As a simple check for any data corruption in your download, you are advised to take a look at the scripts plot_data.py and plot_data.m, modify them if needed to point to the data you've downloaded, and compare the output with what is posted here.
Printout from plot_data.py:
Contents of H2-STRAIN_4096Hz-815045078-256.hdf5 IFO, Npoints = H2, 1048576 Duration = 1.0 * 4096 * 256 min,max,mean,std = -1.83198e-17, 1.83661e-17, 8.40797613955e-23, 4.00000168617e-18 Contents of H2-STRAIN_16384Hz-815043278-2190.hdf5 IFO, Npoints = H2, 35880960 Duration = 1.0 * 16384 * 2190 min,max,mean,std = -7.1032527817e-16, 6.93651633689e-16, -1.80783501127e-21, 1.52450665622e-16 Contents of L1-STRAIN_4096Hz-815045078-256.hdf5 IFO, Npoints = L1, 1048576 Duration = 1.0 * 4096 * 256 min,max,mean,std = -2.5425e-18, 2.53164e-18, -1.86235564404e-25, 5.01073506755e-19 Contents of L1-STRAIN_16384Hz-815043278-2190.hdf5 IFO, Npoints = L1, 35880960 Duration = 1.0 * 16384 * 2190 min,max,mean,std = -1.23971151496e-16, 1.2349742552e-16, 9.44711599361e-23, 2.47410881493e-17
Time series for the first ~10000 points in the files; made by
Histogram of data points, compared with Gaussian.
Comments: These histograms are dominated by the low frequency (~<30 Hz) peaks that are evident in the ASDs (below). The data are only calibrated above 40 Hz. LIGO detector operations and data processing endeavor to achieve "in-band" strain noise (between 40 Hz and 2000 Hz) as stationary and Gaussian as possible.
Noise amplitude spectral densities (noise ASDs) from
compared with the ones stored in
Comment: The data are only calibrated in the sensitive band between 40 Hz and 2000 Hz. The most prominent spectral lines are at harmonics of the power main 60 Hz, the mirror suspension violin modes at 340 Hz, or calibration lines. The difference between the spectrum in the (256 s @ 4096 Hz) data and the (2190 s @ 16384 Hz) data at mid-frequencies are indicative of the non-stationarity of the detector over the 2190 s. The difference starting at ~2000 Hz is due to the different sampling rate.
made by plot_data.py:
Comment: One can see the spectral features evident in the ASDs above, running across the plot in time. There are no prominent short-duration periods of enhanced noise evident in these spectrograms (above 40 Hz); it's a relatively quiet period of detector operation. Periods of enhanced noise are present, but they're hard to see in these plots; they can be seen in omegagrams. LIGO detector operations and data processing endeavor to achieve "in-band" strain noise (between 40 Hz and 2000 Hz) as stationary and Gaussian as possible.
As an illustration of how to interpret the noise amplitude spectral densities, we plot the ASDs from several places:
We also overlay the signal from a binary neutron star (BNS) inspiral at 3.6 Mpc from GRB051103 (with antenna factor 0.49; see below), plotted in the same units as the ASDs. The waveform is plotted up to the nominal end of the inspiral phase (innermost stable circular orbit, or ISCO), which is where the waveform templates used in the Compact Binary Coalescence search end.
This plot is made with the script
At these noise levels, an binary neutron star (BNS) inspiral signal (averaged over sky location and orientation) is expected to give a matched-filter signal-to-noise ratio (SNR) of 8 at distances of 3.6 and 8.0 Mpc in H2 and L1, respectively. This is one of our most useful measures of detector sensitivity.
Antenna factors: The response of a detector to a BNS signal depends not only on its noise spectrum, but also its quadrupole antenna response factor, which in turn depends on the source's sky location and the binary orientation with respect to our line-of-sight. The detection sensitivity is reduced by a factor 0.442 when one averages over source sky location and orientation. In this case, we are considering our sensitivity to a population of sources which we assume to be distributed uniformly in our sensitive volume; so we compute the average of the sensitive distance cubed, then take the cube root.
However, we know the sky location (and time) of GRB051103, so we can compute antenna factors associated with that source. If we assume that the binary is optimally oriented (the orbit is face-on, as one might expect for a beamed GRB), the antenna factors are 0.726 for H2, and 0.657 for L1. If instead we averaged over binary orientation (inclination angle and polarization azimuth) the antenna factors are 0.431 for H2, and 0.389 for L1. In this case we are averaging the sensitive distance, not the sensitive distance cubed.
The table below lists, for the noise spectra around GRB051103 ("H2 SPEC" and "L1 SPEC"), four distances:
|aLIGO HP 0-detune||445.1||196.9|
Since M81 is at ~3.6 Mpc, we see that both H2 and L1 would have BNS signals with SNR of ~8 (or in the case of L1, much larger), making them much louder than the detector noise and easily detectable.