How to Check Candidate Pulsar DMs

When you find what you think is a new pulsar, it is a good idea to check if its DM is realistic. We have models of the Galactic electron density that can tell you what the maximum DM should be in some direction of the Galaxy. If the DM of a candidate pulsar is greater than the maximum, it is unlikely to be real.

You can find out whether your candidate’s DM is realistic using the DM tool on the PSC homepage. This tool is located on the right-hand side of the page under the heading "Calculate the Distance to Your Candidate."

  1. Enter the RA, the Dec, and the DM that the PRESTO software calculated for this plot
  2. Click on the "Estimate Distance" button.
  3. The output will be something like:

    For Galactic longitude 214 degrees and latitude 20.7 degrees, and a dispersion measure of 45.0 cm-3 pc, the estimated distance to your source is 1.9 kpc. The maximum DM predicted by the model for this direction is 71.9 cm-3 pc.

    The first part of the sentence is just reiterating information already known: where your pointing’s location is (in galactic coordinates) and what DM is displayed on your plot. The DM tool has used those two pieces of information plus an electron density model of the galaxy to calculate how far away your candidate is. If we know the density of free electrons in space along the telescope’s "line-of-sight" (the direction it is pointing) and we know how much the radio waves were dispersed, we can figure out how much space the signal has gone through, and thus how far away the source is.

    The second sentence tells you how large the dispersion measure can be, along this line-of-sight, if the candidate is still within the galaxy. If the DM on your plot is less than this number, that’s great! It’s within a reasonable distance. Remember, though, even if your candidate doesn’t pass this test, if the plots look great in ever other way, let us know! Maybe you have found something very new and interesting!

  4. You will also have the option to click on an XY plot and an XZ plot.

    Below, on the left, is an example of the XY plot. The Earth is the red cross, and the candidate is the green X. Generally, you want the green X to be inside spiral arms. On the right, you will find an example of the XZ plot. The Earth is in the right-hand corner, at (0,0). The horizontal axis is the same in this plot as it is in the XY plot, and the vertical axis shows how far above the plane (equator) of the Milky Way the candidate is located.