Thomas John Nelson

Non-local Thermal Equilibrium Model Atmospheres for the Hottest White Dwarfs: Spectral Analysis of the Compact Component in Nova V4743 Sgr

Half a year after its outburst in September 2002, nova V4743Sgr evolved into the brightest supersoft X-ray source in the sky with a flux maximum around 30u We calculated grids of synthetic energy distributions (SEDs) based on NLTE model atmospheres for the analysis of the hottest white dwarfs and present the result of fits to Chandra and XMM-Newton grating X-ray spectra of V4743Sgr of outstanding quality, exhibiting prominent resonance lines of C V, C VI, N VI, N VII, and O VII in absorption. The nova reached its highest effective temperature (Teff =740 ± 70kK) around April 2003 and remained at that temperature at least until September 2003. We conclude that the white dwarf is massive, ≈ 1.1 − 1.2M⊙. The nuclear-burning phase lasted for 2 to 2.5 years after the outburst, probably the average duration for a classical nova. The photosphere of V4743Sgr was strongly carbon deficient (≈ 0.01 times solar) and enriched in nitrogen and oxygen (> 5 times solar). Especially the very low C/N ratio indicates that the material at the white dwarf’s surface underwent thermonuclear burning. Thus, this nova retained some of the accreted material and did not eject all of it in outburst. From March to September 2003, the nitrogen abundance is strongly decreasing, probably new material is already been accreted at this stage.

A CENSUS OF THE SUPERSOFT X-RAY SOURCES IN M31

We examined X-ray, ultraviolet, and optical archival data of 89 supersoft X-ray sources (SSS) in M31. We studied the timescales of X-ray variability and searched UV and optical counterparts. Almost a third of the SSS are known classical or recurrent novae, and at least half of the others exhibit the same temporal behavior as post-outburst novae. Non-stellar objects among SSS seem to be rare: less than 10% of the classified SSS turned out to be supernova remnants, and only one source has been identified with an active galactic nucleus in the background. Not more than 20% of the SSS that are not coincident with observed novae are persistent or recurrent X-ray sources. A few of these long-lasting sources show characteristics in common with other SSS identified as white dwarf (WD) close binaries in the Magellanic Clouds and in the Galaxy, including variability on timescales of minutes, possibly indicating the spin period of a WD. Such objects are likely to be low-mass X-ray binaries with a massive WD. A third of the non-nova SSS are in regions of recent star formation, often at the position of an O or B star, and we suggest that they may be high-mass X-ray binaries. If these sources host a massive hydrogen-burning WD, as it seems likely, they may become Type Ia supernovae (SNe Ia), constituting the star formation dependent component of the SNe Ia rate.

SSS in young stellar populations and the “prompt” component of Type Ia supernovae

We present the results of a search for UV and optical counterparts of the SSS population in M 31. We find that out of the 56 sources we included in our search, 16 are associated with regions of ongoing or recent star formation. We discuss two particularly interesting sources that are identified optically as early type stars, one of which displayed long term X-ray evolution similar to that observed in classical novae. We discuss the physical origin of supersoft X-rays in these and the other SSS in young regions, and their possible link to the so-called “prompt” component of the Type Ia supernova population (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)