Transient Extremely Soft X-ray Emission from the Unusually Bright CV in the Globular Cluster M3: a New CV X-ray Luminosity Record?

TL;DR

This study reports on the highly variable X-ray emission of the white dwarf 1E1339 in M3, including a record high luminosity for a CV, and suggests a possible shift from supersoft to far-UV emission.

Contribution

First detailed X-ray monitoring of 1E1339 revealing extreme variability and record luminosity, indicating complex emission state changes in a globular cluster CV.

Findings

1E1339 reached the highest recorded hard X-ray luminosity for a white dwarf CV.

The source exhibited significant variability in X-ray luminosity over time.

Potential shift from supersoft X-ray to far-UV emission observed.

Abstract

We observed the accreting white dwarf 1E1339.8+2837 (1E1339) in the globular cluster M3 in Nov. 2003, May 2004 and Jan. 2005, using the Chandra ACIS-S detector. The source was observed in 1992 to possess traits of a supersoft X-ray source (SSS), with a 0.1-2.4 keV luminosity as large as 2x10^{35} erg/s, after which time the source's luminosity fell by roughly two orders of magnitude, adopting a hard X-ray spectrum more typical of CVs. Our observations confirm 1E1339's hard CV-like spectrum, with photon index Gamma=1.3+-0.2. We found 1E1339 to be highly variable, with a 0.5-10 keV luminosity ranging from 1.4+-0.3x10^{34} erg/s to 8.5+4.9-4.6x10^{32} erg/s, with 1E1339's maximum luminosity being perhaps the highest yet recorded for hard X-ray emission onto a white dwarf. In Jan. 2005, 1E1339 displayed substantial low-energy emission below 0.3 keV. Although current Chandra responses cannot properly model this emission, its bolometric luminosity appears comparable to or greater than that of the hard spectral component. This raises the possibility that the supersoft X-ray emission seen from 1E1339 in 1992 may have shifted to the far-UV.