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5.2 A zoo of binary pulsars

Nine relativistic binary neutron star systems with orbital periods less than a day are now known. While some are less interesting for testing relativity, some have yielded interesting tests, and others, notably the recently discovered “double pulsar” are likely to produce significant results in the future. Here we describe some of the more interesting or best studied cases; the parameters of the first four are listed in Table 7.

This is a binary pulsar system in our galaxy [24524318Jump To The Next Citation Point]. Its pulses are significantly stronger and narrower than those of B1913+16, so timing measurements are more precise, reaching 3 μs accuracy. The orbital plane appears to be almost edge-on relative to the line of sight (i ≃ 80°); as a result the Shapiro delay is substantial, and separate values of the parameters r and s have been obtained with interesting accuracy. Assuming GR, one infers that the two masses are m1 = 1.335 ± 0.002 M ⊙ and m2 = 1.344 ± 0.002 M ⊙. The rate of orbit decay ˙ Pb agrees with GR to about 15 percent, but the precision is limited by the poorly known distance to the pulsar, which introduces a significant uncertainty into the subtraction of galactic acceleration. Independently of P˙b, measurement of the four other post-Keplerian parameters gives two tests of strong-field gravity in the non-radiative regime [253].

This system appears to be a clone of the Hulse–Taylor binary pulsar, with very similar values for orbital period and eccentricity (see Table 7). The inferred total mass of the system is 2.706 ± 0.011 M ⊙. But because the system is in the globular cluster M15 (NGC 7078), it suffers Doppler shifts resulting from local accelerations, either by the mean cluster gravitational field or by nearby stars, that are more difficult to estimate than was the case with the galactic system B1913+16. This makes a separate, precision measurement of the relativistic contribution to ˙Pb essentially impossible.

J0737-3039A, B
This binary pulsar system, discovered in 2003 [48], was already remarkable for its extraordinarily short orbital period (0.1 days) and large periastron advance (16.88° yr–1), but then the companion was also discovered to be a pulsar [175]. Because two projected semi-major axes can now be measured, one can obtain the mass ratio directly from the ratio of the two values of ap sin i, and thereby obtain the two masses by combining that ratio with the periastron advance, assuming GR. The results are mA = 1.337 ± 0.005M ⊙ and mB = 1.250 ± 0.005 M ⊙, where A denotes the primary (first) pulsar. From these values, one finds that the orbit is nearly edge-on, with sin i = 0.9991, a value which is completely consistent with that inferred from the Shapiro delay parameter (see Table 7). In fact, the five measured post-Keplerian parameters plus the ratio of the projected semi-major axes give six constraints on the masses (assuming GR): All six overlap within their measurement errors. This system provides a unique opportunity for tight tests of strong-field and radiative effects in GR. Furthermore, it is likely that galactic proper motion effects will play a significantly smaller role in the interpretation of P˙b measurements than they did in B1913+16.

This is a case where the companion is probably a white dwarf [2018Jump To The Next Citation Point]. The masses of the pulsar and companion are 1.30 ± 0.02 and 0.986 ± 0.02M ⊙, respectively. P˙ b has been measured to about 25 percent, consistent with the GR prediction. But because of the asymmetry in sensitivities (sNS ∼ 0.2, sWD ∼ 10−4), there is the possibility, absent in the double neutron-star systems, to place a strong bound on scalar-tensor gravity (see Section 5.4).

Discovered in 2004, this pulsar is in a binary system with a probable neutron star companion, with Pb = 7.67 hr, e = 0.18, and ˙ω = 2.585 ± 0.002 deg yr–1 [104].

The discovery of this system was reported in late 2005 [174]. It is a young, 144-ms pulsar in a relativistic orbit with Pb = 3.98 hr, e = 0.085, and ˙ω = 7.57 ± 0.03 deg yr–1.

Table 7: Parameters of other binary pulsars. References may be found in the text; for an online catalogue of pulsars with reasonably up-to-date parameters, see [18].
J0737–3039(A, B)
(i) “Keplerian” parameters:
apsini (s) 3.7294626(8) 2.520(3) 1.85894(1) 1.41504(2) / 1.513(3)
e 0.2736767(1) 0.68141(2) 0.171876(2) 0.087779(5)
Pb (day) 0.420737299153(4) 0.335282052(6) 0.1976509587(3) 0.102251563(1)
(ii) “Post-Keplerian” parameters:
⟨˙ω⟩ (° yr–1) 1.755805(3) 4.457(12) 5.3084(9) 16.90(1)
γ′ (ms) 2.070(2) 4.67 0.72(3) 0.382(5)
˙ Pb (10–12) –0.137(3) –3.94 –0.43(10) –1.21(6)
r (μs) 6.7(1.0) 6.2(5)
s = sini 0.975(7) 0.9995(4)

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