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  Multi-Messenger Probes of Nuclear Physics
  (INT Program July 11—August 5, 2011)

  Reported by B. Link, E. F. Brown, C. L. Fryer, and S. Reddy
  Date posted September 21, 2011

In the coming decade, science for the first time may witness transient astrophysical events, such as the explosion of a massive star, not only across a wide swath of the electromagnetic spectrum, but also in neutrinos and gravitational waves. Events such as supernovae, gamma-ray bursts, and X-ray bursts occur in dense astrophysical environments in which nuclear reactions, the dense matter equation of state, neutrino-matter interactions, and the dynamics of dense matter all play key roles. New observational capabilities are coming online. FERMI is providing new data in X-rays and gamma rays on transient phenomena in compact objects. Optical surveys such as Pan-STARRS, SkyMapper, and the Palomar Transient Factory, along with the future Large Synoptic Survey Telescope (LSST) are opening the time-domain frontier for exploration. Advanced LIGO will provide a new window into the study of supernovae, mergers of compact objects, and rapidly-rotating neutron stars, which will complement current and future studies in the electromagnetic spectrum. Since compact objects are the central engines of these diverse phenomena, and because some of these transients produce both photon (optical, X-ray and gamma-rays) and gravity wave emissions, we are now in the multi-messenger era in astrophysics.

The INT program on “Multi-Messenger Probes of Nuclear Physics” was designed to connect nuclear physicists, observers, and astrophysicists to lay the theoretical groundwork to prepare for the discoveries that will undoubtably emerge from these new observational capabilities. As listed below, discussions at this workshop covered a wide range of topics.

The dense matter equation of state (EOS). Many high-energy transient phenomena involve neutron stars, and a long-standing goal of nuclear astrophysics has been to discern the dense matter equation of state. At low-densities, both chiral effective field theories and quantum Monte Carlo are producing compatible equations of state. The observation of a 1.97 solar mass neutron star suggests that the high-density EOS is rather stiff. As discussed during this program, there has been a great deal of progress on using Bayesian methods to constrain the EOS from a heterogeneous dataset. Observations of thermal emission from cooling transients and X-ray bursts with strong radius expansion tend to suggest small radii, 10—12 km. The systematic uncertainties are not under control, however, and multi-dimensional radiation hydrodynamics calculations will probably be necessary. If the results hold, then the combination of relatively small radii and a high maximum mass implies that the EOS must change rapidly just above nuclear saturation density. Three-body and many-nucleon forces are critically important in this regime. These forces are being accessed by studying nuclei in the laboratory and strong connections between astrophysics and experimental programs on neutron-rich nuclei have emerged.

X-ray image of the supernova remnant Cassiopeia A, which contains a rapidly cooling neutron star. The rapid cooling suggests that the neutrons in the core are becoming a superfluid.
Credit: X-ray: NASA / CXC / UNAM / Ioffe / D.Page, P.Shternin et al; Optical: NASA / STScI;
Illustration: NASA/CXC/M.Weiss)

Cassiopeia A and cooling neutron star

Gravitational radiation from accreting neutron stars. Rossby®-modes on rotating neutron stars are unstable and could produce a significant negative torque via the emission of gravitational waves. If present, they would make accreting neutron stars an attractive source for Advanced LIGO. As presented during the program, however, there is a puzzle: using the current best constraints on the neutron star core temperatures and spins implies that many of the observed accreting neutron stars should be in a short-lived unstable phase, in which the r-mode rapidly grows to saturation and the star rapidly spins down. Possible resolutions discussed at the program include sapping of the r-mode strength via parametric instabilities and other sources of bulk viscosity, such as from compression-induced neutrino emissivity in the crust.

Another possibility for emitting gravitational waves from a rotating neutron star is via a quadrupolar deformation (a “mountain”). The maximum size of a such a deformation is limited by the breaking strain in the crust. Neutron star crusts are “simple” coulomb solids and are very strong: the high pressure inhibits the formation of dislocations and voids, which enable cracking in terrestrial solids.

