Upper Limit Server


Version 0.5: August 7 2019

This is a new implementation of a flux and upper limit server for XMM-Newton and other X-ray astronomy missions.

Purpose and Scope

This tool is designed to return the count rate or upper limit to a count rate and equivalent flux for a given celestial position for XMM-Newton pointed and slew data as well as for other X-ray astronomy missions. It is designed to give the best possible answer and looks for catalogue entries first before calculating upper limits if necessary

How It Works

The system consists of a set of servers, one for each mission. For a given sky position these perform the following tasks:

  • Find the observations which contain that sky position
  • Look in a source catalogue to see if a source has been detected at that position in that observation. If it has then return the catalogued count rate
  • If not, and images are available, then analyse the image to find the upper limit to the count rate at that position, using the Bayesian statistics of Kraft, Burrows and Nousek 1991 (ApJ 374, 344), with a uniform prior. The significance (1,2 or 3-sigma) is user selectable with the default set at 2-sigma (95.4%).
    NB: The count rates are corrected for instrumental effects such as the point spread function and vignetting.
  • Convert the count rate to a flux using a spectral model

A full description of the calculations for each mission is given in uls_missions_full_description.pdf

Catalogue usage

The servers access catalogues from:

How to use it

Usage help is available by pressing the orange question mark. The basic scheme is:

  • Select missions to use
  • Enter either a target name or the RA, DEC of the required sky position. Alternatively enter a list of coordinates from an ASCII file.


The output is a block of results per position, broken down into missions.

Help for the output page can be found by pressing the orange question mark. At the bottom of the page options exists for outputting the results to a text file, .csv file or a latex table. In addition the fluxes may be plotted as a light curve.

Terminal client

As an alternative, the flux and upper limit servers may be queried using a terminal client which runs under Python 2.7.

Change Log

  • VERSION 0.5
    • First release

Future Plans

  • Allow the spectral model for flux conversion to be fully user selectable