Pulsar mass measurements and tests of general relativity


Fig. 1: Neutron star mass measurements presented in the table below, sorted by increasing mass. The letter C in parentheses indicates a companion to a pulsar that is itself not a pulsar. The names in blue are the cases where such a companion could be a NS or a massive WD, the names in red are for confirmed NSs, as the mass values in the main table. The error bars in orange denote multiple recent measurements of the same NS mass. On the right, we have a histogram of the NS masses. Figure created by Vivek V. Krishnan. For a version without the histogram, click here


The 3-part table below only includes systems for which there is either no mass transfer nor significant mass loss, i.e., ``clean'' systems, where tests of general relativity are in principle possible. This page is therefore not meant to be comprehensive, but merely my personal reference on precise NS masses and tests of gravitational theories.

See the latest changes to this page below.

The mass measurements or constraints are highlighted in red for NSs, in blue for objects that can either be NSs or massive WDs, and gray for WDs (and 1 MS star). Each row has one stellar system.

I indicate in boldface the constraints that yield, according to GR, the most precise mass values or limits, or that were used to derive them in the literature. These mass constraints are derived from radio timing and, in some cases, the combination of timing with optical measurements of their white dwarf companions. The 1-σ uncertainties on the masses are indicated by the numbers in parentheses, they apply to the last digits of the values. The types of constraints are explained immediately after the main tables. In cases where there are more than two mass constraints, one might be in the presence of a test of general relativity. For the reference on each mass constraint, click on its symbol.
  1. The top table lists all 60 precise (where the 1-σ uncertainty is smaller than 15% of the measurement) NS mass measurements, plus 4 precise mass measurements for objects that can either be NSs or massive WDs, in a total of 50 stellar systems.
    I list the mass of the binary only if it constrains the individual masses of the components. This happens for several eccentric systems, where the binary mass can be derived directly from the rate of advance of periastron.
  2. The middle table lists 27 systems where the only available post-Keplerian effect is the rate of advance of periastron. In these systems we can only estimate the mass of the binary system, not the individual component masses. The systems listed are those where the binary mass estimates have relative precisions better than 15%. Using the mass functions of these systems and the binary mass, we can estimate maximum limits for the pulsar mass and minimum limits for the companion masses.
  3. The last table lists all confirmed (24) and candidate (12) pulsar - neutron star systems that have been published. The members of this list repeat several systems presented in the previous two parts. It also includes systems for which no mass constraints have been published, apart from the mass functions.
If you find this table useful, then please cite the relevant original references. If not practical, then please cite this review on the topic of NS masses, or this review on the topic of gravity experiments with radio pulsars.

Acronyms used below:
NS - neutron star.
PSR - pulsar, a NS for which we can detect periodic pulsations
yPSR - young pulsar (i.e., no sign of recycling)
MSP - millisecond pulsar (recycled pulsar with F0 > 60 Hz),
WD - white dwarf; He WD - Helium WD
MS - main sequence star,
BH - black hole,
GC - globular cluster,
GR - general relativity.

1. Systems with individual NS mass measurements
PULSAR
F0
(Hz)		 Pb
(days)		 x
(s)		 e MT
(M☉)		 Mp
(M☉)		 Mc
(M☉)		 Mass constraints Geodetic precession Notes
J0218+4232 430.461 002.02885 001.98443 0.0000063(2)   1.49(+23/−20) 0.179(+18/−16) r, s a MSP - He WD system.
J0337+1715 365.953 001.62940
327.25512		 001.21753
074.67234		 0.000699036(+37/−42)
0.0352911721(+20/−28) 		  1.4380(+12/−11) 0.19755(+15/−14)
0.41012(+31/−29)		 i, q a First pulsar in a triple star system. Two companions are He WDs, hence the two companion masses. The two sets of Keplerian parameters are for the orbit of the inner system (containing MSP and WD) and the orbit of the outer WD with the inner system. q is measured independently for inner system. This system yields the best test of the Universality of free fall to date (see also the first and second versions of this test).
J0348+0432 025.561 000.10242 000.14098 0.0000020(10)   2.01(4)
1.806(36)
 0.172(3)
0.154(3) 		q, mWD, GWa
q, GWa
a PSR - He WD system. First massive NS in a relativistic orbit. Latest orbital decay measurements disagrees with prediction derived from spectroscopic mass measurements. If the observed orbital variation is the GW emission predicted by GR, the components masses are significantly smaller than their spectroscopic estimate.
J0437−4715 173.688 005.74104 003.36670 0.000019180(3)   1.418(44) 0.221(4) r, s a MSP - He WD system. Very high timing precision. Full 3-D position, velocity, orbital orientation and masses are known.
J0453+1559 021.843 004.07247 014.46679 0.11251847(8) 2.734(4) 1.559(5) 1.174(4) p, h3 u PSR - NS. Asymmetric DNS. Companion star is lightest NS known.
