Abstract
On April 1st, 2019, the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO), joined by the Advanced Virgo detector, began the third observing run, a year-long dedicated search for gravitational radiation. The LIGO detectors have achieved a higher duty cycle and greater sensitivity to gravitational waves than ever before, with LIGO Hanford achieving angle-averaged sensitivity to binary neutron star coalescences to a distance of 111 Mpc, and LIGO Livingston to 134 Mpc with duty factors of 74.6% and 77.0% respectively. The improvement in sensitivity and stability is a result of several upgrades to the detectors, including doubled intracavity power, the addition of an in-vacuum optical parametric oscillator for squeezed-light injection, replacement of core optics and end reaction masses, and installation of acoustic mode dampers. This paper explores the purposes behind these upgrades, and explains to the best of our knowledge the noise currently limiting the sensitivity of each detector.
Highlights
In 2015, the Advanced LIGO detectors at Hanford, Washington and Livingston, Louisiana achieved unprecedented sensitivity to gravitational waves [1,2]
The purpose of this chain is to reduce coupling of ground motion to the test mass. These pendulums are suspended from seismic isolation platforms [63] which themselves are supported by hydraulically actuated preisolation structures [64]
The output signal of this auxiliary loop is injected into differential arm cavity length (DARM) with this transfer function and opposite sign, canceling the auxiliary noise that normally would appear in DARM
Summary
In 2015, the Advanced LIGO detectors at Hanford, Washington and Livingston, Louisiana achieved unprecedented sensitivity to gravitational waves [1,2]. The third observing run (O3), which ran from April 1 to September 30, 2019 (O3a) and from November 1, 2019 until March 27, 2020 (O3b), has been the most successful search for gravitational waves in history, with greater sensitivity and the permanent addition of the Advanced Virgo detector [13]. During this run, 56 candidate gravitational-wave signals, including at least one new compact binary coalescence in the binary neutron star mass range [14] and a system with record mass ratio [15], were announced [16].
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