Session 2: Pulsar Properties I; Dispersion Measure

Week Two videos are below – there are also some course notes at the bottom of this page to help cement your knowledge!  

Pulsar Properties I (direct video link:

Dispersion Measure
Dr. Maura McLaughlin explains Dispersion Measure

DM Lecture 1 (direct video link:

DM Lecture 2 (Direct video link:

Week Two Homework:

    1. Watch the videos  and:
      1. Try one of Dunc’s questions at the end of the Pulsar Properties Video. Post about iton SLACK or on the forum and try to help each other solve them!
    2. Claim a pulsar from this sheet. What can you learn about its properties?
      1. Try the ATNF Catalog. Its a great place to start, and you will need  to learn it. You can find out your pulsar’s magnetic field (Bsurf), its slow down rate (P1), and lots of other parameters! Here’s a how to video (ignore the bit at the end). Here’s a link to a glossary of all of the parameters.
      2. Look up an article about your pulsar.
      3. Post all of the properties you discover in SLACK or on  the forum.
    3. Try the DM checker. It’s located under tools in the menu up above. Put in the RA and Dec of your adopted pulsar, and the DM (you can find the DM in the ATNF catalog). What is the distance to your pulsar? Add that to your list of properties.

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.