October 17, 2003Globular Clusters and Gravitational Waves1 Gravitational Wave Observations of Globular Clusters M. Benacquista Montana State University-Billings NASA Cooperative Agreement NCC5-579

October 17, 2003Globular Clusters and Gravitational Waves2 Suggested Outline of Topics What are the meeting and session goals? What are the gravitational wave observables? What are the selection effects and the populations observable in gravitational radiation? What are the capabilities of LISA and GB detectors? When will LISA observations be available?

October 17, 2003Globular Clusters and Gravitational Waves3 Why focus on globular clusters? Interesting dynamics –N-body gravitational systems –Tidal interactions with the galactic potential Interesting stellar/binary evolution –Old populations –Binaries formed at different stages of stellar evolution Interesting gravitational radiation –Lots of hard binaries –Lots of evolved objects –Localized populations of sources

October 17, 2003Globular Clusters and Gravitational Waves4 What are the meeting goals? Bring people together from all these disciplines to determine how gravitational radiation can inform us about globular cluster dynamics. The intent is not to have astrophysics predict source rates for gravitational wave detection. The intent is to discover how gravitational wave observations can inform us of new astrophysics or refine present models.

October 17, 2003Globular Clusters and Gravitational Waves5 Outline Gravitational radiation and detection Likely globular cluster sources Foreground sources Selection effects Rampant speculation

October 17, 2003Globular Clusters and Gravitational Waves6 What is a Gravitational Wave? Traveling disturbance in spacetime which manifests itself as a changing relative distance between points. Generated by a time varying quadrupole (or higher) moment Two polarizations “+” and “  ”

October 17, 2003Globular Clusters and Gravitational Waves7 Detection with Interferometers

October 17, 2003Globular Clusters and Gravitational Waves8 Detection with Resonant Bars Gravitational wave deforms the resonant bar Transducer amplifies the resonance

October 17, 2003Globular Clusters and Gravitational Waves9 (Nearly) Current Detectors Interferometers (broadband) –Ground-based (LIGO, Virgo, TAMA300, GEO600): frequency band ~ kHz –Space-based (LISA): frequency band ~ mHz Resonant Bars (narrowband) –Central frequencies ~ 1 kHz –Bandwidths ~ 10 Hz Focus on interferometers and continuous sources

October 17, 2003Globular Clusters and Gravitational Waves10 Modulation by LISA orbit

October 17, 2003Globular Clusters and Gravitational Waves11 Directional Sensitivity All detectors have anisotropic sensitivities All detectors change orientation  directional sensitivity to continuous sources Ground-based interferometers can synthesize aperture for continuous sources

October 17, 2003Globular Clusters and Gravitational Waves12 Major Sources Expected from Globular Clusters Neutron Stars –Non-axisymmetric NS Sunday session Double Degenerate Binaries –DWD Binaries Saturday session –WD-NS Binaries Sunday session Black Holes –Stellar mass binaries –Intermediate mass black holes Monday session

October 17, 2003Globular Clusters and Gravitational Waves13 Binary Sources for LISA Strain amplitude is modulated by detector motion, so

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October 17, 2003Globular Clusters and Gravitational Waves16 Observables Amplitude (chirp mass, distance, frequency) Frequency = 2/(Orbital Period) Angular Position  s and  s Orbital Orientation  L and  L (can give angle of inclination) If orbital period changes due to emission of gravitational radiation (“chirps”), then the amplitude degeneracy is broken and Chirp mass Distance

October 17, 2003Globular Clusters and Gravitational Waves17 Foreground sources Identification of a binary as a member of a globular cluster requires determination of position and distance If there are no foreground sources, position is sufficient Unfortunately there are lots of foreground sources Galactic white dwarf binary population

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October 17, 2003Globular Clusters and Gravitational Waves19 Selection Effects Number density of disk population decreases as frequency increases –Isotropic below ~ 1 mHz Angular resolution increases as frequency increases –Doppler aperture synthesis is effective above ~ 1 mHz 1.More likely to identify high mass, high frequency sources with globular clusters. 2.Depends upon details of galactic population.

October 17, 2003Globular Clusters and Gravitational Waves20 Rampant Speculations Tidal tails: If binaries receive kicks as they are hardened in encounters, then hard binaries may be over- represented in globular cluster tidal tails. Figure from Leon, Meylan and Combes

October 17, 2003Globular Clusters and Gravitational Waves21 Observe high frequency populations See progenitors of low-mass X-ray binaries See progenitors of millisecond pulsars If sufficient numbers are measured for various clusters, can rates of binary burning be inferred? If chirping binaries are measured, will distances be improved? If we measure an inverse chirp, can we infer gravitational potentials from the assumed acceleration?

October 17, 2003Globular Clusters and Gravitational Waves22 Extragalactic Observations High metallicity globular clusters may form during/after galaxy mergers Intermediate mass black holes may form through mergers of stellar mass black holes –Detectable by ground-based interferometers as inspiral events Supermassive black holes are brought together by dynamical friction to merger –Detectable by LISA What are the time scales? Could there be a correlation? What would it tell us about globular cluster evolution?

October 17, 2003Globular Clusters and Gravitational Waves23 Conclusion Consider the potential for testing dynamical models through gravitational radiation observations. Consider how gravitational radiation observations can complement electromagnetic observations. Consider data analysis techniques and statistical methods for measuring globular cluster populations.