Characterization of the LIGO detectors during their sixth science run (1410.7764v2)

Abstract: In 2009-2010, the Laser Interferometer Gravitational-wave Observa- tory (LIGO) operated together with international partners Virgo and GEO600 as a network to search for gravitational waves of astrophysical origin. The sensitiv- ity of these detectors was limited by a combination of noise sources inherent to the instrumental design and its environment, often localized in time or frequency, that couple into the gravitational-wave readout. Here we review the performance of the LIGO instruments during this epoch, the work done to characterize the de- tectors and their data, and the effect that transient and continuous noise artefacts have on the sensitivity of LIGO to a variety of astrophysical sources.

• The paper details key detector upgrades, including increased laser power and improved thermal compensation to enhance sensitivity.
• The paper analyzes transient noise sources and glitch management strategies, notably addressing the spike glitch at L1 through data quality vetoes.
• The paper underscores the S6 run’s impact on advancing gravitational wave astronomy and prepares the groundwork for future aLIGO enhancements.

Characterization of the LIGO Detectors During the Sixth Science Run

The paper provides an in-depth examination of the operations and performance of the LIGO detectors during their sixth science run (S6), spanning from July 2009 to October 2010. During this period, LIGO, comprising the Hanford (H1) and Livingston (L1) Observatories, collaborated with Virgo and GEO600 detectors to form a comprehensive global network dedicated to gravitational wave (GW) detection. This paper focuses on the challenges encountered due to noise and non-Gaussian artefacts, and the subsequent impact on the detection sensitivity.

LIGO Detector Configuration and Enhancements

The LIGO interferometers utilized during S6 were enhanced versions of Michelson interferometers with Fabry-Perot arm cavities, tasked with measuring the GW-induced strain on the 4-kilometer arm lengths. The paper describes the configurations of these detectors, noting several upgrades aimed at improving sensitivity:

• Increased Laser Power: The laser power was upgraded from 10 W to 35 W, enhancing sensitivity in higher frequency ranges and preparing components for upcoming Advanced LIGO (aLIGO) upgrades.
• Thermal Compensation Systems: Improved CO2 laser thermal compensation was implemented to minimize thermal lensing effects, maintaining better optical alignment.
• Optical Filtering: The introduction of an output mode cleaner (OMC) aimed at optimizing the quality of the light reaching the readout.
• Seismic Isolation: Enhanced seismic noise controls were tested at L1 to manage low-frequency disturbances, especially crucial given the site conditions at Livingston.

Detector Sensitivity and Challenges

The sensitivity of the LIGO detectors is contextualized by transient noise and spectral lines that arise from a variety of environmental, mechanical, and electronic sources. The document highlights the detailed spectral analysis of noise sources alongside control measures deployed to mitigate these effects. Key issues identified include:

• Seismic Noise: A fundamental limitation across the low-frequency range, exacerbating transient glitches and contributing upconversion noise at higher frequencies.
• Glitch Management: Numerous instrumental glitches were caused by undesirable mechanical coupling, electronic faults, and environmental disturbances. Notable was the spike glitch at L1, which despite remaining unresolved, was mitigated through data analysis techniques.
• Data Quality Vetoes: Strategies employing data quality vetoes successfully improved search efficacy by correlating transient noise with auxiliary signals allowing portions of corrupted data to be disregarded in analyses. These were crucial in ensuring the integrity of potential GW signals.

Implications and Future Prospects

The efforts to characterize and mitigate noise have enabled a significant improvement in both the stability and sensitivity of the LIGO detectors during S6. This increased sensitivity, alongside improved methods for data analysis and noise vetoing, has allowed LIGO to probe a greater volume of the universe for GW signatures. The comprehensive understanding and handling of these artifacts are instrumental in preparing for the future aLIGO runs, where in-depth diagnostic and real-time detection capabilities will play a central role in multi-messenger astronomy. With expected increases in detection capabilities, these developments herald advancements in both observational cosmology and gravitational physics.

In conclusion, the success of the S6 science run provides critical insights and technical progress necessary for the ongoing evolution and enhancement of gravitational wave astronomy. The groundwork laid in addressing the challenges faced during S6 constitutes an essential component in ensuring the continuing advancement and success of LIGO and its future iterations within the global astronomical community.