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Swift-XRT GRB positions — 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 ground. Positions from the latter two types can sometimes be improved by UVOT-enhancement or (ground data only) X-ray astrometry. This page briefly introduces the data types, positions and improvements. More information is available on the detailed overview page.

The onboard and prompt ground positions are produced in the minutes following a trigger. They are determined from promptly available data products. Ground positions are produced several hours post-trigger, once the full dataset has been telemetered from the spacecraft and processed on the ground.

Data types

Image mode

When Swift slews to a new GRB it takes up to three 0.1 or 2.5 sec X-ray images, and the onboard computer searches these for an X-ray source. (see Hill et al. 2003.) If it finds one, the position of this is automatically downlinked, and distributed as a GCN notice. These images are also telemetered to the ground, so if the onboard algorithm failed to find a source, the XRT team can combine the images into a single 5.1 s exposure. If they find a source they will report this via the GCN Notice system, and in the initial GCN circular.

[More information about image mode positions.]

Prompt Ground

When the XRT enters Photon Counting (PC) mode, single-pixel events are telemetered to the ground. The XRT team search these for a source and, if successful, they report the position in the initial GCN circular which announces the burst, and as a GCN Position (update) notice.

[More information about prompt ground positions]


Once the full dataset from the first few orbits has been downlinked to the ground, all available PC data are used to find the position. Historically this position was found using the xrtcentroid tool, and reported as the "refined" position. Instead, we now generate this position by using a PSF fit, and the exposure map; this corrects for the effects of bad columns and photon pile-up. This position, however, is rarely reported as it is superseded by the Enhanced Position, described next.

[More information about ground positions]

Position improvements

Position Enhancement

By matching field sources detected by the UVOT to the USNO-B1 catalogue, the spacecraft pointing can be determined to greater accuracy than is done otherwise. This allows X-ray positions to be produced with greater accuracy and precision. This can be achieved for prompt-ground and ground data.

[More information about UVOT enhancement]

X-ray astrometry

If serendipitous X-ray sources are found in the field of view and can be associated with optical catalogued objects, the spacecraft pointing can be determined to greater accuracy. This allows X-ray positions to be produced with greater accuracy and precision. This can be only be achieved for ground data.

[More information about X-ray astrometry]