Binaries with comparable mass components

The sources we discuss here are non-EMRI binary sources; the members of these binaries have masses that are typically roughly the same, to within a factor of a few. “Small” binaries of this kind (total mass of a few, a few tens or a few hundreds of solar masses) are targets for ground-based GW detectors like LIGO; “large” binaries (total mass of hundreds of thousands to tens of millions of solar masses) are targets for the space-based antenna LISA. Current prejudice tells us that these binaries are likely to have no eccentricity (though, at least in the case of the LISA targets, this prejudice is now eroding), so all the sounds we present here are for circular systems. It’s worth noting that these sounds were originally encoded about 15 years before the first LIGO discoveries.

LIGO targets

Here some are targets for observation by LIGO. Each of these signals is the “plus” polarization, and the impact of spin is neglected.

In each of these signals, the sound is filtered so that only frequencies from LIGO’s sensitive band are included. The two smaller signals sound fairly similar, though the 5 solar mass binary is much shorter. The larger mass means that the waves are stronger, and so it moves through the band more quickly. (Note also that for the two small mass signals that there is a huge span of dead signal before you get to the “interesting” stuff! We are aware of this, which was an artifact of the routine that filters the signal into LIGO’s band. We are overdue to fix this! In the meantime, jump to the last roughly 5 seconds of these sounds.)

The signal for the 100 solar mass binary is vastly different from the other two. For this signal, the inspiral (when the members are widely separated) occurs outside the band of the detector, so we cannot hear it. Only waves from the final merger and ringdown are in band. Those waves are strong, but rapidly die away; hence the short “pop.”

LISA targets

LISA target signals will be in band for a few to many months. In overall structure, they are very similar to the LIGO target signals (albeit shifted in frequency thanks to the much larger masses). In this case, we have included the impact of spin.

In the first case, the signal is not substantially different from the LIGO low mass signals. Adding spin substantially modifies the signal; the waves become highly modulated in both amplitude and frequency, leaving a clear, audible imprint.

These sounds were produced by alumnus Ryan Lang.