Pulsar Distance by DM

Pulsar Distance from Dispersion Measure (DM)

A pulsar's radio pulse passes through the ionized interstellar medium (ISM) on its way to us, and the lower frequencies show up later than the higher ones. For any two frequencies the delay follows Δt = 4.149 · DM · (ν₁⁻² − ν₂⁻²) in milliseconds, with ν measured in GHz and DM (the Dispersion Measure) in pc/cm³.

DM counts the free electrons stacked up along the line of sight: DM = ∫ nₑ dl. Once you have a measured DM, you feed it into a Galactic electron-density model like NE2001 or YMW16 and read off an estimated distance. Some reference points: the Vela pulsar sits at DM≈68, the Crab pulsar at DM≈57, while Fast Radio Bursts (FRBs) land anywhere from roughly 50 to 1000 pc/cm³.

Applications

Estimating pulsar distances when no parallax is available, mapping the free-electron density of the Galaxy, flagging extragalactic FRBs (their DM exceeds the Galactic maximum), running gravity tests with pulsar timing arrays, probing how cosmic baryons are spread out, and characterizing magnetar emission.

FAQ

What are typical DM values for Galactic pulsars? Nearby objects sit at just a few pc/cm³ (PSR J0437−4715, for instance, comes in around DM≈2.6), while distant pulsars down in the Galactic plane can reach several hundred.

How accurate are NE2001 and YMW16 models? Distances usually carry an uncertainty of 20–40%, and it depends a lot on which way you're looking. Expect the worst errors along spiral arms and toward the Galactic center.

Why do FRBs have such high DM? Most of their dispersion piles up in the intergalactic medium and the host galaxy, which points to cosmological distances.