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2SXPS Catalogue: Upper limit server documentation

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On this page:

  1. Overview.
  2. Options and controls.
  3. Upper limit output.

Other documentation:


Overview

This tool lets you calculate 3-σ upper limits or count rates with 1-σ error bars, for any source not detected in 2SXPS, but covered by its observational footprint. When you request an upper limit, the appropriate dataset or datasets are identified (depending on your input options), and three images associated with each dataset are analysed: the original data, the background map (including the PSF model for detected sources) and the exposure map. These images are analysed over a circular region centred on the requested position, with a radius of 12 pixels (≈28″) in order to determine the total number of measured counts (C) and the expected number of background counts (B). The upper limit on the number of counts the source produced is then calculated from these values. If C or B is over 100 counts, then the limit is simply given as 3√B, otherwise the Bayesian approach of Kraft, Burrows & Nousek (1991) is used. A PSF correction factor is also created, using the exposure map, to correct for the finite size of the circle used, vignetting. and any other effects such as the dead columns on the XRT. The upper limit in counts units is then multiplied by this factor and divided by the dataset exposure time to produce the final upper limit count-rate.

The PSF correction factor includes a vignetting correction, which corrects for the fact that the exposure at the position of interest may be lower than the nominal exposure for the image (by which the counts are divided to get the count rate). For stacked images this correction can be very large, not because of vignetting in the instrument but because the degree of overlap between the observations within the stack can vary a lot and thus the exposure can change by orders of magnitude across the image. In such cases while the PSF correction factor will appear extremely high because of this, the exposure value will also be ‘inflated’ by exactly the same amount and so this large ‘vignetting’ correction has no detrimental impact on the upper limit, count rate or errors.

Important note: this facility is designed to produce upper limits for sources not in 2SXPS. Results for sources already in the catalogue may be incorrect as the background map will contain a model of that source. For 2SXPS sources which were not detected each time they were observed, the light curve on the source web page contains appropriate upper limits.

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Options and controls

By default you are presented with a single input into which you can enter either source name (which will be resolved via SIMBAD) or coordinates. Coordinates should be equatorial in epoch J2000 (FK5), the format is free-form; most common ways of entering coordinates should be supported. When you submit a form an HTML table will be returned showing the total band (0.3—10 keV) upper limit for the dataset with the most exposure at the requested location.

You can gain more control over the upper limit generation by selection the ‘Options…’ link which will display a more detailed control panel.

The first line of options allows you to choose a single upper limit, generated for the dataset with the greatest exposure at the source position (this is the default). You can instead choose to generate an upper limit from every dataset covering the source position, in effect producing a light curve, or an upper limit curve (depending on the other settings) for your object/position.

The second line lets you select which energy bands the limits are calculated for.

The third and final line of options controls how the upper limits will be reported:

Time format

This allows the user to choose how the time covered by each upper limit is reported. There are four formats available:

UTC date
A date in UTF, of the format YYYY-MM-DD HH:MM:SS. This option is not available for ASCII format output.
MET
Swift Mission Elapsed Time. This is seconds in Terrestrial Time (TT) since 2001 January 01.
MJD
Modified Julian Date
TDB
Barycentric Dynamical Time. Note that the barycentric correction was carried out for the mid-point of the observation.

For more details about time systems, we recommend this excellent tutorial on the XTE web pages.

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Output format

You can either receive your upper limits as an HTML table designed for human reading, or as an ASCII formatted light curve file.

In the latter case, the file will contain the following columns:

If multiple bands were selected the limit/rate columns will be given for each band in turn. A header row (starting with #) will identify the columns.

* In the event that you have selected to show a mixture of upper limits and data points, the limits will be shown first, followed by a blank line then a comment line (beginning with a #) and then the bins. The 1-σ uncertainties are only shown for data points, not upper limits.

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Values

The upper limit calculation is effectively a retrospective analysis, and some datapoints may be found where the lower-limit is not zero, i.e. the source is detected with 3-σ significance, despite not being in the catalogue. This is because when considering only a specific location the threshold for detection is lower than in a blind search in which thousands of locations are tested.

You can choose whether to receive upper limits for every bin, or whether a datapoint comprising a count-rate and 1-σ error should be returned for those cases which constitute a 3-σ detection. For the HTML output if ‘Limits and bins’ is selected then an upper limit is always provided, and the count-rate and 1-σ error bars are produced if the source constitutes a retrospective detection. In the ASCII light curve only the limit or bin will be shown for each datapoint, as described above.

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Upper limit output

The first output is a note of the input position and how it was resolved, or parsed into decimal coordinates. If the resultant position was not covered by any 2SXPS datasets, a note of this is given and the page ends.

Assuming the position requested is within 2SXPS, the upper limit server checks whether the input position corresponds to a source within 2SXPS. If it does a warning of this fact appears and a link to the results page for that source is provided. Upper limits are still calculated, but if the 2SXPS source correponds to the object you are interested in, these will be wrong.

The output differs depending on whether HTML output or ASCII output was selected, so these will be described separately here.

HTML output

This output appears as an HTML table, with one row per dataset selected. If multiple energy bands were requested, then some of the columns within a result row are themselves split into multiple rows: one per energy band.

Each row consists of a column giving the identifier of the dataset from which the limit was produced (which links to the dataset page) and the time range covered by that dataset. Note that most datasets will not have had continuous exposure during this time, especially stacked images. The subsequent columns are self-explanatory and supported by the inline help system. They give the energy band, upper limit and the details (measured counts, background level and PSF correction) from which these were calculated (see above). If the ‘Limits and bins‘ output was requested, extra columns also show whether the source was ‘detected’ in the band/dataset shown (i.e. whether its 3-σ lower-limit was above zero) and if so, also report the count-rate and 1-σ error bars.

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ASCII output

If ASCII output is requested then pre-format, tab separated results are returned, for use in data analysis or plotting packages. The first line begins with a # to indicate comment, and contains the headers for the row. These will consist of the time (at the centre of the dataset), the duration of that dataset and the exposure therein. This is then followed by upper limits in each requested band. If ‘Limits and bins‘ output was selected then for each band, as well as the upper limits 4 columns are given indicating whether the source was ‘detected’ in that band/dataset, and if so, giving the count-rate and error.

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