Observational Signatures

Gravitational Waves

Gravitational waves are humankind's newest way to view the heavens and the neutron star mergers and core-collapse supernovae that TEAMS studies are among the strongest sources.

Neutron star mergers have now been observed to produce strong gravitational wave (GW) emission that can total more than a percent of the rest mass energy. Such signals should be observable with advanced LIGO at distances of 100 Megaparsecs or more. Comparison of GW detections with simulations can be used to infer the parameters of the binary and provide important constraints on the physics of nuclear matter.

The GW emission from core-collapse supernovae is expected to be much smaller, and detectable only for events within our own Galaxy, except perhaps for rare cases of rapidly rotating progenitors. Nonetheless, the greater frequency of core-collapse supernovae and the combined physics payload of a Galactic CCSN, with its accompanying neutrino and gravitational wave signals, compels us to push the frontier of gravitational wave prediction for such events forward.

LIGO and VIRGO observations of the gravitational wave GW170817. In the LIGO Hanford and LIGO Livingston panels, GW170817 appears as a blue-green/yellow curve. This "chirp" signal turns sharply upward near time 0 seconds, which is 5:41:04AM PDT on August 17, 2017. The movie converts a combination of the observed stretching and squeezing of the LIGO Hanford and LIGO Livingston detectors into sound at the same frequencies. Credit: LIGO/Virgo/Lovelace, Brown, Macleod, McIver, Nitz


Almost all of the energy from core-collapse supernovae is radiated in the form of neutrinos. This emission is key to both the explosion mechanism itself and to many of the subsequent observables, including the nucleosynthesis. Neutrinos were observed in the case of Supernova 1987a which exploded in the Large Magellanic Cloud. There are several large, underground neutrino detectors (e.g. Super-K and IceCube) are currently operational, and others are planned in the near future (e.g. DUNE), increasing our confidence that a high-statistics SN neutrino signal will eventually be observed.

Observations of supernova neutrinos will shed important light on the physics of the proto-neutron star at the center of a core-collapse supernovae. These observations also have the potential to teach us important lessons about neutrinos themselves. Many properties of neutrinos are now known, including the mass-squared mass differences and vacuum mixing angles. But, the detection of neutrinos from a Galactic core-collapse supernovae (or neutron star merger) could reveal the neutrino mass hierarchy, their Dirac or Majorana nature, and the possibility of sterile states.

Electromagnetic Radiation

By far the most commonly observed signals of stellar explosions are electromagnetic. Wide-field surveys are currently discovering multiple SNe every day, many of which are followed up with panchromatic photometry and spectroscopy. Future surveys such as the Large Synoptic Survey Telescope (LSST) are expected to discover hundreds of thousands of SNe and other explosive transients.

Photons are primarily radiated after the explosion itself, as ejected matter emits thermally in ultraviolet, visible and infrared bands with a luminosity visible for months to years. In addition to the thermal energy deposited by the explosion itself, the supernova light curve can be powered by the decay of radioactive isotopes synthesized in the explosion, and, in some cases, by long-term activity of the central remnant (e.g., a rotating, magnetized neutron star or accreting black hole). Non-thermal radiation from radioactive decay can be detected in x-ray/gamma-ray bands for very nearby events and Galactic SN remnants. The panchromatic light curves and spectra provide a wealth of information as to the mass, kinetic energy, radioactive content, and elemental composition of the material ejected in the explosion.

Simulating the electromagnetic emission from stellar explosion models requires 3-D time-dependent multi-wavelength Boltzmann transport, coupled to the thermodynamics of the expanding, radioactively heated ejecta. Because the timescale for photon diffusion is typically much longer than that of the explosion itself, the problem can usually be treated in post-processing. The initial conditions of the photon transport calculation are the density, velocity, temperature, and composition of the ejecta once hydrodynamical effects have abated. The dynamical evolution thereafter is simple free-expansion, with the gas temperature and ionization state set by a strong matter-radiation coupling that must be calculated implicitly.