samedi 13 février 2016

The real, the true and the plausible (or false, fake and not graduated)

//the title of this post has been slightly changed after the update on Feb. 22 2016

A real blind-injection and fake signal of a direct detection of gravitational waves in 2010...

A rather strong signal was observed on September 16, 2010, within a minute or so of its apparent arrival at the detectors. The scientists on duty at the detector sites immediately recognized the tell-tale chirp signal expected from the merger of two black holes and/or neutron stars, and sprang into action. They knew that it could be a blind injection, but they also knew to act like it was the real thing. The event was beautifully consistent with the expected signal from such a merger. The figures below show the strength of the signal (redder colors indicate more signal power) in time (horizontal axis) and frequency (vertical axis). The signal sweeps upwards in frequency ("chirp") as the stars spiral into one another, approaching merger. The first plot is what was seen in the LIGO Hanford detector, and the second is what was seen at the same time in the LIGO Livingston detector. Despite apparent differences, the two signals are completely consistent with one another. The dark and light blue regions are typical of fluctuating noise in the detectors. 
The loudness of the signal was consistent with it coming from a galaxy at a distance between 60 and 180 million light-years from ours. The detector network is capable of locating the source in the sky only crudely; it seemed to be coming from the constellation Canis Major (the "Big Dog") in the southern hemisphere (the event was dubbed "the Big Dog" shortly thereafter). They sent alerts to partners operating robotic optical telescopes in the southern hemisphere (ROTSE, TAROT, Skymapper, Zadko) and the Swift X-ray space telescope, all of which took images of the sky on that and/or subsequent days in the hope of capturing an optical or X-ray "afterglow".

Blind Injection" Stress-Tests LIGO and VIRGO's Search for Gravitational Waves 2010

Versus the first true signal or false blind-injection five years later...
On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10-21. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410+160-180 Mpc corresponding to a redshift z=0.09+0.03-0.04. In the source frame, the initial black hole masses are 36+5-4M and 29±4M, and the final black hole mass is 62±4 M, with 3.0±0.5M⊙ c2 radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.
                      Signal at LIGO Hanford Observatory            Signal at LIGO Livingston Observatory
B. P. Abbott et al.* 
(LIGO Scientific Collaboration and Virgo Collaboration)
(Received 21 January 2016; published 11 February 2016)

Gravitational waves (GW) ground-based instruments are all-sky monitors with no intrinsic spatial resolution capability for transient signals. A network of instruments is needed to reconstruct the location of a GW in the sky, via time-of-arrival, and amplitude and phase consistency across the network [102]. The observed time-delay of GW150914 between the Livingston and Hanford observatories was 6.9+0.5-0.4 ms. With only the two LIGO instruments in observational mode, GW150914’s source location can only be reconstructed to approximately an annulus set to first approximation by this time-delay [103, 104]. .. the sky map for GW150914 ... corresponds to a projected 2- dimensional credible region of 140 deg2 (50% probability) and 590 deg2 (90% probablity). The associated 3-dimensional comoving volume probability region is ∼10-2 Gpc3; for comparison the comoving density of Milky Way-equivalent galaxies is ∼107 Gpc-3. This area of the sky was targeted by follow-up observations covering radio, optical, near infra-red, X-ray, and gamma-ray wavelengths that are discussed in [105]; searches for coincident neutrinos are discussed in [106].

(Submitted on 11 Feb 2016)

//Update February 22 2016
Compared to a not graduated but plausible second astrophysical signal

Sixteen days of coincident data were used in the analysis of GW150914. This event was by far the most significant found in all transient searches performed. The compact binary coalescences search identified the second most interesting event on the 12th of October 2015. This trigger [designated Ligo-Virgo Trigger 151012] most closely matched the waveform of a binary black hole system with masses 23+18-5M⊙ and 13+4-5M, producing a trigger with a false-alarm rate of 1 event per 2.3 years; far too high to be a strong detection candidate...  
We performed similar in-depth checks of potential noise sources for this trigger. For LIGO-Livingston data, LVT151012 is in coincidence with significant excess power at 10Hz lasting roughly three seconds, a portion of which can be seen in {the} figure {above}. There is no obvious indication of upconversion to the frequency range analyzed by the transient searches, so the low frequency noise is not thought to have caused the signal associated with LVT151012 in the Livingston detector. The data around this event were found to be significantly more non-stationary than those around GW150914. The noise transient rate in the hours around LVT151012 was significantly higher than usual at both LIGO detectors... This was likely due to increased low frequency ground motion associated with ocean waves [55]. The elevated noise transient rate at both sites induced a higher rate of background triggers around the time of LVT151012.

(Submitted on 11 Feb 2016 (v1), last revised 16 Feb 2016 (this version, v2))

... based on two different searches, with different models for the rates at which both noise triggers and astrophysical signals appear in the LIGO detectors, we find posterior probabilities 0.84 and 0.91 that LVT151012 is of astrophysical origin; this is the only other trigger from either search that has probability greater than 50% of being of astrophysical origin. Farr et al. (2015) presented a method by which a set of triggers of uncertain origin like this can be used to produce a rate estimate that is more accurate than that produced by considering only highly significant events.

B. P. Abbott, et al.
(Submitted on 11 Feb 2016)

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