An upper limit server for X-ray astronomy
The success of the Virtual Observatory has made it straightforwad to get flux measurements for catalogued objects in most wavebands. Sometimes, when constructing a SED or searching for variability, it can be of great value to have an upper limit to the flux of a non-detected source. The calculation of such upper limits is currently a laborious and often manual task.
Here we present a web-based application (ULS), developed at the XMM-Newton SOC, which automatically returns the upper limit of the flux at a set of sky positions. ULS has been written to be mission-independent and to work with any counts-based image. It works by querying and retrieving images from a mission's SIAP server and calculating the count-rate upper limit at the requested position, using bayesian statistics. This is later converted to a flux using the PIMMS package. ULS was released publically in 2009 and is currently configured to access XMM-Newton slew (covering 47% of the sky) and XMM-Newton pointed data (1.5% of the sky). The next extension is planned to support the ROSAT and INTEGRAL missions, while further extensions for Chandra and earlier X-ray observatories is a goal.
We briefly discuss how this tool will benefit from the standardisation of X-ray mission calibration files and from a catalogue interrogation standard.
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