band, and • produce gravitational waves detectable by LIGO with noise a little better than SRD LIGO Scientific Collaboration

  4. Context of this talk • People kept saying the r-modes are dead (crust etc) • Lindblom and I kept resurrecting them (melting etc) • We thought the LMXB scenario was bad anyway for GW due to Levin’s argument for thermal runaway • Then we found something (bulk viscosity in hyperon core) that really was deadly (Peter Jones) • Wagoner argued that it’s actually good for GW in the LMXB scenario, though not for the young stars… • But he used the least reliable part of our paper – is it really good if you do it right? LIGO Scientific Collaboration

  5. Thermal run-away 3 months Millions of years LIGO Scientific Collaboration

  6. Thermal sit-there LIGO Scientific Collaboration

  7. Where did that cliff come from? • Hyperons are nucleons with a strange quark (e.g. S- = dds, 1200 MeV), might be formed at high density • Similar things with other exotic matter, but we can measure lab numbers for hyperons • High bulk viscosity by n n p+S- etc • Comes from matching timescales • Bulk viscosity coefficient z goes as t / (1 + w2t2) • Usually wt is large, so long t (superfluid) reduces z • Here wt starts small, so superfluidity increases z! LIGO Scientific Collaboration

  8. Is it really there? • Lindblom & Owen had sloppy treatment of superfluidity – use better one in Haensel et al. • Haensel et al. were sloppy with rest of microphysics – use Lindblom & Owen but fix some minor errors • Explore range of theoretical parameters consistent with laboratory measurements of (hyper)nuclei and astrophysical observations – is the cliff robust? LIGO Scientific Collaboration

  9. New instability curves LIGO Scientific Collaboration

  10. Conclusions • Persistent emission of gravitational waves is robust • Main parameter affecting cliff location is superfluid critical temperature of S- - which is least known! • Oh, and … a very popular equation of state has an error, and turns out to be physically impossible! (Glendenning K = 240 MeV when corrected gives maximum neutron star mass 1.3 solar masses) LIGO Scientific Collaboration

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