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About the automatic XRT spectra

Contents

Overview

This web site provides a repository of Swift/XRT spectra for every GRB this instrument has detected. By default, each GRB will have one spectrum for each of the two instrument modes (Windowed Timing and Photon Counting), provided data in that mode were obtained for the burst.

The interface is similar to that of the light curve repository. To view an object's spectra you can enter the BAT trigger number or the GRB name in the search box and click "Submit". Alternatively, move your mouse over the year and click on the month in the drop-down menu that appears (requires Javascript). Thumbnail images of all GRB light curves available for that month will appear, click on one of these to view the full light curve. There is also a box to the left of the screen showing thumbnail images of the most recently observed objects. These serve as links to the full results page for those bursts.

Once you have selected an object, you will be taken to the products page containing the spectra for that burst. If no spectrum is available the products page should explain why.

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Usage Policy

Users may use any of the data in, or created from, the spectrum repository in their research and publications, provided that the repository is cited. We request this be done both by citing Evans et al. (2009, MNRAS, 2009, 397, 1177) when the data are introduced, and including the following text in the acknowledgements section of the paper (not necessary for ATELs or circulars): This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

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Products Pages

The automatic spectra are presented via a standard-format results page. This should be relatively self-explanatory, and we provide more detail about the features of these pages here.

Underneath the title line the last sequence of data used to create the spectrum is noted, this allows you to determine whether the spectrum is up-to-date or not1. There follow links to other products for this GRB, to this documentation, and a link inviting you to build time-sliced spectra. Next come the actual spectra.

For each spectrum, a title line gives the spectrum name, and the time range covered by the spectrum. Then follows a link to download the spectral data. The download is of a gzipped tar file, containing the following files:

The response matrix is not provided as it is a large file, and is already available from the CALDB. The RESPFILE keyword of the Xspec-ready spectrum expects this file to be in the same directory as the spectrum, so you may wish to modify this.

After the tar file there is a series of links to postscript and gif mode plots for each spectrum, and then a single plot is shown. If WT and PC mode spectra were available, a single plot showing both spectra and their (independent) models and residuals is shown, otherwise the only available mode is shown.

Below the plot we provide the automatic fit results for each mode. The header for this states the mode, and also the "Mean photon arrival" time for that spectrum. This is literally the mean time of all the events in the spectrum. Note that this value may be surprising, as if the GRB is piled up for part of the interval of interest, the annular source region used at these times will reduce the number of photons arriving in this interval, biasing the spectra to later, fainter times: the opposite of what one would naiively expect. It should also be noted that, if the spectra evolve during the time interval, the best fit will be misleading, being an average of a varying spectrum. However, the mean photon arrival time will give some indication as to which times dominate the spectrum.

The spectral values themselves are given in the tables, with the 90% confidence errors (i.e. the maximum range a parameter can cover before the fit statistic increases by 2.706 compared to its best-fit value). It is possible that the automatic fit may have found a local minimum and we thus recommend users examine the plot and residuals and, if in any doubt about the automatic fit results, to download the tar file and fit the spectra themselves (see caveat below, for more information).

Occasionally, a member of the Swift-XRT team may ask the automatic software to produce spectra covering different time intervals to the default one (e.g. to exclude any flares). If this is the case, results for those spectra will appear after the time-averaged spectrum.

1 Note that, for the default 'time-averaged' spectrum only ObsIDs which begin within 12 hours of the first ObsID are included. This is because, for later ObsIDs a typical GRB is too faint to contribute meaningful data to the spectrum. Thus, once data later than this arrive the spectra will not be recreated, and the "last updated" line will not change.

2 This is the name of the spectrum. For time-averaged spectra the name is "interval0". For additional spectra, the name is set by the XRT team member who requests them.

3 i.e. the filename will either contain "pc" or "wt", corresponding to the XRT mode of the spectrum. If both modes were available, there will be 2 files.

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Creating time-sliced spectra

You may wish to define time ranges over which spectra should be created, rather than just use the time-average one. The link at the top of the results page allows you to do this. Following this link leads to a page with two options: either fill in a form or upload a file to specify the times of interest. The GRB light curve is shown at the bottom of this page, to help in choosing the time ranges.

