Max-Planck-Institut für extraterrestrische Physik

- Background Picture -

The background picture used in the MPE home page is originally the poster used to advertise the Seventeenth Texas Symposium on Relativistic Astrophysics and Cosmology which was held at Munich December 11-17, 1994. This symposium was organized by members of the MPE.

This picture shows the (X-ray) sky around midnight at the date of the Symposium above Munich, as seen with X-ray sensitive eyes.

The simulation of this picture, which looks at the first glance like an optical picture, was made possible through the use of the ROSAT all-sky survey data.

At the bottom the skyline of Munich and the alps are indicated.

X-ray Finding Chart , X-ray Picture , Optical Picture

The ROSAT X-Ray Sky Around Orion

(A detailed description how the above pictures were generated)

From August 1990 to February 1991 the German X-ray satellite ROSAT performed the first all-sky survey in soft X-rays with an imaging telescope. We have synthesized the point sources found with a preliminary automatic source detection analysis of these data into a colour image which shows the X-ray sky around the Orion region (5228 objects in an area of approximately 50 deg x 75 deg) in a similar way as we perceive the optical sky.

The colour information was obtained in the following way: for each detected source, the background-subtracted count rates were determined separately in the energy bands 0.1-0.4 keV (`soft') and 0.4-2.4 keV (`hard'), and then normalized into a `hardness ratio' by calculating the quantity

hr_1 = (hard-soft) / (hard+soft), with -1 <= hr_1 <= +1

This quantity, which is specific to the characteristics of the ROSAT PSPC, provides first valuable information on the major properties of the observed X-ray spectra and has some resemblence with the spectral code used in optical stellar astronomy. Therefore, we have adopted a similar colour coding and mapped the [-1,+1] hardness ratio interval onto a color bar extending from orange (soft) over white to blue (hard). To preserve the color information for the whole range of intensities observed, we introduced a synthetical point spread function and distributed some of the intensity into a halo around each source, thus mimicing a photograph taken with a soft-focusing lens.

For this image we have selected that part of the sky which culminated at midnight over Munich during the time of the TEXAS symposium. This is the region around Orion, probably the most beautiful and well-known area of the sky visible from mid-northern latitudes. According to the visual impression, we displayed it in a projection which compresses the areas close to the zenith.

To take the presence of a well-known component of the sky into account, we also included the Moon into the images. Since it is principally not possible to combine the intensities of point-like and extended sources in an unique way, we adjusted the surface brightness of the Moon in the X-ray image so that its shape would be clearly visible. For the X-ray image of the Moon we used that of an actual ROSAT PSPC observation. We scaled this image appropriately and handled it in the same way as the point sources. The position of the Moon (but not its phase) corresponds to December 17, 1994, 04 UT.

Konrad Dennerl and Wolfgang Voges
Max-Planck-Institut für
extraterrestrische Physik

last update: 2002-01-18
editor: Helmut Steinle   (email: