Pulsars and neutron stars/The pulsar population

The globular cluster population
Globular clusters are bound collections of stars in the halo of a galaxy. There are approximately 150 globular clusters in our Galaxy. Globular clusters are often searched for pulsars as 1) they contain a high fraction of millisecond pulsars and 2) their angular size is small allowing for a single, long observation. As globular clusters have a very high star density, close interactions are common. It is thought that the millisecond pulsars arise from such interactions.

Particular globular cluster pulsars of interest
PSR B1620-26 is within the globular cluster M4. This pulsar system contains planetary mass bodies.

Determining the population of globular cluster pulsars
Turk & Lorimer (2013) described a Bayesian method that can be applied to studying the pulsar population in a globular cluster. The number of pulsars in any given cluster depends upon the parameters of the globular cluster. In particular Hui et al. (2010) proposed a relationship between the pulsar abundance in a particular cluster and the stellar encounter rate, $$\Gamma$$.

The best fit model from the Turk & Lorimer (2013) paper suggested that the number of pulsars in a specific globular cluster, $$N_{\rm psr}$$ is

$$\log_e N_{\rm psr} = -1.1 + 1.5\log_{10} \Gamma$$

The number of pulsars expected to be detectable in any given survey is then given by:

$$N_{\rm expected} = \frac{N_{\rm psr}}{2}{\rm erfc}\left(\frac{\log_{10} L_{\rm min} - \mu}{\sqrt{2}\sigma}\right)$$

where $$\mu = -1.1$$ and $$\sigma = 0.9$$ are reasonable values and $$L_{\rm min}$$ is the extrapolated luminosity sensitivity at an observing frequency of 1400 MHz (note that $$L_{\rm min} = S_{\rm min} D^2$$ where $$D$$ is the distance to the globular cluster).

A list of close-by globular clusters is given below:

The Magellanic Clouds
The following pulsars are currently known in the small Magellanic cloud (SMC):

The following pulsars are currently known in the small Magellanic cloud (LMC):

Distant galaxies
McLaughlin et al. (2003) carried out a large-scale search for giant pulses coming from extragalactic pulsars, but did not make any convincing detections. As described below Rubio-Herrera et al. (2013) used Westerbork to search for pulsars in M31. Kondratiev et al. (2013) used the Green Bank and Arecibo telescopes to search for pulsars in nearby galaxies.

M31
Rubio-Herrera et al. (2013) used the Westerbork Synthesis Radio Telescope (WSRT) to search for radio pulsars and fast transients in M31. This search did not find any periodic sources, but they tentatively found some burst events with a dispersion measure of 55cm-3pc which, they argued, placed these objects outside of our Galaxy. These burst signals have not been redetected.

Dwarf galaxies
Dwarf spheroidal galaxies (dSph) are low luminosity galaxies that are companions to our Galaxy (and to M31).

Rubio-Herrera & Maccarone (2012) searched for pulsars in dwarf spheroidal satellite galaxies of the Milky Way using the Green Bank telescope. No unambiguous detections were made.

The following table contains a list of dwarf galaxies that have been searched: