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About the 1SXPS Upper Limit Server

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Upper limit server.


  1. Using the upper limit server.
  2. How the server works.

Other documentation:

Using the upper limit server

The 1SXPS upper limit server calculates a 3-σ (99.7% confidence) upper limit for the count-rate of an undetected source at a given location on the sky. This uses only the data contained in the 1SXPS catalogue: observations of the location more recent than 2012 October 20 are not included when calculating the upper limit.

The simplest way to use the server is to enter an object name (which will be resolved with SIMBAD) or position (free-format) into the text box and click on search. An upper limit in the 0.3—10 keV band will then be calculated from the longest available dataset covering that location on the sky — if any is available. For further controls click on the ‘Options...’ link. A set of controls will appear which allows you to select whether to calculate an upper limit only for the deepest dataset available or for each individual dataset, and which energy bands the upper limit is calculated in. The more datasets or energy bands selected, the longer the calculation takes.

Power-law spectrumAPEC spectrum
BandObservedUnabsorbed ObservedUnabsorbed
Total (0.3—10 keV) 4.1×10-11 4.4×10-11 3.1×10-11 4.6×10-11
Soft (0.3—1 keV) 2.4×10-11 3.1×10-11 3.1×10-11 4.6×10-11
Medium (1—2 keV) 2.1×10-11 2.2×10-11 1.9×10-11 2.0×10-11
Hard (2—10 keV) 8.4×10-11 8.4×10-11 3.2×10-11 3.2×10-11

Table 1. Example counts-to-flux conversion factors, in units of erg cm-2 ct-1. The flux is in the same band as the count-rate. The two models used are a power-law with photon index of 1.7, and an APEC with a temperature of 1 keV. Both are absorbed by a column 3×1020 cm-2.

The 1SXPS catalogue covers ~5% of the sky; if no dataset in the catalogue covers the location requested, this will be reported and no upper limit can be calculated. Otherwise the requested upper limits are listed in a table. These are given in units of counts per second; Table 1 gives some counts-to-flux conversion factors for typical spectra.

As well as calculating an upper limit, this service checks the 1SXPS catalogue for objects within 20′′ of the search position: if any are found they are listed before the upper limit (the upper limit is calculated nonetheless, see below). The table of ‘bad’ detections is also searched and any matches within 20′′ are reported. Bad detections are objects that were detected during the construction of the catalogue, but with such a low significance that they were discarded from the catalogue. There are 275,503 bad detections recorded of which 80% are expected to be false; thus the probability of a bad detection lying by chance with 20′′ of a random location in the 1SXPS catalogue is ~1%.

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How the server works

To create an upper limit in a given energy band and dataset, the software takes both the relevant image and the background map which was constructed by the source detection system. This map comprises a model of the background with the PSF of all sources detected in that specific image (i.e. dataset and energy band) added to that model. A circle with a radius of 12-pixels is placed on the image at the position for which the upper limit is to be determined, and the number of events in that circle is recorded. The same circle is then placed on the background map to determine the expected number of background counts in the circle. If either of these number is greater than 100, then the upper limit is simply calculated as 3√(B) (where B is the expected background level) otherwise, the Bayesian method of Kraft, Burrows & Nousek (1991) is employed to determine the 3-σ upper limit. These are limit on the number of source counts in the image. The exposure map is then used to find the correction factor necessary to convert the limit to an on-axis equivalent value and to account for any bad columns on the detector. The upper limit is then multiplied by this factor and divided by the on-axis exposure time to produce an upper limit on the count-rate of any source at the input location.

If the location requested matches that of a 1SXPS source the calculation proceeds exactly as above. However, in this case the background map will contain the model of the source, thus the number of background counts expected will be high and hence the upper limit is also quite how (i.e. the sensitivity is low). The upper limit in this case in effect represents the limit on a second source at the location of the 1SXPS source. However this many be misleading as it does not account for confusion, or the way artefacts are eliminated: it is possible that a second source was present and above this upper limit, but could not be identified independently of the detected object. Bad detections are not included in the background map, and thus if the input position matches a bad detection, this problem is not encountered.

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