Advanced LIGO Gravitational Wave Detectors Research Articles

Calibration of advanced Virgo and reconstruction of the detector strain h(t) during the observing run O3

The three advanced Virgo and LIGO gravitational wave detectors participated to the third observing run (O3) between 1 April 2019 15:00 UTC and 27 March 2020 17:00 UTC, leading to several gravitational wave detections per month. This paper describes the advanced Virgo detector calibration and the reconstruction of the detector strain h(t) during O3, as well as the estimation of the associated uncertainties. For the first time, the photon calibration technique as been used as reference for Virgo calibration, which allowed to cross-calibrate the strain amplitude of the Virgo and LIGO detectors. The previous reference, so-called free swinging Michelson technique, has still been used but as an independent cross-check. h(t) reconstruction and noise subtraction were processed online, with good enough quality to prevent the need for offline reprocessing, except for the two last weeks of September 2019. The uncertainties for the reconstructed h(t) strain, estimated in this paper in a 20–2000 Hz frequency band, are frequency independent: 5% in amplitude, 35 mrad in phase and 10 μs in timing, with the exception of larger uncertainties around 50 Hz.

Point Absorber Limits to Future Gravitational-Wave Detectors.

High-quality optical resonant cavities require low optical loss, typically on the scale of parts per million. However, unintended micron-scale contaminants on the resonator mirrors that absorb the light circulating in the cavity can deform the surface thermoelastically and thus increase losses by scattering light out of the resonant mode. The point absorber effect is a limiting factor in some high-power cavity experiments, for example, the Advanced LIGO gravitational-wave detector. In this Letter, we present a general approach to the point absorber effect from first principles and simulate its contribution to the increased scattering. The achievable circulating power in current and future gravitational-wave detectors is calculated statistically given different point absorber configurations. Our formulation is further confirmed experimentally in comparison with the scattered power in the arm cavity of Advanced LIGO measured by insitu photodiodes. The understanding presented here provides an important tool in the global effort to design future gravitational-wave detectors that support high optical power and thus reduce quantum noise.

GECKO Optical Follow-up Observation of Three Binary Black Hole Merger Events: GW190408_181802, GW190412, and GW190503_185404

Abstract We present optical follow-up observation results of three binary black hole merger (BBH) events, GW190408_181802, GW190412, and GW190503_185404, which were detected by the Advanced LIGO and Virgo gravitational wave (GW) detectors. Electromagnetic (EM) counterparts are generally not expected for BBH merger events. However, some theoretical models suggest that EM counterparts of BBH can possibly arise in special environments, prompting motivation to search for EM counterparts for such events. We observed high-credibility regions of the sky for the three BBH merger events with telescopes of the Gravitational-wave EM Counterpart Korean Observatory (GECKO), including the KMTNet. Our observation started as soon as 100 minutes after the GW event alerts and covered 29–63 deg2 for each event with a depth of ∼22.5 mag in the R band within hours of observation. No plausible EM counterparts were found for these events, but based on there being no detection of the GW190503_185404 event, for which we covered the 69% credibility region, we place the BBH merger EM counterpart signal to be M g > − 18.0 AB mag within about one day of the GW event. The comparison of our detection limits with light curves of several kilonova models suggests that a kilonova event could have been identified within hours of the GW alert with GECKO observations if the compact merger happened at <400 Mpc and the localization accuracy was on the order of 50 deg2. Our result shows great promise for the GECKO facilities to find EM counterparts within a few hours from GW detection in future GW observation runs.

Reducing scattered light in LIGO’s third observing run

Noise due to scattered light has been a frequent disturbance in the advanced LIGO gravitational wave detectors, hindering the detection of gravitational waves. The non stationary scatter noise caused by low frequency motion can be recognized as arches in the time-frequency plane of the gravitational wave channel. In this paper, we characterize the scattering noise for LIGO and Virgo’s third observing run O3 from April, 2019 to March, 2020. We find at least two different populations of scattering noise and we investigate the multiple origins of one of them as well as its mitigation. We find that relative motion between two specific surfaces is strongly correlated with the presence of scattered light and we implement a technique to reduce this motion. We also present an algorithm using a witness channel to identify the times this noise can be present in the detector.

