Mechanical loss of a multilayer tantala/silica coating on a sapphire disk at cryogenic temperatures: toward the KAGRA gravitational wave detector

We report the results of a new experimental setup to measure the mechanical loss of coating layers on a thin sapphire disk at cryogenic temperatures. Some of the authors previously reported that there was no temperature dependence of the mechanical loss from a multilayer tantala/silica coating on a sapphire disk, both before and after heat treatment, although some reports indicate that Ta<sub>2</sub>O<sub>5</sub> and SiO<sub>2</sub> layers annealed at 600 °C have loss peaks near 20 K. Since KAGRA—the Japanese gravitational-wave detector, currently under construction—will be operated at 20 K and have coated sapphire mirrors, it is very important to clarify the mechanical loss behavior of tantala/silica coatings around this temperature. We carefully investigate a tantala/silica-coated sapphire disk with the new setup, anneal the disk, and then investigate the annealed disk. We find that there is no distinct loss peak both before and after annealing under particular conditions. The mechanical loss for the unannealed disk at 20 K is about 5×10<sup>−4</sup>, as previously reported, while that for the annealed disk is approximately 6.4×10<sup>−4</sup>

Sapphire mirror for the KAGRA gravitational wave detector

KAGRA, the Japanese interferometric gravitational wave detector currently under construction, will employ sapphire test masses for its cryogenic operation. Sapphire has an advantage in its higher thermal conductivity near the operating temperature 20 K compared to fused silica used in other gravitational wave detectors, but there are some uncertain properties for the application such as hardness, optical absorption, and birefringence. We introduce an optical design of the test masses and our recent R&D results to address the above properties. Test polish of sapphire substrate has especially proven that specifications on the surface are sufficiently met. Recent measurements of absorption and inhomogeneity of the refractive index of the sapphire substrate indicate that the other properties are also acceptable to use sapphire crystal as test masses

Coincidence analysis to search for inspiraling compact binaries using TAMA300 and LISM data

Japanese laser interferometric gravitational wave detectors, TAMA300 and LISM, performed a coincident observation during 2001. We perform a coincidence analysis to search for inspiraling compact binaries. The length of data used for the coincidence analysis is 275 hours when both TAMA300 and LISM detectors are operated simultaneously. TAMA300 and LISM data are analyzed by matched filtering, and candidates for gravitational wave events are obtained. If there is a true gravitational wave signal, it should appear in both data of detectors with consistent waveforms characterized by masses of stars, amplitude of the signal, the coalescence time and so on. We introduce a set of coincidence conditions of the parameters, and search for coincident events. This procedure reduces the number of fake events considerably, by a factor $\sim 10^{-4}$ compared with the number of fake events in single detector analysis. We find that the number of events after imposing the coincidence conditions is consistent with the number of accidental coincidences produced purely by noise. We thus find no evidence of gravitational wave signals. We obtain an upper limit of 0.046 /hours (CL $= 90$) to the Galactic event rate within 1kpc from the Earth. The method used in this paper can be applied straightforwardly to the case of coincidence observations with more than two detectors with arbitrary arm directions.Comment: 28 pages, 17 figures, Replaced with the version to be published in Physical Review

Results of the search for inspiraling compact star binaries from TAMA300's observation in 2000-2004

We analyze the data of TAMA300 detector to search for gravitational waves from inspiraling compact star binaries with masses of the component stars in the range 1-3Msolar. In this analysis, 2705 hours of data, taken during the years 2000-2004, are used for the event search. We combine the results of different observation runs, and obtained a single upper limit on the rate of the coalescence of compact binaries in our Galaxy of 20 per year at a 90% confidence level. In this upper limit, the effect of various systematic errors such like the uncertainty of the background estimation and the calibration of the detector's sensitivity are included.Comment: 8 pages, 4 Postscript figures, uses revtex4.sty The author list was correcte

Observation results by the TAMA300 detector on gravitational wave bursts from stellar-core collapses

We present data-analysis schemes and results of observations with the TAMA300 gravitational-wave detector, targeting burst signals from stellar-core collapse events. In analyses for burst gravitational waves, the detection and fake-reduction schemes are different from well-investigated ones for a chirp-wave analysis, because precise waveform templates are not available. We used an excess-power filter for the extraction of gravitational-wave candidates, and developed two methods for the reduction of fake events caused by non-stationary noises of the detector. These analysis schemes were applied to real data from the TAMA300 interferometric gravitational wave detector. As a result, fake events were reduced by a factor of about 1000 in the best cases. The resultant event candidates were interpreted from an astronomical viewpoint. We set an upper limit of 2.2x10^3 events/sec on the burst gravitational-wave event rate in our Galaxy with a confidence level of 90%. This work sets a milestone and prospects on the search for burst gravitational waves, by establishing an analysis scheme for the observation data from an interferometric gravitational wave detector

Stable Operation of a 300-m Laser Interferometer with Sufficient Sensitivity to Detect Gravitational-Wave Events within our Galaxy

TAMA300, an interferometric gravitational-wave detector with 300-m baseline length, has been developed and operated with sufficient sensitivity to detect gravitational-wave events within our galaxy and sufficient stability for observations; the interferometer was operated for over 10 hours stably and continuously. With a strain-equivalent noise level of $h\sim 5 \times 10^{-21} /\sqrt{\rm Hz}$, a signal-to-noise ratio (SNR) of 30 is expected for gravitational waves generated by a coalescence of 1.4 $M_\odot$-1.4 $M_\odot$ binary neutron stars at 10 kpc distance. %In addition, almost all noise sources which limit the sensitivity and which %disturb the stable operation have been identified. We evaluated the stability of the detector sensitivity with a 2-week data-taking run, collecting 160 hours of data to be analyzed in the search for gravitational waves.Comment: 5 pages, 4 figure

KAGRA: 2.5 Generation Interferometric Gravitational Wave Detector

The recent detections of gravitational waves (GWs) reported by LIGO/Virgocollaborations have made significant impact on physics and astronomy. A globalnetwork of GW detectors will play a key role to solve the unknown nature of thesources in coordinated observations with astronomical telescopes and detectors.Here we introduce KAGRA (former name LCGT; Large-scale Cryogenic Gravitationalwave Telescope), a new GW detector with two 3-km baseline arms arranged in theshape of an "L", located inside the Mt. Ikenoyama, Kamioka, Gifu, Japan.KAGRA's design is similar to those of the second generations such as AdvancedLIGO/Virgo, but it will be operating at the cryogenic temperature with sapphiremirrors. This low temperature feature is advantageous for improving thesensitivity around 100 Hz and is considered as an important feature for thethird generation GW detector concept (e.g. Einstein Telescope of Europe orCosmic Explorer of USA). Hence, KAGRA is often called as a 2.5 generation GWdetector based on laser interferometry. The installation and commissioning ofKAGRA is underway and its cryogenic systems have been successfully tested inMay, 2018. KAGRA's first observation run is scheduled in late 2019, aiming tojoin the third observation run (O3) of the advanced LIGO/Virgo network. In thiswork, we describe a brief history of KAGRA and highlights of main feature. Wealso discuss the prospects of GW observation with KAGRA in the era of O3. Whenoperating along with the existing GW detectors, KAGRA will be helpful to locatea GW source more accurately and to determine the source parameters with higherprecision, providing information for follow-up observations of a GW triggercandidate