Supernovae, neutrinos, and nucleosynthesis. There is a growing consensus that multi-dimensional effects and neutrino-matter interactions are critical for producing explosions, with most groups now finding explosions (in 2d) for a range of masses. While the shape of the neutrino spectrum and neutrino oscillations play key roles in the r-process, neutrino-driven wind models fail to produce the r-process. Mergers can produce heavy r-process elements; the heating of the ejecta from radioactive decays is significant and can power an optical transient.

Both the neutrino signal and the gravitational waves from core-collapse supernovae would be good diagnostics of the underlying nuclear physics. At late times, the behavior of the symmetry energy at high-densities affects the timescale of convection, and hence of cooling, in the proto-neutron star. New detectors from 0.1—1~Mt are coming online, and atmospheric and solar neutrinos are now well-characterized. Gravitational waves can discriminate between different explosion mechanisms, and can also detect failed explosions in which the star collapses to a black hole.

Photon observations of supernovae constrain both the progenitor and explosion characteristics of a supernovae. Spectra and light-curve observations can constrain both the initial progenitor and the explosion energy (and, ultimately, the explosion asymmetry). These constraints provide clues into the nature of the explosive engine, removing many uncertainties in neutrino/gravitational-wave studies of nuclear physics.

Image of a borehole for the IceCube neutrino detector near the South Pole.
Image Credit: NSF/B. Gudbjartsson, IceCube Collaboration.

ice-cube string

Magnetar Flares. Magnetars are neutron stars with magnetic fields in excess of 10 14 G. The gamma-ray flares seen in some magnetars are thought to represent large-scale rearrangement of the magnetic field as it decays. Progress is being made on elucidating the physical processes through which the magnetic energy is released. Calculations of the properties of the stellar crust show that it is both strong and brittle. Further work is needed to understand the properties of the densest regions of the crust. Magnetar flares could be associated with starquakes that result from catastrophic failure of the crust under magnetic stresses.

New results presented during the program suggest that the magnetic field cannot break the crust quickly enough to account for the rapid rise time observed in magnetar flares. If these results are correct, magnetar flares cannot be starquakes, but might represent magnetic reconnection events above the stellar surface, similar to solar flares and associated coronal mass ejections.

As was also emphasized at the program, a magnetar’s thermal evolution, magnetic field evolution, and crust dynamics are coupled problems that must be solved together if we are to obtain a realistic picture of how magnetars work. Considerable progress is being made in addressing the coupled thermal evolution and magnetic field evolution in two dimensions. These advances in modeling emphasized the need for input microphysics that can characterize the transport, mechanical, and neutrino cooling properties of the crust under the influence of extreme magnetic fields.

Artist’s rendition of a strongly magnetized neutron star. As the star spins down and the magnetic field decays, stresses are exerted on the rigid crust. Sudden failure of the crust can lead to abrupt rearrangement of the magnetic field and trigger a flare.
Illustration Credit & Copyright: Robert Mallozzi (UAH, MSFC)


Pulsar spin variations and superfluidity. The observed spin glitches of neutron stars and subsequent rotational relaxation provide compelling evidence that most of the neutron star interior is in a superfluid state. Variable coupling of the superfluid to the crust could drive glitches, while affecting observed stochastic spin variations and precession. Progress is being made in the development of the basic hydrodynamic theory of the crust/superfluid dynamics. Superfluid friction coefficients are uncertain due to uncertainties in superfluid gaps as well as poor understanding of the basic interactions that will pin superfluid vortices to nuclei and to magnetic flux tubes. As discussed during the program, realistic modeling of the dynamics of the neutron star interior requires merging of the microscopic problem of vortex motion with the global fluid dynamics. Calculations of vortex pinning with improved microphysics are also crucial.

Behavior of a glitch over time. This span shows two deviations from the uniform spin-down of a pulsar. Such deviations are thought to be due to variable coupling between the crust and the superfluid core. Figure Credit: Kaspi & Gavrill, Astrophys. Jour. 596, L71 (2003)