High-resolution spectroscopy and high-density monitoring in X-rays of Novae

TL;DR

Recent X-ray observatories have provided detailed insights into nova outbursts, revealing complex spectral behaviors during different evolutionary stages, but current models are limited in fully explaining observed spectra.

Contribution

This paper reviews high-resolution X-ray observations of novae, highlighting the capabilities of new observatories and the challenges in modeling complex spectra during the SSS phase.

Findings

Swift observations increased nova sample size

Pre- and post-SSS spectra modeled with plasma models

Complex SSS spectra remain difficult to model accurately

Abstract

The 21st century X-ray observatories XMM-Newton, Chandra, and Swift gave us completely new insights into the X-ray behaviour of nova outbursts. These new-generation X-ray observatories provide particularly high spectral resolution and high density in monitoring campaigns, simultaneously in X-rays and UV/optical. The entire evolution of several nova outbursts has been observed with the Swift XRT and UVOT instruments, allowing studies of the gradual shift of the peak of the SED from UV to X-rays, time scales to the onset and duration of the X-ray brightest supersoft source (SSS) phase, and pre- and post-SSS X-ray emission. In addition, XMM-Newton and Chandra observations can efficiently be scheduled, allowing deeper studies of strategically chosen evolutionary stages. Before Swift joined in 2005, Chandra and XMM-Newton blind shots in search of SSS emission unavoidably led to some underexposed observations taken before and/or after the SSS phase. More systematic Swift studies reduced this number while increasing the number of novae. Pre- and post-SSS spectra at low and high spectral resolution were successfully modelled with collisional plasma models. Pre-SSS emission arises in shocks and post-SSS emission in radiatively cooling thin ejecta. In contrast, the grating spectra taken during the SSS phase are a lot more complex than expected and have not yet been successfully modeled. Available hot WD radiation transport models give only approximate reproduction of the observations, and make some critical assumptions that are only valid in isolated WDs. More grating spectra which would be important to search for systematic trends between SSS spectra and system parameters.