How a gravitational wave is detected?
GWs are not directly detected or observed by telescopes. For this kind of source, the detection happens at an interferometer. Using a beam splitter, a light source is split into two arms. Each of those light beams is reflected back toward the beam splitter which then combines their amplitudes using the superposition principle and results in an interference pattern. When a GW passes through the interferometer, the space-time in the local area is altered. Depending on the source of the wave, this results in an effective change in the length of one or both of the cavities. The cavity will therefore periodically get very slightly out of coherence and the beams, which are tuned to usually be destructively interfering at the detector, will have a very small, periodically varying detuning. This results in a measurable signal.
Automated preliminary alerts are sent for all events that pass a false alarm rate (FAR) threshold. FAR is a statistic that is used to describe the significance of a GW event. It is defined as the rate of accidental events due to detector noise or glitches, in the absence of any astrophysical sources, that are as loud as or louder than the event in question. Alerts were sent out on the Gamma-ray Coordinates Network (GCN), a portal for discoveries and observations of astronomical transients. Historically, GCN has served high-energy satellites but now also other electromagnetic wavelengths and also GW, cosmic ray, and neutrino facilities. LIGO/Virgo uses GCN Circulars to announce detections, and the astronomy community expects participants to promptly disseminate preliminary reports of follow-up observations of LIGO/Virgo counterparts using GCN Circulars as well.
Within a few seconds/minutes, a rapid map of the source’s localization is sent determined by an algorithm which encodes the time shift and amplitude of the signals observed in each interferometer. The more detectors there are and the liu, the greater the precision is. In the hours after the trigger time, a more sophisticated analysis is performed to determine more precisely parameters such as sky location, distance, masses and spins. Thanks to the frequency (and its evolution) of the signal, we can determine the system’s chirp mass (symmetric combination of the primary and secondary component masses) which gives us constraints on the mass of objects and therefore allows us to determine their type (BNS for Binary Neutron Star, BBH for Binary Black Hole, NSBH = Neutron Star-Black Hole, Mass Gap if at least one compact object whose mass between 3 and 5 solar mass an Terrestrial classification for that are of instrumental or environmental origin). Thanks to the duration of the signal, because the interferometers are more sensitive to the frequencies made by objects of low masses, it is also even possible to detect the signal before the collision.
LIGO – How do we detect Gravitational Waves?
LIGO – What types of Gravitational Waves will LIGO detect?
LIGO – How does an Interferometer work?
Gravitational wave localisation and galaxy cross Matching
Talk 21 – Laser Interferometers – Dr. Sendhil Raja
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