A Mid-Infrared Search for the Outer Companion in a Millisecond Pulsar Triple System

Only two systems with pulsars and multiple companions are known, but they offer a wealth of information about dynamics, binary evolution, and the pulsars themselves. We have recently discovered a bright new millisecond pulsar in a very exotic stellar system: a hierarchical triple system with a low-mass He-core white dwarf in a relatively compact 1.6-day orbit and an as-yet-undetermined outer star in a orbit of roughly 327 days. This is a unique system with incredible long-term potential to tell us about detailed Newtonian dynamics and stellar evolution, and it can also provide high-precision pulsar and white dwarf masses. However, our lack of knowledge about the outer companion significantly limits our understanding about the evolution of this system. Evolutionary models suggest a low-mass stellar companion, but current near-infrared data rule out anything earlier than M5. Here we request modest Spitzer/IRAC observations of this system: with 1.1 hours we can obtain precise photometry in the mid-infrared and detect or constrain any cool companion, thus establishing whether the system contains a low-mass star or a white dwarf and helping us unravel its complex evolutionary history.