J0509+3801 013.065 000.37958 002.05046 0.586409(3) 2.81071(14) 1.399(6) 1.412(6) p, γ, GW u PSR - NS. Highly eccentric, similar to the Hulse-Taylor pulsar.
J0514−4002A 200.378 018.78518 036.29028 0.8879771(11) 2.4730(6) 1.25(+5/−6) 1.22(+6/−5) p, γ u MSP - massive compact companion. System formed through exchange encounter in GC NGC 1851. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J0621+1002 034.657 008.31868 012.03207 0.00245744(4) 2.32(8) 1.53(+10/−20) 0.76(+28/−7) p, s a PSR - Massive WD system.
J0737−3039A
J0737−3039B		 044.054
000.360		 000.10225 001.41503
001.516		 0.08777702(6) 2.587052(+9/−7) 1.338185(+12/−14)
1.248868(+13/−11) 		1.338185(+12/−14)
1.248868(+13/−11) 		R, p, γA, s, rA, GW, GB no
yes, yes 		PSR - yPSR. The only double pulsar known. Most highly inclined binary pulsar known. Seven mass constraints, which determine the masses and yield 5 independent GR tests. Most precise measurements of NS masses and most precise measurement of the orbital decay ever. Detection of second-order effects in the rate of advance of periastron (with contribution from Lense-Thirring effect) and Shapiro delay.
J0740+6620 346.532 004.76694 003.97756 0.00000507(4) 2.08(7) 0.253(+6/−5) r, s a MSP- Massive WD system, likely most massive NS.
J0751+1807 287.458 000.26314 000.39661 0.0000005(11)   1.64(15) 0.16(1) s, GWa a MSP - He WD system, only case where mass is derived from s and assumption that GWa is as given by GR.
J0955−6150 500.160 024.57840 013.28248 0.11750575(1) 1.96(2) 1.71(3) 0.254(2) p, h3, ς u Eccentric MSP - He WD system.
J1012−4235 322.462 037.97246 021.26307 0.00034572(1) 1.44(+13/−12) 0.270(+16/−15) h3, ς a MSP - He WD system.
J1012+5307 190.267 000.60467 000.58182 0.0000012(3)   1.72(16) 0.165(15) q, mWD, GWa a MSP - He WD system. Useful constraints on dipolar GW emission (a, b).
J1125−6014 380.173 008.75260 008.33919 0.000000615(11) 1.68+(+17/−15) 0.33(2) r, s a MSP - Light CO WD system.
J1141−6545 002.539 000.19765 001.85892 0.171876(1) 2.28967(6) 1.27(1) 1.02(1) p, γ, GWa yes yPSR, formed after the companion, which is a massive WD. The fast rotation of the WD induces a detectable precession in the orbit, a significant part of which is relativistic (the Lense-Thirring effect).
J1227−6208 028.962 006.72102 023.20067 0.00114918(4) 1.54(15) 1.40(7) p, h3, ς u PSR - massive WD system. WD companion could me most massive in this list.
J1518+4904 024.428 008.63400 020.03949 0.249484383(9) 2.7186(7) 1.470(+30/−34) 1.248(+35/−29) p, s u PSR - NS.
J1528−3146 016.441 003.18035 011.45230 0.0002137(2) 2.94(+21/−20) 1.61(+14/−13) 1.33(+8/−7) r, s, p a PSR - massive WD system. WD companion could me most massive in this list.
B1534+12 026.382 000.42074 003.72946 0.27367740(4) 2.678463(4) 1.3330(2) 1.3455(2) p, γ, r, s, GW, G yes PSR - NS. Orbital decay (GW) has been measured precisely but does not provide a test of GR because of lack of precise knowledge of distance.
J1614−2230 317.379 008.68662 011.29120 0.000001333(8) 1.908(16)
1.94(3)		 0.493(3)
0.495(5)		 r, s
r, s 		a MSP- Massive WD system, massive MSP.
J1713+0747 218.811 067.82513 032.34242 0.0000749402(6) 1.33(+9/−8)
1.35(7) 		0.289(+13/−11)
0.292(11) 		h3, ς
r, s 		a MSP - He WD system. This system surrently provides the best pulsar limit on the variation of the gravitational constant, on violations of the universality of free fall and on the α3 parameter.
J1738+0333 170.937 000.35479 000.34343 0.00000034(11)   1.47(+7/−6) 0.181(+7/−5) q, mWD, GWa a MSP - He WD system. This system yields useful constraints on dipolar GW emission (a, b) and the best limits on igravitational Local Lorenz Invariance violation parameter α1
J1748−2446ap 267.044 021.38817 013.20131 0.905186(4) 1.997(6) 1.700(+15/−45) 0.294(+46/−14) p, γ u MSP - He WD. Located in Terzan 5, system is highly eccentric and likely perturbed by close stellar encounters.
J1756−2251 035.135 000.31963 002.75646 0.1805694(2) 2.56999(6) 1.341(7) 1.230(7) p, γ, s, GW no PSR - NS. Low-mass NS companion. No geodetic precession for pulsar, indicating alignment of its spin axis with orbital angular momentum.
