Pulsar mass measurements and tests of general relativity


Fig. 1: 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 dashed horizontal lines are 95% lower limits for the three most massive pulsars tabulated below, i.e., there is a 5% probability that each is below its respective line. If a particular EOS predicts a maximum mass smaller than the largest measured NS mass then it is excluded.
Figure created by Norbert Wex. EOSs tabulated in Lattimer & Prakash (2001) and provided by the authors.



The first part of the table below lists all 43 precise (i.e, where the 1-σ uncertainty is smaller than 15% of the measurement) NS mass measurements (highlighted in red), plus 4 mass measurements of objects that can either be NSs or massive WDs (highlighted in blue), plus 25 WD and 1 MS star mass measurements (in gray) in a total of 36 stellar systems (one per row).
I only list systems for which there is either no mass transfer nor significant mass loss, i.e., ``clean'' systems, where tests of general relativity are possible. This page is therefore not meant to be comprehensive, but merely my personal reference on precise NS masses and tests of gravitational theories.
I indicate in boldface the two constraints that yield the most precise mass values, 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, they are listed at the bottom of the table. 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 constraint, click on its symbol.
I list the total mass of the binary only if it is determined to better precision than the individual masses of the components. This happens for several eccentric systems, where the total mass can be derived directly from the rate of advance of periastron (assuming GR).
The second part of the table lists 16 systems where the only available mass constraint is the rate of advance of periastron. In these systems we can only estimate the total mass of the system, not the individual masses.

If you find this table useful, then please cite the relevant original references. If not practical, then please cite this recent review on the topic. For a summary on millisecond pulsar masses, please cite this recent work on the topic. Thank you!

Latest changes:

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-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 total mass measurements (from measurement of rate of advance of periastron), where the total mass is known to better than 10%.
2018 May 15: Added pulsar - neutron star 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 pulsar - neutron star system J1757-1854.
2017 June 27: Updated mass of PSR J2222-0137.
2017 June 20: Added PSR J2053+4650.
2016 Nov. 15: Added eccentric MSP-WD system PSR J1946+3417.

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,
GC - globular cluster,
GR - general relativity.

