Conference on Shocks and Particle Acceleration in Novae and Supernovae

On June 23 and 24, 2016, about 60 international researchers working in the fields of novae and supernovae met at the Simons Foundation in New York City to discuss the physics of shocks and particle acceleration in these transient astrophysical events. The fields of novae and supernovae broke off from one another when they were first identified as physically distinct events in the 1930s, by Baade and Zwicky. However, the growing appreciation—based on an ever-expanding array of multi-wavelength observations—that shocks play a crucial role in the appearance of both of these systems motivated us to unite these fields once again. The workshop was comprised of pedagogical overview talks meant to bridge the knowledge gap between these fields, interspersed with long open discussion periods aimed at addressing particular aspects of each phenomena.

The workshop began with an overview by Tommy Nelson (University of Pittsburgh) of the ways that shocks manifest in novae across the electromagnetic spectrum, from radio to gamma rays. Teddy Cheung (NASA Goddard) provided an overview of the unexpected recent discovery by NASA’s Fermi telescope that novae produce luminous ~ GeV gamma-ray emission. This emission is believed to arise either from the Compton scattering of ambient optical photons from electrons accelerated to ultra-relativistic velocities by the nova, or to the decay of π0 produced by collisions between shock-accelerated relativistic protons. Bill Wolf (University of California, Santa Barbara) described the physics of thermonuclear runaway on the surface of accreting white dwarfs, the engine behind novae. Brian Metzger (Columbia University) provoked the audience by suggesting that most nova optical emission are powered indirectly by shocks, contrary to standard theory.

The Thursday afternoon session addressed the signatures of shock waves in the time-dependent emission-line spectra caused by different ejecta components. Raffaella Margutti (Northwestern University) discussed the X-ray emission from shock waves in young, unresolved supernovae, while Steve Reynolds (North Carolina State University) discussed similar emission from shocks in older galactic supernova remnants. On Friday morning, Damiano Caprioli (University of Chicago) described the first particle-in-cell plasma simulations showing the process of diffusive particle acceleration (Fermi acceleration). Our discussions identified a previously neglected discrepancy regarding the low-electron-acceleration efficiency, which is inferred from supernova remnants and PIC simulations, versus the much higher efficiencies inferred from unresolved supernovae. Future methods for analyzing the supernova data were agreed upon to better quantify this discrepancy and its cause.

Overall, the workshop was considered a great success. An informal survey indicated that, for the vast majority of participants, this was the first time they had attended a conference combining novae and supernovae, which effectively brought researchers in each field up-to-speed on the other. Raffaella Margutti commented, “It is definitely one of the conferences where I learned the most.” Steve Reynolds commented, “[The conference] was a real eye-opener.” Each moment of the lengthy discussion sessions was filled with dialogue, revealing many hitherto unrecognized areas of overlap between these research fields.