# What causes the 20 second period in LIGO data?

I download LIGO data on GW150914 (gravitational waves) via the code in Mathematica

H1url = "https://losc.ligo.org/s/events/GW150914/H-H1_LOSC_4_V1-\
1126259446-32.hdf5";
strainH1 = Import[H1url, {"Datasets", "/strain/Strain"}];
attrsH1 = Import[H1url, {"Attributes", "/strain/Strain"}]


To my understanding, the data are those of 4096 seconds with a sample rate of 4 kHz. I plot the full data range to get Figure 1.

I zoom in on a portion of the plot. The result is shown in Fig. 2

My first question is, what causes that sinusoidal pattern of period $T\approx 20 ~\text{s}$?

• I suspect you'll have to ask someone at LIGO. If this is not a gravitational wave detection, it could be a host of other things that really nobody outside of LIGO would be able to know. – enumaris Mar 24 '18 at 2:17