Week 2: Dispersion Measure

This Week’s Videos are below – there are also some course notes at the bottom of this page to help cement your knowledge!   The February 22  Q/A webchat is also available if you missed it: https://vimeo.com/257196122

Celestial Coordinates: Learning our Way around the Sky

Dispersion Measure
Dr. Maura McLaughlin explains Dispersion Measure

DM Lecture 1

DM Lecture 2

Week Three Homework:

    1. Watch the videos and comment in the Forum
    2. Select 2 pulsars from this sheet and observe them with the 20 Meter Telescope.

Here’s how to set up your Pulsar Observation:

Log in to https://skynet.unc.edu > My Observatory > Radio Observing > Add New Observation

Window One:

  • In the Target Lookup Window, type PSR + space + the name of your pulsar exactly. For Example: PSR J0814+7429 (copy and paste  the name of your pulsar from the google sheet)
  • The RA and Dec should fill in.
  • If your pulsar has a Dec. of -40 or less, change the min elevation to 10 or even 5
  • Click “Save and Continue”

Window Two:

  • Receiver Data Acquisition Mode: Low resolution
  • Filter: HI filter
  • Check Box: Pulsar mode
  • Click “Save and Continue”

Window 3:

  • Path type: Track
  • Duration: 300- 1800 seconds– weaker pulsars need longer observations.
  • Integration time: divide the period by 30 and enter that number
  • Click “Save and Continue”
  • Click “Submit”

3. Investigate Dispersion Measure.

Your data will look like this: Here’s a pulsar plot with the different parts labeled. You can find the DM of your pulsar in the Header.

plot anatomy

Use the DM tool located  here (use the bottom tool), to find out the distance to your pulsars, and its location in the Milky Way. (You can see a picture of its location by clicking on the XY plot.)

4. Copy and paste your results into the Google Table:

Course Notes:  More info about dispersion measure: From your research on your adopted pulsar perhaps you have noticed that there is a dispersion measure or DM associated with it.  In the  data plots you will be analyzing there is a subplot called DM as well.   Below is an example of a data plot.  Do you see the DM graph at the bottom of this data plot?  The DM peaks at about 4.9.  As you analyze data you will need to assess whether this DM falls within accepted values for your candidate using the Calculate Distance tool.




Course Notes:  What is DM?  As you learned in Maura’s video, pulsars are located far away from us and hence, their radio waves must travel through many light years of space to get to us and our telescopes. That space, however, is not truly empty, and contains a lot of electrons. When radio waves, such as those from a pulsar, pass through these electrons they  are delayed.

Low-frequency waves are delayed more than the high-frequency waves. So all the frequencies of pulsar’s emission leave the pulsar itself at the same time , but they arrive on Earth at different times : the lower a wave’s frequency, the more its arrival on Earth is delayed. Because of this effect, the pulses we receive are “smeared” over a period of time instead of being straight lines.

This smearing makes it more difficult to detect the pulses. But, if we know how smeared a pulse is, we can correct it and detect the pulse. Dispersion measure tells us how much correction we need to do to line the pulses back up. The higher the DM, the more the pulse was smeared. The more the pulse was smeared, the greater the signal path through the free electrons. So, DM can be thought of as a measurement of distance — the higher the DM, the farther the pulse traveled — but that’s a rough estimate, because some parts of the galaxy have more electrons than others.

Finally, If radio waves are coming from the same object in space, we expect them to have gone through the same parts of space, meaning that they would have interacted with the same number of electrons. For this reason, if a signal comes from a pulsar, we expect it to have a distinct DM. What is a reasonable DM: The maximum dispersion measure depends on where the candidate is in the sky. The galaxy is shaped like a disk and we are about 2/3 from the center of the disk. The DM depends on how much “stuff” is between the pulsar and us. If the pulsar is in the plane of the disk, we have to look through a lot of stuff to see it and so, the DM can be very high. If the pulsar is outside the plane of the galaxy (like above the disk), then we don’t look through a lot of stuff to see the pulsar and therefore, the maximum DM is much lower. Hence, the maximum dispersion measure depends on where you are looking.