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, ${\displaystyle \Gamma }$.

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

${\displaystyle \log _{e}N_{\rm {psr}}=-1.1+1.5\log _{10}\Gamma }$

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

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

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

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

Cluster Right ascension Declination Distance (kpc) ${\displaystyle \Gamma }$ ${\displaystyle N_{\rm {known}}}$
Terzan 5 17:48:05 -24:46:48 6.9 6800 34
NGC 7078, M15 21:29:58.33 +12:10:01.2 10.4 4510 8
Terzan 6 6.8 2470
NGC 6441 11.6 2300 4
NGC 6266, M62 17:01:12.60 -30:06:44.5 6.8 1670 6
NGC 1851 05:14:06.76 -40:02:47.6 12.1 1530 1
NGC 6440 8.5 1400 6
NGC 6624 18:23:41 -30:21:39 7.9 1150 6
NGC 6681, M70 18:43:12.76 -32:17:31.6 9.0 1040
47 Tucanae, NGC 104 00:24:05.67 -72:04:52.6 4.5 1000 23
Pal 2 04:46:05.91 +31:22:53.4 27.2 929 0
|NGC 2808 09:23:03.10 -64:51:48.6 9.6 923
NGC 6388 17:36:17.461 -44:44:08.34 9.9 899
NGC 6293 17:10:10.4 -26:34:54 9.5 847
NGC 6652 10.0 700 1
NGC 6284 17:04:28.7 -24:45:52 15.3 666
NGC 6626, M28 18:24:32.89 -24:52:11.4 5.5 648 12
M80 16:17:02.42 -22:58:33.9 10.0 532
NGC 7089, M2 21:33:27.02 -00:49:23.7 11.5 518
NGC 5286 13:46:26.81 -51:22:27.3 11.7 458
NGC 6517 18:01:50.52 -08:57:31.6 10.6 4
NGC 6539 18:04:49.68 -07:35:09.1 7.8 1
NGC 6760 19:11:12.01 +01:01:49.7 7.4 2
NGC 5904, M5 15:18:33.22 +02:04:51.7 7.5 5
NGC 5024, M53 13:12:55.25 +18:10:05.4 17.9 1
NGC 6838, M71 19:53:46.49 +18:46:45.1 4 1
NGC 5272, M3 13:42:11.62 +28:22:38.2 10.2 4
NGC 6205, M13 16:41:41.24 +36:27:35.5 7.1 5
NGC 5986 15:46:03.00 -37:47:11.1 10.4 1
M4, NGC 6121 16:23:35.22 -26:31:32.7 2.2 1
NGC 6342 1
NGC 6397 17:40:42.09 -53:40:27.6 2.2 1
NGC 6522 3
NGC 6544 2
NGC 6656, M22 18:36:23.94 -23:54:17.1 3 2
NGC 6749 1
NGC 6752 19:10:52.11 -59:59:04.4 4.0 5
NGC 7099, M30 21:40:22.12 -23:10:47.5 8.3 2
Omega Centuari, NGC 5139 13:26:47.28 -47:28:46.1 4.84

The extra-Galactic population

The Magellanic Clouds

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

Name Period (s) Dispersion measure (cm-3pc)
J0045-7042 0.632 70
J0045-7319 0.926 105.4
J0100-7211 8.02 - (XRS)
J0111-7131 0.689 76
J0113-7220 0.326 125.49
J0131-7310 0.348 205.2

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

Name Period (s) Dispersion measure (cm-3pc)
J0449-7031 0.479 65.83
J0451-67 0.245 45
J0455-6951, B0456-69 0.320 94.89
J0456-69 0.117 103
J0456-7031 0.800 100.3
J0457-69 0.231 91
J0458-67 1.134 97
J0502-6617, B0502-66 0.691 68.9
J0519-6932 0.263 119.4
J0521-68 0.433 136
J0522-6847 0.675 126.45
J0529-6652, B0529-66 0.976 103.2
J0532-6639 0.642 69.3
J0534-6703 1.818 94.7
J0535-66 0.211 75
J0535-6935 0.201 93.7
J0537-69 0.113 273
J0537-6910 0.016 -
J0540-6919, B0540-69 0.050 146.5
J0542-68 0.425 114
J0543-6851 0.709 131
J0555-7056 0.828 73.4

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:

Galaxy Right ascension Declination Distance (kpc) Reference
Ursa Minoris 15:09:08.04 +67:13:21.36 60 Rubio-Herrera & Maccarone (2012)
Draco 17:20:12.12 +57:54:55.80 80 Rubio-Herrera & Maccarone (2012)
Leo I 10:08:27.00 +12:18:27.36 250 Rubio-Herrera & Maccarone (2012)