Novae after the gamma-ray emission phase: X-ray studies of the continuing evolution of shocks in V959 Mon
Abstract
More than a dozen novae have been detected as GeV gamma-ray sources for several weeks around the time of visible light maximum. This requires particle acceleration in powerful shocks, whose thermal particles are expected to emit X-rays. Indeed, X-rays from such shocks are observed in many novae, usually weeks or months after the optical peak, and suggests a universal picture in which initial, slow ejecta collide with later, faster, outflows. Here we present Neil Gehrels Swift, Suzaku, and Chandra data on V959 Mon (Nova Mon 2012), which was discovered first as a transient Fermi/LAT source in 2012 June, while the object was too close to the Sun for optical or X-ray observations. The optical nova was discovered about two month later, when X-ray observations started. Our data are among the most comprehensive monitoring of X-rays from novae, along with those on V392 Vel (Nova Vel 1999). In V959 Mon, we observe a clear trend of decreasing NH with time, which we model as due to the expansion of the unshocked slow ejecta ahead of the shock. Our analysis suggests that the slow ejecta did not start its secular expansion until about 30 days after the initial detection of the gamma-rays. The temperature and the emission measure of the X-ray emitting gas evolve in more subtle ways, roughly in line with what we expect in this complex situation involving the continuous addition of freshly shocked matter and relatively inefficient cooling of shocked gas. We suggest that both reverse and forward shock contributed to the observed Chandra grating spectra of V959 Mon. Finally, we discuss the implications for our understanding of the shock in the earliest days of the nova, during its gamma-ray emission phase.
- Publication:
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AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019
- Bibcode:
- 2019HEAD...1740306M