J1757−1854 046.518 000.18354 002.23781 0.6058174(3) 2.732876(8) 1.3412(4) 1.3917(4) p, γ, h3, ς GW yes PSR - NS. Most accelerated binary, largest relative velocity, five measured PK parameters. Second-formed NS significantly more massive than first-formed NS.
J1802−2124 079.066 000.69889 003.71885 0.00000248(5) 1.24(11) 0.78(4) r, s a MSP - massive WD system.
J1807−2500B 238.881 009.95667 028.92039 0.747033198(40) 2.57190(73) 1.3655(21) 1.2064(20) p, ς, h3 u MSP - massive compact companion. System formed through exchange encounter in GC NGC 6544. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J1829+2456 024.384 001.17603 007.23684 0.1391435(3) 2.60551(38) 1.306(7) 1.299(7) p, s u PSR - NS.
B1855+09 186.494 012.32717 009.23078 0.00002170(3) 1.37(+13/−10)
1.59(+21/−18) 		0.244(+14/−12)
0.268(+22/−19)
 r, s,
h3, ς 		a MSP - He WD system. First measurement of Shapiro delay in a binary pulsar.
J1903+0327 465.135 095.17412 105.59346 0.436678409(3) 2.697(29)* 1.667(21)* 1.029(8)* p, h3, ς, q u First MSP outside GCs with eccentric orbit. MS companion. System likely originated as a triple. *Masses indicated to 99.7% C.L. as uncertainties are not Gaussian.
J1906+0746 006.941 000.16599 001.41995 0.0852996(6) 2.6134(3) 1.291(11) 1.322(11) p, γ, GW, G yes yPSR - massive compact companion. The pulsar (only 110 kyr old) is the second-formed compact object in the system. Companion could be a recycled NS as in most other PSR - NS systems, but it could also be a massive WD, as in the cases of J1141−6545 and B2303+46.
J1909−3744 339.316 001.53345 001.89799 0.000000115(7) 1.48(3)
1.492(14)
1.45(3) 		0.208(2)
0.209(1)
0.205(3) 		r, s
r, s
r, s 		a MSP - He WD system. First MSP with a precise mass. Very high timing precision and high inclination. Useful constraints on dipolar GW emission (a, b).
J1910−5958A 306.167 000.83711 001.20605 0.00000082(3)   1.55(7) 0.202(6) r, s, q, mWD a MSP - He WD system in GC NGC 6572.
J1913+1102 036.650 000.20625 001.75462 0.089531(2) 2.8887(6) 1.62(3) 1.27(3) p, γ, GW u PSR - NS. Most massive among these systems, pulsar is most massive NS in a PSR - NS system. First known member of new population of merging, asymmetric DNSs.
B1913+16 016.940 000.32299 002.34178 0.6171340(4) 2.828378(7) 1.438(1) 1.390(1) p, γ, GW, h3, ς yes PSR - NS. The Hulse-Taylor binary. First binary pulsar discovered. First NS mass measurements. Orbital decay was the first GR test in a binary pulsar, and first radiative test anywhere. This showed that gravitational waves exist.
J1918−0642 130.790 010.91318 008.35047 0.000020340(18) 1.29(+10/−9) 0.231(10) r, s a MSP - He WD system.
J1933−6211 282.212 012.81941 012.28157 0.00000126(2) 1.4(+3/−2) 0.43(5) r, s a MSP - CO WD system.
J1943+2210 077.699 000.37205 002.57980 0.0000016(4) 1.84(+11/−9) 1.03(+4/−3) r, s u PSR - Massive WD system. Massive pulsar, compact orbit.
J1946+3417 315.445 027.01995 013.86907 0.134495389(17) 2.094(22) 1.828(22) 0.2556(19) p, h3, ς u Eccentric MSP - He WD system. Massive MSP.
J1949+3106 076.114 001.94954 007.28865 0.000043122(35) 1.34(+17/−15) 0.81(+6/−5) h3, ς, p a MSP - massive WD system.
J1950+2414 232.300 022.19137 014.21994 0.07981173(4) 1.779(25) 1.496(23) 0.2975(+46/−38) p, h3 u Eccentric MSP - He WD system.
J2023+2853 088.270 000.71823 004.00222 0.0000131(6) 1.28(+6/−6) 0.85(2) r, s u PSR - Massive WD system.
J2043+1711 420.189 001.48229 001.62396 0.00000489(13) 1.38(+12/−13) 0.173(10) r,s a MSP - He WD system.
J2045+3633 031.564 032.29784 046.94080 0.017212447(6) 2.127(31) 1.251(21) 0.873(+16/−14) p, h3, ς a MSP - massive WD system.
J2053+4650 079.652 002.45250 008.80430 0.0000089(1) 1.40(+21/−18) 0.86(+7/−6) h3, ς a MSP - massive WD system.
