We have described a procedure developed to time-screen X-ray observations conducted with the ROSAT PSPC and HRI detectors which can be easily generalized to any photon counting detector with negligible internal background. The aim of this procedure is to maximize the SNR of faint sources in the image, introducing a trade-off between cutting high background time intervals and keeping the observation interval the longest possible.
Test have been conducted in order to optimize the screening algorithm. It turns out that a convenient choice for the PSPC is to tailor the algorithm to the faintest detectable sources in the field at 30' off-axis angle.
We have compared results of analysis on a PSPC moderately crowded field (pointed towards the Hyades open cluster) with no time-screening and time-screening applied according to our procedure. We have verified that a significant increment in the number of detected sources is obtained, as well as a gain in terms of the SNRs of detected sources.
While the finding of new true sources is the most important result of our screening procedure, the systematic increase of detected sources SNR is also important. In fact, since the detection algorithms operate at a fixed SNR, to which correspond a given number of expected spurious sources, the observed gain yields a decrease of contamination by fake sources, which is important especially in large area surveys.
Even though optimized for point sources, the screening procedure
described here is applicable to automatic pipeline procedure, such as
the one described in Mackie et al. (1996), since it does not
fail in case of extended sources, and in particular
when extended sources occupies a large fraction of the FOV. In fact, in
these cases, our algorithm tend to reject a very small fraction of the
total exposure time (
%) with respect to the case of sequences
with point sources, because the extended source increases the background
level and makes the SNR ratio almost insensitive to the presence of
contaminating spikes in the light curve.
Acknowledgements
The authors wish to thank the suggestion and comments from S. Serio. This work was partially supported by Ministero della Università e delle Ricerca Scientifica e Tecnologica, GNA-CNR and a contract of Italian Space Agency (ASI).