Testing the Universality of Free Fall towards Dark Matter with Radio Pulsars.

The violation of the weak equivalence principle (EP) in the gravitational field of Earth, described by the Eötvös parameter η_{⊕}, was recently constrained to the level |η_{⊕}|≲10^{-14} by the MICROSCOPE space mission. The Eötvös parameter η_{DM}, pertaining to the differential couplings of dark matter (DM) and ordinary matter, was only tested to the level |η_{DM}|≲10^{-5} by the Eöt-Wash group and lunar laser ranging. This test is limited by the EP-violating driving force in the solar neighborhood that is determined by the galactic distribution of DM. Here we propose a novel celestial experiment using the orbital dynamics from radio timing of binary pulsars, and obtain a competing limit on η_{DM} from a neutron-star-white-dwarf (NS-WD) system, PSR J1713+0747. The result benefits from the large material difference between the NS and the WD and the large gravitational binding energy of the NS. If we can discover a binary pulsar within ∼10 pc of the galactic center, where the driving force is much larger in the expected DM spike, precision timing will improve the test of the universality of free fall towards DM and constrain various proposed couplings of DM to the standard model by several orders of magnitude. Such a test probes the hypothesis that gravity is the only long-range interaction between DM and ordinary matter.

A pulsar in a binary with a compact object in the mass gap between neutron stars and black holes.

Some compact objects observed in gravitational wave events have masses in the gap between known neutron stars (NSs) and black holes (BHs). The nature of these mass gap objects is unknown, as is the formation of their host binary systems. We report pulsar timing observations made with the Karoo Array Telescope (MeerKAT) of PSR J0514-4002E, an eccentric binary millisecond pulsar in the globular cluster NGC 1851. We found a total binary mass of 3.887 ± 0.004 solar masses (M⊙), and multiwavelength observations show that the pulsar's binary companion is also a compact object. The companion's mass (2.09 to 2.71 M⊙, 95% confidence interval) is in the mass gap, indicating either a very massive NS or a low-mass BH. We propose that the companion formed in a merger between two earlier NSs.

A massive pulsar in a compact relativistic binary.

Many physically motivated extensions to general relativity (GR) predict substantial deviations in the properties of spacetime surrounding massive neutron stars. We report the measurement of a 2.01 ± 0.04 solar mass (M⊙) pulsar in a 2.46-hour orbit with a 0.172 ± 0.003 M⊙ white dwarf. The high pulsar mass and the compact orbit make this system a sensitive laboratory of a previously untested strong-field gravity regime. Thus far, the observed orbital decay agrees with GR, supporting its validity even for the extreme conditions present in the system. The resulting constraints on deviations support the use of GR-based templates for ground-based gravitational wave detectors. Additionally, the system strengthens recent constraints on the properties of dense matter and provides insight to binary stellar astrophysics and pulsar recycling.