B2127+11C 032.755 000.33528 002.51845 0.681395(2) 2.71279(13) 1.358(10) 1.354(10) p, γ, GW yes PSR - massive compact companion. Formed through exchange encounter in GC NGC 7078 (M15), companion could be a massive WD, but generally assumed to be a NS given the large mass. Acceleration of system in the cluster precludes further improvements in precision of radiative test of GR.
J2222−0137 030.471 002.44576 010.84802 0.00038092(1) 3.150(14) 1.831(10) 1.3194(40) r, s, p, GWa a PSR - massive WD system. Useful constraints on dipolar GW emission (a, b).
J2234+0611 279.597 032.00140 013.93737 0.129274035(8) 1.6518(+33/−35) 1.353(+14/−17) 0.298(+15/−12) h3, p u Eccentric MSP - He WD system. Full 3-D position, velocity, orbital orientation and masses are known.
B2303+46 000.937 012.33954 032.6787 0.658369(9) 2.64(5) 1.34(10) 1.30(10) p, mWD u yPSR. Formed after the companion, which is a massive WD.
2. Systems with binary mass measurements only
PULSAR F0
(Hz)		 Pb
(days)		 x
(s)		 e MT
(M☉)		 Mp
(M☉)		 Mc
(M☉)		 Mass constraints Geodetic precession Notes
J0024−7204H 311.493 002.35770 002.15281 0.0705585(7) 1.665(7) < 1.49 > 0.175 f, p u MSP with likely He WD companion. Located in GC NGC 104 (47 Tuc).
J0514−4002E 178.701 007.44790 027.8192 0.70793232(85) 3.8870(45) < 1.79 > 2.09 f, p u MSP - massive compact companion. Most massive system in this list. System formed through exchange encounter in GC NGC 1851. Companion is either a very massive NS or a light BH. Limits on the individual masses derived from non-detection of the Shapiro delay.
J0528+3529 012.782 011.72618 031.43468 0.2901088(10) 2.90(12) < 1.8 > 1.2 f, p u PSR - NS.
J1018−1523 012.026 008.98393 026.15662 0.227749(2) 2.3(3) < 1.2 > 1.1 f, p u PSR - NS.
J1208−5936 034.826 000.63157 004.2570(4) 0.347988(1) 2.586(5) < 1.48 > 1.10 f, p u PSR - NS.
J1325−6253 034.520 001.81559 007.57391 0.0640091(7) 2.57(6) < 1.59 > 0.98 f, p u PSR - NS.
J1411+2551 016.012 002.61586 009.20514 0.1699308(4) 2.538(22) < 1.62 > 0.92 f, p u PSR - NS.
B1516+02B 125.835 006.85845 003.04826 0.137839(2) 2.157(28) < 2.05 > 0.14 f, p u MSP - WD system, located in GC NGC 5904 (M5).
J1618−3921 083.422 022.74560 010.27828 0.02741231(1) 1.42(+20/−19) < 1.39 > 0.17 f, p u Eccentric MSP - He WD system. Very likely a member of a triple system.
J1748−2446I 104.491 001.328 001.818 0.428 2.17(2) < 1.96 > 0.24 f, p u MSP - WD system, located in GC Terzan 5.
J1748−2446J 012.447 001.102 002.454 0.350 2.20(4) < 1.96 > 0.38 f, p u PSR - WD system, located in GC Terzan 5.
J1748−2446am 340.853 000.80011 000.93782 0.204736(9) 1.85(2) < 1.7 > 0.15 f, p u MSP - WD system, located in GC Terzan 5.
J1748−2446ao 439.681 057.55568 062.31393 0.32488898(8) 3.166(24) < 2.23 > 0.93 f, p u MSP with massive companion. System formed through exchange encounter in Terzan 5. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J1748−2446au 219.866 005.97946 006.54572 0.025695(1) 1.82(7) < 1.53 > 0.30 f, p u MSP - WD system, located in GC Terzan 5.
J1748−2021B 059.665 020.55001 004.46699 0.5701606(15) 2.92(15) < 3.24 > 0.11 f, p u PSR - WD system, located in GC NGC 6440. Possibly massive pulsar.
J1750−37A 008.961 017.33428 024.39312 0.712431(2) 1.97(15) < 1.65 > 0.53 f, p u PSR - WD system, located in GC NGC 6441.
J1759+5036 005.681 002.04298 006.82461 0.30827(12) 2.679(12) < 1.84 > 0.83 f, p u PSR - NS.
B1802−07 043.288 002.61676 003.92059 0.21206(2) 1.62(7) < 1.33 > 0.29 f, p u PSR - WD system, located in GC NGC 6539.
J1811−1736 009.599 018.77917 034.7827 0.828011(9) 2.57(10) < 1.74 > 0.93 f, p u PSR - NS.
J1823−3021G 164.169 001.54014 003.00331 0.380466(6) 2.65(7) < 2.4 > 0.44 f,p u MSP - massive compact companion. System formed through exchange encounter in GC NGC 6624. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J1824−2452C 240.484 008.07781 007.35739 0.8470437(8) 1.616(4) < 1.367 > 0.245 f, p u MSP - WD, located in NGC 6626 (M28). System is highly eccentric and likely perturbed by close stellar encounters.
