Download Nuclear physics experiments with gravitational wave interferometers

Nuclear physics experiments with gravitational wave interferometers
Andrew MELATOS, University of Melbourne
Neutron stars are currently the only natural settings where one can study experimentally
the fundamental physics of bulk nuclear matter in the true many-body limit. Gravitational
waves in the hectohertz band will provide unprecedented opportunities to explore this
physics by measuring directly the internal motions of neutron stars. In this review,
examples are given of how gravitational-wave data can be inverted to infer the
thermodynamic coefficients (e.g. compressibility, viscosity), state of superfluidity, and
electrical properties (e.g. resistivity, state of magnetization) of nuclear matter. Experiments
are identified which would benefit especially from the availability of a worldwide detector
network.
(1) With respect to isolated neutron stars, it is shown that rotational glitches observed in
radio pulsars are promising sources which, under some scenarios, approachthe
detectability limit of Advanced LIGO. Glitches emit a two-part signal, a sub-second burst
followed by a continuous tone lasting for days/weeks. The burst maps the motion of
quantized superfluid vortices in the star; if detected, it would be the first direct proof that
bulk nuclear matter is indeed a quantum fluid. The continuous tone arises as the star
responds hydrodynamically to the glitch; its spectrum and polarization depend in a
calculable way on the nuclear compressibility and viscosity. Results are presented from
recent microscopic (Gross-Pitaevskii) and macroscopic (two-fluid) numerical simulations
to illustrate these effects as well as their astrophysical implications, e.g. quantifying
differential rotation inside neutron stars and tying down the glitch mechanism, which
remains an unsolved problem after 40 years. A detector network will assist these studies,
because the first detections are likely to be radio-silent objects nearby, whereupon
gravitational-wave triangulation is critical.
(2) With respect to accreting neutron stars, it is already possible to place interesting
gravitational-wave-based limits on nuclear physics. Strong indirect evidence exists (e.g.
from X-ray data on spins) that neutron stars in low-mass X-ray binaries have sizeable
quadrupoles, with Sco X-1 plausibly detectable by Advanced LIGO. A detection will
constrain the elasticity and breaking strain of nuclear matter in its crystalline state (in the
crust) as well as its composition (e.g. hyperon-rich matter sustains larger quadrupoles
than normal). If the quadrupole arises from asymmetric electron capture reactions, the
nuclear reaction rate, thermal conductivity, and state of superfluidity can be inferred in
principle. If the quadrupole arises from magnetic funneling, the electrical resistivity and
crustal adiabatic index can be inferred. It is argued that indirect (gravitational-wave
spin-down) limits already challenge theoretical resistivity calculations. Detection of a
magnetic mountain quadrupole, e.g. in Sco X-1, holds out the prospect of measuring
directly the star's crustal magnetic field strength and geometry for the first time, thereby
probing the origin and nature of high magnetization generally in compact astrophysical
objects. Finally, if the quadrupole arises from r-modes, the gravitational-wave frequency
depends on the equation of state, while the long-term persistence of the mode implies a
lower limit on the strange quark fraction. The main benefit of a detector network for these
sources is better sensitivity. (3) With respect to neutron star inspirals, tidal signatures in
the gravitational-wave signal constrain the pressure at twice nuclear density to 10% or so,
independently measuring the isospin asymmetry energy and nuclear compressibility. A
detector network enhances these experiments by improving source localization, thus
aiding the hunt for faint electromagnetic counterparts with good positions, so that the
gravitational-wave detection can be refined to reduce the uncertainties in the nuclear
quantities inferred.