Advanced LIGO Laser Systems for O3 and Future Observation Runs

The advanced LIGO gravitational wave detectors need high power laser sources with excellent beam quality and low-noise behavior. We present a pre-stabilized laser system with 70 W of output power that was used in the third observing run of the advanced LIGO detectors. Furthermore, the prototype of a 140 W pre-stabilized laser system for future use in the LIGO observatories is described and characterized.

Are stellar-mass binary black hole mergers isotropically distributed?

ABSTRACT The Advanced LIGO and Advanced Virgo gravitational wave detectors have detected a population of binary black hole mergers in their first two observing runs. For each of these events, we have been able to associate a potential sky location region represented as a probability distribution on the sky. Thus, at this point we may begin to ask the question of whether this distribution agrees with the isotropic model of the Universe, or if there is any evidence of anisotropy. We perform Bayesian model selection between an isotropic and a simple anisotropic model, taking into account the anisotropic selection function caused by the underlying antenna patterns and sensitivity of the interferometers over the sidereal day. We find an inconclusive Bayes factor of 1.3: 1, suggesting that the data from the first two observing runs are insufficient to pick a preferred model. However, the first detections were mostly poorly localized in the sky (before the Advanced Virgo joined the network), spanning large portions of the sky and hampering detection of potential anisotropy. It will be appropriate to repeat this analysis with events from the recent third LIGO observational run and a more sophisticated cosmological model.

Advanced LIGO squeezer platform for backscattered light and optical loss reduction

The Advanced LIGO gravitational-wave detectors are limited by optical quantum noise in most of their detection band. To overcome this limit, squeezed vacuum states have been injected into the Advanced LIGO detectors during the third observing run (O3), leading to an increase of their detection rate by about 40% to 50%. Here we present a key element of LIGO’s squeezed vacuum source: the seismic isolation platform that houses core components placed in ultra-high vacuum. This paper describes the architecture of the isolation platform as well as the active control system, tuned to minimize backscattered light that otherwise deteriorates the sensitivity of the detectors. This architecture permits fewer optical Faraday isolators in the optical path of the squeezing system, minimizing optical losses to maximize the quantum noise improvement. The system reliably operated throughout LIGO’s O3 with no evidence of noise from backscattered light. The innovative architecture of this platform makes it ideal for straightforward reshaping and adaptation to other gravitational-wave detector subsystems and low-noise optical instrumentation.

Analysis and visualization of the output mode-matching requirements for squeezing in Advanced LIGO and future gravitational wave detectors

The sensitivity of ground-based gravitational wave (GW) detectors will be improved in the future via the injection of frequency-dependent squeezed vacuum. The achievable improvement is ultimately limited by losses of the interferometer electromagnetic field that carries the GW signal. The analysis and reduction of optical loss in the GW signal chain will be critical for optimal squeezed light-enhanced interferometry. In this work we analyze a strategy for reducing output-side losses due to spatial mode mismatch between optical cavities with the use of adaptive optics. Our goal is not to design a detector from the top down, but rather to minimize losses within the current design. Accordingly, we consider actuation on optics already present and one transmissive optic to be added between the signal recycling mirror and the output mode cleaner. The results of our calculation show that adaptive mode-matching with the current Advanced LIGO design is a suitable strategy for loss reduction that provides less than 2% mean output mode-matching loss. The range of actuation required is +47 uD on SR3, +140 mD on OM1 and OM2, +50 mD on the SRM substrate, and -50 mD on the added new transmissive optic. These requirements are within the demonstrated ranges of real actuators in similar or identical configurations to the proposed implementation. We also present a novel technique that graphically illustrates the matching of interferometer modes and allows for a quantitative comparison of different combinations of actuators.

Improved fused silica fibres for the advanced LIGO monolithic suspensions

To further increase the sensitivity of the advanced LIGO (aLIGO) gravitational wave detectors, two major upgrades of the monolithic fused silica suspension are considered: a higher stress in the suspension fibres and a longer final suspension stage. One of the challenges for this upgrade will be producing thinner and longer fibres that can hold the test mass safely. We demonstrate that laser power fluctuations during the fibre fabrication process can produce diameter variations and potentially weak fibres. We present a laser intensity stabilisation technology for fused silica fibre fabrication using a camera system to monitor heating. Fibres fabricated with this new technique showed a 34% decrease in the interquartile range of measured breaking stress, which indicates that the application of intensity stabilisation technology can improve the statistical strength of fused silica fibres by tightening the spread of values. As the aLIGO detectors upgrade plan (A+) proposes to use thinner fibres, it is essential to enhance the performance of fused silica fibres.

