Jan-Uwe Ness, Astronomer, juness@sciops.esa.int

Main research field: X-ray observations of Classical Nova outbursts.
 Nova explosions occur in accreting cataclysmic binary systems and are powered
by explosive nuclear burning of hydrogen-rich material that has previously
been accreted by the white dwarf primary. The energetics of the explosion
lead to the ejection of material at high velocities. Since the ejected material
forms an optically thick envelope, all observations of novae are dominated by
the characteristics of the outer regions of the ejecta, which behave similar to
a stellar atmosphere. The outflow of mass decreases with time, and as a
consequence the opacity of the envelope changes. This allows successively
deeper views into the outflow where it is hotter. The peak of the observable
spectrum then changes, starting in the optical and then moving to successively
shorter wavelengths (=higher energies). After a few weeks to months (sometimes
years), the envelope becomes bright in X-rays with a spectrum of an X-ray
atmosphere. During this phase, the X-ray spectrum of the nova resembles those
of the class of Super Soft X-ray Binary Sources (SSS).

My work involves XMM-Newton, Chandra, and Swift X-ray and UV spectroscopy and
monitoring observations of novae during the SSS phase. The X-ray spectra can
be fitted with Blackbody curves, but they are a lot more complex. The
high-resolution spectra contain deep absorption lines that are broadened and
oftentimes significantly blue-shifted. The Swift monitoring light curves show
extremely high degrees of variability, especially during the early SSS phase.
The UV and X-ray brightness should be anticorrelated, but this is not always
observed.

Keywords
- Classical Nova Outbursts
- Super Soft X-ray Binary Sources
- Single-degenerate Supernova Ia progenitors
- High-Resolution X-ray spectroscopy: Absorption line spectra

Ongoing collaborations (outside ESAC)
- University of Hamburg (Germany: P. Hauschildt, D. vanRossum)
- Harvard University (CfA, USA: J.J. Drake)
- University of Leicester (UK: J. Osborne)
- Liverpool John Moores University (UK: M. Bode)
- Arizona State University (USA: S. Starrfield)
- West Chester University (USA: G. Schwarz)
- Japan Aerospace Exploration Agency (JAXA: M. Tsujimoto)

External Supervision of Students/Postdocs:
- University of Technology in Bratislava (Slovak Republic: A. Dobrotka)
- Rikkyo University (Japan: D. Takei)
- University of Muenchen (Germany: R. Schoenrich)

Secondary Research: Stellar Activity and Stellar Coronae

 The formation and heating of the Solar Corona to 1000 times the
photospheric temperature is still an outstanding problem. One
approach is to study the coronae around other stars in order to
find systematic trends between coronal properties and stellar
parameters. High-Resolution X-ray spectra taken with XMM-Newton
RGS and Chandra LETGS/HETGS allow measurements of temperatures
densities, and elemental abundances.

Keywords
- High-resolution X-ray spectroscopy of Stellar Coronae: Emission line spectra
  (Temperatures, Densities, Emission Measure Distributions)
- Flare monitoring and spectral changes during flares
- Simultaneous Doppler Imaging and X-ray spectroscopy

Ongoing collaborations (outside ESAC)
- University of Hamburg (J. H.M.M. Schmitt, C. Liefke, U. Wolter)
- University of Oxford (C. Jordan)