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:

Cluster	Right ascension	Declination	Distance (kpc)	$\Gamma$	$N_{\rm {known}}$
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)