Stepping-stone sampling algorithm for calculating the evidence of gravitational wave models

Bayesian statistical inference has become increasingly important for the analysis of observations from the Advanced LIGO and Advanced Virgo gravitational wave detectors. To this end, iterative simulation techniques, in particular nested sampling and parallel tempering, have been implemented in the software library LALInference to sample from the posterior distribution of waveform parameters of compact binary coalescence events. Nested sampling was mainly developed to calculate the marginal likelihood of a model but can produce posterior samples as a byproduct. Thermodynamic integration is employed to calculate the evidence using samples generated by parallel tempering but has been found to be computationally demanding. Here we propose the stepping-stone sampling algorithm, originally proposed by Xie et al. (2011) in phylogenetics and a special case of path sampling, as an alternative to thermodynamic integration. The stepping-stone sampling algorithm is also based on samples from the power posteriors of parallel tempering but has superior performance as fewer temperature steps and thus computational resources are needed to achieve the same accuracy. We demonstrate its performance and computational costs in comparison to thermodynamic integration and nested sampling in a simulation study and a case study of computing the marginal likelihood of a binary black hole signal model applied to simulated data from the Advanced LIGO and Advanced Virgo gravitational wave detectors. To deal with the inadequate methods currently employed to estimate the standard errors of evidence estimates based on power posterior techniques, we propose a novel block bootstrap approach and show its potential in our simulation study and LIGO application.

Demonstration of the optical AC coupling technique at the advanced LIGO gravitational wave detector

Lasers for gravitational wave detectors need to fulfill tight requirements in amplitude stability, which can only be met by means of feedback control loops. Ultimately, power stabilization control loops are limited by the shot noise of their sensor. The power noise increases linearly with the amount of detected power, while the shot noise grows with the square root. Increasing the detected power is therefore a suitable means to reach a lower sensing noise but it is limited by the power handling capabilities of the photodiodes.An alternative way of improving the sensitivity is the optical AC coupling technique, which exploits the high pass behavior of an optical resonator to reduce the optical power on the detector without compromising its sensitivity above the corner frequency.In this paper we investigate the optical AC coupling technique at the aLIGO Livingston gravitational wave detector. We measured an optical AC coupling gain of 10 dB in the gravitational wave detection band, which offers the potential to improve the laser power stability by the same factor.

Mechanical characterisation of the TorPeDO: a low frequency gravitational force sensor

Newtonian noise is likely to be a future challenge at low frequencies for Advanced LIGO and other second generation gravitational wave detectors. We present the TorPeDO system: a dual torsion pendulum sensor designed to measure local gravitational forces to high precision. Gravitational forces induce a differential rotation between the two torsion beams, which is measured with an optical read-out. Both torsion pendulums have a common suspension point, tunable centre of mass, and resonant frequency. This produces a high level of mechanical common mode noise cancellation. We report on a controls prototype of the TorPeDO system, presenting the frequency response and tuning range of both pendulums. A noise budget and mechanical cross-coupling model for this system are also presented. We demonstrate frequency tuning of the two torsion pendulums to a difference of 4.3 μHz.

