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Swift-XRT GRB positions — Detailed Overview


Contents

Introduction

GRBs positions can be determined from three types of Swift XRT data. In the order that they become available after a trigger, these are: image mode, prompt-ground and the full ground dataset. Positions from the latter two types can sometimes be improved by UVOT-enhancement or X-ray astrometry (full-ground data only). An overview of these data types, positions and improvements is available on the introduction page, here we describe them in more detail. The data types will be reviewed in the chronological order in which they are available, then the improvements will be explained.

The absolute sky co-ordinates of standard XRT data are subject to a systematic error of 3.5′′ (90% confidence), arising from the spacecraft's star trackers. This error is not included when the UVOT enhancement or X-ray astrometry processed are applied. These methods calculate the spacecraft pointing without reference to the star trackers.

NOTE In the text below, the "initial circular" refers to the GCN circular produced by the Swift team shortly after the trigger, announcing the detection of the burst.

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General Caveats

The onboard and prompt-ground positions assume that the brightest object in the BAT error circle is the GRB: this is generally true. However, if there is a bright serendipitous source within the BAT error circle it may initially be mistaken for the GRB. This is very rare, and when it happens the Swift team endeavour to issue a correction at the earliest opportunity. If a prompt position was determined, the full ground position will be taken as the source closest to this position. If no prompt position was determined, the full ground position reverts, like the other positions, to the brightest object in the BAT error circle. Under such circumstances, the enhanced position is not automatically disseminated as a GCN circular.

If the GRB lies close to the bad columns, the position determined is less reliable than normal, unless the centroiding algorithm is aware of the bad columns. The PSF fitting used with the prompt-ground data and by the UVOT-enhancement process does account for the bad columns, however the barycentric fit used in other positions does not.

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Data types

Image mode

When Swift slews to a new GRB the XRT takes a 0.1 s exposure in image mode, and the onboard software searches this image for a source. If successful, it performs a barycentric centroid, and then telemeters the image and the position to the ground. A GCN XRT position Notice is automatically produced. Should the onboard software be unable to find a source in the initial image, the process is repeated on up to two 2.5-s images. If it still fails, a Nack-Position GCN notice is sent. For full details of the onboard centroiding process, see Hill et al. 2003.

The images are also telemetered to the ground. If no position was found automatically onboard, XRT Burst Support scientist (XBS) will combine these images and search for a source. If one is found a barycentric centroid is carried out. If no prompt-ground data (see below) are received, this position will be incorporated into the initial circular and a GCN position notice.

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Prompt Ground

If the XRT enters Photon Counting (PC) mode during its first snapshot of the new GRB, any single pixel (grade 0) events above 0.5 keV that occur within the central 200x200 pixel region of the XRT are telemetered periodically to the ground as SPER data (which usually contains many more photons than the short images). Software at the UKSSDC automatically combines these data and searches the resultant image for sources. PSF-fitting, weighted by the SPER data's exposure map, issued to localise the source. If possible, this positions is also enhanced.

The most recent prompt-ground position will always be available from the SPER webpages, and the initial circular will include the best position (enhanced if possible) available when the circular is submitted. Should more prompt data be received after this, the web page will be updated. If no onboard position was found, an XRT Position GCN Notice will be produced as soon as a prompt-ground position is found. After this, or if an onboard position was found, an XRT Position UPDATE GCN Notice is automatically produced either when an enhanced prompt-ground position is found. If no enhanced position is found, an update notice may still be produced once all prompt data have been received, if the position is notably better than that in the original position notice.

Occasionally the initial circular may be produced with only an onboard position (for example, if the source is so bright that the XRT tays in WT mode for a long time). In such cases, should the prompt-ground data become available after the circular is submitted, and an enhanced position be determined, an automatic circular will be produced to announce this position.

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Full Ground

In the minutes post-trigger only limited data products are telemetered to the ground. The full dataset is downlinked to the Malindi groundstation, passes occur every few hours. Once the data are on the ground they are automatically reprocessed at the UKSSDC using the latest release of the Swift software. Then an image and exposure map are created, and an exposure-map-weighted PSF fit is performed to localise the source. This localisation occurs in the XRT coordinate system (i.e. with an absolute systematic error of 3.5′′). This result is pushed to the XRT GRB positions website, but is rarely used more widely since, in around 90% of cases, the full ground dataset can be enhanced, and the position thus produced is better and is disseminated instead. If no enhanced position is produced, the unenhanced position will be reported in the refined analysis circular.

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Position improvements

UVOT-enhancement

If both XRT and UVOT data covering approximately the same time interval are available, the UVOT data can be used to astrometrically correct the X-ray position. Stars in the UVOT field of view are matched with the USNO-B1 catalogue, allowing the UVOT pointing to be calculated more accurately than is done onboard. Using the known mapping between the XRT and UVOT, the XRT pointing can thus be enhanced, correcting the position and reducing the systematic error to 1.4′′ (90% confidence).

This correction can be calculated for each UVOT exposure (in the White, V and B filters), provided PC mode X-ray data are taken which overlap in time with the UVOT exposure. If no such overlaps exists, or if the X-ray source cannot be found in the XRT data, no enhanced position is produced. The exact implementation of the method depends on whether it is applied to prompt ground or ground data. The prompt ground and enhanced ground documentation pages give full details.

Enhanced prompt-ground positions are distributed as described in the prompt ground positions section above. When an enhanced ground position is first produced for a GRB, a GCN Circular is automatically dispatched, to advise the community.

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X-ray astrometry

If serendipitous X-ray sources are found in the X-ray field of view, these can be compared with ground catalogues for optical counterparts. These matches can then be used to astrometrically correct the X-ray position and reduce the systematic error. These positions are occasionally manually produced by the XRT team, and reported in the refined circular, or in a special circular. These positions are also automatically produced by Nat Butler, and are available on his website.

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