Variable Hard X-ray Emission From the Central Star of the Eskimo Nebula

TL;DR

The central star of NGC 2392 exhibits hard X-ray emission likely caused by accretion processes involving a companion, with short-term variability suggesting complex interactions, but not due to coronal activity or wind shocks.

Contribution

This study provides detailed spectral and temporal analysis of X-ray emission from the CSPN of NGC 2392, highlighting accretion as the probable origin and ruling out other mechanisms.

Findings

X-ray emission characterized by a plasma temperature of ~26 MK.

No long-term variability detected in X-ray emission.

Short-term variability with a period of ~0.26 days observed.

Abstract

The central star of NGC 2392 shows the hardest X-ray emission among central stars of planetary nebulae (CSPNe). The recent discovery of a spectroscopic companion with an orbital period of 1.9 days could provide an explanation for its hard X-ray emission, as well as for the collimation of its fast outflow. Here we analyse the available Chandra and XMM-Newton X-ray observations to determine accurately the spectral and temporal variation properties of the CSPN of NGC 2392. The X-ray emission can be described by an absorbed thermal plasma model with temperature 26$^{+8}_{-5}$ MK and X-ray luminosity (8.7$\pm$1.0)$\times$10$^{30}$ erg s$^{-1}$. No long-term variability is detected in the X-ray emission level, but the Chandra light curve is suggestive of short-term variations with a period $\sim$0.26 days. The possible origins of this X-ray emission are discussed. X-ray emission from the coronal activity of a companion or shocks in the stellar wind can be ruled out. Accretion of material from an unseen main-sequence companion onto the CSPN or from the CSPN wind onto a white dwarf companion are the most plausible origins for its hard X-ray emission, although the mismatch between the rotational period of the CSPN and the modulation time-scale of the X-ray emission seems to preclude the former possibility.