X-RAY FLASHES POWERED BY THE SPINDOWN OF LONG-LIVED NEUTRON STARS

ABSTRACT X-ray flashes (XRFs) are a class of high-energy transients whose nature is still open to question. Similar in many aspects to common gamma-ray bursts (GRBs), their strong X-ray emission is accompanied by very low or absent emission in the gamma-ray band. Despite this key difference, a number of indications have consolidated the idea that XRFs and GRBs share a common origin, including a number of potential XRF/supernova associations and the consistency of some XRFs with the Amati relation for long GRBs. However, the difficulties in explaining XRFs as off-axis or intrinsically weak GRBs still cast doubts on this interpretation. Here we explore the possibility that some XRFs are instead powered by the spindown of a long-lived neutron star (NS) formed in a binary NS (BNS) merger or, possibly, in a core-collapse supernova. Focusing on XRF 020903 and a few other cases observed by HETE-2, we show that their lack of gamma-ray emission, spectral properties, duration and X-ray luminosity find a natural explanation within our hypothesis. Moreover, we point out that the agreement of XRF 020903 with the Amati and Ghirlanda relations for long GRBs is respectively only marginal and problematic. Assuming a BNS merger origin for the long-lived NS, we use XRF observations to estimate a lower limit on the rate of BNS mergers accompanied by a potentially observable XRF signal. Within the reach of the advanced LIGO and Virgo gravitational wave detectors, we find . Finally, we discuss the implications of a supernova association for the XRF events considered.

The effect of time on optical coating mechanical loss and implications for LIGO-India

We report on the persistence of mechanical loss with time of ion beam sputtered dielectric coatings made from alternating layers of Ta2O5 and SiO2 deposited onto fused silica substrates. From this, we predict the coating thermal noise in gravitational wave interferometers, after the coated optics have been stored for years. We measured the modal mechanical quality factor, Q, of two coated fused silica samples in 2015. These samples also had their modal Q's measured in 2002. We conclude that storing the coated silica disks for 13 years does not change their mechanical loss and thus the storage of Advanced LIGO gravitational wave detector optics until their future installation in India will not degrade their achievable thermal noise.

A DARK ENERGY CAMERA SEARCH FOR AN OPTICAL COUNTERPART TO THE FIRST ADVANCED LIGO GRAVITATIONAL WAVE EVENT GW150914

ABSTRACT We report the results of a deep search for an optical counterpart to the gravitational wave (GW) event GW150914, the first trigger from the Advanced LIGO GW detectors. We used the Dark Energy Camera (DECam) to image a 102 deg2 area, corresponding to 38% of the initial trigger high-probability sky region and to 11% of the revised high-probability region. We observed in the i and z bands at 4–5, 7, and 24 days after the trigger. The median 5σ point-source limiting magnitudes of our search images are i = 22.5 and z = 21.8 mag. We processed the images through a difference-imaging pipeline using templates from pre-existing Dark Energy Survey data and publicly available DECam data. Due to missing template observations and other losses, our effective search area subtends 40 deg2, corresponding to a 12% total probability in the initial map and 3% in the final map. In this area, we search for objects that decline significantly between days 4–5 and day 7, and are undetectable by day 24, finding none to typical magnitude limits of i = 21.5, 21.1, 20.1 for object colors (i − z) = 1, 0, −1, respectively. Our search demonstrates the feasibility of a dedicated search program with DECam and bodes well for future research in this emerging field.

The advanced LIGO input optics.

The advanced LIGO gravitational wave detectors are nearing their design sensitivity and should begin taking meaningful astrophysical data in the fall of 2015. These resonant optical interferometers will have unprecedented sensitivity to the strains caused by passing gravitational waves. The input optics play a significant part in allowing these devices to reach such sensitivities. Residing between the pre-stabilized laser and the main interferometer, the input optics subsystem is tasked with preparing the laser beam for interferometry at the sub-attometer level while operating at continuous wave input power levels ranging from 100 mW to 150 W. These extreme operating conditions required every major component to be custom designed. These designs draw heavily on the experience and understanding gained during the operation of Initial LIGO and Enhanced LIGO. In this article, we report on how the components of the input optics were designed to meet their stringent requirements and present measurements showing how well they have lived up to their design.

Advanced LIGO

The Advanced LIGO gravitational wave detectors are second-generation instruments designed and built for the two LIGO observatories in Hanford, WA and Livingston, LA, USA. The two instruments are identical in design, and are specialized versions of a Michelson interferometer with 4 km long arms. As in Initial LIGO, Fabry–Perot cavities are used in the arms to increase the interaction time with a gravitational wave, and power recycling is used to increase the effective laser power. Signal recycling has been added in Advanced LIGO to improve the frequency response. In the most sensitive frequency region around 100 Hz, the design strain sensitivity is a factor of 10 better than Initial LIGO. In addition, the low frequency end of the sensitivity band is moved from 40 Hz down to 10 Hz. All interferometer components have been replaced with improved technologies to achieve this sensitivity gain. Much better seismic isolation and test mass suspensions are responsible for the gains at lower frequencies. Higher laser power, larger test masses and improved mirror coatings lead to the improved sensitivity at mid and high frequencies. Data collecting runs with these new instruments are planned to begin in mid-2015.

