Probing the neutron star interior and the Equation of State of cold dense matter with the SKA

Abstract

With an average density higher than the nuclear density, neutron stars (NS) provide a unique testground for nuclear physics, quantum chromodynamics (QCD), and nuclear superfluidity. Determination of the fundamental interactions that govern matter under such extreme conditions is one of the major unsolved problems of modern physics, and -since it is impossible to replicate these conditions on Earth -a major scientific motivation for SKA. The most stringent observational constraints come from measurements of NS bulk properties: each model for the microscopic behaviour of matter predicts a specific density-pressure relation (its 'Equation of state', EOS). This generates a unique mass-radius relation which predicts a characteristic radius for a large range of masses and a maximum mass above which NS collapse to black holes. It also uniquely predicts other bulk quantities, like maximum spin frequency and moment of inertia. The SKA, in Phase 2 and particularly in Phase 2 will, thanks to the exquisite timing precision enabled by its raw sensitivity, and surveys that dramatically increase the number of sources: 1) Provide many more precise NS mass measurements (high mass NS measurements are particularly important for ruling out EOS models); 2) Allow the measurement of the NS moment of inertia in highly relativistic binaries such as the Double Pulsar; 3) Greatly increase the number of fast-spinning NS, with the potential discovery of spin frequencies above those allowed by some EOS models; 4) Improve our knowledge of new classes of binary pulsars such as black widows and redbacks (which may be massive as a class) through sensitive broad-band radio observations; and 5) Improve our understanding of dense matter superfluidity and the state of matter in the interior through the study of rotational glitches, provided that an ad-hoc campaign is developed.Advancing Astrophysics with the Square Kilometer Array

Testing Gravity with Pulsars in the SKA Era

“…Measuring two PK parameters, we can use a specific theory of gravity to determine the component masses of the system; these are important to study stellar evolution theories and, in some cases, to constrain EOS (Watts et al 2015). If more PK parameters are measured, then the theory can be tested by a self-consistent argument -using the masses derived in the first stage one should be able to predict the subsequent PK measurements.…”

Understanding the Neutron Star Population with the SKA

“…the chapter by Watts et al 2015) sets a maximum mass stability limit, hunting for massive pulsars can provide an efficient way of constraining its nature. With a significant sample of NS masses, however, one can statistically constrain their EoS.…”

“…From the distribution of NS masses and eccentricities it is even possible to place limits on the gravitational binding energy of NSs (Podsiadlowski et al 2005), released via neutrinos during the SN event. Thus future SKA observations can also via this method help to narrow down the EoS of dense nuclear matter (see the chapter by Watts et al 2015).…”

Very Long Baseline Interferometry with the SKA

“…Achieving this for a large fraction of the radio pulsar population detected in the SKA Galactic pulsar census will enable strong field tests of gravity in a broad range of relativistic binary systems (Kramer & Stappers 2015;Shao et al 2015), the detection of the gravitational wave background (Janssen et al 2015), tomographic modelling of the large scale Galactic magnetic field and mapping the ionized interstellar plasma in the Galaxy (Han et al 2015), and constraining the physics of neutron stars (Watts et al 2015). Curiously, the gravitational wave background might be constrained independently by observing its subtle effect on the apparent position of quasars (e.g.…”