J1846−0513 042.791 000.61302 004.75647 0.2085888(9) 2.6287(35) < 1.35 > 1.28 f, p u PSR - NS.
J1901+0658 013.202 014.45477 032.40204 0.3662392(5) 2.79(7) < 1.68 > 1.11 f, p u PSR - NS.
J1930−1852 005.390 045.06000 086.89027 0.39886334(15) 2.54(3) < 1.25 > 1.30 f, p u PSR - NS. Largest orbital period for this type of system.
J1946+2052 058.962 000.07849 001.15432 0.063848(9) 2.50(4) < 1.31 > 1.18 f, p yes PSR - NS. Shortest orbital period for this type of system.
J2140−2311B 076.983 006.21565 019.5222(7) 0.87938(2) 2.53(8) < 1.43 > 1.11 f,p u MSP - massive compact companion. System formed through exchange encounter in GC NGC 7099 (M30). Companion is either a massive WD or NS, so this could be a MSP - NS system.
J2150+3427 001.528 010.59213 025.48802 0.601494(2) 2.59(13) < 1.67 > 0.98 f, p u Pulsar with massive companion in eccentric orbit. Pulsar is very slow, and was likely (mildly) recycled. If it is the second-formed object in the system, then the companion (the primary) could be a massive WD.
3. Confirmed and possible pulsar - neutron star systems
(Important: most systems were already listed above)
PULSAR F0
(Hz)		 Pb
(days)		 x
(s)		 e MT
(M☉)		 Mp
(M☉)		 Mc
(M☉)		 Mass constraints Geodetic precession Notes
J0453+1559 021.843 004.07247 014.46679 0.11251847(8) 2.734(4) 1.559(5) 1.174(4) p, h3 u Confirmed. Asymmetric DNS. Companion star is lightest NS known.
J0509+3801 013.065 000.37958 02.05046(3) 0.586409(3) 2.81071(14) 1.399(6) 1.412(6) p, γ, GW u Confirmed. Highly eccentric, similar to the Hulse-Taylor pulsar.
J0514−4002A 200.378 018.78518 036.29028 0.8879771(11) 2.4730(6) 1.25(+5/−6) 1.22(+6/−5) p, γ u Candidate. MSP - massive compact companion. System formed through exchange encounter in GC NGC 1851. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J0514−4002E 178.701 007.44790 027.8192 0.70793232(85) 3.8870(45) < 1.79 > 2.09 f, p u Candidate. MSP - massive compact companion. System formed through exchange encounter in GC NGC 1851. Companion is either a very massive NS or a light BH.
J0528+3529 012.782 011.72618 031.43468 0.2901088(10) 2.90(12) < 1.8 > 1.2 f, p u Confirmed.
J0737−3039A
J0737−3039B		 044.054
000.360		 000.10225 001.41503
001.516		 0.08777702(6) 2.587052(+9/−7) 1.338185(+12/−14)
1.248868(+13/−11) 		1.338185(+12/−14)
1.248868(+13/−11) 		R, p, s, γA, rA, GW, GB no
yes, yes 		Confirmed. The only double pulsar known. Companion is a young, non-recycled pulsar.
J1018−1523 012.026 008.98393 026.15662 0.227749(2) 2.3(3) < 1.2 > 1.1 f, p u Confirmed.
J1155−6529 012.68 003.67 015.34 0.26 - [1.4] > 1.27 f u Confirmed.
J1208−5936 034.826 000.63157 004.2570(4) 0.347988(1) 2.586(5) < 1.48 > 1.10 f, p u Confirmed.
J1325−6253 034.520 001.81559 007.57391 0.0640091(7) 2.57(6) < 1.59 > 0.98 f, p u Confirmed.
J1411+2551 016.012 002.61586 009.20514 0.1699308(4) 2.538(22) < 1.62 > 0.92 f, p u Confirmed.
J1518+4904 024.428 008.63400 020.03949 0.249484383(9) 2.7186(7) 1.470(+30/−34) 1.248(+35/−29) p, s u Confirmed.
B1534+12 026.382 000.42074 003.72946 0.27367740(4) 2.678463(4) 1.3330(2) 1.3455(2) p, γ, r, s, GW, G yes Confirmed. Orbital decay (GW) has been measured precisely but does not provide a test of GR because of lack of precise knowledge of distance.
J1748−2446ao 439.681 057.55568 062.31393 0.32488898(8) 3.166(24) < 2.23 > 0.93 f, p u Candidate. MSP with massive companion. System formed through exchange encounter in Terzan 5. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J1748−2021B 059.665 020.55001 004.46699 0.5701606(15) 2.92(15) < 3.24 > 0.11 f, p u Candidate. Total mass consistent with PSR - NS system. If companion is a NS, then the system formed through exchange encounter. This would imply a very low orbital inclination. Located in GC NGC 6440.
