8. What is the LIGO S3 data set?

Construction of the LIGO and GEO instruments began in the mid-1990s. When the construction phase of the project was completed, the LIGO instruments were officially inaugurated on November 11 and 12, 1999. Since that time, the commissioning of the instruments has proceeded in a sequence of engineering (E) and science (S) runs at increasing sensitivity.

The five LIGO/GEO science runs to date are:

S1: August 23 - September 9, 2002

S2: February 14 - April 14, 2003

S3: October 31, 2003 - January 9, 2004

S4: February 22 - March 23, 2005

S5: Nov 4, 2005 - (planned August 2007)

The sensitivity of the instruments has increased dramatically between these runs, as shown in the following figure. The current S5 science run will last for about one and a half years, with the LIGO detectors operating essentially at design sensitivity.

Figure 8.1: The curves show the level of instrumental noise at different frequencies for the LIGO Hanford and Livingston 4km instruments, and how it has decreased during the commissioning process. The design goal is the curve labeled SRD (Science Requirements Document) [32].

The following figure is a similar plot for GEO600. As shown in the figure, GEO600 was less sensitive than LIGO during the S3 run. Therefore, Einstein@Home used only LIGO data for the S3 analysis. The results reported in the following sections are the final Einstein@Home analysis results for LIGO S3 data.

Figure 8.2: The curves show the level of instrumental noise in the GEO600 instrument (Figure courtesy GEO600). The arm length of GEO600 is 600m, which is shorter than the 4km length of the LIGO arms. In general, the sensitivity of a laser interferometric gravitational wave detector increases with arm length. But the GEO600 project has been investigating alternative advanced techniques to achieve improved sensitivity. These will benefit future gravitational wave detection projects, including a planned upgrade to LIGO at the end of this decade [7].

The quantity h(f) that appears on the vertical axis of these graphs is a spectral representation of the so-called "strain" h(t) measured by the detector over different frequencies f. To appreciate the remarkable sensitivity of these instruments, refer back to the schematic diagram of an interferometric gravitational wave detector shown earlier. The strain h(t) is the fractional change in the apparent arm length that would be caused by the passage of a gravitational wave at time t. Thus a strain $h=10^{-21}$ in the LIGO 4km arms corresponds to a change in the arm length of $4 \times 10^{-18}$ meters. This is about one thousand times smaller than the size of a proton!

Some of the LIGO Scientific Collaboration analysis results obtained from the S1 and S2 data are reported in [33,34,35,36]. Einstein@Home is using similar methods and code but began its analysis work with S3 data. Einstein@Home host machines completed the first-pass analysis of the LIGO S3 data in early August 2005, and then began the first-pass search of S4 data. The post-processing work on the S3 data, which is based on the S3 first-pass results, began a few days later.

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Einstein@Home S3 Analysis Summary	Last Revised: 2007.03.28 08:59:23 UTC
Copyright © 2005 Bruce Allen for the LIGO Scientific Collaboration	Document version: 1.132