What means "Standard Sirens" associated to Gravitational Waves observable(s)? We know the Gravitational Waves (GW) provide a new way to observe the Universe. Now, we face a new era for cosmological and astrophysical researches. I know that it is possible to obtain the gravitational redshift from some astrophysical phenomena like merge of black holes and other sources. Furthermore, I understand the meaning of Standard Ruler (@ Baryonic Acustic Oscillations) and Standard Candle (@ Surpernovae etc). However, I do not understand what is the meaning of Standard Sirens for GW observable(s). Thanks!
 A: The meaning is, that by measuring the amplitude and frequency of the gravitational waves received from an inspiralling binary system, and by also measuring the rate of change of that frequency, we can determine both the"chirp mass" and the luminosity distance to the gravitational wave source.
In an analogous way, the observed brightness of a standard candle depends on how far away it is. Here, because
it is the pitch and the rate of change of pitch that is being measured$^*$, we call these inspiralling binaries "standard sirens". A distance can be estimated directly from the measurements without any appeal to external calibrations or cosmological parameters.
This explanation is to first order. There is another complication, which is the amplitude also depends on the inclination of the plane of the binary orbit to the line of sight. However, this can also be estimated/eliminated by measuring the amplitude of each of the two possible gravitational wave polarisations separately. This is best done by having two independent detectors (like LIGO and VIRGO) with different orientations.
In addition, if the source redshift is known (e.g. it is located in a galaxy with a known redshift), then cosmological parameters like the Hubble constant can themselves be independently estimated.
A good popular account of this can be read here.
Abbott et al. (2017) describes how a standard siren can be used to estimate the distance to a merging neutron star binary in an identified galaxy, leading to an estimate of the Hubble constant.
$^*$ There is also the fact that the frequency of the binary gravitational wave sources that are detected by ground-based interferometers is actually in the range that would be audible (if they were sound waves).
