Capabilities of a Gravitational Wave Network Bernard F Schutz Albert Einstein Institute (Potsdam, Germany) and School of Physics and Astronomy, Cardiff University, UK

2 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | Why Networks? All operating GW interferometers pool their data LIGO/H, LIGO/L, GEO600 part of LSC, joint operation, partnership LSC and VIRGO pool all data, do joint analysis and publication Exceptional in physics: competition is the rule GW science demands cooperation: Verification: signals are transient, so no single detector can securely claim a burst detection (CW signals are the exception) Information: positions, polarization require triangulation among  3 detectors (exceptions: CW, coincident optical events). Added accuracy in parameter determination (eg sky location) is where the science payoff is: optical identifications, population studies of BHs or NSs, Hubble constant, etc. Sensitivity: because we observe coherent amplitudes, coherent analysis increases range, event rate. Sky coverage: extra detectors fill in holes in the antenna pattern of others. Duty cycle: interferometers typically have high-quality science data for ~80% of the time. That means that a 3-detector network only operates in triple coincidence 50% of time.

3 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | Joint Analysis In practice, joint analysis addresses all issues of verification, information, sensitivity, coverage, and duty cycle. Searches typically apply thresholds to individual data streams, identifying possible candidate events. eliminates most spurious noise-generated events must be done in close cooperation with experimentalists, using all data Coincidences studied coherently, by adding weighted data with appropriate time- and phase-shifts. If shot noise limits sensitivity, this essentially synthesizes a detector with all the light in one. At present interferometers operate with about 80% duty cycle in each detector Three detectors operate three-way 50% of the time Four detectors would operate three-way about 80% of the time: for 33% extra expenditure, a fourth detector increases the science by 60%. Three detectors is minimum, not optimum.

4 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | Information Extracting information from multiple detectors requires All detectors must receive event with reasonable SNR There must be more data than unknowns: to determine sky position ( ,  ) plus polarization (  ) plus amplitude (h), we need three detectors: 2 time-delays plus three amplitudes provides some redundancy. Existing network has a near degeneracy: the two large LIGO detectors are nearly aligned, so their amplitudes are not independent. This provides a check for a two-detector coincidence, but reduces the information available with three detectors. Degeneracy also reduces sky coverage: bigger holes in antenna pattern. This is a major reason for big improvements brought by a 4th detector.

5 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | Comparing Networks Consider the following configurations of identical detectors located where the real detectors are (or are proposed): LIGO/Hanford and LIGO/Livingston LIGO/H, LIGO/L and VIRGO LIGO/H, LIGO/L, VIRGO, and LCGT Get an idea of sky coverage by adding antenna patterns.

6 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | LIGO-LIGO Each detector has a typical “peanut” pattern (sensitivity to randomly linearly polarized waves from a given sky direction) L/H and L/L not too different Sum L/H+L/L similar. Figure of merit f 50 : One interferometer has more than 50% of its peak sensitivity over 33% of sky: f 50 = 0.33 L/H+L/L have combined f 50 = 0.34

7 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | LIGO-LIGO-VIRGO When VIRGO is added to the network, its overall sensitivity is increased. This describes the expected situation for S6. Maximum goes up by 9% Coverage increases more: f 50 = 72% This means doubling the number of 2-way coincidences. L/H+L/L+VL/H+L/L+V 50%

8 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | LIGO-LIGO-VIRGO-LCGT There are further strong gains if LCGT is added. The maximum sensitivity increases by a further 13% above that of L/H+L/L+V. Coverage at 50% of the new maximum now 100%. This suggests the number of 3- way coincidences goes up by 28%. This is in addition to the 3- detector duty cycle improvement of 60%! L/H+L/L+V+LCGT 50%

9 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | Summary Combined Antenna Patterns 50% Sensitivity Coverage L/HL/H+L/L L/H+L/L+V L/H+L/L+V+LCGT L/H+L/L L/H+L/L+VL/H+L/L+V+LCGT

10 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | Truly Global Network The current development of three large detectors is a minimum for good science, but the increase in science from adding a 4th large detector is very significant: duty cycle up by 60%, sky coverage for 3-way coincidences up by 30%. Payoffs in source detection, more accurate locations, more identifications. Achieving this payoff requires joint coherent data analysis, along with close cooperation with other observatories: large optical surveys, rapid-response telescopes, transient event monitors (gamma-ray, X-ray, radio,...).

11 B F Schutz Albert Einstein Institute Fujihara Seminar | Networks of GW Detectors | Thank you!