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Home > Data Analysis > Build Swift-XRT products > Glossary
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.
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.
- Sum all of the selected data, and produce a
single position.
- Use only the first snapshot of data to get a
position.
- 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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