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XRT position for GRB 081008 enhanced by UVOT field astrometry

Related pages: Burst Analyser | Light curve | Spectrum | Download obs data | SPER analysis

RA (J2000):	18 39 49.87
Dec (J2000):	-57 25 54.8
90% Error radius:	1.4"
Number of overlaps included in position:	8
Exposure time in final position:	8661 s
Total number of overlaps considered:	10
Total overlap time:	9153 s
Number of overlaps where no X-ray source was found:	1
Number of low-X-ray-count fits rejected:	0
Number of spurious fits rejected:	0
Number of bad astrometric corrections:	0
Number of outliers removed:	1

For details of how these positions are produced, and why some positions are rejected, see the online documentation.

All enhanced positions

GCN E-mail

The following paragraph should be used to announce this position in a GCN circular:

Using 8661 s of XRT Photon Counting mode data and 8 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching to the USNO-B1 catalogue): RA, Dec = 279.95779, -57.43188 which is equivalent to:

RA (J2000): 18 39 49.87
Dec (J2000): -57 25 54.8

with an uncertainty of 1.4 arcsec (90% confidence).

This position may be improved as more data are received. The latest position can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401 http://www.swift.ac.uk/xrt_positions/Goad.pdf), the current algorithm is an extension of this method.

Final Position

The image below shows the UVOT field with the final, astrometrically corrected XRT refined position. The image is 50" to a side, and contains 542 s of data in the WHITE filter.

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Individual Frame Positions

The final position (magenta) is the weighted mean of 8 positions (green). A further 1 positions were rejected, either because they had too few photons to be secure (red) or because they are more than 3 sigma away from the mean position, more than 40" from the early mean, or had a problematic astrometric correction (yellow). The image is 50" to a side.