Jan-Uwe Ness: Projects at Arizona State University

X-ray observations of Classical Novae

Classical Nova explosions occur in binary systems (=two stars) where one of the two components is a normal star like our Sun and the second star is a very compact star, a so-called White Dwarf. A White Dwarf has about the size of the Earth (a radius of 6000km, which is 3700 miles), but is as massive as the Sun (which is about 300,000 times as massive as Earth). The very compact White Dwarf attracts material from the normal star (= Main Sequence star) and this material spirals onto the surface of the White Dwarf. This process is called accretion. Because of the extreme conditions on the surface of the White Dwarf the material gathered with time will eventually start to react in nuclear reactions. The main process is hydrogen burning, a process where hydrogen nuclei are effectively combined into He nuclei with the release of vast amounts of energy. This is an explosive event similar to a hydrogen bomb. The energy is released in the form of highly energetic radiation, i.e., gamma and X-ray emission. This explosion is very energetic, but not energetic enough to tear the White Dwarf apart. Instead only the outer layers of the White Dwarf are ejected into space. In the very early phase of the explosion the energetic radiation cannot pass through the ejected material, because it is too thick and not transparent to radiation. After a period of several months the ejected material forms an expanding shell, which becomes thinner and allows the radiation to pass through and we on Earth can measure this radiation.

A few technical terms

The thermonuclear reaction can only take place at extremely high temperatures (14 Million degrees Kelvin) and the radiation produced will be X-ray emission. X-ray emission is a form of light, only much more energetic. Light is an electromagnetic wave and can be described in terms of waves (with a wavelength) or in terms of photons (particles with no mass but with energy). The visible light will apear blue to the human eye when the wavelengths are short or, when described in terms of photons, when the energy of each photon is high. The number of photons determines the intensity of light, while the energy of each individual photon determines the color of the radiation. Long wavelengths (or low energies) will apear red. X-rays are much more energetic than blue light. The term count rate refers to a measurement device which counts the number of events, usually directly proportional to the number of photons emitted by the source. The higher a count rate the brighter the source will be.

The energetic Nova explosions are ideal candidates to be observed with the powerfull X-ray satellite telescopes Chandra and XMM-Newton. These missions have devices which allow the compilation of X-ray spectra, thus sort out how many photons we have at each X-ray energy. Spectra are not regarded very spectacular in general, because they are abstract constructions, but when trying to understand physical processes, the interpretation of spectra is an absolute must.

V4743 Sagittarius (2002)

Exciting observations of the Classical Nova outburst of V4743 Sgr (2002) have been carried out with the Chandra X-ray satellite. The first observation after the energetic radiation can pass through the expanding shell was taken in March 2003. The development of light is shown in the two figures. The most exciting event of the observation of this nova is the dramatic decay in intensity during the observation. The movie in the left figure below shows this quite well. In the middle the development of the spectrum is shown following the evolution of brightness.
click to view again

Chandra observation of Nova V4743 Sgr with evolution of brightness and spectrum
(Note the different scale by the end of the evolution)

Chandra lightcurve of Nova V4743 Sgr

In the left is the X-ray image of the Nova, and the light does not appear to shine regularly but to flicker. After a while of observation (about 5 hours) the Nova suddenly becomes fainter and fainter until it almost disappears. This is a most unusual event and nothing like this has ever been seen. The count rate, which represents the brightness of a source, is shown in the right. It can be seen that the nova is periodically changing the brightness and then drops all the way to essentially zero.

Spectrum during bright phase

Spectrum during faint phase

In the top right figure I marked the first part of the observation with blue and the last phase with green. The photons collected during these times were used to compile two spectra (graphical representations of the energy distribution). A spectrum tells us how much light is gathered at any energy. The spectrum obtained from the bright phase of the observation is entirely different from the spectrum obtained from the faint phase. The bright spectrum has a broad intensity feature around 30Å. This feature represents continuum emission produced from the high temperature of the nuclear burning. The location of the maximum is a direct tracer for the temperature. If the material was cooler we would see the maximum at longer wavelengths, while hotter material would emit at shorter wavelengths. But the continuum emission is not smooth and shows depressions of brightness at very specific wavelengths. These dips are caused by atomic transitions triggered by the light provided from the continuum. The atoms and ions between the nuclear burning zone and us observers (i.e, on the line of sight) are capable of selectively absorbing light and this process produces these dips, which are called absorption lines. The presence of these lines allows us to tell which atoms are there and therefore, what the composition of the material is. The faint spectrum shows no continuum, but narrow peaks at certain wavelengths. These peaks are also produced by atomic transitions. In this case the atoms and ions absorb kinetic energy from collisions and dissipate the absorbed energy in the form of radiation. These are also transitions in the atoms and ions and are therefore again selective in wavelength (or energy) and emitt only at very specific energies; these peaks are called emission lines. The emission lines allow us to determine the composition of the material.

The Nova V4743 Sgr has oscillations in X-ray intensity and experienced a most unusual decay into an extremely faint phase. The reason for this event is that the nuclear burning that takes place on the surface of the White Dwarf has somehow turned off. We know this from the analysis of the spectra, where the spectrum during the bright phase shows the nuclear burning in the form of continuum emission, while the spectrum during the faint phase lacks this continuum emission. The analysis of the composition of the material is possible, but rather complicated, so I do not explain this here. The scientific results have been published in scientific papers (Ness et al. 2003, ApJL 594, 127 and Petz et al. 2005, A&A, in press), and more detailed papers are in preparation.

Nova V382 Velorum (1999)

Another interesting example is V382 Velorum (1999), which was observed in X-rays with a CCD detector aboard Chandra (ACIS) while it was still actively burning and another time with a high-resolution grating spectrometer after nuclear burning has ended. Although the spectra have quite different resolution it is clear that during nuclear burning the spectrum was dominated by strong continuum emission and after nuclear burning has switched off, only emission lines remained. The continuum spectrum is probably a blackbody spectrum and has likely also absorption lines which cannot be seen because the spectral resolution of CCD detectors are not sufficient to do that.
A paper on this has been accepted by MNRAS and is available under  http://www.hs.uni-hamburg.de/DE/Ins/Per/Ness/Pubs/v382.pdf

Spectrum during bright phase

Spectrum after nuclear burning ended