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 four 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

The sensitivity of the instruments has increased dramatically between these runs, as shown in the following figure.

Figure 8.1: The curves show the level of instrumental strain noise in the LIGO Hanford 4km instrument as a function of gravitational wave frequency, and how it has decreased during the commissioning process. The design goal is the curve labeled SRD (Science Reference Design) [30].

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 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 [6].

The quantity $h$ that appears on the vertical axis of these graphs is called ``strain''. To appreciate the remarkable sensitivity of these instruments, refer back to the schematic diagram of an interferometric gravitational wave detector shown earlier. The strain $h$ is the fractional change in the apparent arm length that would be caused by the passage of a gravitational wave. 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 an atomic nucleus!

Currently, LIGO is the most sensitive gravitational wave detector operating anywhere in the world, and is operating close to design sensitivity. A long data taking run (S5) at design sensitivity is anticipated to begin late in 2005.

Some of the LIGO Scientific Collaboration analysis results obtained from the S1 and S2 data are reported in [31,32,33]. Einstein@Home is using similar methods and code but began its analysis work with S3 data. Einstein@Home completed analysis on the LIGO S3 data in early August 2005, and then began searching the S4 data.

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Einstein@Home S3 Analysis Summary	Last Revised: 2005.09.11 16:22:17 UTC
Copyright © 2005 Bruce Allen for the LIGO Scientific Collaboration	Document version: 1.97