Once the full dataset gathered by Swift has been telemetered the ground, we redetermine the source position. We create two types of position:
The XRT Positions index by default shows the enhanced position where available, and the unenhanced position otherwise. Using the controls above the table you can choose to instead view only the (un)enhanced positions.
When quoting enhanced XRT positions of GRBs (even if those positions were given in a GCN Circular) please cite Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 2009, 397, 1177). If you use these positions in a paper, please include the following text in the acknowledgements section: This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.
The basic principle is that the GRB is located within the XRT's field of view, its position is transformed to the corresponding position on UVOT and then the UVOT field of view is matched to the USNO-B1 catalogue to provide the correct 'enhanced' position. Since the exact direction in which the spacecraft is pointing will change from one snapshot to the next this process has to be performed for each snapshot separately. Further, the UVOT observes through various filters with different boresights, so the process must be applied separately to the different filters. The data are thus split into 'overlaps' (where XRT and UVOT data are both obtained), a position calculated for each overlap, and then these positions combined.
The first step is thus to split the observations into overlaps. We take any observation which began within 12 hours of the first observation, and create one overlap for each valid UVOT exposure which is at least partly covered by the XRT operating in Photon Counting (PC mode). We take as much XRT data as possible without combining snapshots or extending into another valid UVOT exposure. Fig. 1 (below) demonstrates this. A 'valid' UVOT exposure is one taken through the V, blue or white filters. We do not use the UV filters since there are fewer stars and the astrometric corrections can become unreliable.
Fig 1. An example of how overlaps are defined. In the first
snapshot, there are two overlaps. The first XRT exposure starts with the
snapshot, and ends midway between the UVOT exposures. The second XRT
exposure starts immediately, and continues to the end of the snapshot.
For the second snapshot there is only one valid UVOT image, so XRT data are taken from the entire snapshot.
Once the overlaps have been defined, the algorithm can be summarised thus:
For each overlap, when the UVOT field stars are matched with the USNO-B1 catalogue, a residual error is returned. This is added in quadrature to the error in the PSF fit, to give the error in the overlap. These errors are then propagated through the weighted mean calculation to give an error on the final position (90% confidence). A systematic error of 1.5" is then also added in quadrature; this arises primarily from the uncertainty in the XRT to UVOT transformation.
Warning: Very occasionally there may be a serendipitous source within the BAT GRB error circle which is brighter than the GRB even in the first images. In such cases the enhanced position will likely be calculated for the wrong source. As soon as this is known, the process will be manually overridden and the position recalculated for the real afterglow.
The index page shows, by default, the XRT position for every GRB. Where an enhanced position is available this is shown (on a green background). Otherwise the unenhanced (unenhanced) position is shown (on a yellow background; the final column in the table, labelled, "Source" also indicates whether the source is enhanced or not). If for some reason no position has been determined, the table entry has a red background. Sometimes the words, "No PC/UVOT overlaps" also appear. This means that there were no usable XRT and UVOT data which were contemporaneous.
For each burst for which a UVOT-enhanced position was found, a detailed results page is produced. This can be accessed either directly, via the URL: http://www.swift.ac.uk/xrt_positions/xxxxxxxx where xxxxxxxx is the target ID of the burst (i.e. the trigger number padded to 8 digits with leading zeroes), or by clicking on the burst name on the main results page.
The first table on this page reports the enhanced position and error, and also indicates on how many overlaps and how much exposure time this position is based. The second table then reports how many overlaps there were in total, and the overall exposure time, before detailing how many overlaps were discarded and why. The reasons for discarding overlaps are summarised below. See also the algorithm section, above.
Finally, there are two images. The first shows a summed, aspect corrected UVOT image centred on the position of the burst, with the enhanced XRT position and error drawn in magenta. The second image has a larger field of view, and shows the positions returned by each overlap. Any which contributed to the final enhanced position are shown in green, while those rejected are shown in red if they contained fewer than 10 photons, and yellow for any other reason.
For each burst with an unenhanced position, there is a web page giving details of the position. This can be accessed either directly, via the URL: http://www.swift.ac.uk/xrt_unenh_positions/xxxxxxxx where xxxxxxxx is the target ID of the burst (i.e. the trigger number padded to 8 digits with leading zeroes), or by clicking on the burst name on the main results page, provided the position is listed as, "Unenhanced". (To view only unenhanced positions on the main results page, select the relevant option from the "Show which positions?" drop-down menu above the results table).
The page thus accessed shows first the position produced, and then reports which of the PSF profiles gave the best fit. In most cases, this will be, "Non piled-up" otherwise it will be something like "2.6 count sec-1". This means that the PSF profile used was determined from a source with a raw (i.e. not corrected for pile-up) count-rate of 2.6 count sec-1.
In order to confirm the accuracy of the enhanced positions and their errors, we compared the positions produced with the positions of Swift-detected optical GRB counterparts. For 59 bursts with both UVOT and enhanced-XRT positions, we calculated the angular offset between the two positions, and divided it by the uncertainties in the positions (added in quadrature). If this ratio is <=1, the two positions agree. We found that an extra systematic error of 0.4" (in quadrature), was necessary to achieve 90% agreement with the UVOT, thus the overall systematic error is 1.36" The plot below shows the cumulative proportion of GRBs plotted against this ratio; as can be seen, 90% have a ratio <=1.
There was a small possibility that, during the gyro anomaly of 2007 August, the XRT to UVOT map may have shifted slightly. However, using calibration and GRB observations, taken since the return to normal operations, we have confirmed that this was not the case, and the positions and their accuracy are still valid.
GRB 080319C demonstrates the improvement for bright XRT sources yielded by using piled-up PSF profiles where appropriate. Note that this is an extreme case. The images below show a UVOT image of GRB 080319C -- the optical afterglow is obvious. The magenta circle shows the enhanced XRT position. In the left panel (version 2.0 of the software, no pile-up awareness) the XRT position is clearly offset. In the right panel (version 2.1, with pile-up awareness), the position agrees with the UVOT.