Very close binary, strongest steady source of gravitational waves

1. Mar 13, 2010

bcrowell

Staff Emeritus
This paper http://arxiv.org/abs/1003.0658 nails down the interpretation of a binary white dwarf system with an orbital period of only 5.4 minutes, versus 8 hours for the celebrated Hulse-Taylor binary neutron star system. This would probably be the strongest source in the sky of steady sinusoidal gravitational waves. The shortening of the period has been measured with good precision (although not as spectacular as the precision of the Hulse-Taylor system). The masses of the stars are only roughly determined, so it doesn't provide the same kind of high-precision test of GR that the Hulse-Taylor system does.

2. Mar 14, 2010

Frame Dragger

Hmmm... still the paper makes the point at the end that this is well within the sensitivity of LISA. To me, detecting ANYTHING is step 1, then calibration would be 2. An early success would be politically expedient.

On a scientific note, I enjoyed that paper, thanks bcrowell, it was very convincing.

3. Mar 14, 2010

Mentz114

It is an interesting look into the world of pixel gathering. What is an 'ephemeris' ?

I enjoyed it too.

4. Mar 14, 2010

bcrowell

Staff Emeritus
Direct detection of gravitational waves would be spectacular. I hadn't realized that there were so many known systems that were actually expected to be detectable by LISA. This paper http://arxiv.org/abs/0901.1778 gives estimates of signals strengths that ought to be detectable (fig. 2 on p. 6). Some of them are predicted to be 100 times stronger than LISA's sensitivity, and a factor of 1000 or more above background.

One thing I don't understand is that the frequencies on the x axis of that figure don't seem to correspond directly to the periods of the binaries. E.g., RX J0806 is believed to have a period of 5.35 min ( http://eprints.ucl.ac.uk/9415/ ), but it's plotted with a period of about 2.7 min.

I hope LISA gets funded. It would be really cool.

5. Mar 14, 2010

Frame Dragger

http://en.wikipedia.org/wiki/Ephemeris

I didn't expect this. You ask good questions Mentz, thanks!

@bcrowell: Makes you want to eat plenty of fiber and exercise so as to live a good long life and see the data, doesn't it? ;)

6. Mar 14, 2010

nicksauce

"One thing I don't understand is that the frequencies on the x axis of that figure don't seem to correspond directly to the periods of the binaries. E.g., RX J0806 is believed to have a period of 5.35 min ( http://eprints.ucl.ac.uk/9415/ ), but it's plotted with a period of about 2.7 min."

In the simplest model, in the gravitational wave point of the view, the two stars are indistinguishable. Therefore after half a period the system looks exactly the same. Therefore the gravitational wave period is half of the system's physical period.

7. Mar 14, 2010

bcrowell

Staff Emeritus
Ahhhh! Thanks! I guess if the system is asymmetric you should get Fourier component at both f and 2f.

If the orbit is elliptical, presumably you'd also get components at higher multiples of f as well?

8. Mar 14, 2010

nicksauce

"If the orbit is elliptical, presumably you'd also get components at higher multiples of f as well? "

I'd assume so, yes. However, it is my understanding that eccentricity is damped out on very short time scales for such strongly gravitational systems. I'm not totally sure about this, though.

9. Mar 14, 2010

bcrowell

Staff Emeritus
The Hulse-Taylor binary has an eccentricity of 0.617131. Maybe this depends on what is meant by "very short time scales" and "such strongly gravitational systems"...?

10. Mar 14, 2010

nicksauce

Oh I didn't realize it was that high. I suppose about eccentricity damping then.