J1753−2240 010.511 013.63757 018.11537 0.303582 - [1.4] > 0.49 f - Confirmed. Pulsar is recycled.
J1755−2550 003.173 009.69633 012.28441 0.08935(2) - [1.3] > 0.39 f - Candidate. yPSR - massive compact companion. The pulsar (only 2.1 Myr old) is the second-formed compact object in the system. Companion could be a recycled NS as in most other PSR - NS systems, but it could also be a massive WD, as in the cases of J1141−6545 and B2303+46.
J1756−2251 035.135 000.31963 002.75646 0.1805694(2) 2.56999(6) 1.341(7) 1.230(7) p, γ, s, GW no Confirmed. Low-mass NS companion. No geodetic precession for pulsar, indicating alignment of its spin axis with orbital angular momentum.
J1757−1854 046.518 000.18354 002.23781 0.6058171(3) 2.732882(12) 1.3406(5) 1.3922(5) p, γ, h3, ς GW yes Confirmed. Most accelerated binary, largest relative velocity, five measured PK parameters. Second-formed NS significantly more massive than first-formed NS.
J1759+5036 005.681 002.04298 006.82461 0.30827(12) 2.679(12) < 1.84 > 0.83 f, p u Confirmed.
J1807−2500B 238.881 009.95667 028.92039 0.747033198(40) 2.57190(73) 1.3655(21) 1.2064(20) p, ς, h3 u Candidate. MSP - massive compact companion. System formed through exchange encounter in GC NGC 6544. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J1811−1736 009.599 018.77917 034.7827 0.828011(9) 2.57(10) < 1.74 > 0.93 f, p u Confirmed.
J1823−3021G 164.169 001.54014 003.00331 0.380466(6) 2.65(7) < 2.4 > 0.44 f,p u Candidate. MSP - massive compact companion. System formed through exchange encounter in GC NGC 6624. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J1829+2456 024.384 001.17603 007.23684 0.1391435(3) 2.60551(38) 1.306(7) 1.299(7) p, s u Confirmed.
J1835−3259A 257.147 009.2460 019.6(3) 0.968(5) - [1.4] > 0.76 f u Candidate. MSP - massive compact companion. System formed through exchange encounter in GC NGC 6652. Companion is either a massive WD or NS, so this could be a MSP - NS system.
J1844−0128 034.312 010.60032 020.96779 0.234949(5) - [1.4] > 0.74 f, p u Confirmed.
J1846−0513 042.791 000.61302 004.75646 0.2085888(9) 2.6287(35) < 1.35 > 1.28 f, p u Confirmed.
J1901+0658 013.202 014.45477 032.40204 0.3662392(5) 2.79(7) < 1.68 > 1.11 f, p u Confirmed.
J1906+0746 006.941 000.16599 001.41995 0.0852996(6) 2.6134(3) 1.291(11) 1.322(11) p, γ, GW, G yes Candidate. yPSR - massive compact companion. The pulsar (only 110 kyr old) is the second-formed compact object in the system. Companion could be a recycled NS as in most other PSR - NS systems, but it could also be a massive WD, as in the cases of J1141−6545 and B2303+46.
J1913+1102 036.650 000.20625 001.75462 0.089531(2) 2.8887(6) 1.62(3) 1.27(3) p, γ, GW u Confirmed. Most massive among these systems, pulsar is most massive NS in a PSR - NS system. First known member of new population of merging, asymmetric DNSs.
B1913+16 016.940 000.32299 002.34178 0.6171340(4) 2.828378(7) 1.438(1) 1.390(1) p, γ, GW, h3, ς yes Confirmed. The Hulse-Taylor binary. First binary pulsar discovered. First NS mass measurements. Orbital decay was the first GR test in a binary pulsar, and first radiative test anywhere. This showed that gravitational waves exist.
J1930−1852 005.390 045.06000 086.89027 0.39886334(15) 2.54(3) < 1.25 > 1.30 f, p u Confirmed. Largest orbital period for this type of system.
J1946+2052 058.962 000.07849 001.15432 0.063848(9) 2.50(4) < 1.31 > 1.18 f, p yes Confirmed. Shortest orbital period for this type of system.
B2127+11C 032.755 000.33528 002.51845 0.681395(2) 2.71279(13) 1.358(10) 1.354(10) p, γ, GW yes Candidate. PSR - massive compact companion. Formed through exchange encounter in GC NGC 7078 (M15), companion could be a massive WD, but generally assumed to be a NS given the large mass. Acceleration of system in the cluster precludes further improvements in precision of radiative test of GR.
J2140−2311B 076.983 006.21565 019.5222(7) 0.87938(2) 2.53(8) < 1.43 > 1.11 f,p u Candidate. MSP - massive compact companion. System formed through exchange encounter in GC NGC 7099 (M30). Companion is either a massive WD or NS, so this could be a MSP - NS system.
