12 Jun 2007 6:48:00 UTC

Topic 192839

(moderation:

Has anyone gone through the "Hands-On GW Astronomy: Extraction of Astrophysical Information from Simulated Signals - Student Worksheet" paper? I found it to be an excellent educational tool for those wanting to better understand gravitational wave astronomy ... BUT I believe I found some problems and errors in the paper. I'd like to discuss with others.

Thank you!!

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## Hands-On GW Astronomy - Student Worksheet

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Here's a link to the page for downloading the worksheet and plots of the activity from the CGWP, in case others are wondering where it's from. I had visited the site previously but don't recall seeing this exercise, so I've looked it over just now rather quickly. What did you find?

## Hi, for starters in Problem

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Hi, for starters in Problem #4 which calculates the tangential velocity of the binary system, they seem to have used the gravitational wave period instead of the orbital period. So their answer is: v=2.1x10E6m/s whereas it should be v=1.0710E6m/s. Maybe somebody can confirm this.

I found other issues, but let's start with this one.

_dan

## The only problem I see is

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The only problem I see is that there is no mention of Einstein@home. This is very common on both sides of the Atlantic Ocean.

Tullio

## RE: Hi, for starters in

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Looks like you might be right, Dan. I sort of worked it backwards using the answer provided and came up with P_orb ~ 1017 seconds, but if you look at the 'far' plot, the GW period looks like ~1000 seconds which means that P_orb should be 2000 seconds, so it does look to be off by a factor of 2.

## Hi again, that's the point.

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Hi again, that's the point. The gravitational wave period from the "Far from coalescence" plot shows 1,000s, then if you use the equation provided in the paper that relates orbital period to GW period: Porb(t)=2Pgw(t) the value is then 2,000s. So the computation for tangential velocity s/b 1.07x10E6 m/s.

Glad you agree!

_dan

## Hi! Here's another, much

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Hi!

Here's another, much bigger, problem with the paper:

Problem #6 computes the distance r to the binary system. The answer they get is pretty big: 2.4x10E20 meters. That is something like 8.0x10E11 light years. I don't think so. This is way outside LIGO sensitivity range.

Then in Problem #7, they compute what percentage of r is the distance to Proxima Centauri which is 4.22 l.y. They show .0037% but clearly the answer is MUCH smaller like 5.064x10E-10%. Same thing with the distance to the galactic center at 26,000 l.y. Their answers are way off.

So there's a serious problem with Problems #6 and #7. Do you find the same?

_dan

## RE: Here's another, much

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I haven't computed the distance, but using 2.5 E20 meters, and OnlineConversion.com, I get:

250 000 000 000 000 000 000 meter = 26 443.120 179 965 light year [traditional],

which looks pretty close to something that will give the proper answers for #7. Maybe recheck your calculation for distance conversion to light years?

## Hi! You're right, I had a

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Hi! You're right, I had a conversion problem, but after examining further, I now get .01595% versus their answer for #7: .0037%. This is what you'd get if you used your converted value as well. Their answer for the Galactic Center is off too. Can you confirm? Thanks.

## RE: Hi! You're right, I had

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Yup. I get:

The distance to Proxima Centauri is ~ 0.016% of the distance to the binary system, and

the distance to Galactic Center is ~ 98.3% of the distance to the binary system.

That's using 26,443 light years as the distance to the binary system.

Well spotted, Dan. I still haven't worked out any of the answers, yet. I was content to see how some of the calculations are done once a signal is discerned out of everything else (including all other signals and all the noise), and some of the specific formulas that are used and how they relate to physical interpretations of the event.

Did you send an errata note to the website feedback link? Any more issues with the remaining questions?

- - -

Some general questions I was wondering about:

If the orbit it elliptical, does it become more circular as the stars approach coalescence? What's the equation relating P_orb to P_gw when the orbit is elliptical, and what is the value for the eccentricity when the orbit can be considered circularized enough to use the simpler equation, P_orb (t) = 2 P_gw (t) ?

## There ya go, we match our

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There ya go, we match our results. Good to hear.

The reason I've spent time to confirm these results is because I'm impressed with all that can be discerned from a gravitational waveform. This leads me to believe that the astronomy field will indeed be enhanced signficantly once GW are detected. That day may have to wait until after the S5 run however.

Another reason I'm spending this time, is because I'm building some methods to use inside of an Evolutionary Algorithm to run against some live LIGO time series data, and for the Mock LISA Data Challenge.

I am submitting errata to the authors of the outreach paper. I'll let you know their response.

I have a couple of other issues; let me summarize in another post.

All of the formulas assume a circular orbit for the binary system, but from what I gather binary systems do not generally follow elliptical orbits anyway, so the math is similar as a result.

Cheers,

_dan