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The Swift-XRT data products generator - glossary of options

This glossary presents a short summary of the fields on the product creator form. Users with javascript enabled browsers can see this information on the actual generator page; click on the field name and the glossary entry will appear in a box at the top left of your window.

Contents



Bin length

The duration of light curve bins. Specified for WT and PC mode individually

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Binning Method

There are 4 binning methods to choose from:

Time
Fixed-duration bins.
Counts
Fixed counts-per-bin.
Snapshot
One bin per snapshot.
Observation
One bin per observation.

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Build image

Whether to build an image/images of the specified field. This does not affect how long other products will take to build; the processes are independent.

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Build light curve

Whether to build a light curve. This does not affect how long other products will take to build; the processes are independent.

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Build spectrum

Whether to build a spectrum. This does not affect how long other products will take to build; the processes are independent.

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Centroid?

Whether or not the software should try to refine the position from that given in your co-ordinates.Ideally, set this to "yes", because the software requires the best position in the XRT co-ordinate frame. However, for faint sources the centroiding may fail (you will be told no source was found) in which case set centroid to "no" and the coordinates you supplied will be used.

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Coordinates

The coordinates of the object of interest. Free format, but if you use sexagesimal at least arc-minute precision is required.

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Counts per bin

The minimum number of counts a bin must contain to be considered complete.

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Determine position

Whether to determine a position. Both unenhanced and enhanced positions will be attempted. This does not affect how long other products will take to build; the processes are independent.

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Dynamic binning

If this is turned on, the mininum counts per bin entered on the form is valid when the count rate is 1 count s-1. When the count rate changes by the "rate factor" the necessary counts for a bin to be full changes by the "bin factor". For more information see the dynamic binning documentation.

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E-mail

If you supply an e-mail address you will be e-mailed when each of your products has been built, or if the build fails.

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Energy and grade selection

By default light curves use all events in the range 0.3-10 keV and all avaible event grades. The soft and hard bands cover 0.3-1.5 and 1.5-10 keV respectively. You can override these settings if you desire. Note that the soft and hard bands must be fully enclosed by the main light curve energy band.

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Energy range

The energy range, in keV, the main light curve should be built over. Note that this must cover both the soft and hard bands.

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Grade range

Which events should be included in the product, either the default (grades 0-12 in PC mode and 0-2 in WT) or only grade zero (single pixels). Unless you know what this means and why you may want to change it, leave it at "default".

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Hard energy band

The energy range, in keV, for the hard-band light curve. This must be within the range for the complete light curve.

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Hardness ratio bin length

The duration of bins on the hardness ratio. Specified for WT and PC mode individually. This can be tied to the main light curve bin size, but that this is not advisable as the hardness ratio usually contains fewer counts per band that the total curve.

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Image energy bands

A comma-separated list of energy bands, in keV, for which you would like images to be created. The format for each entry is low_en-high_en.

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Inclusion radius

A conservative error in the input position. Any sources detected further from the input position than this are rejected; the brightest source within this radius is assumed to be your object. We recommend setting this to 20" or higher.

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Min Bin length

The minimum duration of a bin before it can be considered full.

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Min WT counts

Bins with fewer than this number of counts in WT mode are not included in the light curve. This is because the XRT sometimes enters WT mode for reasons other than source brightness, such as earth limb contamination. This can result in spurious WT data points with non Gaussian errors which ruin the scaling of the light curve plots and distort fitting.

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Min fractional exposure

You can choose to reject bins with a lower fractional exposure. The default minimum acceptable value is 0, i.e. no bins are rejected..

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Minimum counts per bin

Even with dynamic binning on, the minimum counts per bin cannot fall below this. We recommend that this is at least 15, so that the Gaussian method of calculating the errors is valid.

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Minimum sigma

The minimum significance (source counts/sqrt(bg counts in source area)) a bin must have to be considered complete. Note that this does not apply to the the final bin. That bin will always be plotted if the detection is 3-sigma, otherwise a 3-sigma upper limit will be plotted.

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Name

The name of the object - mainly cosmetic, to be used in the plots. However, if you click the "Find" button next to the name, an attempt is made to fill in the Target ID, start time and position information automatically.

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PSF method

There are 3 ways of getting a PSF fit.

  1. Sum all of the selected data, and produce a single position.
  2. Use only the first snapshot of data to get a position.
  3. Split the data into snapshots and calculate a position for each, then report the weighted mean.

See the documentation for more details.

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Redshift

The redshift to be used for the absorption component in the automatically-fitted spectral model.

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Search radius

If you said "yes" to centroid, the centroiding will take the brightest object within this radius of your input position as the source (needs to be at least 1 arcmin).

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Soft energy band

The energy range, in keV, for the soft-band light curve. This must be within the range for the complete light curve.

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Specify observations

By default a light curve will be built for all available observations. If you prefer, you can specify which are used.

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Start time

What to use as the zeropoint in time. If not specified, the TRIGTIME or TSTART keyword of the earliest select observation will be used. This number forms the zeropoint of the time axis for light curves, and the time relative to which user defined time regions for spectra are determined. See the time format page for a description of accepted date/time formats.

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Statistical error threshold

If you choose the "multiple snapshots" PSF fit method you can force the centroid to finish once the position has a statistic error lower than this value (default: 0").

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Target ID

The 8-digit target ID (e.g. 00037062) of the object you are interested in. This is the ObsID, minus the last 3 digits. If your object has multiple target IDs which you wish to use, list them separated by commas.

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Time for spectrum

By default a single spectrum of all selected data will be built. If you select "User defined" you can enter up to 4 time intervals. Each interval is a comma separated start-stop list. The times can be any valid time format, seconds since your Start Time or an obsID. One spectrum can cover multiple time sections; separate the selection with commas, e.g. "100-350, 2009/07/25-2009/08/12T12:32:45" .

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Use redshift

If you supply a redshift the spectrum will be automatically fitted with two absorbers and a power-law. One absorber will be frozen at the Galactic NH, the other will have NH free and a redshift, fixed at the value you supplied. If you say "No" here there will be a single, unconstrained, absorption component at z=0.

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Which observations

You can enter either a list of obsODs, or a list of start-stop times; only observations matching these criteria will be included in the light curve.

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