(Superconducting Omni-directional Gravitational Radiation Observatory) SOGRO (Superconducting Omni-directional Gravitational Radiation Observatory) Ho Jung Paik Department of Physics, University of Maryland ICGAC-XIII, Seoul, July 4, 2017

GW detector bandwidths Missing frequency band Merger of IMBHs and inspiraling stellar mass BHs are expected to produce signals at 0.1 to 10 Hz bandwidth. Could a terrestrial antenna be built to fill this missing band? Paik

Analyzed three detector options: 1. Atom-laser interferometer 2. TOBA with laser interferometer 3. Michelson interferometer Would be astrophysically interesting, if one can reach Sh½(f ) = 1020 Hz1/2 in 0.1-10 Hz band. Detecting and removing NN appears to be extremely challenging. Paik

Long-baseline resonant-mass detector In the Newtonian limit, Gravity gradiometer is a GW detector. Paik

SOGRO (Superconducting Omni-directional Gravitational Radiation Observatory) Each test mass has 3 DOF. Combining six test masses, tensor GW detector is formed. Source direction (, ) and wave polarization (h+, h) can be determined by a single antenna. “Spherical” Antenna x polarization + polarization Paik

Antenna pattern  polarization LIGO + polarization Sky location of GW150914 rms sensitivity SOGRO Diagonal Off-diagonal Total Sky location by SOGRO Paik

Design philosophy of SOGRO Extremely low detector noise is required. Low T, high Q, nearly quantum-limited amplifier. Test mass suspension frequency should be lowered to below the signal bandwidth. Almost free test masses by magnetic levitation. Seismic noise is very difficult to isolate at low frequencies. High CM rejection in a superconducting differential accelerometer. Newtonian noise increases sharply below 10 Hz and cannot be shielded. Tensor detector can help mitigate the NN. H. J. Paik, C. E. Griggs, M. V. Moody, K. Venkateswara, H. M. Lee, A. B. Nielsen, E. Majorana and J. Harms, Class. Quantum Grav. 33, 075003 (2016) Paik

SGG (Superconducting Gravity Gradiometer) Sensitive SGGs have been under development for over 30 years at UM. 3 x 10-11 Hz-1/2 Moody et al., RSI 73, 3957 (2002) Test masses are mechanically suspended (fDM ~ 10 Hz). CM platform vibration noise is rejected to 3 parts in 108. Paik

Tensor SGG with levitated test masses More sensitive SGG is under development with NASA support. Test masses are magnetically suspend (fDM ~ 0.01 Hz). 102-103 times higher sensitivity Test masses are levitated by a current induced along a tube. Six test masses mounted a cube form a tensor gradiometer. Paik

Magnetic levitation To provide large area for levitation, test mass is made in the shape of square Nb shell with flanges. Horizontal DM frequencies can all be tuned to 0.01 Hz. However, due to nonlinearities, strong levitation fields will cause vertical DM frequencies > 1 Hz. Vertical accelerometers will be noisier. Since the platform is isolated from tilt of the ground, hxz and hyz can also be obtained from horizontal motions of test masses only. Paik

Tuned capacitor-bridge transducer Near quantum-limited SQUIDs have 1/f noise below 10 kHz. Signal needs to be upconverted to  10 kHz by using active transducer. Capacitor bridge coupled to nearly quantum-limited SQUID thru S/C transformer. (Cinquegrana et al., PRD 48, 448 (1993)) Bridge is driven at LC resonance frequency p . Oscillator noise is rejected by the bridge balance. Maintain precise balance by feedback. Paik

Achievable detector noise Parameter SOGRO aSOGRO Method employed (/aSOGRO) Each test mass M 5 ton Nb shell Arm-length L 50 m Over “rigid” platform Antenna temp T 1.5 K 0.1 K Liquid He / He3-He4 dilution refrigerator Platform temp Tpl Large-scale cryogenics Platform Q factor Qpl 105 106 Square Al tube construction DM frequency fD 0.01 Hz Magnetic levitation (horizontal only) DM quality factor 107 108 Surface polished pure Nb Pump frequency fp 50 kHz Tuned capacitor bridge transducer Amplifier noise no. n 20 5 Two-stage dc SQUID cooled to 0.1 K Detector noise Sh1/2(f ) 1  1020 Hz1/2 3  1021 Hz1/2 Computed at 1 Hz aSOGRO requires QD ~ 108 for test masses and Qpl ~ 106 for the platform. aSOGRO requires improvement by a factor of 2 over Italian SQUIDs. Paik

Various noise contributions The greatest challenge appears to be platform design and construction. Platform needs to be rigid with all DM modes > 10 Hz and Q > 106. Paik

Suspension and seismic noise Go underground (> 500 m) to reduce the seismic and Newtonian noise, as well as to be far away from moving objects. Infrasound is attenuated to 0.15 at 0.2 Hz and to 108 at 1 Hz. To reach the intrinsic noise limit, 200 dB isolation + rejection is required at 1 Hz. 25-m pendulum: fp = 0.1 Hz for pendulum modes and f ≤ 103 Hz for torsional mode. Completely decouples ground tilt and provides 40 dB isolation for horizontal and 120 dB for torsional mode at 1 Hz . Nodal support prevents odd harmonics from being excited. Pendulum suspension from center CMRR = 104 is required for angular accelerations, but CMRR = 109 and 1010 for horizontal and vertical accelerations. Paik

50 m platform with braces 3 m diameter 1 cm thick Al tubes with braces, Mpl = 235 tons, Qpl = 107 xx mode xy mode

CM rejection on non-rigid platform Response of non-rigid platform to the seismic noise is not identical at the test mass positions due to asymmetry. Platform appears to be rigid enough to achieve CMRR = 1010. Thermal noise requirement may be satisfied with Q = 106. Asymmetry: 103 ( 2.5 cm over 25 m) Combined with vibration isolation (f/fp)2

Newtonian gravity noise GWs are transverse whereas near-field Newtonian gradient is not. Tensor measurement is insufficient to remove NN from multiple waves. First remove Rayleigh NN by using seismometers only, then remove infrasound NN by using microphones and cleaned-up SOGRO outputs. NN due to Rayleigh waves removed by using 7 seismometers with SNR = 103 at R = 5 km. NN due to infrasound, 15 mikes of SNR = 104, 1 at detector, 7 each at R = 600 m and 1 km. Harms and Paik, PRD 92, 022001 (2015) Paik

Astrophysics with SOGRO SOGRO would fill 0.1-10 Hz frequency gap between the terrestrial and future space interferometers. SOGRO would be able to detect stellar mass BH binaries like GW150914. Could alert interferometers days before merger. SOGRO could detect IMBH binaries with 103-104M◉ at a few billion light years away, and WD binaries within the Local Group. With two SOGROs, stochastic GW background could be searched. Better limit than aLIGO and AdV since h ~ 1/f 3. Paik

Conclusion and summary By using six levitated superconducting test masses, low-frequency tensor GW detector can be constructed. A single SOGRO could locate source and determine polarization. SOGRO is uniquely capable of CM rejection and full-tensor detection. SOGRO could overcome the seismic and Newtonian noise. SOGRO could detect IMBH binaries at a few billion light years away, as well as stellar mass BH binaries like GW150914. SOGRO could alert laser interferometers days before merger. With two SOGROs, stochastic GW background could be searched. The universe 1035–1020 s after the Big Bang could be studied. In Korea, KASI-KISTI-NIMS (KKN) collaboration is undergoing a pilot study on SOGRO. Paik