Gravity Tests with Radio Pulsars

The discovery of the first binary pulsar in 1974 has opened up a completely new field of experimental gravity. In numerous important ways, pulsars have taken precision gravity tests quantitatively and qualitatively beyond the weak-field slow-motion regime of the Solar System. Apart from the first verification of the existence of gravitational waves, binary pulsars for the first time gave us the possibility to study the dynamics of strongly self-gravitating bodies with high precision. To date there are several radio pulsars known which can be utilized for precision tests of gravity. Depending on their orbital properties and the nature of their companion, these pulsars probe various different predictions of general relativity and its alternatives in the mildly relativistic strong-field regime. In many aspects, pulsar tests are complementary to other present and upcoming gravity experiments, like gravitational-wave observatories or the Event Horizon Telescope. This review gives an introduction to gravity tests with radio pulsars and its theoretical foundations, highlights some of the most important results, and gives a brief outlook into the future of this important field of experimental gravity.

On the origin of GW190425

The LIGO/Virgo collaborations recently announced the detection of a binary neutron star merger, GW190425. The mass of GW190425 is significantly larger than the masses of Galactic double neutron stars known through radio astronomy. We hypothesize that GW190425 formed differently from Galactic double neutron stars, via unstable ‘case BB’ mass transfer. According to this hypothesis, the progenitor of GW190425 was a binary consisting of a neutron star and a ∼4–$5\, {\mathrm{ M}_\odot }$ helium star, which underwent common-envelope evolution. Following the supernova of the helium star, an eccentric double neutron star was formed, which merged in ${\lesssim }10\, {\rm Myr}$. The helium star progenitor may explain the unusually large mass of GW190425, while the short time to merger may explain why similar systems are not observed in radio. To test this hypothesis, we measure the eccentricity of GW190425 using publicly available LIGO/Virgo data. We constrain the eccentricity at $10\, {\rm Hz}$ to be e ≤ 0.007 with $90{{\ \rm per\ cent}}$ confidence. This provides no evidence for or against the unstable mass transfer scenario, because the binary is likely to have circularized to e ≲ 10−4 by the time it was detected. Future detectors will help to reveal the formation channel of mergers similar to GW190425 using eccentricity measurements.

Observational diversity of magnetized neutron stars

Young and rotation-powered neutron stars (NSs) are commonly observed as rapidly-spinning pulsars. They dissipate their rotational energy by emitting pulsar wind with electromagnetic radiation and spin down at a steady rate, according to the simple steadily-rotating magnetic dipole model. In reality, however, multiwavelength observations of radiation from the NS surface and magnetosphere have revealed that the evolution and properties of NSs are highly diverse, often dubbed as 'NS zoo'. In particular, many of young and highly magnetized NSs show a high degree of activities, such as sporadic electromagnetic outbursts and irregular changes in pulse arrival times. Importantly, their magnetic field, which are the strongest in the universe, makes them ideal laboratories for fundamental physics. A class of highly-magnetized isolated NSs is empirically divided into several subclasses. In a broad classification, they are, in the order of the magnetic field strength (B) from the highest, 'magnetars' (historically recognized as soft gamma-ray repeaters and/or anomalous x-ray pulsars), 'high-B pulsars', and (nearby) x-ray isolated NSs. This article presents an introductory review for non-astrophysicists about the observational properties of highly-magnetized NSs, and their implications. The observed dynamic nature of NSs must be interpreted in conjunction with transient magnetic activities triggered during magnetic-energy dissipation process. In particular, we focus on how the five fundamental quantities of NSs, i.e. mass, radius, spin period, surface temperature, and magnetic fields, as observed with modern instruments, change with evolution of, and vary depending on the class of, the NSs. They are the foundation for a future unified theory of NSs.

The prospects of pulsar timing with new-generation radio telescopes and the Square Kilometre Array

Pulsars are highly magnetized and rapidly rotating neutron stars. As they spin, the lighthouse-like beam of radio emission from their magnetic poles sweeps across the Earth with a regularity approaching that of the most precise clocks known. This precision combined with the extreme environments in which they are found, often in compact orbits with other neutron stars and white dwarfs, makes them excellent tools for studying gravity. Present and near-future pulsar surveys, especially those using the new generation of telescopes, will find more extreme binary systems and pulsars that are more precise 'clocks'. These telescopes will also greatly improve the precision to which we can measure the arrival times of the pulses. The Square Kilometre Array will revolutionize pulsar searches and timing precision. The increased number of sources will reveal rare sources, including possibly a pulsar-black hole binary, which can provide the most stringent tests of strong-field gravity. The improved timing precision will reveal new phenomena and also allow us to make a detection of gravitational waves in the nanohertz frequency regime. It is here where we expect to see the signature of the binary black holes that are formed as galaxies merge throughout cosmological history.This article is part of a discussion meeting issue 'The promises of gravitational-wave astronomy'.