Astronomy: Measuring Radio Wave Distance from Universe

[psuedoben]

when astronomers receive bursts of high energy radio waves from distant parts of the universe, how are they able to tell how far they have traveled? does it have to do with the wavelength?

 

[Simon Bridge]

Well they can tell which direction they came from and then look back along that direction and see what is there.
There is a good chance that the strong radio source off thataway was where the radio burst came from, so the distance to the source is the distance the waves have travelled.

You get the distance to the source by a number of methods - one of which is the red-shift (for very distant sources) but mostly triangulation is used as well as comparing the brightness of the source compared with similar sources nearby.

That's a very simple picture ... the actual approach fills textbooks with the details.
Did you have particular radio bursts in mind?

 

[davenn]

hi there psuedoben

no, it doesn't have to do with wavelength.
In most ( not all) cases there is an optically observable source at that point in "the sky"
eg, the Crab Nebula which is a supernova remnant with a pulsar in the middle, that pulsar is putting out bursts of radio signal at ~ 30 times a second
http://en.wikipedia.org/wiki/Crab_Nebula

There are lots and lots of SN remnants with all sorts of varying speed pulsars at their centre

In other cases its a galaxy that is emitting intense radio signals ... sometimes called radio galaxies
Centaurus A radio galaxy is an example
http://en.wikipedia.org/wiki/Centaurus_A

Generally, its the optical observations and measurements and will give the distance to the object
well let's say, it will give the MOST accurate distance measurements
Cepheid variable stars are commonly used for this purpose
http://en.wikipedia.org/wiki/Cepheid_variable

cheers
Dave

 

[psuedoben]

Well they can tell which direction they came from and then look back along that direction and see what is there.
There is a good chance that the strong radio source off thataway was where the radio burst came from, so the distance to the source is the distance the waves have travelled.

You get the distance to the source by a number of methods - one of which is the red-shift (for very distant sources) but mostly triangulation is used as well as comparing the brightness of the source compared with similar sources nearby.

That's a very simple picture ... the actual approach fills textbooks with the details.
Did you have particular radio bursts in mind?

this is the one that sparked my interest http://www.sciencemag.org/content/318/5851/777.full

 

[psuedoben]

hi there psuedoben

no, it doesn't have to do with wavelength.
In most ( not all) cases there is an optically observable source at that point in "the sky"
eg, the Crab Nebula which is a supernova remnant with a pulsar in the middle, that pulsar is putting out bursts of radio signal at ~ 30 times a second
http://en.wikipedia.org/wiki/Crab_Nebula

There are lots and lots of SN remnants with all sorts of varying speed pulsars at their centre

In other cases its a galaxy that is emitting intense radio signals ... sometimes called radio galaxies
Centaurus A radio galaxy is an example
http://en.wikipedia.org/wiki/Centaurus_A

Generally, its the optical observations and measurements and will give the distance to the object
well let's say, it will give the MOST accurate distance measurements
Cepheid variable stars are commonly used for this purpose
http://en.wikipedia.org/wiki/Cepheid_variable

cheers
Dave

Awesome! thanks dave, that answered my question

 

[davenn]

from that link you gave

A Bright Millisecond Radio Burst of Extragalactic Origin
Pulsar surveys offer a rare opportunity to monitor the radio sky for impulsive burst-like events with millisecond durations. We analysed archival survey data and found a 30-jansky dispersed burst, less than 5 milliseconds in duration, located 3° from the Small Magellanic Cloud.
Pulsar surveys offer a rare opportunity to monitor the radio sky for impulsive burst-like events with millisecond durations. We analysed archival survey data and found a 30-jansky dispersed burst, less than 5 milliseconds in duration, located 3° from the Small Magellanic Cloud. The burst properties argue against a physical association with our Galaxy or the Small Magellanic Cloud. Current models for the free electron content in the universe imply that the burst is less than 1 gigaparsec distant. No further bursts were seen in 90 hours of additional observations, which implies that it was a singular event such as a supernova or coalescence of relativistic objects. Hundreds of similar events could occur every day and, if detected, could serve as cosmological probes.

