Surveying the X-ray Behavior of Novae as They Emit $\gamma$-rays

TL;DR

This survey analyzes X-ray and gamma-ray emissions from novae, revealing that hot plasma X-ray evidence typically appears after gamma-ray emission fades, with embedded novae showing concurrent X-ray and gamma-ray signals.

Contribution

It provides the first systematic comparison of X-ray and gamma-ray emissions in novae, highlighting differences based on companion star types and shock environments.

Findings

Most novae show delayed X-ray emission after gamma-ray fade

Embedded novae with giant companions show concurrent X-ray and gamma-ray emission

Lower density environments in novae with giant companions facilitate external shocks

Abstract

The detection of GeV $\gamma$-ray emission from Galactic novae by $Fermi$-LAT has become routine since 2010, and is generally associated with shocks internal to the nova ejecta. These shocks are also expected to heat plasma to $\sim 10^7$ K, resulting in detectable X-ray emission. In this paper, we investigate 13 $\gamma$-ray emitting novae observed with the Neil Gehrels $Swift$ Observatory, searching for 1-10 keV X-ray emission concurrent with $\gamma$-ray detections. We also analyze $\gamma$-ray observations of novae V407 Lup (2016) and V357 Mus (2018). We find that most novae do eventually show X-ray evidence of hot shocked plasma, but not until the $\gamma$-rays have faded below detectability. We suggest that the delayed rise of the X-ray emission is due to large absorbing columns and/or X-ray suppression by corrugated shock fronts. The only nova in our sample with a concurrent X-ray/$\gamma$-ray detection is also the only embedded nova (V407 Cyg). This exception supports a scenario where novae with giant companions produce shocks with external circumbinary material and are characterized by lower density environments, in comparison with novae with dwarf companions where shocks occur internal to the dense ejecta.