High quantum efficiency over a broad spectral range is one of the main properties of the EPIC pn camera on-board XMM-Newton. The quantum efficiency rises from ~75% at 0.2 keV to ~100% at 1 keV, stays close to 100% until 8 keV, and is still ~90% at 10 keV. The EPIC pn camera is attached to an X-ray telescope which has the highest collecting area currently available, in particular at low energies (more than 1400 cm2 between 0.1 and 2.0 keV). Thus, this instrument is very sensitive to the low-energy X-ray emission. However, X-ray data at energies below ~0.2 keV are considerably affected by detector effects, which become more and more important towards the lowest transmitted energies. In addition to that, pixels which have received incorrect offsets during the calculation of the offset map at the beginning of each observation, show up as bright patches in low-energy images. Here we describe a method which is not only capable of suppressing the contaminations found at low energies, but which also improves the data quality throughout the whole EPIC pn spectral range. This method is then applied to data from the Vela supernova remnant.
The Cern Axion Solar Telescope - CAST - uses a prototype 9 Tesla LHC superconducting dipole magnet to search for a hypothetical pseudoscalar particle, the axion, which was proposed by theory in the 1980s to solve the strong CP problem and which could be a dark matter candidate. In CAST a strong magnetic field is used to convert the solar axions to detectable photons via inverse Primakoff effect. The resulting X-rays are thermally distributed in the energy range of 1-7 keV and can be observed with conventional X-ray detectors. The most sensitive detector system of CAST is a pn-CCD detector originally developed for XMM-Newton combined with a Wolter I type X-ray mirror system. The combination of a focusing X-ray optics and a state of the art pn-CCD detector which combines high quantum efficiency, good spacial and energy resolution, and low background improves the sensitivity of the CAST experiment such that for the first time the axion photon coupling constant can be probed beyond the best astrophysical constraints. In this paper we report on the performance and status of the X-ray telescope and pn-CCD detector of CAST.
DUO and ROSITA are two future X-ray astronomy missions observing in the energy band from about 0.3 keV to 10 keV. While the NASA satellite DUO will scan selected areas of the X-ray sky with high sensitivity, the German ROSITA mission shall perform an all-sky survey. Both missions apply an array of seven Wolter telescopes with separated field of views and seven dedicated PN-CCD focal plane detectors. The focal plane detectors are a further development of the flight-proven PN-CCD applied for the XMM-Newton observatory. The advanced device, called 'frame store PN-CCD', is designed and fabricated in the semiconductor laboratory of the Max-Planck-Institute for extraterrestrial physics. An introduction into the detector concept and design are presented as well as the promising results which have been achieved with the prototype devices. This includes an overview about the performance of the PN-CCD and in detail the recent measurements with the detector. An example is the low energy response of the optimized photon entrance window with integrated optical light filter. As the CAMEX analog signal processor chip is a main component of the detector module, we describe its development status. Furthermore, we report about the application of the mesh experiment to the PN-CCD which allows for a study of the electric potential characteristics in the detector bulk, in particular in the charge transfer depth. The information is of great importance for an accurate knowledge about the drift of the generated signal electrons into the potential wells of the pixels.
The fully depleted PN-CCD detector is meanwhile field-tested in several experiments on ground and in space. Its application as focal plane detector aboard ESA's XMM-Newton observatory can be considered as the most impressive one. The further development of this detector type including its readout chip in the MPI semiconductor laboratory is presented here. The new device, called frame store PN-CCD, shows substantial improvement of performance, in particular concerning the energy resolution and the probability of out of time event occurrence. Moreover, the detector offers features which are of great importance for its application in space. This is, besides the radiation hardness of the CCD, the variety of feasible pixel sizes and the high frame rates in combination with the small power consumption of the detector. Because of the thin radiation entrance window and the full depletion of the chip, the detector provides a high quantum efficiency for soft X-rays as well as for optical light and the near infrared. The frame store PN-CCD detector will be applied for the proposed X-ray astronomy missions DUO and ROSITA.
The pn-CCD was developed as focal plane detector for the XMM-Newton mission and operates successfully for more than 30 months in orbit without performance degradation. In order to match the new requirements of the future ROSITA mission which will perform a broad band X-ray all-sky survey, we propose an advanced type of pn-CCD. The concept and the new features of this frame store pn-CCD as part of the imaging X-ray spectrometer of ROSITA are described. First
measurements with prototype devices show the improvement of detector performance in comparison to the pn-CCD on XMM-Newton. We suggest as optical filter for the observations of the X-ray sky, a thin aluminum layer deposited on the photon entrance window of the device.
The pn-CCD is the focal plane detector of one of the three X-ray
telescopes aboard the XMM-Newton observatory. During
revolution #156 more than 30 individual bright pixels lightened
up out of approximately 150,000 pixels of the 6 cm × 6 cm
large detector area. The amount of leakage current generated
in the pixels cannot be explained by single heavy ions impact,
however. We suggest that a micrometeoroid scattered off the
mirror surface under grazing incidence reached the focal plane
detector and produced the bright pixels. This proposal was
studied by us experimentally at the Heidelberg dust accelerator.
Micron-sized iron particles were accelerated to speeds of the
order of 5 km/s impinging on the surface of an X-ray mirror
under grazing incidence. Scatter products have been found with
detectors placed behind the mirror. They have been analyzed by
various methods to characterize their properties and the effects
produced by them in the pn-CCD. Micrometeoroid damage to
semiconductor detectors in the focus of grazing incidence optics
might be of concern for future space projects with very large
collecting area and are proposed to be studied in detail.
The observation of comet C/2000 WM1 (LINEAR) with XMM-Newton is a
highlight in the field of cometary research. During 17 hours of almost
uninterrupted observations, more than one million photons were recorded with unprecedented spectral resolution. The results obtained so far clearly demonstrate that the X-ray emission is caused by charge exchange reactions between highly charged heavy ions in
the solar wind - mainly oxygen and carbon - and cometary gas. Due to the high number of detected photons, an investigation of the X-ray morphology is also possible in considerable detail. The observation was also a highlight from the technological point of view, demonstrating that XMM-Newton has more capabilities than are
currently utilized. In order to follow the comet, the orientation of the satellite had to be adjusted several times. This was done by just changing the guide star offsets, leaving the X-ray instruments switched on. Compared to the usual procedure with regular slews, where the X-ray instruments are reinitialized afterwards, this new technique increased the time available for scientific observations by 34% for MOS and by 83% for PN. The capability of performing quick attitude adjustments during an observation may be of interest also for other astrophysical applications.