Note: Time-sliced spectra are not updated when new data are received.

Specifying time slices via the form

The form allows you to specify up to 4 time intervals. For each interval you must supply a name and time information. The name is the label under which the spectrum will appear on the results page, and the stem of the files produced (i.e. replacing "interval0" in the file list above).

The times should be be specified in the form tstart-tstop (no spaces), or for multiple time ranges in one spectrum, start1-stop1,start2-stop2 etc. Times before 108 s are assumed to be relative to T0 of the burst (as shown in the light curve), times after 108 s are assumed to be in Swift MET. This is evaluated for each number, so 100-1.987E8 is a valid input.
You can also select entire ObsIDs, by preceding tstart with "OBS" e.g. OBS00282445001-00282445003, in this case both tstart and tstop are treated as ObsIDs.
You can specify multiple time ranges for a single spectrum (for example, to avoid flares), e.g. 100-500,800-1200, or 100-300,OBS00282445001-00282445003 and so on.

By file upload

If you prefer, you can upload a file listing the spectra you wish to have built. The file should contain one line per spectrum, with the following format for each line: 

spec_name   tstart-tstop ! mode

Fields are separated by the regular expression /\s+/, i.e. any whitespace. The fields are:

spec_name
A label by which the spectrum will be known. This must contain no whitespace, and only alphanumeric characters. If you are sharing these spectra, please choose a descriptive name like, "2nd_Orbit", or "flare".
tstart-tstop
The time range the spectrum should cover. Times before 108 s are assumed to be relative to T0 of the burst (as shown in the light curve), times after 108 s are assumed to be in Swift MET. This is evaluated for each number, so 100-1.987E8 is a valid input.
You can also select entire ObsIDs, by preceding tstart with "OBS" e.g. OBS00282445001-00282445003, in this case both tstart and tstop are treated as ObsIDs.
You can specify multiple time ranges for a single spectrum (for example, to avoid flares), e.g. 100-500,800-1200, or 100-300,OBS00282445001-00282445003 and so on.
Mode (optional)
The XRT mode for which you wish to generate spectra. This must be PC, WT or BOTH, anything else will raise an error. If no mode is specified do not include the ! on the line. The mode will be treated as BOTH (if data from only one mode are available during this time only that spectrum will be built. This will not raise any errors).

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Caveats

  1. Redshifts — These are not automatically parsed from GCN Circulars, but are manually input by members of the XRT team, thus there may be some delay between a redshift being provided and the fit recalculated. For uniformity, only spectroscopic redshifts reported in GCN Circulars or papers will be used.
  2. Automatic fitting — The Swift-XRT team have conducted extensive testing and have verified that the automatic spectra are reliable, accurate and of publication quality. However, as any X-ray astronomer will know, when fitting spectra there is always a danger that one will find a local minimum of the fit statistic rather than the true best-fitting parameters. While we have taken steps to reduce the chances of this, it may still happen from time to time. Users are thus advised to examine the spectral plots and determine for themselves whether they believe the best-fitting values. Since we do provide the data for download, users can easily perform fitting themselves if they so choose.
    Note also that, if the model is incorrect (e.g. there are other emission components present), the fit parameters would be wrong.
    If a Swift-XRT team member determines that an automatic fit did not find the true minimum, they will upload the best-fitting parameters and these will replace the automatically determined ones.
  3. Non-standard spectra — As noted above, occasionally XRT team members may ask the software to produce extra spectra, in addition to the time-average one. These spectra are not updated as more data are received from the spacecraft.

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Automation

For bursts which trigger the Swift-BAT the generation of spectra is fully automated. For other GRBs observed by Swift, an XRT team member must upload details of the burst to register it for automatic processing. Thus, for non-BAT triggered bursts, there may be a short delay between the observations and the product generation.

For either BAT-triggered bursts, or those manually registered, the XRT data are first reprocessed locally with the latest version of the XRT software (currently HEADAS 6.4.1) and then spectra are built. Both of these tasks are performed by CRON jobs running every 5 minutes, thus typically spectra should be available within 5-10 minutes of data being received. As Swift continues to observe the GRB the spectrum will be automatically updated after each data downlink1.

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Change log

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