If they only got that one burst, its going to be almost impossible to locate it, distance wise
as to whether it was within our galaxy or if it was actually associated with an event in the SMC
if as suggested it may be from a supernova event, then there is a chance that neutrino detectors also recorded a burst
They may possibly have already looked for that correlation ? :)

cheers
Dave

 

[psuedoben]

from that link you gave
If they only got that one burst, its going to be almost impossible to locate it, distance wise
as to whether it was within our galaxy or if it was actually associated with an event in the SMC
if as suggested it may be from a supernova event, then there is a chance that neutrino detectors also recorded a burst
They may possibly have already looked for that correlation ? :)

cheers
Dave

amateur questions here, but 1) what do they mean when they refer to the "free-electron content in the universe" in the article 2) what is the SMC?

 

[davenn]

amateur questions here, but 1) what do they mean when they refer to the "free-electron content in the universe" in the article 2) what is the SMC?

not sure about the "free-electron content in the universe" ?

I would have to google it, have you tried, any results ? SMC = Small Magellanic Cloud.
closer reading of that text says they are suggesting much further distances that something within our galaxy or the SMC
I didn't pick up on that earlier in my quick read

That infers it was possibly in a galaxy along the same approx. line of sight as the SMC but much further beyond
The SMC is ~ 170,000 light years and 1Gigaparsec = approx. 3.26 billion light years

so the distance range within which it could be is pretty huge ;)

Dave

 

[Simon Bridge]

Lorimer B. Bailes M. (2007) A bright millisecond radio burst of extragalactic origin
Full text: http://arxiv.org/abs/0709.4301

The universe is full of stuff, although there are big gaps between the bits.
Some of the bits are electrons - and there are electrons between the stars and between galaxies just flying around on their own. The paper talks about this as the galactic and intergalactic plasma (p2 etc).

This is what they mean by the free electron content of the Universe.

Radiation interacts with these electrons (and other charged particles) ... the electrons are very sparse, but over very long distances the effects add up.

Basically it's like looking at a light through a mist - you can tell how away it is from it's brightness and diffuseness if you have some idea about how light is affected by mist and how dense mists are. The researchers are basically saying they don't know how far away the source of the burst was, but the character of the burst was consistent with the best current guesses for how the intergalactic plasma affects radiation. Ergo: extragalactic origin - but still close enough to see it as well as we did.

Similarly, you can tell that a light you see outside the window was not in you backyard, because it looked like it was clearly affected by mist and the mist in your backyard is too diffuse for that.

 

[psuedoben]

Lorimer B. Bailes M. (2007) A bright millisecond radio burst of extragalactic origin
Full text: http://arxiv.org/abs/0709.4301

The universe is full of stuff, although there are big gaps between the bits.
Some of the bits are electrons - and there are electrons between the stars and between galaxies just flying around on their own. The paper talks about this as the galactic and intergalactic plasma (p2 etc).

This is what they mean by the free electron content of the Universe.

Radiation interacts with these electrons (and other charged particles) ... the electrons are very sparse, but over very long distances the effects add up.

Basically it's like looking at a light through a mist - you can tell how away it is from it's brightness and diffuseness if you have some idea about how light is affected by mist and how dense mists are. The researchers are basically saying they don't know how far away the source of the burst was, but the character of the burst was consistent with the best current guesses for how the intergalactic plasma affects radiation. Ergo: extragalactic origin - but still close enough to see it as well as we did.

Similarly, you can tell that a light you see outside the window was not in you backyard, because it looked like it was clearly affected by mist and the mist in your backyard is too diffuse for that.

thank you that explains a great deal, and that analogy at the end did a wonderful job of helping me visualize what exactly they are dealing with when they try to calculate the distance!

 

[Simon Bridge]

No worries - please bear in mind that it is a simplification to give you the idea.
The paper is actually pretty easy to read for it's type - you should go through it. Some of the questions above are actually answered, drcetly, in the main text and, anyway, it is a good idea to get used to reading the lit.
Enjoy.