J2150+3427 001.528 010.59213 025.48802 0.601494(2) 2.59(13) < 1.67 > 0.98 f, p u Candidate. Pulsar is very slow, and was likely (mildly) recycled. If it is the second-formed object in the system, then the companion (the primary) could be a massive WD.


The mass constraints and the requirements for their measurement are:
f: The mass function. If only this is listed, then we indicate the minimum companion mass obtained assuming the value of the pulsar mass that appears in square brackets. If this is listed together with the rate of advance of periastron, then we can obtain a maximum estimate for the pulsar mass and a minimum estimate for the companion mass.
i: Mass estimate based on measurement of deviations from Keplerian orbits caused by many-body interactions via pulsar timing. This requires a number of massive components in the system larger than 2.
R: Mass ratio derived from pulsar timing. This requires a double pulsar system, only one case known (the J0737−3039 system).
q: Mass ratio derived from pulsar timing and optical spectroscopy. This requires a companion WD bright enough for spectroscopic radial velocity measurements.
mWD: White dwarf mass (optical). This requires a companion WD bright enough for detailed spectroscopic modeling.
p: Rate of advance of periastron (timing). This requires significant orbital eccentricity.
γ: Einstein delay (timing). This requires a combination of significant orbital eccentricity and compactness.
s: Shape parameter of Shapiro delay (timing). All Shapiro delay parameters require the system to have a high orbital inclination and good timing precision.
r: Range parameter of Shapiro delay (timing).
ς: Orthometric ratio of Shapiro delay (timing) - From the Freire & Wex (2010) reparameterization of the Shapiro delay.
h3: Orthometric amplitude of Shapiro delay (timing) - From the Freire & Wex (2010) reparameterization of the Shapiro delay.
GW: Orbital decay (timing), caused by energy loss due to gravitational waves. Detecting this requires a compact orbit.
GWa: Same as GW, but for an asymmetric system, like a pulsar-WD system. This introduces stronger constraints on alternative theories of gravity.
G: Rate of geodetic precession.
A subscript refers to the pulsar for which we measure the parameter (this is only an issue for the J0737−3039 system).
For the column on the detection of geodetic precession, the meaning of the entries is:
a: The spin of this pulsar is expected to be aligned with the orbital angular momentum of the system. This means that geodetic precession should not cause detectable changes in the pulse profile.
u: The spin of this pulsar is not expected to be aligned with the orbital angular momentum of the system, but geodetic precession is too slow to cause detectable changes in the pulse profile.
In the following cases, the pulsar spin is not expected to be necessarily aligned with the orbital angular momentum of the system and geodetic precession should be fast enough to produce detectable changes in the pulse profile:
yes: Such changes are indeed observable, confirming the spin-orbit misalignment.
no: No changes in the pulse profile are detected. This suggests the pulsar spin is aligned with the orbit after all.



Fig. 2: Precise neutron star mass measurements are useful for a variety of purposes. One of them is to constrain the macroscopic behaviour (in particular the relation between density and pressure, known as the equation of state, or EOS) of the cold, super-dense matter at the center of a neutron star. For each EOS (named in the figure) the relation between mass and radius for all neutron stars is indicated by its related curve. The horizontal lines are 68.3% confidence limits for the two most massive pulsars tabulated below. If a particular EOS predicts a maximum mass smaller than the largest measured NS mass then it is excluded.
The green bar corresponds to the 90% confidence limit range of radii for 1.4 solar mass neutron stars, this results from the multi-messenger analysis of Dietrich et al. (2020).

Figure created by Norbert Wex. EOSs tabulated in Lattimer & Prakash (2001) and provided by the authors.



Latest changes to this page:

2024 Dec. 5: 2024 Nov. 21: Updated masses for the PSR J0337+1715 triple system based on the latest and most precise test of the strong equivalence principle with this system, which take into account a likely orbiting planet.
2024 Sept. 8: Added mass measurements for the PSR J1618−3921 system, which is likely a member of a triple system.
2024 Jul. 19: Added mass measurements for the PSR J1227−6208 system.
2024 Jul. 15: Added update on masses for PSR J0437−4715.
2024 Mar. 27: 2024 Mar. 19: Added binary mass measurement for a new PSR - NS system, PSR J1901+0658.
2024 Mar. 16: Added binary mass measurement for a new PSR - NS system, PSR J1846−0513.
2024 Feb. 14: Added mass measurements for PSR J0218+4232 and individual NS mass measurements for PSR J1518+4904.
2024 Jan. 18: Added reference on the timing of NGC 1851E. System formed through exchange encounter in GC NGC 1851. With a total mass of 3.8870 ± 0.0045 solar masses, it is by far the most massive system consisting of a pulsar and a degenerate companion that we know of. Companion is either a very massive NS or a light BH, so this could be the first pulsar - black hole system.
2023 Dec. 13: Added much improved measurements of the masses of PSR J0509+3801, a measurement of the orbital decay of this system and a much improved measurement of the binary mass of PSR J1759+5036.
