UVOT Standard Products

This page documents the UVOT standard products that are created for each new Swift gamma-ray burst (GRB). The products are produced automatically whenever more data from Segment 0 for the GRB are processed at the SDC. The products are not made if there is no accurate position for the afterglow. After all the data for Segment 0 have been processed, the results are checked by a UVOT scientist and, if correct, sent to the GCN, and posted on the Swift Burst Ground Analysis page.

UVOT & DSS Thumbnails

The standard products include two small images (thumbnails) centered at the position of the GRB afterglow. One is taken from the first UVOT finding chart, which is typically a 150-second exposure with the white filter. If only a BAT position is available, then the image is 6' on a side. Otherwise the image is 2' on a side. The error circle for BAT is shown as a green circle, and the XRT error circle is shown in blue. If available, UVOT positions and ground-based optical positions are indicated with red and black reticles respectively. The image is labelled with the filter used, the date of the GRB, the trigger number, and the start time of the exposure in both UTC and the delay from the burst trigger. The other image is produced from the Digital Sky Survey (DSS), which uses the same approach for displaying the BAT, XRT, UVOT, and ground-based optical positions. An example of the UVOT image is shown below.

A light curve is produced for each of the visible (White, V, B, and U) and UV (UVW1, UVM2, and UVW2) filters. The light curves for all the visible filters are shown on one plot, those for the UV filters shown on a second plot, and all the light curves is shown on a third plot. The plot with all the light curves is often difficult to read. The curves plot magnitude vs the log of the time since the GRB trigger. A separate data point is produced for each UVOT exposure, but adjacent points are often combined if the individual points are not significant detections.

The light curves are produced using the FTOOL uvotproduct. The source region is centered at the best known position for the GRB. A UVOT position is preferred, followed by a ground-based optical position, and finally the XRT position. If only a BAT position is available, no light curves are produced. The background region is an annulus centered at the source position with 6-arc second holes around detected sources removed. The magnitudes have all the usual corrections used in UVOT photometry (Poole et al. 2008). The plotted points show 1-σ error bars or 2-σ upper limits.

There are several situations in which the automated process will produce invalid results. An example is a source unrelated to the afterglow contributing counts to the source region. Another is a very bright source in or near the background region.

The light curves for the visible filters for GRB 091020 are shown below.

The qdp files used to produce the three plots of light curves are saved and made available. The files can be downloaded, modified, and used to produce new plots and fits to the light curves. The data points used to make the light curves are available as either a text file or a FITS file.