Getting the hubble constant from time delay in a gravitational lens

[cegec]

Hi everyone,

First time posting, sorry if this is in the wrong format or wrong place. For a project in a cosmology course I need to find the Hubble constant from from time delay in a gravitational lens. the one we are using is QSO 0957+561. My question is how to I find the time delay from a set of data containing the flux densities (for each of the two images the lens created) and the julian day for each data entry?

thanks
ps please let me know if I need to be more specific in this question.

 

[marcus]

I've seen the QSO 0957+561 lightcurves and especially in the g-band there is this obvious spike and drop-off that occurs earlier in the A source (the direct straightline source) and then a certain number of Julian days later it occurs in the B source, where the light was bent and traveled two sides of a triangle to get to us.

It's obvious by eyeball pattern recognition. You can't miss the pattern. So all you have to do is count the Julian days. And that is the time delay.

Maybe you don't have a visual plot of the lightcurves? So you can't eyeball and see the pattern, and its shifted twin pattern? then my advice would be to make a plot so your visual brain can spot the pattern.

To me the interesting part is after you have determined the delay, which is easy to do, how do you determine from that how far away the quasar is!

After you have found the delay why don't you come back and tell us about it and tell us how you are going to find the distance. Nice project.

Officially your post belongs in homework help forum, not cosmo. They know what to say and not to say there. We shouldn't give away too much in hints. Here is the link to homework help forum:
https://www.physicsforums.com/forumdisplay.php?f=152
If you find you need more help than this you should probably go there.

 

[cegec]

thanks marcus that makes perfect sense! i was making it out to be more complicated than it was. how would you use the time delay to find the distance? the equation i have for D_ls depends only on the redshift for the lens and the redshift for the source. should i be working with something different? appreciate the help

 

[marcus]

I'm curious: do you have the angle between the two images?
I am picturing a skinny triangle. I don't know if that is the right picture.

Ideally you should go to Homework forum and ask the same question. They are skilled at helpful questions without giving away. So then you learn more.

Your real problem is finding DISTANCE from the grav. lensing time delay. If you knew the distance along the straightline path (image A) then you could easily compare that with the redshift and find H.

Your job is to find the distance, along the long straight side of the triangle.

 

[cegec]

yes they are 6 arcseconds apart

 

[marcus]

yes they are 6 arcseconds apart

Well what are you waiting for? You know the approximate proportions, and you know an angle. Make a schematic picture of what you know. The first step is always to get some rough solution---you can make it more accurate later.
BTW what are the two redshifts?