Characterization of transient noise in Advanced LIGO relevant to gravitational wave signal GW150914

Abstract

On September 14, 2015, a gravitational wave signal from a coalescing black hole binary system was observed by the Advanced LIGO detectors. This paper describes the transient noise backgrounds used to determine the significance of the event (designated GW150914) and presents the results of investigations into potential correlated or uncorrelated sources of transient noise in the detectors around the time of the event. The detectors were operating nominally at the time of GW150914. We have ruled out environmental influences and non-Gaussian instrument noise at either LIGO detector as the cause of the observed gravitational wave signal.

Figure A1:

The effectiveness of the veto criteria designed to flag h(t) non-stationarity due to the malfunction of the 45MHz driver over a six hour period on September 21, 2015. The top panel shows the witness channel (a monitor for amplitude fluctuations in the signal used to generate the 45 MHz optical sidebands) over a 6 hour period with non-stationary data in h(t). Due to variation in its mean value, a band limited root-mean-square (BLRMS) of this channel over 60 seconds was a better indicator of the targeted behavior, shown in the middle panel. Thresholds of this BLRMS were tested over 11 days during the analysis period for efficiency in identifying periods of high trigger rate in h(t), and the threshold shown in the middle figure was found to be optimal for the analysis time removed. The bottom panel shows Omicron h(t) triggers over the same 6 hour time period. Times removed by the veto are shaded out in gray.

Figure A2:

The rate of Omicron triggers with and without vetoes applied to 11 days of data, a subset of the analysis period. The veto is effective at removing excess triggers with a SNR between 15 and 100. When applied to the full GW150914 analysis period, this data quality veto removed 42% of noise transients of an SNR of 20 or greater, at the expense of 2.6% of coincident data.

Figure B1:

The physical environment monitor (PEM) array at the Livingston detector, as seen on http://pem.ligo.org [10]. Gray dashed lines enclose instrumentation in separate structures: the corner station building located at the vertex of the laser-interferometric detector, the two end stations located at the end of the 4km detector arms, and the ‘vault’, which houses PEM sensors away from all buildings to measure noise due to the external environment. Purple dashed lines indicate rooms within structures, or spaces just outside of structures. For example, the corner station and both end stations have PEM sensors in electronics rooms containing computers that sense and control the detector as well as PEM equipment mounted on a mast on the roof. See [4, 6] for detailed description of the optical layout shown.

Figure 1:

The average measured strain-equivalent noise, or sensitivity, of the Advanced LIGO detectors during the time analyzed to determine the significance of GW150914 (Sept 12 - Oct 20, 2015). LIGO-Hanford (H1) is shown in red, LIGO-Livingston (L1) in blue. The solid traces represent the median sensitivity and the shaded regions indicate the 5th and 95th percentile over the analysis period. The narrowband features in the spectra are due to known mechanical resonances, mains power harmonics, and injected signals used for calibration [4, 5, 6].

Figure 2:

Noise coupling example: determining magnetic field coupling for a location at LIGO-Hanford. The top panel shows the output of a magnetometer installed in the corner station (see Figure B1) during the injection of a series of single frequency oscillating magnetic fields at 6 Hz intervals (in red) and at a nominally quiet time (in blue). The middle panel shows h(f) during this test (in red) and during the same nominally quiet time (in blue). The heights of the induced peaks in h(f) can be used to determine the magnetic coupling (in m/T) at those frequencies, as shown in the bottom panel. The points in the bottom panel above 80 Hz were determined in a different test with a stronger magnetic field needed to produce discernible peaks in h(f). The green points in the middle panel are an estimate of the contribution to h(f) from the ambient magnetic noise during the nominally quiet time, calculated using the coupling function from the bottom panel. Injection tests also induced strong magnetic fields above 200 Hz. At higher frequencies, coupling was so low that the injected fields did not produce a response in h(f), but were used to set upper limits on the coupling function. This figure only shows data for one (typical) location, but similar injections were repeated at all locations where magnetic coupling might be of concern.

Figure 3:

A normalized spectrogram of the LIGO-Livingston h(t) channel at the time of a blip transient. The color scale indicates excess signal energy of data normalized by an estimated power spectral density.

