Spin 99.8% of maximum

Why 99.8%? Well, we wanted something that was about as fast as is physically plausible. 100% of maximum is mathematically the maximum possible; but, physics tells us that black holes are unlikely to spin up quite that fast. According to a 1974 analysis by Kip Thorne, when a black hole accretes hot matter 99.8% of maximum is the value at which the spin tends to saturate. The physics of this is pretty cool: The hot matter radiates a roughly isotropic glow of photons, and those photons carry both energy and angular momentum. The capture cross section for counter-rotating photons is significantly larger than that for co-rotating photons. The retrograde angular momentum of the counter-rotating photons buffers the spin at 99.8% of the maximum. Thorne's model uses a very thin disk of material, now regarded as unlikely to be astrophysically relevant. When thicker disk models are used, the spin is buffered to an even smaller value. But 99.8% is still a nice number to have in our head if we want to really see how spin effects impact our waves.

In these sounds, the two GW polarizations are encoded via stereo: The "plus" polarization is in one ear, the "cross" is in the other. You can sometimes hear a cool evolution in the two polarizations, especially as the end is approached. Also, note that there are two angles describing the waves: Orbital plane and viewing angles. The orbital plane describes how the orbit is tilted with respect to the equator of the black hole (where the "equator" is 90° from the spin axis); the viewing angle describes the orientation of the system to our line of sight. Zero degrees viewing angle means that we are in the equatorial plane of the binary.

In all cases, the system's mass ratio is set to 10,000:1; the total mass was then adjusted so that it lies nicely in the band of the human ear. The total mass of the system turns out to be roughly 100 solar masses in this case.

Orbital plane 20°

0° viewing angle: iota_20_10000_4_4_00_h
30° viewing angle: iota_20_10000_4_4_30_h
60° viewing angle: iota_20_10000_4_4_60_h
90° viewing angle: iota_20_10000_4_4_90_h

Orbital plane 40°

0° viewing angle: iota_40_10000_4_4_00_h
30° viewing angle: iota_40_10000_4_4_30_h
60° viewing angle: iota_40_10000_4_4_60_h
90° viewing angle: iota_40_10000_4_4_90_h

Orbital plane 50°

0° viewing angle: iota_50_10000_4_4_00_h
30° viewing angle: iota_50_10000_4_4_30_h
60° viewing angle: iota_50_10000_4_4_60_h
90° viewing angle: iota_50_10000_4_4_90_h

Orbital plane 60°

0° viewing angle: iota_60_10000_4_4_00_h
30° viewing angle: iota_60_10000_4_4_30_h
60° viewing angle: iota_60_10000_4_4_60_h
90° viewing angle: iota_60_10000_4_4_90_h

These sounds were all made by former UROP alumna Pei-Lan Hsu using code written by Hughes.