Parameter estimation for compact binaries with ground-based gravitational-wave observations using the LALInference software library

The Advanced LIGO and Advanced Virgo gravitational wave (GW) detectors will begin operation in the coming years, with compact binary coalescence events a likely source for the first detections. The gravitational waveforms emitted directly encode information about the sources, including the masses and spins of the compact objects. Recovering the physical parameters of the sources from the GW observations is a key analysis task. This work describes the LALInference software library for Bayesian parameter estimation of compact binary signals, which builds on several previous methods to provide a well-tested toolkit which has already been used for several studies. We show that our implementation is able to correctly recover the parameters of compact binary signals from simulated data from the advanced GW detectors. We demonstrate this with a detailed comparison on three compact binary systems: a binary neutron star, a neutron star black hole binary and a binary black hole, where we show a cross-comparison of results obtained using three independent sampling algorithms. These systems were analysed with non-spinning, aligned spin and generic spin configurations respectively, showing that consistent results can be obtained even with the full 15-dimensional parameter space of the generic spin configurations. We also demonstrate statistically that the Bayesian credible intervals we recover correspond to frequentist confidence intervals under correct prior assumptions by analysing a set of 100 signals drawn from the prior. We discuss the computational cost of these algorithms, and describe the general and problem-specific sampling techniques we have used to improve the efficiency of sampling the compact binary coalescence parameter space.

Architecture, implementation and parallelization of the software to search for periodic gravitational wave signals

The parallelization, design and scalability of the PolGrawAllSky code to search for periodic gravitational waves from rotating neutron stars is discussed. The code is based on an efficient implementation of the F-statistic using the Fast Fourier Transform algorithm. To perform an analysis of data from the advanced LIGO and Virgo gravitational wave detectors’ network, which will start operating in 2015, hundreds of millions of CPU hours will be required—the code utilizing the potential of massively parallel supercomputers is therefore mandatory. We have parallelized the code using the Message Passing Interface standard, implemented a mechanism for combining the searches at different sky-positions and frequency bands into one extremely scalable program. The parallel I/O interface is used to escape bottlenecks, when writing the generated data into file system. This allowed to develop a highly scalable computation code, which would enable the data analysis at large scales on acceptable time scales. Benchmarking of the code on a Cray XE6 system was performed to show efficiency of our parallelization concept and to demonstrate scaling up to 50 thousand cores in parallel. Program summaryProgram title: parallel PolGrawAllSkyCatalogue identifier: AEUX_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEUX_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 163747No. of bytes in distributed program, including test data, etc.: 28989030Distribution format: tar.gzProgramming language: C.Computer: Any parallel computing platform supporting MPI standard.Operating system: Linux as well any other supporting MPI standard.Has the code been vectorized or parallelized?: Yes, using MPI. Tested with up to 50208 processorsRAM: 1 Gigabyte per parallel taskClassification: 1.5.External routines: MPI v.2 or newer, FFTW v.3 or newerNature of problem:Search for periodic gravitational waves from rotating neutron stars.Solution method:The F-statistic method using the Fast Fourier Transform algorithm.Running time:The example provided takes approximately 30 mins with 256 processors.

Enhanced characteristics of fused silica fibers using laser polishing

The search for gravitational wave signals from astrophysical sources has led to the current work to upgrade the two largest of the long-baseline laser interferometers, the LIGO detectors. The first fused silica mirror suspensions for the Advanced LIGO gravitational wave detectors have been installed at the LIGO Hanford and Livingston sites. These quadruple pendulums use synthetic fused silica fibers produced using a CO2 laser pulling machine to reduce thermal noise in the final suspension stage. The suspension thermal noise in Advanced LIGO is predicted to be limited by internal damping in the surface layer of the fibers, damping in the weld regions, and the strength of the fibers. We present here a new method for increasing the fracture strength of fused silica fibers by laser polishing of the stock material from which they are produced. We also show measurements of mechanical loss in laser polished fibers, showing a reduction of 30% in internal damping in the surface layer.