Steven Harris

Research Interests

Neutron star mergers, supernovae, magnetars
Transport, bulk viscosity, cooling, Urca process, axions, pions

In Cairns, Australia for the 2024 Confinement conference.

About Steven Harris

I did my PhD research on transport in neutron star mergers with Prof. Mark Alford at Washington University in St. Louis, graduating in 2020. I moved to the Institute for Nuclear Theory (INT) at the University of Washington to do my first postdoc from 2020-2023. As part of my current NP3M fellow position, I am based at Indiana University and later plan to spend time at Iowa State University.

My research uses neutron star mergers and supernovae to better understand the nature of dense matter, as well as to investigate possible physics beyond the standard model. Matter in neutron star mergers and supernovae is incredibly hot and dense, probing the interior of the QCD phase diagram. Between them, mergers and supernovae produce gravitational waves, electromagnetic signals, and neutrinos that can be measured here on Earth and used to understand the internal dynamics of these events. In paricular, transport properties like thermal conductivity and viscosity may play important roles. Because of their high temperatures and their multimessenger signals, mergers and supernovae also present an intriguing environment in which physics beyond the standard model may be explored.

Research projects

1. Bulk viscosity in neutron star mergers

When the dense matter in neutron stars is compressed, it is pushed out of beta equilibrium. Urca processes help to restore equilibrium, leading to bulk viscous energy dissipation. We have calculated the bulk viscosity of neutron-proton-electron matter in neutron star merger conditions, showing that it can damp density oscillations in tens of milliseconds. We have extended this calculation to matter including a population of thermal pions. Also, I have worked with merger simulators to include the Urca process in neutron star merger simulations to determine the impact of bulk viscosity on neutron star mergers.

As two neutron stars merge, their matter gets heated up to several tens of MeV. If particles like keV- or MeV-scale axions exist, they would be copiously produced during the merger. Their emission could preemptively cool the merger remnant and if they escape the system, they could decay into photons, generating a long-lasting electromagnetic signal. We calculated the nature of this axion-induced gamma-ray signal and put constraints on axions from the Fermi-LAT observations of GW170817.