Jan-Uwe Ness

Astronomer, ESAC Faculty Sentinel (Chair), XRISM Operations Scientist, INTEGRAL Project Scientist


Main Research Fields

X-ray observations of Classical Nova in outburst; Super-Soft Sources, Stellar Activity and Stellar Coronae

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 explosion ejects material at high velocities which forms an optically thick envelope 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, shifting the peak of the observable spectrum 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).

Magnetically induced activity of cool stars

The outer atmosphere of the Sun is filled with tenuous plasma of kinetic temperatures ranging over a million degrees. During solar eclipses, straylight scattered by this plasma can be seen appearing like a crown around the sun. For this reason this plasma is called the Solar Corona. In X-rays, the corona is the prime emission source, and for other stars, any X-ray emission can thus be assumed to probe the stellar coronal plasma. During my PhD thesis, I have performed spectral analyses of X-ray emission lines in stellar coronae yielding plasma temperatures and densities from which the spatial extent can be derived assuming loop scaling laws.


  • Classical Nova Outbursts
  • Super Soft X-ray Binary Sources
  • Supernova Ia progenitors
  • High-Resolution X-ray spectroscopy
  • Monitoring and spectral changes during stellar flares
  • Simultaneous Doppler Imaging and X-ray spectroscopy
  • X-ray emission in the Solar System
  • N-Body simulations of Interacting galaxies

Ongoing collaborations

  • University of Hamburg (Germany: J. H.M.M. Schmitt, C. Schneider)
  • Slovak University of Technology in Bratislava (A. Dobrotka)
  • Harvard University (CfA, USA: J.J. Drake)
  • Massachusets Institute for Technologies (MIT, USA: M. Guenther)
  • University of Leicester (UK: J. Osborne, K.L. Page, A. Beardmore)
  • University of Oxford (UK: C. Jordan)
  • Institut de Ciencies de l'Espai (M. Hernanz)
  • Universitat Politècnica de Catalunya.BarcelonaTech (G. Sala)
  • Liverpool John Moores University (UK: M. Bode)
  • Arizona State University (USA: S. Starrfield)
  • University College London, MSSL (UK: R. Schoenrich)
  • DAA, TIFR, Mumbai (India: K.P. Singh)
  • University of Wisconsin (USA) and Padova University (IT: M. Orio)


ORCID: 0000-0003-0440-7193

ADS filters:

See also:

Project/mission at ESA

INTEGRAL Project Scientist

Chair of ESAC Science Faculty (until fall 2023)

European XRISM Science Operations Centre

ESA Athena Science Study Team

Latest results

RS Oph 2006 and 2021 outbursts

XMM-Newton RGS spectra of RS Oph, 55 days after the 2006 outburst (orange) and 54 days after the 2021 outburst (grey shadings). Given same system parameters like mass of white dwarf, plus the same epoch, the spectra should be the same, but in 2021, RS Oph was much fainter in soft X-rays. The red line is the product of the (orange) 2006 spectrum and an absorption model (parameters given in the legend). It fits so well to the (grey-shaded) 2021 spectrum that it can be seen as proven that there was simply more line-of-sight absorption in 2021 without the need to invoke processes leading to a lower luminosity.

Fitting data to data is an unconventional approach, normally a source model is multiplied with an absorption model, but in the absence of a good source model, replacing a model by the observed data is innovative allowing the identification of the reason for lower emission without necessarily understanding the nature of the source emission itself.


Spectral Time Map of V3890 Sgr

Spectral Time Map of XMM-Newton RGS spectrum of the Recurrent Nova V3890 Sgr (1962, 1990, 2019). The four panels share x/y axes as follows:

1) Top left: photon flux versus wavelength: Average spectrum (black line with blackbody approximation with red dotted line) and two spectra extracted from different time intervals (see below)

2) Top right: Colour code along photon flux axis used in:

3) Bottom left: Time (from top down) versus wavelength: Time resolved spectra, the colours correspond to photon fluxes as in top right panel. Horizontal dashed lines mark the time intervals over which the shaded spectra of same colour in the top left panel are integrated

4) Bottom right: Light curve turned around by 90 degrees with time along same axis as 3) and count rate horizontally. The shaded areas mark the time intervals over which the shaded spectra of same colour in the top left panel are integrated



Current Research

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.

As a secondary topic I am interested in 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.



Main Hobby: Church Organ

Here my YouTube Channel

Participation at 2021 Fete de la Musique, here the Playlist

Some highlight recordings:

Spontaneous improvisation: Impro I (2:26): Plenum, with flowing character

Mattheus-Final, J.S. Bach, Orchestration de Ch. Widor (1844-1937); Played on Great Organ in Magdeburg Cathedral​​​​​​

Toccata and Fuge D-minor BWV 565 (J.S. Bach)