5 MIDAS/EXSAS implementation

The X-ray group at the Astrophysical Institute Potsdam (AIP) has developed some MIDAS/EXSAS routines to correct for the ROSAT wobble effect. The routines can be obtained by anonymous ftp from ftp.aip.de at directory pub/users/rra/wobble. The correction procedure works interactively in five main steps:

• Choosing of a constant roll angle interval.
• Folding the data over the 402 s wobble period.
• Creation of images using 5 or 10 phase intervals.
• Determining the centroid for the phase resolved images.
• Shifting the photon X/Y positions in the events table.

We have tested the wobble correction procedures for 21 stars and 24 galaxies of the ROSAT Bright Survey using archival HRI data. The procedures work successfully down to an HRI source count rate of about 0.1 counts/s. In the case of lower count rates the determination of the centroid position failed because of the few photons available in the phase-binned images. The number of phase bins which can be used is of course dependent on the X-ray brightness of the source.