Neutron stars are composed of the densest form of matter known to exist in our Universe, the composition and properties of which are still theoretically uncertain. Measurements of the masses or radii of these objects can strongly constrain the neutron star matter equation of state and rule out theoretical models of their composition. The observed range of neutron star masses, however, has hitherto been too narrow to rule out many predictions of 'exotic' non-nucleonic components. The Shapiro delay is a general-relativistic increase in light travel time through the curved space-time near a massive body. For highly inclined (nearly edge-on) binary millisecond radio pulsar systems, this effect allows us to infer the masses of both the neutron star and its binary companion to high precision. Here we present radio timing observations of the binary millisecond pulsar J1614-2230 that show a strong Shapiro delay signature. We calculate the pulsar mass to be (1.97 ± 0.04)M(⊙), which rules out almost all currently proposed hyperon or boson condensate equations of state (M(⊙), solar mass). Quark matter can support a star this massive only if the quarks are strongly interacting and are therefore not 'free' quarks.

We report results from continued timing observations of PSR J0740+6620, a high-mass, 2.8 ms radio pulsar in orbit with a likely ultracool white dwarf companion. Our data set consists of combined pulse arrival-time measurements made with the 100 m Green Bank Telescope and the Canadian Hydrogen Intensity Mapping Experiment telescope. We explore the significance of timing-based phenomena arising from general relativistic dynamics and variations in pulse dispersion. When using various statistical methods, we find that combining ∼1.5 yr of additional, high-cadence timing data with previous measurements confirms and improves on previous estimates of relativistic effects within the PSR J0740+6620 system, with the pulsar mass m p = 2.08 − 0.07 + 0.07 M ⊙ (68.3% credibility) determined by the relativistic Shapiro time delay. For the first time, we measure secular variation in the orbital period and argue that this effect arises from apparent acceleration due to significant transverse motion. After incorporating contributions from Galactic differential rotation and off-plane acceleration in the Galactic potential, we obtain a model-dependent distance of d = 1.14 − 0.15 + 0.17 kpc (68.3% credibility). This improved distance confirms the ultracool nature of the white dwarf companion determined from recent optical observations. We discuss the prospects for future observations with next-generation facilities, which will likely improve the precision on m p for J0740+6620 by an order of magnitude within the next few years.

We report 21-year timing of one of the most precise pulsars: PSR J1713+0747. Its pulse times of arrival are well modeled by a comprehensive pulsar binary model including its three-dimensional orbit and a noise model that incorporates short-and long-timescale correlated noise such as jitter and red noise. Its timing residuals have weighted root mean square ∼92 ns. The new data set allows us to update and improve previous measurements of the system properties, including the masses of the neutron star (1.31 ± 0.11 M e ) and the companion white dwarf (0.286 ± 0.012 M e ) as well as their parallax distance 1.15 ± 0.03 kpc. We measured the intrinsic change in orbital period, P b Inṫ , is −0.20 ± 0.17 ps s −1 , which is not distinguishable from zero. This result, combined with the measured P bInṫ of other pulsars, can place a generic limit on potential changes in the gravitational constant G. We found that G Ġ is consistent with zero [(−0.6 ± 1.1) × 10 −12 yr −1 , 95% confidence] and changes at least a factor of 31 (99.7% confidence) more slowly than the average expansion rate of the universe. This is the best G Ġ limit from pulsar binary systems. The P b Inṫ of pulsar binaries can also place limits on the putative coupling constant for dipole gravitational radiation 0.9 3.3 10´-(95% confidence). Finally, the nearly circular orbit of this pulsar binary allows us to constrain statistically the strong-field post-Newtonian parameters Δ, which describes the violation of strong equivalence principle, and 3 a , which describes a breaking of both Lorentz invariance in gravitation and conservation of momentum. We found, at 95% confidence, 0.01 D < and 2 10 3 20 a <´-based on PSR J1713+0747.

We searched for radio pulsars in 25 of the non-variable, unassociated sources in the Fermi LAT Bright Source List with the Green Bank Telescope at 820 MHz. We report the discovery of three radio and γ-ray millisecond pulsars (MSPs) from a high Galactic latitude subset of these sources. All of the pulsars are in binary systems, which would have made them virtually impossible to detect in blind γ-ray pulsation searches. They seem to be relatively normal, nearby (≤2 kpc) millisecond pulsars. These observations, in combination with the Fermi detection of γ-rays from other known radio MSPs, imply that most, if not all, radio MSPs are efficient γ-ray producers. The γ-ray spectra of the pulsars are power-law in nature with exponential cutoffs at a few GeV, as has been found with most other pulsars. The MSPs have all been detected as X-ray point sources. Their soft X-ray luminosities of ∼10 30−31 erg s −1 are typical of the rare radio MSPs seen in X-rays.