Non-LTE Model Atmosphere Analysis of the Large Magellanic Cloud Supersoft X-Ray Source CAL 83

• Published in: The Astrophysical journal Vol. 619; no. 1; pp. 517 - 526
• Main Authors: Lanz, Thierry, Telis, Gisela A, Audard, Marc, Paerels, Frits, Rasmussen, Andrew P, Hubeny, Ivan
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 20.01.2005; University of Chicago Press
• Subjects: Astronomy; Binary and multiple stars; Earth, ocean, space; Exact sciences and technology; Faint blue stars (including blue stragglers), white dwarfs, degenerate stars, nuclei of planetary nebulae; Normal stars (by class): general or individual; Stars; X-ray binaries; stars: atmospheres; X-rays: individual (CAL 83); White dwarf stars; Atmosphere model; X-rays: binaries; Binary X ray source; Cosmic x-ray sources; binaries: close; Non-LTE; white dwarfs; X-ray binary stars; Physical parameter; Close binary stars; stars: fundamental parameters; Large Magellanic Cloud; Extragalactic X ray source
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/426382

We present a non-LTE (NLTE) model atmosphere analysis of Chandra High Resolution Camera (HRC-S) and Low Energy Transmission Grating (LETG) and XMM-Newton Reflection Grating Spectrometer (RGS) spectroscopy of the prototypical supersoft source CAL 83 in the Large Magellanic Cloud. Taken with a 16 month interval, the Chandra and XMM-Newton spectra are very similar. They reveal a very rich absorption-line spectrum from the hot white dwarf photosphere but no spectral signatures of a wind. We also report a third X-ray off-state during a later Chandra observation, demonstrating the recurrent nature of CAL 83. Moreover, we found evidence of short-timescale variability in the soft X-ray spectrum. We completed the analysis of the LETG and RGS spectra of CAL 83 with new NLTE line-blanketed model atmospheres that explicitly include 74 ions of the 11 most abundant species. We successfully matched the Chandra and XMM-Newton spectra assuming a model composition with LMC metallicity. We derived the basic stellar parameters of the hot white dwarf, but the current state of atomic data in the soft X-ray domain precludes a detailed chemical analysis. We have obtained the first direct spectroscopic evidence that the white dwarf is massive (M sub(WD) 1 M sub( )). The short timescale of the X-ray off-states is consistent with a high white dwarf mass. Our analysis thus provides direct support for supersoft sources as likely progenitors of Type Ia supernovae (SNe Ia).