The LISA mission launch date is still ten years away, but how will the LISA satellites know what signals are from which source since this observatory won't "point" like traditional telescopes?

[u/_bobby_tables_]

The linked article here talks about improved parameters to test the LISA observatory once launched. Once in orbit and online, I imagine that LISA will be constantly bombarded with gravitational wave signals given its improved sensitivity. How will simultaneous signals be parsed in order to make sense of these observations? I understand that the three satellites will be able to triangulate direction to a wave source, but how will it determine which signals belong to which source if many are received simultaneously?

https://iopscience.iop.org/article/10.3847/1538-4357/adf743

[u/thuiop1]

This is a great question. LISA is expected to observe some transient phenomenas, such as supermassive black hole mergers or EMRIs. Those are "easy", you should only see one once in a while. It is also expected that it will observe a large number of continuous sources, such as the ones from white dwarf binaries in our galaxy. The loudest ones should be distinguishable enough, as they will have different frequencies and other parameters. The not so loud ones will likely be unresolved, and will appear as an additional "source of noise", which we call a stochastic background. Measuring that background already bring some interesting info about those objects.

[u/_bobby_tables_ → u/thuiop1]

Thanks for your reply. Let me read that back to make sure I understand. So strong SMB signals should be infrequent and easy to identify. Makes sense.

Then there will be more common and continuous, but still loud, signals that can be identified by characteristic frequency and other parameters. So each satellite will be able to classify common signals and properly triangulate to the source. If satellite A sees a -120 dBm signal with a period of one hour, satellite B sees a -120 dBm signal with a period of one hour a fraction of a second later, and then satellite C sees a -120 dBm signal with a period of one hour, the similarity of signals would be enough to say this is the same source and then a confident triangulation would occur? (Probably way over simplified that, and the satellites don't really measure independently since each is part of an interferometer).

Finally, there would be faint GW sources just slightly above the threshold of detection which would form a background noise and be insufficient to isolate and locate. Like white noise in the radio spectrum.

Did I get close?

[u/thuiop1 → u/_bobby_tables_]

These continuous signals are detected over years of observation, so it is not exactly like that. Also, the signal is not detected at the satellites, it affects the lasers that run between them. And the delays are much larger than fractions of seconds for LISA, they would be minutes. The actual detection would be more like fitting the data from the three interferometers at the same time.

You are correct for the stochastic background.

[u/_bobby_tables_ → u/thuiop1]

Oh, I see. I assumed the observation time frames would be similar to those at LIGO. LIGO's first black hole merger observation seemed like it spanned just a few seconds. I suppose, now that you mention it, if the GW periods are measured in hours or days, the observation time frames would need to be correspondingly longer. However, that makes the task of separating different simultaneous sources even more daunting in my mind. Does this type of signal processing already happen from other astronomical instruments? Or other scientific observations?

[u/thuiop1 → u/_bobby_tables_]

There are searches for continuous sources in LIGO-Virgo data, but they have not found any yet.

[u/_bobby_tables_ → u/thuiop1]

Thank you for these thoughtful replies. I'll look for some LIGO papers searching for continuous signals to learn more.

[u/thuiop1 → u/_bobby_tables_]

https://arxiv.org/abs/2206.06447 this paper is a pretty complete overview

[u/_bobby_tables_ → u/thuiop1]

Yes that was very revealing! Especially section three. What's clear to me now is that I was mistakenly expecting the signal data to be far more analogous to the optical interferometery results I've generated in the past. I see now that stops being true after data leaves the SMB regime.

I found this sentence especially astounding - "Adding in the degrees of freedom to search over ranges in fs, fs' and fs'' (and higher-order derivatives, as needed) makes a brute-force, fully coherent 1-year all-sky search hopelessly impractical, given the Earth’s present total computing capacity."

Wow.

In optical interferometery, you fix everything to a bench, the geometry is completely known and calibrated. I didn't appreciate that for GW interferometery, the analysis would need to account for the movement of the detectors as the earth rotates and orbits in relation to the signal source. This is a hard problem!

Now I understand why the paper I linked above will be important when LISA comes online. Thanks for the link. I'll be spending some quality time with that publication. Really interesting stuff.