Figure 4:

The maximum sensitivity of LIGO-Hanford (red) and LIGO-Livingston (blue) during the analyzed period (September 12 - October 20 2015) to a binary black hole system with the same observed spin and mass parameters as GW150914 for optimal sky location and source orientation and detected with an SNR of 8. Each point was calculated using the PSD as measured for each analysis segment (2048 seconds) of the CBC search. The times of events GW150914 and LVT151012 are indicated with vertical dashed and dot-dashed lines respectively. The LIGO-Livingston detector entered observation mode roughly 30 minutes prior to GW150914 after completing PEM injection tests in a stable, operational state. The LIGO-Hanford detector had been in observation mode for over an hour.

Figure 5:

The rate of single interferometer background triggers in the CBC search for H1 (above) and L1 (below), where color indicates a threshold on the detection statistic, χ²-weighted SNR. Each point represents the average rate over a 2048 second interval. The times of GW150914 and LVT151012 are indicated with vertical dashed and dot-dashed lines respectively.

Figure 6:

The behavior of cWB background triggers in frequency and coherent network SNR over the duration of the analysis period (right) and the frequency distribution of these triggers by week from September 12 to October 20, 2015 (left). For each time-shifted background trigger, the time for the Livingston detector is indicated. The time of GW150914, recovered with a coherent network SNR of 20, is indicated with a dashed vertical line in the right panel. (LVT151012 was not identified by cWB.) Overall, the background distribution is consistent throughout the analysis period.

Figure 7:

The impact of data-quality vetoes on the CBC background trigger distribution for (a) LIGO-Hanford and (b) LIGO-Livingston. The single-detector χ²-weighted SNR of GW150914 is indicated for each detector with a dashed line (19.7 for Hanford and 13.3 for Livingston), and for event LVT151012 with a dot-dashed line (6.9 for Hanford and 6.7 for Livingston).

Figure 8:

The impact of data-quality vetoes and signal consistency requirements on the background trigger distribution from the cWB search for gravitational-wave bursts by coherent network SNR. The multi-detector coherence required by cWB greatly reduces the rate of outlier events relative to the single-detector triggers shown in Figure 9. Note that the background rate is much lower than for single-interferometer triggers because it is normalized by the entire duration of the time-shifted analysis, not only the analysis period. The detected coherent network SNR of GW150914 is indicated with a dashed line. Note the background distributions shown here were selected to illustrate the effect of data quality vetoes and differ from those in Figure 4 of [1].

Figure 9:

The impact of data-quality vetoes on the single-detector burst triggers detected by the Omicron burst algorithm for (a) LIGO-Hanford and (b) LIGO-Livingston. The SNR of GW150914 in each detector is indicated with a dashed line.

Figure 10:

Normalized spectrograms of GW150914 in LIGO-Hanford (left) and LIGO-Livingston (right) h(t) data with the same central GPS time. The data at both detectors exhibited typically low levels of noise around the time of the event; the signal, offset by ~7 ms between detectors, was recovered by a matched-filter CBC search with a combined detector signal-to-noise ratio of 24 [1, 2], by the coherent burst search with a coherent network SNR of 20 [3], and by Omicron with a single-detector SNR of 12 in Hanford and 9 in Livingston. The time-frequency morphology of the event is distinct from the known noise sources discussed in Section 3.

Figure 11:

A normalized spectrogram centered around the time of GW150914 of a Streckeisen STS-2 seismometer located near the Y-end test mass. An air compressor turns on at −75 seconds and off at +100 seconds.

Figure 12:

A blip transient in LIGO-Livingston strain data that produced a significant background trigger in the CBC analysis in orange, and the best-match template waveform (amplitude-scaled for comparison) in black, which exhibits a few more low-SNR cycles but otherwise quite similar morphology. The best-match waveform for the GW150914 signal, in gray, is quite distinct from both the blip transient and the neutron-star-black-hole (NSBH) waveform that most closely matches it, with more than 10 distinct cycles shown and a significant increase in frequency over time. All three time series have the same zero-phase band-pass filter applied.

Figure 13:

Normalized spectrograms of LVT151012 in LIGO-Hanford (left) and LIGO-Livingston (right) h(t) data with the same central GPS time. Note these spectrograms have a much smaller normalized energy scale than those in Figure 10.

Figure 14:

The rate of transient noise as witnessed by the single detector burst algorithm Omicron for the LIGO Hanford (above) and LIGO-Livingston (below) detectors. Each dot represents the average trigger rate over a 600 second interval. Green dots show triggers with an SNR above 5, and blue crosses show triggers with an SNR above 10. Time vetoed from the analysis period is indicated in gray. The time of GW150914 is indicated with a vertical dashed line and LVT151012 with a dot-dashed line.