2023 Dec. 6: Updated binary mass measurement for PSR B1516+02B.
2023 Nov. 23: Added mass measurement for PSR J1012−4235, a MSP - He WD system.
2023 Nov. 22: Added binary mass measurement for PSR J2150+3427, a pulsar with a massive companion where the pulsar is unusually slow.
2023 Sept. 1: Added reference on the timing solution and binary mass of PSR J1208−5936.
2023 May 25: Updated mass measurements for PSR J1757−1854.
2023 April 19: Added mass measurements for PSR J1933−6211, a MSP - CO WD system.
2023 April 14: Added mass measurements for PSR J1528−3146, a pulsar - massive WD system, and new mass estimates for PSRs J1125−6014, J1614−2230 and J1909−3744 from MeerKAT data.
2023 April 12: Added the discovery of two new pulsar - NS systems, PSRs J1155−6529 and J1208−5936, by the MMGPS survey.
2023 January 13: Added binary mass measurement for PSR J2140−2311B, an eccentric binary MSP in M30 that could potentially be a MSP - NS system.
2023 January 11: Updated mass measurements for PSR J1910−5958A from Shapiro delay (pulsar is designated as PSR J1910−5959A).
2022 December 12: Added PSR - NS system PSR J1018−1523.
2022 October 31: Added mass measurement for PSR J1125−6014, a MSP - CO WD system.
2022 March 31: Updated masses for PSR J1757−1854.
2022 March 30: Added PSR - NS system J1325−6253.
2022 March 2: Added PSR J0955−6150, an eccentric MSP - He WD system.
2021 Dec. 14: Updated mass and GR tests with the double pulsar, PSR J0737−3039A/B.
2021 July 21: Updated masses of PSR J2222−0137 and its very massive WD companion.
2021 April 5: Updated masses of the PSR J0740+6620 system.
2021 March 9: Added binary mass measurement for a new system, PSR J1823−3021G, an eccentric binary MSP in NGC 6624 that could potentially be a MSP - NS system.
2021 March 5: Added third part of the table, with list of PSR - NS systems, both confirmed (mainly by stellar evolution arguments) and candidates.
2021 Feb. 27: Added binary mass measurement for a new PSR - NS system, PSR J1759+5036.
2020 Nov. 5: Updated mass measurements for the two NSs in the PSR J1829+2456 system.
2020 Sept. 29: Updated mass measurements for PSR J1909−3744.
2020 Sept. 28: Added PSR J2045+3633, a pulsar - massive WD system.
2020 July 16: Added mass measurements for the two NSs in the PSR J1829+2456 system.
2020 July 8: Added mass measurements for the two NSs in the PSR J1913+1102 system. The system is asymmetric and demonstrates the existence of a new population of merging, asymmetric double NSs.
2020 May 8: Updated masses for the PSR J0337+1715 triple system based on the latest and most precise test of the strong equivalence principle with this system.
2020 April 9: Added updated mass measurement for the PSR J1012+5307 system and new limits on dipolar GW emission derived from these binaries.
2020 Jan. 31: Added detection of Lense-Thirring precession in the PSR J1141−6545 system.
2019 Sept. 25: Added measurement of the rate of geodetic precession for PSR J1906+0746.
2019 Sept. 16: Individual masses for both components of PSR J0514−4002A have been measured. System very likely formed via exchange encounter in the GC NGC 1851. The companion is either a massive WD or a NS.
2019 June 24: Added MSP - WD systems PSR J1949+3106 and PSR J1950+2414. The latter is an eccentric MSP - He WD system.
2019 April 16: Added MSP - WD system PSR J0740+6620, likely the most massive NS known!
2018 Sept. 14: Added eccentric MSP - He WD system PSR J2234+0611. Also, added link to the test of the universality of free fall with PSR J0337+1715.
2018 July 23: Added MSP - WD system J1748−2446am, located in GC Terzan 5.
2018 May 29: Added data for PSR J1824−2452C, from Steve Bégin's M.Sc. Thesis, University of British Columbia, 2006.
2018 May 22: Added systems with binary mass measurements (from measurement of rate of advance of periastron), where the binary mass is known to better than 10%.
2018 May 15: Added PSR - NS system J0509+3801.
2018 March 6: Replaced the ``double neutron star system'' (DNS) term by the more precise ``Pulsar - Neutron star system'' (PSR-NS), in order to avoid confusion with the LIGO/Virgo systems.
2018 March 1: Updated parameters for PSR J1713+0737, in particular the new limits on violation of gravitational symmetries.
2018 Jan. 08: Updated masses from the NANOGrav Eleven-year Data Set (PSRs J1614−2230, J1713+0737, B1855+09, J1909−3744, J1918−0642 and J2043+1711).
2017 Nov. 22: Added PSR - NS system J1757−1854.
2017 June 27: Updated mass of PSR J2222−0137.
2017 June 20: Added PSR J2053+4650.
2016 Nov. 15: Added PSR J1946+3417, and eccentric MSP - He WD system.


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