Skip banner and navigation tools.

 |  site map

About the automatic XRT spectra

Back to spectrum repository.



This web site provides a repository of Swift-XRT spectra for every GRB this instrument has detected. By default, each GRB will have one or two spectra for each of the two instrument modes (Windowed Timing and Photon Counting), provided data in that mode were obtained for the burst. There are four ways provided of selecting a GRB to view: a page of thumbnail images for each GRB, a search box (where you can enter a GRB number, or BAT trigger number), a year/month menu and a panel showing the 5 most recently observed GRBs. Each of these links to the products page for that burst.

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.

The process of creation the spectra is described in full in Evans et al. (2009, MNRAS, 397, 1177). Most of the questions we are commonly asked about the spectracan be answered by reading this. An overview of the algorithm is also given online.

Back to contents | Back to repository

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.

Back to contents | Back to repository

Products Pages

The products page begins a title, and then indicates which dataset triggered the most recent update of the spectrum. This is followed by a series of links to any other products which exist for this burst, a link to this page, and a link to create timesliced spectra. If more than one spectrum exists for this object, links to each spectrum are given, and this is followed by the spectra.

For every GRB detected by the XRT there is at least one spectrum: a time-averaged spectrum1. If Swift began observing the GRB promptly, and has also observed the GRB more than 4 ks after the trigger, a "late-time" spectrum will also exist: this contains the same data as the time-averaged spectrum, except that the data from the first Swift snapshot are excluded. This is because spectral evolution is often present at early times, but is rarely seen in the late-time data. In exceptional circumstances additional spectra, requested by the Swift XRT Burst Specialist, may also be present.

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:

(For the late-time spectra, interval0 is replaced with late-time, likewise any additional spectra will have the interval0 replaced with some other text).

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, and is not always intuitive: 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 naïvely 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-fit 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).

Spectra are fitted in xspec using the command statistic cstat, however since the background spectrum is included, xspec automatically selects the w-statistic (see The XSPEC statistics manual under For Poisson data with Poisson background (cstat)).

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.

Back to contents | Back to repository

Multi-source Web Pages

For most GRBs it is immediately obvious from the XRT data which X-ray object is the afterglow. However, in some cases this is not true. If the afterglow is faint (either intrinsically, or because of a delay in observing it with the XRT) it may be difficult to tell whether it is fading, for example. Also, for some GRBs with large error circles (e.g. Fermi-LAT bursts) Swift may have to perform a tiled series of observations to cover the error circle. It is inevitable in such cases that a number of serendipitous sources will be found.

In cases with more than one candidate afterglow, the products page will show thumbnail spectra of each candidate (which serve as links to the standard products page for that afterglow), and then an image of the field, with each of the sources marked on it (for fields which are part of a tiled campaign, the sources in the image will contain a letter which refers to the field, e.g. "A1"). Should one of the sources be confirmed as the afterglow by the XRT Team, the main products page for this GRB will revert to being the spectrum page for that candidate.

Note that, for sources which are subject to a tiled campaign, the image shown on this page contains only data for the field with the target ID of that URL. To see an image of the entire tiled dataset, you will need to visit the results page for that tiled campaign. This will be linked to from the light curve page.

Back to contents | Back to repository

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:

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".
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).

Back to contents | Back to repository


  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.

Back to contents | Back to repository


When a GRB is detected by the Swift-BAT, or a ToO for a GRB detected by another mission is uploaded, the UKSSDC servers automatically register the object for analysis. The arrival of data on our quick-look site automatically triggers the creation of products including the light curve. First, however, the XRT data are reprocessed locally with the most recent version of the Swift software. (There will be a short delay between a new software release and its incorporation into our system. Note also that we will not as standard rebuild all historical data products after each software release).

Back to contents | Back to repository

Change log

Back to contents | Back to repository