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Advanced interferometers for astronomical observations

Advanced interferometers for astronomical observations. Lee Samuel Finn Center for Gravitational Wave Physics, Penn State. Goals and Outline. Design sensitivity limits and next-generation technologies to overcome them Astrophysics and analysis implications of increased sensitivity.

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Advanced interferometers for astronomical observations

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  1. Advanced interferometers for astronomical observations Lee Samuel Finn Center for Gravitational Wave Physics, Penn State

  2. Goals and Outline • Design sensitivity limits and next-generation technologies to overcome them • Astrophysics and analysis implications of increased sensitivity 2nd Gravitational Wave Phenomenology Workshop

  3. What limits LIGO’s sensitivity? • Initial LIGO detectors: • Different f, different limit • < ~50Hz : seismic noise • 50 - 200Hz : thermal noise • > 200Hz : “shot” noise • Facility limits • Gravity gradients • Stray light • Residual gas 2nd Gravitational Wave Phenomenology Workshop

  4. Building a better interferometer:Advanced LIGO • Seismic isolation • Thermal noise mitigation; high power optics • High power lasers • Tuning ifo response 40kg 2nd Gravitational Wave Phenomenology Workshop

  5. Thermal noise contributions • Suspensions: • kT energy in taut suspension wire violin modes • Test masses: • Normal modes: kT energy in mirror modes • Thermoelastic: Temperature fluctuations and thermal expansion coefficient • Noise proportional to mechanical losses: reduce losses • Initial LIGO: mirrors rest on wires • Advanced: mirrors bonded to ribbons 2nd Gravitational Wave Phenomenology Workshop

  6. Thermal noise mitigation: test masses • Material properties problem • Normal modes: • Increase Young modulus: less motion for same thermal energy • Thermoelastic: • Decrease coefficient thermal expansion a: less motion for same thermal fluctuations • Laser spot diameter, profile • Fluctuations averaged over effective spot area • Increase area, reduce effective fluctuation • Initial LIGO: 25cm • Advanced LIGO: 35cm 2nd Gravitational Wave Phenomenology Workshop

  7. Signal recycling mirror Tuning the detector response • Undisturbed interferometer operates on dark fringe • Response to gravitational waves is light at output port • Introduce partially reflecting mirror at output port cavity end mirror Interferometer arm (4km long) • Make resonant cavity with rest of interferometer • Resonance enhances power at output port for excitation at resonant frequency • Higher power: lower shot noise • Mitigate shot noise in (relatively) narrow band cavity input mirrors cavity end mirror laser Interferometer arm (4km long) photodetector 2nd Gravitational Wave Phenomenology Workshop

  8. Fused Silica v. Sapphire: Two Alternatives • Fused silica • Pros: • Broader bandwidth • Better low-frequency performance • Cons: Higher in-band noise • Sapphire • Pros: • Lower in-band noise • Better high frequency performance • Cons: Narrower bandwidth • What does this mean for astrophysics? 2nd Gravitational Wave Phenomenology Workshop

  9. “Ignorance” Sensitivity • Ignorance? • Specific sources? Specific amplitude in specific bands • General considerations • Generic burst character low-Q damped SHO • Measure? [f Sh(f)]1/2 • Fused silica • Pros: Broader bandwidth; better low-f performance • Cons: Higher in-band noise • Sapphire • Pros: Lower in-band noise, better high-f performance • Cons: Narrower bandwidth 2nd Gravitational Wave Phenomenology Workshop

  10. Compact Binary Inspiral • Signal spectrum known • Measure: [dr2/dlnf ]1/2 • Favor low-frequency sensitivity • But balance against bandwidth • CBI range • Silica: • 130 Mpc (M/3M8)5/6 • Sapphire: • 190 Mpc (M/3M8)5/6 • But… • Chirp fmax ~ M-1 • Silica IFO more sensitive than sapphire when M > 26M8 (2x30M8 binary) 2nd Gravitational Wave Phenomenology Workshop

  11. Stochastic signal • Detection involves pair of detectors • Sensitive to wavelengths greater than separation between detectors (f < 100 Hz for LIGO) • Stochastic signal: prefer Silica • Silica: WGWh-2 < 2.6x10-9 • Sapphire: WGWh-2 < 5.0x10-9 • Note! • CBI: prefer sapphire • Stoch: prefer silica 2nd Gravitational Wave Phenomenology Workshop

  12. Pulsar periods • Normal, millisecond pulsars are different populations • “Normal”: P > 100 ms • “MS”: P < 100 ms • More normal than ms pulsars • Expect 160K normal, 40K ms pulsars in galaxy • Preference? Name your game: • Aim for closest NS? Favor low-f performance • Aim for detected pulsar? Favor high-f performance • Fold in prejudice regarding e for normal, ms pulsars 2nd Gravitational Wave Phenomenology Workshop

  13. Conclusions, or What does this all mean? • Advanced LIGO is not a blunt instrument! • Subtle difference in science goals begin to make difference • Ground-based “ifos” on-track for • Stochastic background sensitivity Wh-2<10-9 @ 100Hz • (3x4Km IFO) inspiral sensitivity • NS/NS to ~330 Mpc • 2x10 M8 BH/BH to z~0.3 • 2x30 M8 BH/BH to z~0.5 • Pulsars @ 100 pc in 1 yr obs: e95% < • 3-5x10-8 @ 100 Hz • 3-5x10-9 @ 200 Hz • 1-2x10-9 above 300 Hz 2nd Gravitational Wave Phenomenology Workshop

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