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
J0337+1715 365.953 001.62939
327.25685
001.21753
074.67263
0.00069437(20)
0.03535596(3)
  1.4359(3) 0.19730(4)
0.40962(9)
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. Most precise MSP mass.
J0348+0432 025.561 000.10242 000.14098 0.0000020(10)   2.01(4) 0.172(3) q, mWD, GWa a PSR - He WD system. First massive NS in a relativistic orbit. GWa yields a radiative test of GR in a new extreme gravity regime.
J0437-4715 173.688 005.74104 003.36670 0.000019180(3)   1.44(7) 0.224(7) r, s a MSP - He WD system. Very high timing precision.
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. Largest mass asymmetry for a DNS. Companion star is lightest NS known. Pulsar is most massive NS known in a DNS.
J0509+3801 013.065 000.37958 02.0506(3) 0.586400(6) 2.805(3) 1.36(8) 1.46(8) p, γ 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+0.05−0.06 1.22+0.06−0.05 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+0.10-0.20 0.76+0.28-0.07 p, s a PSR - Massive WD system.
J0737-3039A
J0737-3039B
044.054
000.360
000.10225 001.41503
001.516
0.0877775(9) 2.58708(16) 1.3381(7)
1.2489(7)
1.3381(7)
1.2489(7)
R, p, γA, s, rA, GW, GB no
yes, yes
PSR - yPSR. The only double pulsar known. Seven mass constraints, which determine the masses and yield 5 independent GR tests. No geodetic precession for older pulsar (A), indicating alignment of its spin axis with orbital angular momentum.
J0740+6620 346.532 004.76694 003.97756 0.00000507(4) 2.14+0.10-0.09 0.258(8) 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.
J1012+5307 190.267 000.60467 000.58182 0.0000012(3)   1.64(22) 0.16(2) q, mWD, GWa a MSP - He WD system. Useful constraints on dipolar GW emission.
J1141-6545 002.539 000.19765 001.85892 0.171884(2) 2.2892(3) 1.27(1) 1.01(1) p, γ, GWa yes yPSR, formed after the companion, which is a massive WD.
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) 0.493(3) r, s a MSP- Massive WD system, massive MSP.
J1713+0747 218.811 067.82513 032.34242 0.0000749402(6) 1.33+0.09-0.08
1.35(7)
0.289+0.013-0.011
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+0.07-0.06 0.181+0.007-0.005 q, mWD, GWa a MSP - He WD system. GWa yields the most stringent limits on scalar-tensor theories of gravity. This system also yields the best limits on Local Lorenz Invariance violation parameter α1
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 0.6058142(10) 2.73295(9) 1.3384(9) 1.3946(9) p, γ, h3, ς GW u 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.
B1855+09 186.494 012.32717 009.23078 0.00002170(3) 1.37+0.13-0.10
1.59+0.21-0.18
0.244+0.014-0.012
0.268+0.022-0.019
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 339.315 001.53345 001.89799 0.000000092(13) 1.48(3)
1.54(3)
0.208(2)
0.213(2)
r, s
h3, ς
a MSP - He WD system. First MSP with a precise mass. Very high timing precision and high inclination. Useful constraints on dipolar GW emission.
J1910-5958A 306.167 000.83711 001.20605 <0.00001   1.3(2) 0.180(18) q, mWD, r, s a MSP - He WD system in GC NGC 6572.
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+0.10-0.09 0.231(10) r, s a MSP - He WD system.
J1946+3417 315.444 027.01995 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+0.17−0.15 0.81+0.06−0.05 h3, ς, p u MSP - massive WD system.
J1950+2414 232.300 022.19137 014.21994 0.07981173(4) 1.779(25) 1.496(23) 0.2975+0.0046−0.0038 p, h3 u Eccentric MSP - He WD system.
J2043+1711 420.189 001.48229 001.62396 0.00000489(13) 1.38+0.12-0.13 0.173(10) r,s a MSP - He WD system.
J2053+4650 079.652 002.45250 008.80430 0.0000089(1) 1.40+0.21-0.18 0.86+0.07-0.07 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 0.000380940(3)   1.76(6) 1.293(25) r, s, p, GWa u PSR - massive WD system. Largest NS birth mass; most massive WD companion in this list, most massive system in this list. Useful constraints on dipolar GW emission.
J2234+0611 279.597 032.00140 013.93737 0.129274035(8) 1.6518+0.0033-0.0035 1.353+0.014-0.017 0.298+0.015-0.012 h3, p u Eccentric MSP - He WD system. First system where 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.
Systems with total mass measurements
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 p u MSP with likely He WD companion. Located in GC NGC 104 (47 Tuc).
J1411+2551 016.012 002.61586 009.20514 0.1699308(4) 2.538(22) < 1.62 > 0.92 p u PSR - NS.
B1516+02B 125.835 006.85845 003.04857 0.137845(10) 2.29(17) < 2.52 > 0.13 p u MSP with likely He WD companion. Located in GC NGC 5904 (M5).
J1518+4904 024.428 008.63401 020.04401 0.24948451(3) 2.7183(7) < 1.42 > 1.29 p u PSR - NS.
J1748-2446I 104.491 001.328 001.818 0.428 2.17(2) < 1.96 > 0.24 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 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 p u PSR - 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 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 p u PSR - WD system, located in GC NGC 6441.
B1802-07 043.288 002.61676 003.92059 0.21206(2) 1.62(7) < 1.33 > 0.29 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 p u PSR - NS.
J1824-2452C 240.484 008.07781 007.35739 0.8470437(8) 1.616(4) < 1.367 > 0.245 p u MSP - WD, located in NGC 6626 (M28).
J1829+2456 024.384 001.17603 007.238 0.1391412(19) 2.59(2) < 1.34 > 1.26 p u PSR - NS.
J1913+1102 036.650 000.20625 001.75462 0.08954(1) 2.875(14) < 1.84 > 1.04 p u PSR - NS. Most massive among these systems, possibly asymmetric.
J1930-1852 005.390 045.06000 086.89027 0.39886334(15) 2.54(3) < 1.25 > 1.30 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 p u PSR - NS. Shortest orbital period for this type of system.


The mass constraints and the requirements for their measurement are:
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: Neutron star mass measurements presented in the table above, sorted by increasing order. 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. Figure created by Vivek V. Krishnan.



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