XMM-Newton Trainee Project 2004

Gabriele Schönherr

I was born in July 1979 in Düsseldorf, Germany. After four years of undergraduate studies of physics and astronomy at the University of Bonn, Germany, including a half year stay at the Universidad Autónoma de Madrid, Spain, I worked during one year on my diploma thesis at the Research Center Jülich in the field of theoretical statistical physics. After my graduation in February 2004, I returned to Madrid for a 5 months traineeship at ESAC. In January 2005 I started working on my PhD project at the Institute of Astronomy and Astrophysics at the University of Tübingen, Germany, in collaboration with the University of Warwick, UK, and ESAC, Spain.

I worked as a trainee at ESAC for 5 months in 2004. The main scientific objective of my project was a pulse phase resolved spectral analysis of observations of the Crab pulsar with XMM-Newton. Moreover, a new method would be developed into how to combine data from different camera modes in order to achieve a optimal phase and space resolved spectroscopy of the Crab pulsar and nebula.

Since its discovery (Staelin & Reifenstein 1968, Science 162, 1481), the Crab is one of the best studied objects and one of the most luminous sources at the X-ray sky. It consists of a compact object, the so-called Crab pulsar, and a region of nebulosity, the Crab nebula. The pulsar, a spinning neutron star with a period of only 33ms, has been observed over nearly all energy bands. Its pulse profile is characterized by a double-peaked structure with a phase difference of 0.4 between the first and second pulse.



Fig. 1: X-ray image of the Crab as seen with EPIC-pn in Small Window mode.


The X-ray Crab has been observed several times with the European Photon Imaging Camera (EPIC) of XMM-Newton. EPIC consists of three single cameras: MOS1, MOS2 and pn, which are denominated according to the semiconductor techniques employed. The high time resolution of EPIC-pn in its fastest readout mode, the 'Burst' mode, in combination with its large effective area and the high quantum efficiency of the pn-CCD, allows a high quality phase resolved spectroscopy over an energy range of 0.6-12keV. However, the fast readout procedure in the Burst mode leads to a loss of spatial information in readout direction. Crab data with two-dimensional spatial information, collected in a different EPIC-pn mode ('Small Window' mode) could not be used, because it suffers from serious pile-up problems, which leads to an underestimation of the flux and a change of spectral characteristics (pile-up may happen in the case of very bright sources: several photons which hit the CCD within the same time frame are interpreted as a single photon event).



Fig. 2: pulse profile of the Crab pulsar (over two phases) in blue (Burst mode data);
variations of the photon index with the phase of pulsation depicted in red


I combined several XMM-Newton observations in EPIC-pn Burst and Small Window mode in order to achieve a two dimensional spatial and a high quality phase resolved spectroscopy of the Crab nebula and pulsar. All data were processed with the SAS (Software Analysis System) version 6.0.0. Moreover, I assessed the pile-up level of the Small Window data by different approaches in order to estimate the effects on the spectral parameters. A spatially resolved analysis of the Crab spectrum was obtained with both types of data sets (one-dimensional results from Burst mode data and two-dimensional analysis of SW mode data). The Small Window results have yet to be corrected with appropriate pile-up simulations of the Crab spectrum but already give a good qualitative impression of the spectral variations. The typical Crab spectrum was fitted with an absorbed power law model. The varying slope of the power law becomes manifest in the variations of the photon index as shown in figure 3.



Fig. 3: Spatial variations of the photon index (colour coded) along part of the CCD;
flux is overplotted as contours (Small Window data)