# Is there a significant population of neutron stars outside the galactic plane?

Pulsar kicks originating from supernovae can impart neutron stars with speeds of 100-500 km/s, often at or close to the escape velocity of the Milky Way. Even if a pulsar fails to escape the galaxy, it can be launched into unusual orbits. Assuming that the velocity distribution is isotropic, a decent fraction of such pulsars should have large orbital inclinations after the kicks, such that they rise above the galactic plane.

I have two questions regarding this:

1. Is there a large - i.e. noticeable - population of such neutron stars, distinguishable by modern telescopes, or is the fraction of total neutron stars relatively small?
2. Does observing a pulsar - at any wavelength - in such an orbit have any advantages over observing an identical pulsar firmly in the galactic disk? I would assume that some kicks could propel them high enough that scattering along the line of sight could be lessened, leading to a smaller dispersion measure smearing, and could also isolate the pulsars from ambient radio sources in the disk, such as a the galactic center.

I understand that "noticeable", at the least, is a bit subjective, but I can't come up with any non-arbitrary criteria to change that.

• It's perhaps worth asking as a related question (or perhaps a separate question), how long, or how many orbits, a star can maintain a high inclination in a spiral galaxy. – userLTK Dec 30 '16 at 21:42

I'll partly answer your question with the following plot from the ATNF pulsar database (which is easy to play around with). The plot shows the Galactic z coordinate of the pulsars (distance from the Galactic plane in kpc) versus their estimated age (from their spin down rates, in years).

The basic picture is that young pulsars are all near the Galactic plane, because they are born from high mass stars that are born, live and die near the plane and even if the neutron star got a kick, there has been insufficient time to move far from the plane. A typical kick might be 500-1000 km/s, roughly 0.5-1 kpc in a million years.

Once pulsars reach a million years old, then indeed some reach a kpc above/below the Galactic plane and then a few even older examples reach 10 kpc. Most normal pulsars only show pulsar activity for 10-100 million years (the older ones on the plot are probably a different population of pulsars in binaries), so they aren't observed at much greater distances (though there will be old neutron stars out there).

• This makes a lot of sense; thank you. I guess I overestimated pulsar lifetimes. – HDE 226868 Dec 31 '16 at 16:49