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.

Nearly a century after Einstein first predicted the existence of gravitational waves, a global network of Earth-based gravitational wave observatories [1, 2, 3, 4] is seeking to directly detect this faint radiation using precision laser interferometry. Photon shot noise, due to the quantum nature of light, imposes a fundamental limit on the attometre-level sensitivity of the kilometre-scale Michelson interferometers deployed for this task. Here, we inject squeezed states to improve the performance of one of the detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) beyond the quantum noise limit, most notably in the frequency region down to 150 Hz, critically important for several astrophysical sources, with no deterioration of performance observed at any frequency. With the injection of squeezed states, this LIGO detector demonstrated the best broadband sensitivity to gravitational waves ever achieved, with important implications for observing the gravitational-wave Universe with unprecedented sensitivity

Radio pulsars with millisecond spin periods are thought to have been spun up by the transfer of matter and angular momentum from a low-mass companion star during an x-ray-emitting phase. The spin periods of the neutron stars in several such low-mass x-ray binary (LMXB) systems have been shown to be in the millisecond regime, but no radio pulsations have been detected. Here we report on detection and follow-up observations of a nearby radio millisecond pulsar (MSP) in a circular binary orbit with an optically identified companion star. Optical observations indicate that an accretion disk was present in this system within the past decade. Our optical data show no evidence that one exists today, suggesting that the radio MSP has turned on after a recent LMXB phase.

Gravitationally bound three-body systems have been studied for hundreds of years 1, 2 and are common in our Galaxy 3, 4 . They show complex orbital interactions, which can constrain the compositions, masses, and interior structures of the bodies 5 and test theories of gravity 6 , if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, B1620−26 7, 8 (with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13) M , where M is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15) M and 0.4101(3) M ), as well as the inclinations of the orbits (both ∼39.2• ). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity. Millisecond pulsars (MSPs) are neutron stars that rotate hundreds of times per second and emit radio waves in a lighthouse-like fashion. They are thought to form in binary systems 9 and their rotation rates and orbital properties can be measured with exquisite precision via the unambiguous pulse-counting methodology known as pulsar timing. As part of a large-scale pulsar survey 10, 11 with the Green Bank Telescope (GBT), we have discovered the only known MSP in a stellar triple system. The pulsar has a spin period of 2.73 ms, is relatively bright (∼2 mJy at 1.4 GHz), and has a complex radio pulse profile with multiple narrow components.Though initial timing observations showed a seemingly typical binary MSP system with a 1.6-day circular orbit and a 0.1−0.2 M white dwarf (WD) companion, large timing systematics quickly appeared, strongly suggesting the presence of a third body. There are two other MSPs known to have multiple companions: the famous pulsar B1257+12 which hosts at least 3 low-mass planets 12,13 , and the MSP triple system B1620−26 in globular cluster M4 with a WD inner companion and a roughly Jupiter-mass outer companion 7, 8 . The timing perturbations from J0337+1715 were much too large to be caused by a planetary mass companion.We began an intensive multi-frequency radio timing campaign (Methods) using the GBT, the Arecibo telescope, and the Westerbork Synthesis Radio Telescope (WSRT) to constrain the system's position, orbital parameters, and the nature of the third body. At Arecibo, we achieve median arrival time uncertainties of 0.8 µs...